Accepted Papers

Abstract: It is usually essential to have a common reference frame between two communication parties to perform quantum communication. Notably,  Reference-Frame-Independent Quantum Key Distribution (RFI-QKD) provides a practical way to generate secret keys between two remote parties without sharing standard reference frames. Here, we have shown that the RFI-QKD protocol can be expanded into a multiparty system with Greenberger-Horne-Zeilinger (GHZ) entangled state. We derive the asymptotic key rate and perform the proof-of-principle experiment to verify the proposed multiparty protocols feasibility. Considering that sharing a common reference frame becomes more difficult as the number of parties increases, our protocol provides a new path to implement multipartite quantum communication in real world.

Abstract: Quantum key distribution (QKD) based on the fundamental laws of quantum physics can allow the distribution of secure keys between distant users. However, the imperfections in realistic devices may lead to potential security risks, which must be accurately characterized and considered in practical security analysis. High-speed optical modulators, being as one of the core components of practical QKD systems, can be used to prepare the required quantum states. Here, we find that optical modulators based on LiNbO3, including phase modulators and intensity modulators, are vulnerable to photorefractive effect caused by external light injection. By changing the power of external light, eavesdroppers can control the intensities of the prepared states, posing a potential threat to the security of QKD. We have experimentally demonstrated the influence of light injection on LiNbO3-based optical modulators and analyzed the security risks caused by the potential green light injection attack, along with the corresponding countermeasures.

Abstract: Verifying the quality of a random number generator involves performing computationally intensive statistical tests on large data sets commonly in the range of gigabytes. Limitations on computing power can restrict an end-user's ability to perform such verification. There are also applications where the user needs to publicly demonstrate that the random bits they are using pass the statistical tests without the bits being revealed. We report the implementation of an entanglement-based protocol that allows a third party to publicly perform statistical tests without compromising the privacy of the random bits.

Abstract: Continuous Variable (CV) quantum key distribution (QKD) is a promising candidate for practical implementations due to its compatibility with the existing communication technology. A trusted device scenario assuming that an adversary has no access to imperfections in the detector is expected to provide significant improvement in the key rate, but such an endeavor so far was made separately for specific protocols and for specific proof techniques. Here, we develop a simple and general treatment that can incorporate the effects of Gaussian trusted noises for any protocol that uses homodyne/heterodyne measurements. In our method, a rescaling of the outcome of a noisy homodyne/heterodyne detector renders it equivalent to the outcome of a noiseless detector with a tiny additional loss, thanks to a noise-loss equivalence well-known in quantum optics. Since this method is independent of protocols and security proofs, it is applicable to Gaussian-modulation and discrete-modulation protocols and to any proof techniques developed so far and yet to be discovered as well.

Abstract: This paper presents a new method for quantum identity authentication (QIA) protocols. The logic of classical zero-knowledge proofs (ZKPs) due to Schnorr is applied in quantum circuits and algorithms. This novel approach gives an exact way with which a prover $P$ can prove they know some secret by encapsulating it in a quantum state before sending to a verifier $V$ by means of a quantum channel - allowing for a ZKP wherein an eavesdropper or manipulation can be detected with a fail-safe design. This is achieved by moving away from the hardness of the Discrete Logarithm Problem towards the hardness of estimating quantum states. This paper presents a method with which this can be achieved and some bounds for the security of the protocol provided. With the anticipated advent of a `quantum internet', such protocols and ideas may soon have utility and execution in the real world.

Abstract: The bounded quantum storage model aims to achieve security against computationally unbounded adversaries that are restricted only with respect to their quantum memories. In this work, we provide everlasting and information-theoretic secure constructions in this model for the following powerful primitives: (1) CCA1-secure symmetric key encryption, message-authentication, and one-time programs. These schemes require no quantum memory for the honest user, while they can be made secure against adversaries with arbitrarily large memories by increasing the transmission length sufficiently. (2) CCA1-secure asymmetric key encryption, encryption tokens, signatures, and signature tokens. These schemes are secure against adversaries with roughly $e^{\sqrt{m}}$ quantum memory where $m$ is the quantum memory required for the honest user. All of the constructions additionally satisfy notions of disappearing and unclonable security.

Abstract: Trapdoor claw-free functions (TCFs) are immensely valuable in cryptographic interactions between a classical client and a quantum server. Typically, a protocol has the quantum server prepare a superposition of two-bit strings of a claw and then measure it using Pauli-X or Z measurements. In this paper, we demonstrate a new technique that uses the entire range of qubit measurements from the XY-plane. We show the advantage of this approach in two applications. First, building on (Brakerski et al. 2018, Kalai et al. 2022), we show an optimized two-round proof of quantumness whose security can be expressed directly in terms of the hardness of the LWE (learning with errors) problem. Second, we construct a one-round protocol for blind remote preparation of an arbitrary state on the XY-plane up to a Pauli-Z correction.

Abstract: In the permutation inversion problem, the task is to find the preimage of some challenge value, given oracle access to the permutation. This is a fundamental problem in query complexity, and appears in many contexts, particularly cryptography. In this work, we examine the setting in which the oracle allows for quantum queries to both the forward and the inverse direction of the permutation—except that the challenge value cannot be submitted to the latter. Within that setting, we consider two options for the inversion algorithm: whether it can get quantum advice about the permutation, and whether it must produce the entire preimage (search) or only the first bit (decision). We prove several theorems connecting the hardness of the resulting variations of the inversion problem, and establish lower bounds for them. Our results indicate that, perhaps surprisingly, the inversion problem does not become significantly easier when the adversary is granted oracle access to the inverse, provided it cannot query the challenge itself.

Abstract: Device-independent (DI) protocols have experienced significant progress in recent years, with a series of demonstrations of DI randomness generation or expansion, as well as DI quantum key distribution. However, existing security proofs for those demonstrations rely on a typical assumption in DI cryptography, that the devices do not leak any unwanted information to each other or to an adversary. This assumption may be difficult to perfectly enforce in practice. While there exist other DI security proofs that account for a constrained amount of such leakage, the techniques used are somewhat unsuited for analyzing the recent DI protocol demonstrations. In this work, we address this issue by studying a constrained leakage model suited for this purpose, which should also be relevant for future similar experiments. Our proof structure is compatible with recent proof techniques for flexibly analyzing a wide range of DI protocol implementations. With our approach, we compute some estimates of the effects of leakage on the keyrates of those protocols, hence providing a clearer understanding of the amount of leakage that can be allowed while still obtaining positive keyrates. Our results and techniques should also be relevant in proving security of device-dependent QKD against constrained leakage.

Abstract: Much of the strength of quantum cryptography may be attributed to the no-cloning property of quantum information. We construct three new cryptographic primitives whose security is based on uncloneability, and that have in common that their security can be established via a novel monogamy-of-entanglement (MoE) property: -- We define interactive uncloneable encryption, a version of the uncloneable encryption defined by Broadbent and Lord [TQC 2020] where the receiver must partake in an interaction with the sender in order to decrypt the ciphertext. We provide a one-round construction that is secure in the information-theoretic setting, in the sense that no other receiver may learn the message even if she eavesdrops on all the interactions. -- We provide a way to make a bit string commitment scheme uncloneable. The scheme is augmented with a check step chronologically in between the commit and open steps, where an honest sender verifies that the commitment may not be opened by an eavesdropper, even if the receiver is malicious. Our construction preserves the assumptions of the original commitment while requiring only a polynomial decrease in the length of the committed string. -- We construct a receiver-independent quantum key distribution (QKD) scheme, which strengthens the notion of one-sided device independent QKD of Tomamichel, Fehr, Kaniewski, and Wehner (TFKW) [NJP 2013] by also permitting the receiver's classical device to be untrusted. Explicitly, the sender remains fully trusted while only the receiver's communication is trusted. We provide a construction that achieves the same asymptotic error tolerance as the scheme of TFKW. To show security, we prove an extension of the MoE property of coset states introduced by Coladangelo, Liu, Liu, and Zhandry [Crypto 2021]. In our stronger version, the player Charlie also receives Bob's answer prior to making his guess, thus simulating a party who eavesdrops on an interaction. To make use of this property, we express it as a new type of entropic uncertainty relation which arises naturally from the structure of the underlying MoE game.

Abstract: Uncloneable encryption, first introduced by Broadbent and Lord (TQC 2020) is a quantum encryption scheme in which a quantum ciphertext cannot be distributed between two non-communicating parties such that, given access to the decryption key, both parties cannot learn the underlying plaintext. In this work, we introduce a variant of uncloneable encryption in which several possible decryption keys can decrypt a particular encryption, and the security requirement is that two parties who receive independently generated decryption keys cannot both learn the underlying ciphertext. We show that this variant of uncloneable encryption can be achieved device-independently, i.e., without trusting the quantum states and measurements used in the scheme. Moreover, we show our variant of uncloneable encryption works just as well as the original definition in constructing quantum money, and can be used to get uncloneable bits without using the quantum random oracle model. Finally, we show that a simple modification of our scheme yields a single-decryptor encryption scheme, which was a related notion introduced by Georgiou and Zhandry. In particular, the resulting single-decryptor encryption scheme achieves device-independent security with respect to a standard definition of security against random plaintexts.

Abstract: Quantum Key Distribution (QKD) enables two parties to establish common secret keys by transmitting bits encoded in quantum systems (qubits), which provides unconditional security. QKD introduces errors during quantum communication, which need to be corrected post-key exchange. Typical error-correcting codes in the context of QKD include Cascade, Low-density parity check (LDPC) codes, and polar codes. In our work, we use polar codes, which are state-of-the-art error-correcting codes meeting the requirements of a QKD system. We provide an implementation of an encoder for polar codes based on reliability sequence, which is computationally efficient and can be implemented in QKD postprocessing. Our work on improving the efficiency of QKD postprocessing is highly relevant for the commercialization of QKD.

Abstract: At its core quantum key distribution (QKD) offers information theoretical security based on the laws of physics. In deployments one has to take into account practical security and resilience. The latter includes the localization of a possible eavesdropper after an anomaly has been detected by the QKD system to avoid denial-of-service. In this work, we present a novel approach to eavesdropper localization inside a quantum channel based on opto-acoustic interaction. Employing localized stimulated Brillouin scattering, we are able to localize common eavesdropping approaches such as evanescent outcoupling as low as 1% of optical transmission power to the cm level. Furthermore we are capable to distinguish multiple nominally indistinguishable fibers from different manufacturers, paving the way for high security applications. Finally we show, that this approach surpasses traditional OTDR technology.

Abstract: We propose and demonstrate an upgraded quantum key distribution protocol based on time-bin entanglement source with access control through introducing phase randomization. The upgraded source can protect users from memory attacks at a negligible cost.

Abstract: A multidimensional optimisation analysis for CV-QKD systems with practical constraints is presented. We demonstrate secret-key-rates >1Mb/s for 30km transmission with arbitrary discrete modulation, utilising 10dB receiver clearance and 100kHz summedlinewidth as a cost-effective implementation.

Abstract: We implement a simulation of a recent device-independent quantum key distribution (DIQKD) protocol to investigate its features, especially with respect to the effect of imperfections such as noise or loopholes. The simulation is based on a RESTful API recently introduced by us, capable of implementing nonlocal no-signaling correlations via communication with a server instead of making measurements on quantum systems. The presented framework can be used in development projects for testing and experimenting, before putting a DIQKD-based solution into production, replacing the API with actual quantum devices.

Abstract: The no-cloning theorem is a cornerstone of quantum cryptography. Here we generalize and rederive under weaker assumptions various upper bounds on the maximum achievable fidelity of probabilistic and deterministic cloning machines. Building on ideas by Gisin [Phys.~Lett.~A, 1998], our results hold even for cloning machines that do not obey the laws of quantum mechanics, as long as remote state preparation is possible and the non-signalling principle holds. We apply our general theorem to several subsets of states that are of interest in quantum cryptography.

Abstract: We show the security of multi-user key establishment on a single line of quantum communication. More precisely, we consider a quantum communication architecture where the qubit generation and measurement happen at the two ends of the line, whilst intermediate parties are limited to single-qubit unitary transforms. This network topology has been previously introduced to implement quantum-assisted secret-sharing protocols for classical data, as well as the key establishment, and secure computing. This architecture has numerous advantages. The intermediate nodes are only using simplified hardware, which makes them easier to implement. Moreover, key establishment between arbitrary pairs of parties in the network does not require key routing through intermediate nodes. This is in contrast with quantum key distribution networks for which non- adjacent nodes need intermediate ones to route keys, thereby revealing these keys to intermediate parties and consuming previously established ones to secure the routing process. Our main result is to show the security of key establishment on quantum line networks. We show the security using the framework of abstract cryptography. This immediately makes the security composable, showing that the keys can be used for encryption or other tasks.

Abstract: Global-scale quantum communication networks will require efficient long-distance distribution of quantum signals. Optical fibre communication channels have range constraints due to exponential losses in the absence of quantum memories and repeaters. Satellites enable intercontinental quantum communication by exploiting more benign inverse square free-space attenuation and long sight lines. However, the design and engineering of satellite quantum key distribution (QKD) systems are difficult and characteristic differences to terrestrial QKD networks and operations pose additional challenges. The typical approach to modelling satellite QKD (SatQKD) has been to estimate performances with a fully optimised protocol parameter space and with few payload and platform resource limitations. Here, we analyse how practical constraints affect the performance of SatQKD for the Bennett-Brassard 1984 (BB84) weak coherent pulse decoy state protocol with finite-key size effects. We consider engineering limitations and trade-offs in mission design including limited in-orbit tunability, quantum random number generation rates and storage, and source intensity uncertainty. We quantify practical SatQKD performance limits to determine the long-term key generation capacity and provide important performance benchmarks to support the design of upcoming missions.

Abstract: Measurement-device-independent (MDI) QKD removes all side-channel attacks on detectors. Continuous variable (CV) MDI-QKD based on coherent states is a promising candidate for integration into existing telecom infrastructure. Despite previous demonstrations of the concept and the potential for secure communication offered by CV MDI-QKD, a practical implementation of the system for real-world data encryption has yet to be achieved. Here, we introduce a simple and practical CV MDI-QKD system that can coexist with classical telecommunications channels. This is achieved through the use of a new relay structure, a real-time phase locking system and a well-designed digital signal-processing pipeline. Our design demonstrates the first practical CV MDI-QKD system, operating at a symbol rate of 20 MBaud and generating keys that are secure against collective attacks in both the finite-size and asymptotic regimes. This sets an important milestone towards in-field implementation and integration of high-performance CV MDI-QKD into telecom networks.

Abstract: Quantum key distribution (QKD) is introduced to make encryption and transmission of data over any public channel unconditionally secure. A key requirement of such a promise is to have access to an encryption key with a similar length as the message and data itself. While QKD has become mature and the key rate significantly increased over the past 20 years, there is still a notable gap between data transmission and key generation rates. High-dimensional QKD is proposed as a method to respond to this demand. Here, we demonstrate a 4-dimensional path-\&-time encoding QKD system with more than 100\% improvement compared to a standard 2D system in the same test-bed, a 52-km deployed multicore fiber link.

Abstract: This work is motivated by the following question: can an untrusted quantum server convince a classical verifier of the answer to an efficient quantum computation using only polylogarithmic communication? We show how to achieve this in the quantum random oracle model (QROM), after a non-succinct instance-independent setup phase. We introduce and formalize the notion of post-quantum interactive oracle arguments for languages in QMA, a generalization of interactive oracle proofs (Ben-Sasson--Chiesa--Spooner). We then show how to compile any non-adaptive public-coin interactive oracle argument (with private setup) into a succinct argument (with setup) in the QROM. To conditionally answer our motivating question via this framework under the post-quantum hardness assumption of LWE, we show that the XZ local Hamiltonian problem with at least inverse-polylogarithmic relative promise gap has an interactive oracle argument with instance-independent setup, which we can then compile. Assuming a variant of the quantum PCP conjecture that we introduce called the weak XZ quantum PCP conjecture, we obtain a succinct argument for QMA (and consequently the verification of quantum computation) in the QROM (with non-succinct instance-independent setup) which makes only black-box use of the underlying cryptographic primitives. The full version of this preprint is available at: https://eprint.iacr.org/2023/421

Abstract: Quantum key distribution (QKD) enables two remote parties to share encryption keys with security based on physical laws. Continuous variable (CV) QKD based on coherent states and coherent detection is a suitable scheme for integration into existing telecom networks. However, thus far, long-distance CV-QKD has only been demonstrated using a highly complex transmitted local oscillator scheme, opening security loopholes for eavesdroppers and limiting its potential applications. Here, we report a long-distance CV-QKD experiment with a locally generated local oscillator over a 100 km fiber channel. This record-breaking distance is enabled by controlling the phase-noise component of excess noise, using a machine-learning framework for carrier recovery and optimizing the modulation variance. We consider the full CV-QKD protocol implementation and demonstrate the generation of keys secure against collective attacks in asymptotic and finite-size regimes. Our results set an essential milestone for CV quantum access networks realization, where a high loss budget is required, and pave the way for large-scale deployment of secure QK.

Abstract: With the recent availability of cloud quantum computing services, the question of verifying quantum computations delegated by a client to a quantum server is becoming of practical interest. While Verifiable Blind Quantum Computing (VBQC) has emerged as one of the key approaches to address this challenge, current protocols still need to be optimised before they are truly practical. To this end, we establish a fundamental correspondence between error-detection and verification and provide sufficient conditions to both achieve security in the Abstract Cryptography framework and optimise resource overheads of all known VBQC-based protocols. As a direct application, we demonstrate how to systematise the search for new efficient and robust verification protocols for BQP computations. While we have chosen Measurement-Based Quantum Computing (MBQC) as the working model for the presentation of our results, one could expand the domain of applicability of our framework via direct known translation between the circuit model and MBQC.

Abstract: Secure multi-party computation (SMPC) protocols allow several parties that distrust each other to collectively compute a function on their inputs. In this paper, we introduce a protocol that lifts classical SMPC to quantum SMPC in a composably and statistically secure way, even for a single honest party. Unlike previous quantum SMPC protocols, our proposal only requires very limited quantum resources from all but one party; it suffices that the weak parties, i.e. the clients, are able to prepare single-qubit states in the X-Y plane. The novel quantum SMPC protocol is constructed in a naturally modular way, and relies on a new technique for quantum verification that is of independent interest. This verification technique requires the remote preparation of states only in a single plane of the Bloch sphere. In the course of proving the security of the new verification protocol, we also uncover a fundamental invariance that is inherent to measurement-based quantum computing.

Abstract: The present study analyzes the efficiency of employing quantum error correction codes (QECC) to encode quantum information states in the context of Quantum Key Distribution (QKD). Specifically, the possibility of enhancing the security and reliability of QKD systems by adding a secondary layer of quantum coding to the states emitted by Alice in the \textit{Prepare-and-Measure} protocols is exhaustively quantified. Such an encoding scheme would be expected to be achievable by means of quantum hardware potentially available in the mid-term. This last statement refers to the assumed reasonable interconnectivity and scalability requirements that may be imposed on the physical encoding capabilities of a quantum processor for the case here considered, since only 1-qubit states are used in QKD. The model for quantum states transmission here considered does not impose any restrictions on the quantum channel, but does assume that the noise and errors to which qubits may be subject in QKD links can be characterized by discrete transformations. That is, depending on the physical encoding scheme chosen for photon's polarization, errors such as bit-flip or phase-shift errors (among others) can be corrected through logical gates derived from Pauli operators, which, along with identity, form the basis $\{I,X,Y,Z\}$ for 1-qubit discrete error operators of the form: \begin{equation} E = \left(\begin{array}{cc} \alpha_0 & \alpha_1\\ \alpha_2 & \alpha_3 \end{array}\right) \end{equation} \medskip Such a consideration imposes the need to be able to identify and correct up to a total of $k = 1 + 3n$ different types of errors (including no error at all, bit-flip, phase-shift, and combinations of the previous) that may affect a QKD state (encoded in an $n-$qubit physical state). In line with previous scalability arguments, that requires for the number of physical qubits needed to achieve such encoding to be lower bounded by the product of the previous magnitude and the dimension of the quantum code $C$ used (which, in the context of QKD, shall be $\mathrm{dim}(C) = 2$). Therefore, if $m=1$ is the number of qubits to be encoded for each transmitted state in QKD, the condition: \begin{equation} 2^n \geq dim(C)(1+3n) \end{equation} imposes a a minimum of $n=5$ physical qubits in a quantum algorithm to carry out encoding and correction of a 1-qubit quantum state. However, it should be noted that, beyond the anticipated error types, the efficiency of identifying errors in a key distilled through QKD (i.e., for all purposes, the correctable QBER associated with each transmission) will be all the more efficient the greater the number n of physical qubits available in a processor for such encoding (of the order of $2^{n-1}$). Thus, the minimum requirements of the quantum hardware topology for the feasibility of this type of encoding are specified, as well as the optimal trade-off in terms of the assumable QBER against different types of attacks, supported by future advances in quantum processor scalability. In this sense, beyond the security considerations associated with QKD implementations of this nature, the goal of this analysis is to parallelly discern the potential speed-up of employing quantum algorithms to carry out error correction of QKD keys and their potential superiority over classical error correction processes in the future. In this regard, two types of QECC are tested in this work. On the one hand, the widespread use of low-density parity check (LDPC)-type linear codes (being linearity a requirement that quantum error correction codes must necessarily satisfy) naturally leads to considering their use in Quantum CSS (Calderbank, Shor \& Steane) codes. The efficiency and performance benefits of LDPC codes applied to QKD are therefore as well transferable to a quantum processor in this context. The performance of these codes applied in QKD is contrasted, secondly, with stabilizer codes. It can be anticipated that the latter may present challenges in the initial algorithm for the encoding of the states emitted by Alice, however the decoding circuit algorithms can be implemented with relative simplicity -albeit scalability limitations- through 1-qubit logical gates (such is also the case with CSS codes once the parity matrix of the LDPC code is known, whose speed advantages over the classical use of belief-propagation algorithms are showed here). On the previous precepts, this study focuses on carrying out a comparative analysis of the convenience of potentially benefitting from the performance of either type of code, while analyzing technical considerations derived from the experimental implementation of QECC protocols in this QKD hybrid approach. The most important considerations are the following: -Complexity. From the point of view of reliability of these types of implementations, potential disadvantages are analyzed in terms of complexity added to real physical systems. Not only is the experimental complexity increase of combining quantum hardware with QKD optical transmissions estimated, but also the anticipation of additional error sources, considering the acceptable threshold values of decoding techniques and calibration errors for real applications and security proofs. - Efficiency. In terms of efficiency and overall code performance, estimated times (for different prospective states of quantum processor advancement) for quantum key generation through these techniques are simulated, and the circumstances under which each may be most convenient are identified. - Components demands. Increased demand for quality of the optics involved in the QKD protocol is expected. Protocols of this nature further increase the demand for high-quality transmissions, especially regarding photon sources, which may have a significant impact on both implementability and its associated costs. - Overhead. Additional overhead needs are projected in terms of code design, number of qubits required depending on the use case, as well as measurement operators necessary for error detection and correction. Consequently, partial limits have been found on the amount of data that can be transmitted in a QKD system that integrates this methodology, which is projected to be overcome when widely available quantum hardware reaches sufficient maturity. - Side channel attacks. Possible vulnerabilities to quantum hacking are preliminarily identified, and a testing method is suggested for this type of QECC-based QKD systems. In addition to the previous analyses, the authors note that one of the most significant features of -both of- the codes here analyzed is that they carry out the identification of errors that affect quantum states at the time of reception, while preserving the encoded quantum information in photons. In this sense, a protocol of these characteristics allows to anticipate, in some applications, the error correction process to the security analysis (although the syndromes of each of the states can be stored classically and the correction processed once the QBER estimation is finished). This can constitute a significant disadvantage in unnecessary computational energy costs when the transmission is not considered secure, but may also be exploited for beneficial applications on certain use cases. With all of the above, the work here presented collects the results on the aforementioned considerations, quantitative cost analysis and future feasibility prospects of this QECC-QKD proposal, as well as details on design and integration considerations, and requirements of both the QKD and quantum hardware components that support this type of implementation.

Abstract: Quantum Algorithms reduce the computational complexity or solve certain difficult problems that were originally impossible to solve with classical computers. Grover's search algorithm is a Quantum computation algorithm that can find target elements from a set of unstructured data with the best possible, O(√N ) queries. Grover's search Quantum circuits implemented accurately can be used to successfully search and find the keys of Symmetric ciphers. However, very few demonstrations of such practical cryptanalysis are available. In this paper, practical Quantum cryptanalysis circuits for Affine Cipher are proposed and demonstrated, that successfully break the cipher by finding the keys.

Abstract: Quantum supremacy poses that a realistic quantum computer can perform a calculation that classical computers cannot in any reasonable amount of time. It has become a topic of significant research interest since the birth of the field, and it is intrinsically based on the efficient construction of quantum algorithms. It has been shown that there exists an expeditious way to solve the noisy linear (or learning with errors) problems in quantum machine learning theory via a well-posed quantum sampling over pure quantum states. In this paper, we propose an advanced method to reduce the sample size in the noisy linear structure, through a technique of randomizing quantum states, namely, $\varepsilon$-random technique. Particularly, we show that it is possible to reduce a quantum sample size in a quantum random access memory (QRAM) to the linearithmic order, in terms of the dimensions of the input-data. Thus, we achieve a shorter run-time for the noisy linear problem.

Abstract: In our work, we provide a clean security analysis of a new high-dimensional QKD setup with a Franson interferometer in the asymptotic limit and calculate secure key rates using a recent method developed. We argue that our new protocol is not only experimentally easier, as it does not require tomography of the polarization degree of freedom, but also allows for a clean security analysis without assumptions that were implicitly hidden in earlier analyses of similar and related protocols. We build a realistic noise model that takes environmental photons, dark counts, channel losses and non-unit detection efficiency into account and show that our new protocol allows secure key rates for twice as many environmental photons than comparable protocols available in literature. We want to highlight that while the security analysis of our protocol is rigorous and clean, the compared key rates for the compared protocol are actually only an upper bound (due to the assumptions implicitly hidden described earlier), so our new protocol outperforms previous settings by at least a factor of 2. Current free-space QKD implementations are only operable during night when environmental photons are low, but fail to provide secure keys during twilight and daytime, which is a major obstacle towards broad practical usage. Thus, doubling the robustness against environmental photons marks an important step forwards towards daylight-independent Quantum Key Distribution implementations.

Abstract: In our work, we explore various multi-user scenarios for Continuous Variable Quantum Key Distribution with discrete modulation. We propose and analyse DM CV-QKD protocols for various different multi-user scenarios such as * One Alice to $n$ Bobs, where the Bobs do not trust each other, * One Alice to $n$ Bobs, where $m<n$ Bobs trust each other, * Conference Key Agreement between one Alice and $n$ Bobs. One common feature of all protocols that we study is that Alice's source does not need any additional expensive components except state-of-the-art beamsplitters, therefore we call it `cheap source'. This makes the transmitter of our proposed protocols easily implementable in experiments and demonstrations. In our work, we calculate asymptotic secret key rates for a range of parameters and different trust scenarios and show that in the asymptotic limit multi-user DM CV-QKD is possible for distances relevant for mid-sized urban area networks between at least 16 user. This highlights, that DM CV-QKD can be extended to the multi-user scenario and remains a feasible candidate also for early implementations of Quantum Key Distribution in local networks.

Abstract: Nonlocal tests on multipartite quantum correlations can certify randomness in a device-independent (DI) way. Such correlations admit a rich structure, making the task of choosing an appropriate witness, known as a Bell inequality, difficult. For example, extremal Bell inequalities are tight witnesses of nonlocality, however achieving their maximum violation places constraints on the underlying quantum system, which are often incompatible with optimal randomness generation. As a result we find a trade-off between maximum randomness and Bell violation. Understanding this trade-off for more than two parties has not been explored, and would inform the best way to generate DI randomness in this setting. Moreover, suitable techniques that enable maximum randomness certification for arbitrarily many parties are missing. Here, we study the maximum amount of randomness that can be certified by correlations exhibiting a violation of the Mermin-Ardehali-Belinskii-Klyshko (MABK) inequality. We find that maximum quantum violation and maximum randomness are incompatible for any even number of parties, with incompatibility diminishing as the number of parties grow, and conjecture the precise trade-off. We also show that maximum MABK violation is not necessary for maximum randomness for odd numbers of parties. To obtain our results, we derive new families of Bell inequalities certifying maximum randomness from a new technique for randomness certification, which we call "expanding Bell inequalities". Our technique allows one to take a bipartite Bell expression, known as the seed, and transform it into a multipartite Bell inequality tailored for randomness certification, showing how intuition learned in the bipartite case can find use in more complex scenarios.

Abstract: The key relay protocol (KRP) plays an important role in improving the performance and the security of quantum key distribution (QKD) networks. On the other hand, there is also an existing research field called secure network coding (SNC), which has similar goal and structure. We here analyze differences and similarities between the KRP and SNC rigorously. We found, rather surprisingly, that there is a definite gap in security between the KRP and SNC; that is, certain KRPs achieve better security than any SNC schemes on the same graph. We also found that this gap can be closed if we generalize the notion of SNC by adding free public channels; that is, KRPs are equivalent to SNC schemes augmented with free public channels.

Abstract: In this work, we present the design considerations and architecture of an optical ground station being developed on National University of Singapore campus. The primary objective of the station is to enable quantum key distribution and facilitate other free space communication protocols. The development of the optical ground station is underway and it is projected to be commissioned by 2023. We elaborate on the building blocks and design techniques of the optical ground station in Singapore that can receive i.e downlink weak quantum signals from a satellite and perform necessary analysis to generate secret keys in a quantum key distribution experiment. We emphasize on the different subsystems namely the telescope system, quantum receiver, polarization correction system, and the pointing, acquisition and tracking system. We envision our ground station to support a range of beacon wavelengths to ensure its compatibility with various similar satellite missions. The working lab-configuration of the station is able to receive and analyse state of photons around 800 nm. To achieve a global quantum network, cross-compatibility among optical ground stations and quantum satellites is crucial. To facilitate this, we have initiated a collaboration with various academic groups involved in satellite based quantum key distribution research to standardize the configuration of an optical ground station. This collaboration aspires to create cross-compatibility among multiple optical ground stations and quantum satellites to enhance the efforts of a global quantum network.

Abstract: We present experimental findings of a versatile quantum key distribution (QKD) system for diverse application scenarios such as long-distance, metropolitan, and last-mile/in-house links. This is enabled by the system’s dual-wavelength support, automatic initialization, stabilizing feedback loops, and modular design, which allows for usage of commercial detectors and encryptors.

Abstract: Twin-Field Quantum Key Distribution (TF-QKD) is an innovative family of protocols characterized by a weaker dependence of the achievable secret key rate on the channel loss, with respect to conventional QKD solutions. In this work, we discuss several important aspects encountered in TF-QKD when transitioning from point-to-point links to a network configuration. 1) The effects of path length mismatch between the two arms of the link (A-C and B-C) is discussed in several configurations. 2) The noise contributions (stronger in in-field deployment) are meticulously analyzed, their effect on the final key rate is estimated and solutions to mitigate the problem are implemented. 3) The topic of building complex and large networks with TF-QKD is tackled to find advantageous configurations. Interconnected macro-star networks based on TF-QKD are simulated by means of the “qkdnetsim” package of the network simulator “ns3”. The upcoming deployment of national QKD networks requires dedicated studies in this direction to build efficient and long-range solutions, compatible with current telecom standards.

Abstract: In the context of telecommunications-wavelength fiberoptic resources, quantum-classical coexistence is considered an economical approach for efficient quantum networking, such as through (dense) wavelength-division multiplexing. However, inadequate filter isolation can introduce unwanted crosstalk noise. In this study, we investigate polarization-entangled photons contaminated by highly polarized classical signals, mapping them to maximally entangled mixed states (MEMS). Notably, MEMS can be effectively concentrated using a local filtering technique commonly referred to as the Procrustean method. To achieve this, we employ programmable polarization-dependent loss emulators (PDLEs), resulting in significant enhancements in the measured state fidelities.

Abstract: As our lives and interactions become more dependent on the internet, our security needs continue to evolve. Future transactions will likely be secured by quantum means such as point-to-point quantum key distribution and more complex quantum protocols. Quantum key distribution has the potential to revolutionize secure communication, but it is often limited by device imperfections and environmental factors such as weather conditions. Currently, quantum key distribution schemes based on orbital angular momentum-carrying optical beams employ conventional settings. As a result, various attacks, such as detector side-channel attacks, are possible, and these beams are subject to spatial aberrations because of atmospheric turbulence and poor weather conditions. As a result, we present a novel approach to measurement device-independent quantum key distribution scheme using vortex vector modes and scalar beams that is capable of achieving high key rates even under diverse weather conditions, including clear skies, light rain, and fog. Furthermore, adopting this approach maximizes the advantages of both orbital angular momentum states and measurement device-independent quantum key distribution. According to our implementation, a secure key can be transmitted up to a maximum distance of approximately 178 kilometers under clear conditions, and we can transmit signals up to a comparable distance of approximately 160 kilometers under adverse weather conditions. Since these distances are comparable, this work presents a significant advance, illustrating how measurement device-independent quantum key distribution can be implemented using vortex vector modes. Most significantly, results demonstrate the effectiveness of this approach, opening up new possibilities for secure long-distance communication under adverse weather conditions.

Abstract: Twin-field quantum key distribution (TF-QKD) and its variants provide a promising solution for sharing information-theoretic secure keys between intercity peers since they are able to overcome the fundamental rate-transmittance bound without quantum repeaters. In this paper, we propose to improve the key rate at long distances and the maximum achievable distance for TF-QKD by deriving the error rates under three mutually unbiased bases, i.e., σX, σY , and σZ in two-dimensional Hilbert space. Moreover, learning these error rates, one can add noisy preprocessing to further improve its performance. We also observe that higher bit error rates do not necessarily imply lower key rates when noisy preprocessing is added. Our method does not change the existing physical implementation or experimental operation, but only requires simple postprocessing of the experimental data, which can be directly used to improve the key rate performance of the existing QKD system. The simulation results demonstrate its notable enhancements in terms of key rate at long distances and the maximum achievable distance for the phase-encoded TF-QKD protocol.

Abstract: As the volume of data and connections exchanged across telecom/datacom networks continues to increase, there is a growing need for technologies that deploy quantum key distribution (QKD) on a large scale in a practical and sustainable manner. To realize high-speed, real-time communication of large-volume data using one-time pad cryptography with QKD modules, it will be important to multiplex QKD modules in the future. Furthermore, it is necessary to consider the physical size of the device for the practical application of multiplexed QKD modules. In this study, we focused on miniaturizing the key distillation process required at the back end of the QKD chip. To reduce the size of the device, it is necessary to estimate as accurately as possible the minimum computing power required to run the key distillation process for the target secret key rate (SKR). However, the performance of the key distillation process requires computing power and involves the exchange of messages via classical channels. Therefore, we evaluate the performance by a network simulator before performing evaluations on the actual equipment. In this paper, we focus on the behavior of classical communication paths in the multiplexed QKD system, which is a problem in studying the key distillation process, and we evaluate it with the simulator. Specifically, we clarify the relationship between the required performance of the key distillation process (i.e., throughput) and the target SKR, which is necessary to realize a part of the key distillation process in hardware.

Abstract: The no-go theorem regarding unconditionally secure Quantum Bit Commitment protocols is a relevant result in quantum cryptography. The impossibility proof for Quantum Bit Commitment has been used to prove the impossibility of unditional security for other protocols, such as Quantum Oblivious Transfer or One-Sided Two Party Computation. In this paper, we extend the same proof to the non-deterministic version of Quantum Private Queries, a protocol addressing the Symmetric-Private Information Retrieval problem. Moreover, we prove the equivalence between Quantum Private Queries and Quantum Bit Commitment and One-Sided Two Party Computation protocols.

Abstract: The ideal Quantum random number generator (QRNG) is a black box which allows the users to test the quantum nature of the generated numbers. Producing a device which is close to this ideal is very demanding and will yield a low rate of random bits. Here we propose a simple setup which is self-testing on the detection part, meaning that only the source has to be characterized. We expect the implementation of this device to yield a random bit rate of around 10 Mpbs.

Abstract: To construct a large-scale quantum key distribution network (QKDN) as future secure infrastructure, it is necessary interwork many QKDNs. Here, we demonstrate an interoperable key relay between two different types of QKDNs: a centralized QKDN and a distributed QKDN. In the demonstration, we build an experimental environment for interworking by using physical QKDNs and implement three fundamental functions (key relay, delivery confirmation, and status information collection) for performing key relay between heterogeneous QKDNs.

Abstract: Satellite-based implementations are essential to realise QKD systems with global reach. Our current work aims to develop a consistent protocol that specifies the individual procedural steps of Decoy-State BB84 for space applications, accompanied by a rigorous security analysis. To this end, we are bringing together the results of decades of fundamental research and patching gaps where necessary to make it ready for application in accredited systems. On the poster, we will present interim results as well as the main challenges we are facing. For a more detailed abstract, please see the submitted pdf file above.

Abstract: In this work, we investigate the effect of the Kalman filter, an algorithm predicting future values of a system, for reducing pointing errors and improving the tracking performance of the coarse tracking system. We present the pointing error based on the angular velocity of the target when the Kalman filter is applied to the tracking algorithm. The tracking system is mounted on a fixed tripod, while the mobile platform moves around the system at a constant speed as a target. The effect of the Kalman filter on the performance of the tracking system and future work will be given.

Abstract: In this work, we analyze the security of a QKD repeater chain where some, but not all, repeaters and fiber links are under the control of an adversary. We show how to bound the quantum min-entropy for this scenario, needed to compute key-rates in the finite-key scenario. Our proof methods may also have numerous applications in other areas of QKD and quantum cryptographic research. Finally we evaluate our new bound and show that positive key-rates are possible even in noisy scenarios. Since early quantum repeaters are bound to be noisy, yet also bound to be partially trustworthy in some scenarios, our work shows improved bit generation rates are possible for early QKD networks.

Abstract: Without access to robust quantum memory or gates, long distance QKD relies upon trusted relays. Several implementations place these relays on satellites, however they are limited in computational power and numerically intensive tasks such as privacy amplification cause bottlenecks for continuous key exchange. Currently, one solution is the simplified trusted relay which leaves all privacy amplification to the end parties at a potentially significant cost to key rate. We developed a post processing technique called pre-privacy amplification which performs a small and efficient post processing step to boost key rates without any additional rounds of communication. For a simplified trusted relay running an asymptotic qubit six-state protocol, we demonstrate an increase to the maximum tolerable QBER from 9.05% to 11.7%. We also identify several sufficient conditions to determine functionally unique pre-privacy amplification maps, and connect it to the graph isomorphism problem.

Abstract: An important goal in quantum key distribution (QKD) is the task of providing a finite-size security proof without assuming that the states across the protocol rounds are independent and identically distributed (IID). For prepare-and-measure QKD, one recently developed approach for obtaining such proofs is the generalized entropy accumulation theorem (GEAT), but thus far it has only been applied to study a small selection of protocols. In this work, we present techniques for applying the GEAT in finite-size analysis of generic prepare-and-measure protocols, incorporating several methods to optimize the min-tradeoff function and minimize the second-order term in the GEAT. As a particular focus, we analyze decoy-state protocols and present a method for generically obtaining min-tradeoff functions for such protocols, even those where a closed-form expression for the asymptotic rate is not known. Furthermore, we highlight that the techniques we develop in the process should also yield improved bounds on the keyrates of decoy-state protocols even in the asymptotic limit.

Abstract: Quantum computers has a major influence on our modern computing platforms. New way of delegated quantum computation solutions continues to be introduced by researchers. The basic functionality of delegated quantum computation enables a classical client to delegates quantum computation related jobs to remote untrusted server with appropriate security measures. However, only very few techniques are addressed the security challenges and its feasibility to implement in practice. One of the solution is quantum trusted execution environment (Q-TEE), which ensures a secure and practical way to build a remote quantum computing server with classical client. In this work, we explore some new features of a quantum-TEE (QTEE), which can be seen as aiding secure computation on a quantum computer. It is reasonable to expect that a QTEE may be required to authenticate classical entities relating to a particular quantum computation. For example, a client, which has submitted a particular job, may require a proof that the quantum computation was indeed executed in that particular computer. Such a QTEE may be envisaged to be using a post-quantum signature scheme like DILITHIUM or Falcon. The quantum computing platform provider would use its secret key to sign various classical entities. The signature can be verified by using the provider's public key. We propose a design of a QTEE which uses Tokenized Signature Scheme (TSS). We also point out that such a QTEE has certain advantages over the naive DS-based ones. Ben-David and Sattath introduced the primitive called (public key) Tokenized Signature Scheme, which can be used in a situation where a owner wants to delegate the power to sign to a signer. The owner, after generating the signing and verification keys (using PPT called KeyGen) (similar to key generation in a DS), creates a certain number of quantum tokens (using QPT called TokenGen) and gives them to designated signers. The signers authenticate classical messages (using QPT called Sign) by generating a classical string called signature, on behalf of the owner and at her behest, using the owner-provided tokens. The verification (using PPT called Vrfy) can be run by anyone using the public key, the signature and the message. The authors also give a construction of TSS using subspace states. A quantum computation platform provider can generate its own key pair and generate tokens. The computers owned by the service provider may be equipped with a QTEE based on a candidate TSS scheme. The quantum tokens are loaded onto the QTEE, which are used for signing. We point out some advantages of such a construction. Firstly, the secret key of the owner is never revealed and all the computers controlled by the provider authenticate in the same manner. Secondly, the trust assumption on the QTEE may be relaxed. A secure TSS is expected to have the following unforgeability property. An adversarial signer can not sign n+1 messages if it has only n tokens. Thirdly, in a situation where the client pays for such authentication services, quantum tokens can be budgeted and monetized. A complete design of a QTEE, supporting various secure quantum computation related requirements, may be achived with a TSS at its core. A TSS supporting aggregation and aggregated-verification brings in added advantage. Meaningful analogues of remote attestation (RA) and direct anonymous attestation (DAA) in this setting may also be explored. The development of quantum- based TEE techniques enables service providers to implement proprietary quantum computing devices in practice. Also, it allows classical users to perform remote quantum computation at very high security levels.

Abstract: This research introduces a simplified variation of the time-based BB84 protocol, employing time-bin encoding and one decoy state. The proposed approach significantly simplifies the security analysis, enabling the identification of potential vulnerabilities by avoiding interference in the transmission of specific state combinations. This simplification reduces the reliance on finite key analysis and allows us to better characterize the source imperfections without much compromise on the secret key rate (SKR).

Abstract: Advances in the security analysis of continuous-variable quantum key distribution (CVQKD) protocols with true discrete modulation aim to unlock the same performance as that obtained from `traditional' protocols based on Gaussian modulation. We report a CVQKD experiment using 4 states that utilizes a composable security proof to generate a secret key fraction of $5.6 \times 10^{-3}$ bits/symbol over 10 km channel, while providing security against collective attacks.

Abstract: Dense coding is known as an attractive quantum information protocol. While the original study considers the noiseless setting, many subsequent studies extended this result to more general settings. However, all of them focused only on the communication speed in various noisy settings. While dense coding with the noiseless setting realizes twice communication speed, it also realizes quantum secure direct communication (QSDC) as follows.In dense coding, the sender, Alice, and the receiver, Bob, share perfect Bell states and Alice encodes her message by application of a unitary operation. Since Alice's local state is a completely mixed state, the eavesdropper, Eve, cannot obtain any information about the message even when Eve intercepts the transmitted quantum state. However, it is not easy to share a perfect Bell state. Hence, we need to consider secure communication under imperfect shared state. Specifically, we study secure direct communication by using a general preshared quantum state and a generalization of dense coding. In this scenario, Alice is allowed to apply a unitary operation on the preshared state to encode her message, and the set of allowed unitary operations forms a group. To decode the message, Bob is allowed to apply a measurement across his own system and the system he receives. In the worst scenario, we guarantee that Eve obtains no information for the message even when Eve access the joint system between the system that she intercepts and her original system of the preshared state. For a practical application, we construct a modular wiretap code by concatenating inverse universal hashing and an arbitrary error correcting code. Combining the wiretap code with error verification, we propose a concrete protocol for the private dense coding model and derive an upper bound of information leakage in the finite-length setting. We also discuss how to apply our scenario to the case with discrete Weyl-Heisenberg representation when the preshared state is unknown. In this case, Pauli encoding operation and Pauli channel are considered. Hence, our protocol can be applied many similar tasks.

Abstract: Properties of quantum mechanics have enabled the emergence of quantum cryptographic protocols achieving important goals which are proven to be impossible classically. Unfortunately, this usually comes at the cost of needing quantum power from every party in the protocol, while arguably a more realistic scenario would be a network of classical clients, classically interacting with a quantum server. In this paper, we focus on copy-protection, which is a quantum primitive that allows a program to be evaluated, but not copied, and has shown interest especially due to its links to other unclonable cryptographic primitives. Our main contribution is to show how to dequantize existing quantum copy-protection from hidden coset states, by giving a construction for classically-instructed remote state preparation for coset states. We also present the first secure copy-protection scheme for point-functions in the plain model, to which our dequantizer can be applied.

Abstract: The reliability of quantum resources can be compromised in practice due to the complexity of their generation processes and/or the potential manipulations by untrusted parties during transmission. When performing an information task with an unreliable quantum resource, it is incorrect to treat the random variables associated with repeated experimental trials as independent and identically distributed (i.i.d.). To certify the performance of such a task, one can make a random decision in each trial, either to spot-check some property of the quantum resource or to utilize the resource for the task. The task considered can be quantum key distribution, quantum randomness expansion, verifiable quantum computation, or resource allocation in quantum networks. Unfortunately, existing methods for certifying quantum performance through spot-checking are not suitable for non-i.i.d. repeated trials without additional assumptions. Here we present a novel method to address this challenge. The method works efficiently with a finite number of non-i.i.d. trials. Furthermore, our method can be adapted to estimate quantum properties in situations where the quantum resource is spot-checked and destroyed by a measurement during each non-i.i.d. repeated trial.

Abstract: Quantum key distribution (QKD) systems provide a method for two users to exchange a provably secure key that can be used to establish an unconditionally secure communication channel. Here we present an FPGA-controlled prepare-and-measure BB84 polarization-based decoy state protocol using light-emitting diodes (LEDs). Our setup uses three separate LEDs driven by a field-programmable gate array (FPGA) that go through different optical paths that set the state of polarization. Each LED is connected to two GPIO pins via a different resistive path. By setting one pin to high impedance and driving the other with a nanosecond-scale electrical signal, we can choose between signal and decoy states. We can thus send 3 signal states, 3 decoy states, and 3 vacuum states. To prevent side-channel attacks multi-source QKD systems require that each state is indistinguishable from the others in the spatial, spectral, and temporal degrees-of-freedom on the photon. We do this by passing the 3 photonic wavepackets through the same single-mode fiber and 1-nm-bandwith spectral filter and use dynamic shifting of the FPGA phase-locked-loops to control the phase and the width of the electrical pulses that drive the LEDs, which allows us to control the optical pulses produced by the LEDs. Both spectral and temporal profiles are shown in Figure 1. We control the timing of the photonic wavepackets to a resolution of 78 ps. Additionally, we use the FPGA to generate true random states as required by the BB84 protocol. To quantify the indistinguishability of Alice’s various states, we use the mutual information to calculate the fraction of the final sifted key that an eavesdropper would know after making temporal and/or spectral measurements on every state that is sent. We are able to achieve 2.39e-05 and 4.31e-05 mutual information fraction leaked in the spectral and temporal waveforms, respectively. Furthermore we put our scheme into practice with a simple tabletop QKD setup where we are able to achieve 1.7% quantum bit-error rate (QBER) in the L/R bases and 2.1% QBER in the H/V bases. Additionally, our system's SWaP restrictions make it very desirable for highly mobile platforms such as drones.

Abstract: Decoy state methods improve the feasibility of quantum key distribution (QKD) by enabling the use of simple, robust sources, and techniques have been developed to allow for the use of decoy analysis in the regime where only a finite number of signals are sent. We present an iid security proof for finite-size key rates of prepare-and-measure protocols with probabilistic testing, including decoy state methods, within a composable security framework that allows for future extensions to device imperfections. Additionally, we improve the acceptance set over previous works through the use of entrywise constraints, allowing us to efficiently perform decoy state protocols. Moreover, we introduce a new figure of merit, the expected key rate, to capture the tradeoff between aborting too often and achieving high key rates, which allows for increased practicality of QKD implementations.

Abstract: Quantum key distribution (QKD) provides a method for two users to exchange a provably secure key, which requires synchronizing the user’s clocks. Qubit-based synchronization protocols directly use the transmitted quantum states and thus avoid the need for additional classical synchronization hardware, but previous approaches sacrifice secure key either directly or indirectly. Here, we introduce a Bayesian probabilistic algorithm that incorporates all published information to efficiently find the clock offset without sacrificing any secure key [1]. Additionally, the output of the algorithm is a probability, which allows us to quantify our confidence in the synchronization. Our experimental system employs an efficient three-state BB84 prepare-and-measure protocol with decoy states. Our algorithm exploits the correlations between Alice’s published basis and mean photon number choices (which must already be published for the protocol) and Bob’s measurement outcomes to probabilistically determine the most likely clock offset. We perform cross-correlations using Fast Fourier Transforms to count the number of each type of event pairing for each potential offset (e.g., how many times Alice sent a decoy state in the horizontal/vertical polarization basis and Bob registered a click in the horizontal detector). Taking these along with a lookup table for the probabilities of the different event pairings, we determine the synchronization probability of the different potential offsets using Bayesian analysis. To demonstrate the robust nature of this algorithm, we tracked its performance using simulated data with varying parameters. We find that we can achieve a 95% synchronization confidence using a string length of only 4,140 communication bin widths, meaning we can tolerate clock drift approaching 1 part in 4,140 in this example when simulating this system with a dark count probability per communication bin width of 8⨉10-4 and a received mean photon number of 0.01. The relationship between the received mean photon number and the number of communication bin widths required to achieve a 95% synchronization confidence is shown in Fig. 1. We applied this algorithm to data collected from our drone-to-done QKD experiments, with a received mean photon number of 0.043, achieving quantum bit error rates of 0.0106, 0.0287, and 0.0361 for our 3 states.

Abstract: Here we report on the experimental results implementing robust anonymous quantum conference key agreement using GHZ states. Results confirm the advantage when allowing for the use of multipartite entanglement along with bipartite entanglement.

Abstract: Quantum key distribution offers unconditionally secure keys for communicating parties. Although using high-dimensional quantum systems in QKD protocols does offer considerable advantages, which has been extensively shown in different experiments, analytical security proofs for high-dimensional protocols are not abundant. This is partly because many QKD protocols lack certain ``symmetry'' in terms of the parties' capabilities and responsibilities, which complicates security proofs. In this work, we consider one such protocol and provide analytical security proof and compare our results against prior work showing an advantage of our method. We also develop a continuity bound for conditional quantum entropies which is pertinent to our analysis here and may have applications in other scenarios also.

Abstract: Quantum dot-based entangled photon sources are promising candidates for quantum key distribution (QKD), as they can in principle emit deterministically, with high brightness and low multiphoton contribution. However, quantum dots (QD) often inherently possess a fine structure splitting (FSS). Since the entangled photonic state in the presence of non-zero FSS is oscillating, one must settle for a lower efficiency source through temporal post-selection or a lower measured entanglement fidelity. In both cases, the overall key rate is reduced. Our QKD analysis shows that this trade-off can be overcome by constructing a time-resolved QKD protocol where all photon pairs emitted by a QD with non-zero FSS can be used in secret key generation. This protocol works only when the detection system's temporal resolution is much smaller than the FSS period. By implementing our protocol, higher key rates can be achieved as compared to previous QKD experiments with QD entangled photon pair sources. Additionally, unlike previous security analyses that assume perfect qubit states, we rigorously bound the effect of any multi-photon components of the optical state on the key rate, which is more applicable to practical implementations.

Abstract: The postselection technique is a widely used tool to lift the security of Quantum Key Distribution (QKD) protocols against IID collective attacks to coherent attacks. While various other approaches for proving security against coherent attacks exist, they have limitations that make them less suitable for typical optical prepare-and-measure protocols. We identify and address some limitations of the postselection technique as applied to optical prepare-and-measure QKD protocols. We extend this analysis to decoy-state protocols, which are essential for long-distance QKD. Finally, we also improve the practical applicability of the postselection technique. Thus, we argue that the postselection technique, with the relevant modifications, is the only lift to coherent attacks that can be broadly applied to optical implementations of generic prepare-and-measure QKD protocols.

Abstract: We point out a critical flaw in the analysis of Quantum Key Distribution (QKD) protocols that employ the two-way error correction protocol Cascade. Specifically, this flaw stems from an incom-plete consideration of all two-way communication that occurs during the Cascade protocol. We present a straightforward and elegant alternative approach that addresses this flaw and produces valid key rates. We exemplify our new approach by comparing its key rates with those generated using older, incorrect approaches, for Qubit BB84 and Decoy-State BB84 protocols. We show that in many practically relevant situations, our rectified approach produces the same key rate as older, incorrect approaches. However, in other scenarios, our approach produces valid key rates that are lower, highlighting the importance of properly accounting for all two-way communication during Cascade.

Abstract: Quantum information technologies that utilize entangled photon pairs assume a single- photon source. While this assumption poses no significant issues when the channel loss is low, high loss can have a detrimental impact on the system's performance. To overcome high loss, the most intuitive solution is to increase the gain of entangled photon pairs by sending a large quantity of them. However, high-gain sources tend to degrade the quantum quality of entangled photon pair sources. We derived the density matrix of the quantum state in the distribution of polarization-entangled photon pairs under the non- symmetric channel losses with threshold detectors. We analyzed the variation of the CHSH inequality parameter S and the effective photon state transfer probability 𝑁𝑚 by changing the non-linear gain γ. The increase and subsequent decrease in Nm with increasing γ can be interpreted as follows: when γ is small, the state is not properly transmitted due to high loss, but as γ increases, the error probability, such as double-click events, increases due to the influence of multi-photon events, leading to a decrease in Nm. This result indicates the need to optimize the brightness of the light source for practical implementation in quantum information technologies. This study is expected to contribute to the analysis of discrete variable quantum key distribution(DVQKD) systems like BBM92, E91, and long- distance quantum imaging systems in the future.

Abstract: In recent times, field trials of quantum key distribution (QKD) have been conducted using the existing optical fiber infrastructure. However, one significant challenge faced during these trials is ensuring the stability of QKD operation. The instability of QKD operation is caused by the two factors: random fluctuations in polarization of photon over time and time drift of the photon as it traverses the deployed optical fiber. These issues are unavoidable due to the inability to accurately estimate and control factors such as temperature, vibration, and stress in the deployed optical fiber. To address this instability, various solutions based on active or passive optics have been proposed. In this paper, we present an active optics-based simple polarization stabilizer utilizing an optical polarizer, an active polarization controller, and a single photon detector. For the fast operation, we utilize only 2 out of the 4 axes of the polarization controller for the stabilizer. The experimental results verify the stability of the stabilizer.

Abstract: The coherent one-way (COW) protocol is a promising commercial solution to practical quantum key distribution (QKD) due to its simple optical implementation. However, the non-IID structure of COW due to its inter-signal coherence makes standard security analysis inapplicable. Recently, it has been shown that a modified COW setup allows standard IID analysis, but at the cost of imposing extra limitations and increasing the number of pulses required for each bit. Here we propose a variant that possesses the IID structure and completely retains the optical setup of COW, but with a different data processing scheme that ignores inter-signal information. We obtain key rate lower bound close to analysis for the previously proposed IID variant, and achieves a higher number of key bits transmitted per second.

Abstract: We report a method for continuously stabilizing the polarization change of photons when propagating across fibers. This technique operates at single-photon-level intensity and therefore imposes minimal noise onto the quantum channel, allowing for un-interrupted operation of a quantum network.

Abstract: Succinct Non-interactive Arguments (SNARGs) are cryptographic protocols that enable a prover to demonstrate the validity of an $\NP$ statement to a verifier using a single message of size poly-logarithmic in the size of the $\NP$ statement and witness. Currently, SNARGs are only known to exist based on non-standard cryptographic assumptions, and were shown to be inherently challenging to obtain from standard assumptions by the work of \cite{STOC:GenWic11}. The work proved that standard (black-box) proof techniques are insufficient to prove the security of a SNARG based on any standard (falsifiable) cryptographic assumption. We extend the result of \cite{STOC:GenWic11} to the quantum setting, where parties can perform quantum computations and communicate using quantum information. The result of \cite{STOC:GenWic11} uses the meta-reduction paradigm, which is a general technique for obtaining cryptographic impossibility results. To obtain our result, we extend the above paradigm to the quantum setting, which we believe to be of independent interest.

Abstract: Measurements in quantum physics are inherently random. Moreover, it is possible to certify quantum randomness from systems that are only partially characterized by the user. Here, we propose a simple quantum random number generator (QRNG) that requires only a single photodiode and one laser. We trust only the quantum efficiency of the photodiode and the characterization of the detector, leaving the laser in control of the eavesdropper. Such a QRNG is source-device-independent and its optical setup is among the simplest setups achieving source-device independence.

Abstract: Besides discrete-variable QKD, where single photon detection is used, continuous-variable (CV) protocols are using homodyne detection and are thus promising to be compatible with existing classical coherent communication technology. Originally, the research on CV QKD protocols mostly focused on Gaussian modulation (see review [1]), where one assumes that Alice can continuously displace coherent states according to a 2D Gaussian distribution. This modulation allows the security proofs to take advance of Gaussian optimality conditions, but experimental implementations can only reach this pattern up to some finite discretization. Another approach is to directly use a discrete-modulated (DM) CV QKD protocol. Here, Alice is required to prepare a finite number of displaced coherent states, aiming for a higher experimental simplicity, with the drawback of higher theoretical complexity. Recently, new security proofs such as [2] and corresponding experiments [3,4] could show the feasibility of systems using quadrature amplitude modulation (QAM) with 64 and 256 displaced states. However, the security proof was limited to the asymptotic regime and since the experiments did not use implemented error correction codes, one could only estimate the achievable key rates, but could not generate the secret key itself. In this poster, we demonstrate experiments with a protocol with a smaller constellation size of four coherent states that share the same amplitude but are shifted by 90° in phase (QPSK modulation). We exploit a recently published security proof providing tight secret key rates for collective attacks even in the finite size regime [5]. Furthermore, we show that the QPSK data is compatible with our implemented low density parity check (LDPC) codes for binary symmetric channels. This allows us to perform the full QKD protocol from experimental quantum state exchange to classical post processing and to generate a secret key shared between Alice and Bob. For this purpose, we use a laboratory system based on polarization encoding in the Stokes parameters which is equivalent to a QPSK pattern in phase space. This scheme is designed to cope with the challenges of a turbulent atmospheric channel. While the fluctuating nature of such a channel can be targeted by sub-binning the transmission channels [6], the atmosphere is in general non-birefringent, allowing for atmospheric quantum communications [7]. [1] F. Laudenbach et al., Adv. Quantum Technol. 1, 1800011 (2018) [2] A. Denys et al., Quantum 5, 540 (2021) [3] F. Roumestan et al., arXiv:2207.11702 (2022) [4] Y. Pan et al., Optics Letters 47, 3307-3310 (2022) [5] F. Kanitschar et al., arXiv:2301.08686v1 (2023) [6] V. Usenko et al., New J. Phys. 14, 093048 (2012) [7] B. Heim et al., New J. Phys. 16, 113018 (2014)

Abstract: This work experimentally investigates the impact of residual phase noise on CVQKD systems using phase profiles obtained through simulated Wiener phase processes and experimental measurements, and compares the experimental measurements to the theoretical calculation.

Abstract: Decoy-state quantum key distribution (QKD) represents nowadays the best countermeasure to attacks exploiting multi-photon emissions in realistic sources. A fundamental requirement is the uniform and independent distribution of phases of the transmitted pulses. However, this can not be true for lasers working under high-speed gain-switching conditions, as residual photons in the cavity can induce phase correlations across consecutive pulses. A security proof robust against such imperfections has been recently proposed, which requires knowledge of a parameter that quantifies how close the conditional distribution of each phase is to a uniform distribution. In this work we propose an experimental method to characterise this parameter in realistic setup conditions and we extend the application to the case of arbitrary length of correlations, aiming to enable experimental verification of the implementation security.

Abstract: Two new concepts of quantum random number generators (QRNG) are presented. The first one is related with the application of quantum entanglement to producing several mutually coupled in a random manner bit sequences, which can be used in cryptographic applications and verified in a parallel manner allowing for entropy measurement in real time in public domain using arbitrary large resources for patterns detection, but without compromising the secrecy of coupled by quantum entanglement dual random binary sequences. This is a new concept for verification of fidelity of random bit sequences in a fully non-destructive way, allowing for various applications of generated random bits for which secrecy is important (e.g. in cryptograhic applications). The idea is the development of former our proposal [1]. The second concept is reletad to our progress in prototyping of miniaturized QRNG utilizing the quantum transitions allong the Fermi golden rule as the entropy source, developed for application to quantum cryptography (QKD) systems based on continuous variables. The prototype exploiting, as the source of the entropy, the photoelectric process in a photodiode coupled to a small LED is miniaturized to size of 2 cm [2] and produces the random sequence with a rate of 1 Mb/s. We present current developments of the concept towards its further miniaturization to sizes suitable for using this QRNG device in portable computers, mobile phones and miniaturized terminals for QKD using non-entangled photons. 1. Janusz E. Jacak, Witold A. Jacak, Wojciech A. Donderowicz, Lucjan Jacak, Quantum random number generators with entanglement for public randomness testing, Scientific Reports, (2020) 10:164, https://doi.org/10.1038/s41598-019-56706-2 2. Marcin M. Jacak, Piotr Jóźwiak, Jakub Niemczuk, Janusz E. Jacak, Quantum generators of random numbers, Scientific Reports, (2021) 11:16108, https://doi.org/10.1038/s41598-021-95388-7

Abstract: Validation of the randomness of a quantum random number generator (QRNG) can be performed via robust statistical testing, which generally reduces to the problem of finding long range patterns in the generated random bit sequence. This problem is computationally exhaustive and poses one of important challenges for industrial implementation of self-testing integrated QRNG devices. Furthermore, classical statistical testing cannot in principle confirm the quantum non-determinism (from which the QRNG device can deviate due to its implementation imperfections). Instead, classical testing can confirm that up to certain parameters threshold, deterministic patterns were not detected. The device independent QRNG schemes are based on quantum entanglement, which is a non-classical resource that can be verified in terms of quantum measurements non-classical correlations statistically violating Bell type (e.g. CHSH) inequalities for classical limits on such correlations. This reults in a fundamental (independent from a technical implementation) confirmation that the process used to generate randomness based on entangled quantum states is indeed non-deterministic. In this paper we describe a series of recent experimental developments focused on generating quantum entanglement based randomness in a quntum optics device-independent approach, with validation of the randomness through experimentally verified violation of the CHSH inequality [1]. The experimental setup for entanglement based QRNG involves generation of entanglement in photon polarizations in the SPDC type II process with a single-photon detectors (SPAD) for quantum measurements of entangled photons. Statistical processing of the measurements outcomes shows violation of the classical limits on the correlations, violating the CHSH inequality and hence proving that the QRNG generated randomness is based on a quantum, non-deterministic process. The further direction for this research is towards miniaturization of the robust quantum optics setups to be more adequate for integrated entanglement QRNG devices. This work is part of the NCBR research and development project (contract no. POIR.01.01.01-00-0173/15) aimed at advancing QRNG setups with technical achievements reported in the SeQre.net platform [2]. 1. J.F. Clauser, M.A. Horne, A. Shimony, R.A. Holt, Proposed experiment to test local hidden-variable theories, Phys. Rev. Lett., 23 (15): 880–4, doi: https://doi.org/10.1103%2FPhysRevLett.23.880, (1969) 2. SeQre.net, Quantum Cryptography R&D Platform managed by the Department of Quantum Technology at WUST and CompSecur / SeQre, https://seqre.net/qrng

Abstract: Quantum key distribution (QKD) aims to extract secret keys from correlations between quantum systems. Most QKD research focuses on "device-dependent" protocols whose security is conditioned on their quantum devices operating within specified tolerances. These assumptions on device operation render device-dependent protocols vulnerable to attacks that exploit the differences in real devices and their models in security proofs, and hence threaten the security of such protocols. Alternatively, Device-independent (DI) QKD seeks to achieve security with minimal assumptions on quantum devices by relying on quantum correlations that violate Bell inequalities, overcoming this short-coming of device-dependent QKD. Our work is motivated by the following two observations. First, DIQKD is more secure but has worse noise and loss tolerances than device-dependent QKD. This point has motivated investigations into new techniques to improve these tolerance thresholds such as random key generation, random post-selection, noisy pre-processing and advantage distillation, the last of which we investigate, and which describes a two-way communication procedure in the error correction step of the protocol. Second, the precise circumstances in which DIQKD is possible are unclear, since not all correlations that violate Bell inequalities can be used to distill a secret key in DIQKD. Under the independent and identically distributed (IID) collective attacks framework, previous work sought to resolve both problems by implementing DIQKD with an advantage distillation protocol called the repetition-code protocol. The authors derived both a sufficient and a conjectured necessary condition for security based on the fidelity between some states in the protocol. However, the significance of their results was limited by a gap between the two security conditions, which prevented the calculation of tight noise tolerance bounds and suggested that the fidelity is not the right quantity to consider to characterize exactly when key distillation in DIQKD is possible. Furthermore, in our work we replace the fidelity in the security proofs with the quantum Chernoff divergence, a measure of distinguishability in symmetric hypothesis testing, and achieve equivalent sufficient and necessary conditions for security for the repetition-code DIQKD protocol under the i.i.d collective attacks framework. Consequently, our work strongly indicates that quantum Chernoff divergence is the relevant quantity to describe the security of the repetition-code DIQKD protocol. With our new security condition, we show that the noise tolerance thresholds of the repetition-code DIQKD protocol outperform even one-way DIQKD protocols implemented with noisy pre-processing and random key measurements.

Abstract: Recent years have seen tremendous progress in increasing distances for distribution of quantum states and quantum entanglement, most notably in quantum key distribution. Even though these advances point towards breaching 1000 km and more in the near future, true global connectivity for secure intercontinental quantum links will likely require the operation of trusted networks based on quantum repeaters. To overcome associated losses in even the best optical fibers on ground, operating repeater nodes in space to utilize low-loss inter-satellite links may prove to be the only viable strategy. Successfully deployed QKD experiments and quantum technology in space, brings this idea closer to realization. Nevertheless, conceptual designs [9, 10] and component development are still in their infancy and it will require extraordinary engineering achievements to materialize robust space-based quantum networks. Here, we present recent efforts at the German Aerospace Center (DLR) to investigate the realization of robust global quantum networks. We are developing a holistic approach which bundles expertise on the necessary components for space-based quantum repeaters, i.e. photon sources, quantum memories, optical links, laser terminals, and orbital simulations. From this, we derive a common set of requirements to push concrete technological implementation. The long-term goal of this project is to develop space-hardened components for successful operation of intercontinental space-based quantum networks.

Abstract: High brightness, low-g2 single-photon sources (SPSs) are an alternative to commonly employed weak coherent pulse (WCP) sources for discrete variable quantum key distribution (QKD) and offer potential key-rate and finite-block scaling advantages. However, the loss tolerance of SPS-based QKD is compromised by photon number splitting (PNS) attacks against non-negligible multiphoton emissions. Decoy state (DS) techniques mitigate against PNS attacks, with WCP-DS QKD over several hundred km in fibre being demonstrated. Here, we adapt the DS method to any practical SPS that can easily generate multiple photon number distributions (PND) by attenuating its original photon emissions. Hence, we provide finite-key security bounds for a Multi-PND (adapted 2-Decoy) protocol using Efficient BB84 with optimised parameters. We use a particular true quantum dot source to compare its key rate generation with a Single-PND (adapted Non-Decoy) protocol for several finite block sizes. As expected, the Multi-PND gives higher key rates than the Single-PND, except for considerably small blocks. Moreover, the Multi-PND protocol goes beyond 200 km of tolerable fibre distance for high acquisition times. In this work, we set a generalised method to employ the DS techniques with any realistic SPS and further research may be done implementing distinct SPS characteristics.

Abstract: It is well-known in classical cryptography that standard (black-box) proof techniques are insufficient to establish the security of statistical NIZK arguments for NP based on any standard (falsifiable) cryptographic assumption. In this work, we extend this impossibility result to a quantum scenario where quantum computations and communications are incorporated into the protocol. The classical result is demonstrated using the meta-reduction paradigm, which is a typical technique employed to generate cryptographic impossibility results. In our work, we extend this technique to the quantum setting to prove our results.

Abstract: Quantum entanglement is a preliminary requirement for many protocols in quantum computing, communication, and sensing. Entanglement is typically achieved by having two particles created from the same source [1]. However, creating quantum networks and internet requires distributing and manipulating quantum states between remote nodes through protocols such as quantum entanglement. Here we report high-fidelity entanglement swapping using time-bin qubits, with the aim of distributing entanglement between national laboratories in the United States. References: [1] Zhang, W., Xu, D., amp; Chen, L. (2023). Polarization entanglement from parametric down-conversion with an LED pump. Physical Review Applied, 19(5). https://doi.org/10.1103/physrevapplied.19.0540

Abstract: We detail our experiments towards generating GHZ states encoded into time-bin qubits using a 2x2 optical switch. We present a theoretical model founded on phase-space techniques to corroborate our experimental findings.