Poster Session 2

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: 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: 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: 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: 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: 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: 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: 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: 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: 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: 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: 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: 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: 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: 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 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: 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 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: 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: 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: 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: 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: 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: 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: 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: 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: 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: 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: 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: 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: 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: 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: 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.