The performance of quantum key distribution protocols is evaluated based on the ease of implementation and key generation rate. Among major protocols, the differential-phase-shift (DPS) protocol has the advantage of simple implementation using a train of coherent pulses and a passive detection unit. Unfortunately, however, its key rate is known to be at least proportional to η 2 with respect to channel transmission η → 0 . If one can only prove the rate proportional to η 2 and cannot improve the analysis beyond that, then the DPS protocol will be deemed inferior to other major protocols, such as the decoy BB84 protocol. In this paper, we consider a type of DPS protocol in which the phase of each emitted block comprising n pulses is randomized and significantly improves the analysis of its key rate. Specifically, we reveal that the key rate is proportional to η 1 + 1 n − 2 and this scaling is tight. This implies that the DPS protocol can achieve a key rate proportional to η for a large number of n , which is the same scaling as the decoy BB84 protocol. Our result suggests that the DPS protocol can achieve a combination of both advantages of ease of implementation and a high key generation rate. Published by the American Physical Society 2024
The performance of quantum key distribution protocols is evaluated based on the ease of implementation and key generation rate. Among major protocols, the differential-phase-shift (DPS) protocol has the advantage of simple implementation using a train of coherent pulses and a passive detection unit. Unfortunately, however, its key rate is known to be at least proportional to η 2 with respect to channel transmission η → 0 . If one can only prove the rate proportional to η 2 and cannot improve the analysis beyond that, then the DPS protocol will be deemed inferior to other major protocols, such as the decoy BB84 protocol. In this paper, we consider a type of DPS protocol in which the phase of each emitted block comprising n pulses is randomized and significantly improves the analysis of its key rate. Specifically, we reveal that the key rate is proportional to η 1 + 1 n − 2 and this scaling is tight. This implies that the DPS protocol can achieve a key rate proportional to η for a large number of n , which is the same scaling as the decoy BB84 protocol. Our result suggests that the DPS protocol can achieve a combination of both advantages of ease of implementation and a high key generation rate. Published by the American Physical Society 2024
Differential-phase-shift (DPS) quantum key distribution (QKD) is one of the major QKD protocols that can be implemented with a simple setup using a laser source and a passive detection unit. Recently, an information-theoretic security proof of this protocol was established by Mizutani et al. [npj Quantum Inf. 5, 87 (2019)], assuming an infinitely large number of emitted pulses. To implement the DPS protocol in a real-life world, it is indispensable to analyze the security with the finite number of emitted pulses. The extension of the security proof to the finite-size regime requires the accommodation of the statistical fluctuations to determine the amount of privacy amplification. In doing so, Azuma's inequality is often employed, but unfortunately we show that in the case of the DPS protocol, this results in a substantially low key rate. This low key rate is due to a loose estimation of the sum of probabilities regarding three-photon emission whose probability of occurrence is very small. The main contribution of our work is to show that this obstacle can be overcome by exploiting the recently found novel concentration inequality, Kato's inequality. As a result, the key rate of the DPS protocol is drastically improved. For instance, assuming typical experimental parameters, a 3-Mbit secret key can be generated over 77 km for 8.3 hours, which shows the feasibility of DPS QKD under a realistic setup.
One of the simplest methods for implementing quantum key distribution over fiber-optic communication is the Bennett-Brassard 1984 protocol with phase encoding (PE-BB84 protocol), in which the sender uses phase modulation over double pulses from a laser and the receiver uses a passive delayed interferometer. Using essentially the same setup and by regarding a train of many pulses as a single block, one can carry out the so-called differential-quadrature-phase-shift (DQPS) protocol, which is a variant of differential-phase-shift (DPS) protocols. Here we prove the security of the DQPS protocol based on an adaptation of proof techniques for the BB84 protocol, which inherits the advantages arising from the simplicity of the protocol, such as accommodating the use of threshold detectors and simple off-line calibration methods for the light source. We show that the secure key rate of the DQPS protocol in the proof is eight-thirds as high as the rate of the PE-BB84 protocol.
The differential-phase-shift (DPS) quantum-key-distribution (QKD) protocol was proposed aiming at simple implementation, but it can tolerate only a small disturbance in a quantum channel. The round-robin DPS (RRDPS) protocol could be a good solution for this problem, which in fact can tolerate even up to 50 of a bit error rate. Unfortunately, however, such a high tolerance can be achieved only when we compromise the simplicity, i.e., Bob's measurement must involve a large number of random delays (∣R∣ denotes its number), and in a practical regime of ∣R∣ being small, the tolerance is low. In this paper, we propose a DPS protocol to achieve a higher tolerance than the one in the original DPS protocol, in which the measurement setup is less demanding than the one of the RRDPS protocol for the high tolerance regime. We call our protocol the small-number-random DPS (SNRDPS) protocol, and in this protocol, we add only a small amount of randomness to the original DPS protocol, i.e., $2\ensuremath{\le}|\mathcal{R}|\ensuremath{\le}10$. In fact, we found that the performance of the SNRDPS protocol is significantly enhanced over the original DPS protocol only by employing a few additional delays such as ∣R∣=2. Also, we found that the key generation rate of the SNRDPS protocol outperforms the RRDPS protocol without monitoring the bit error rate when it is less than 5 and $|\mathcal{R}|\ensuremath{\le}10$. Our protocol is an intermediate protocol between the original DPS protocol and the RRDPS protocol, and it increases the variety of the DPS-type protocols with quantified security.
Abstract Differential-phase-shift (DPS) quantum key distribution stands as a promising protocol due to its simple implementation, which can be realized with a train of coherent pulses and a passive measurement unit. To implement the DPS protocol, it is crucial to establish security proofs incorporating practical imperfections in users’ devices, however, existing security proofs make unrealistic assumptions on the measurement unit using a Mach–Zehnder interferometer. In this paper, we enhance the implementation security of the DPS protocol by incorporating a major imperfection in the measurement unit. Specifically, our proof enables us to use practical beam splitters with a known range of the transmittance rather than the one with exactly 50%, as was assumed in the existing security proofs. Our numerical simulations demonstrate that even with fluctuations of ± 0.5 % in the transmittance from the ideal value, the key rate degrades only by a factor of 0.57. This result highlights the feasibility of the DPS protocol with practical measurement setups.
In quantum key distribution (QKD), eavesdropping is generally detected based on bit errors resulting from eavesdropper attacks. This study proposes a QKD scheme for detecting eavesdropping based on the photon-detection rate instead of the bit error rate. The differential-phase-shift (DPS) QKD protocol, which transmits lasting weak coherent pulses, is modified such that decoy pulses with a large mean photon number are randomly inserted into a signal pulse sequence. With this modification, eavesdropping can be detected by monitoring the photon-detection rates at the time slots around the decoy pulses. The key-creation performance of the modified DPS-QKD system is evaluated, and the results indicate that the QKD distance is increased with the modification.
Differential phase-shift (DPS) quantum key distribution (QKD) is a unique QKD protocol that is different from traditional ones, featuring simplicity and practicality. In this work, we simulated the DPS-QKD experiment conducted by (Liu et al., 2013), using OptiSystem 7. To the best of our knowledge, this is the first simulation work on DPS-QKD using a single photon source.We used a random number generator to get the phase modulation pattern of N=5, 7,9,11 and 13, while for the 3 and 15 pulse cases, the pattern adopted in the experiment was used. When the number of pulse (N) was 3, a quantum bit error rate (QBER) of 3.0%, which is lower than the minimum QBER of 4.12% required for unconditional security, was obtained. The key creation efficiency increases with the increase in the number of pulse up to 15, as it reaches 93.4% but at the expense of the increment in QBER. The result of our simulation is, on some aspect, in agreement with the experimental result. However, we were able to extend the transmission distance from 3 meter, as in the experiment, to 10 meter. The coincidence count obtained was also in total agreement with the one obtained from the experiment. The result of the average QBER indicated that increase in the pulse number N causes the QBER to raise up due to longer rise and fall time of phase modulation step which affect the MZ inference. Therefore, we suggest using a faster waveform generator with shorter rise and fall times will remarkably lower the QBER. Extending the transmission coverage to a longer distance while, at the same time reducing the QBER with full unconditional security will part of the future research.