Quantum conference key agreement based on differential-phase-shift quantum key distribution

The version of record of this article, first published in Quantum Information Processing, is available online at Publisher’s website: https://doi.org/10.1007/s11128-024-04453-3.A quantum conference key agreement (QCKA) protocol based on differential-phase-shift quantum key distribution is presented, which provides a common secret key for secure communication between more than two parties. In the proposed protocol, one party simultaneously broadcasts a weak coherent pulse train with {0, π} phases to multiple parties that measure the phase differences between adjacent pulses using a delay interferometer followed by photon detectors, and the transmitter and receivers share secret key bits from the coincident counts in the receivers. The system setup and operation are simpler than those of conventional QCKA schemes that use a multipartite quantum entanglement state. The key creation performance is evaluated by considering the eavesdropping probability. The results indicate that the proposed scheme offers better performance than the conventional entanglement-based QCKA system

Multiuser Differential-Phase-Shift Quantum Key Distribution System on a Ring Network

Inoue K., Honjo T.. Multiuser Differential-Phase-Shift Quantum Key Distribution System on a Ring Network. IEEE Photonics Technology Letters 36, 989 (2024); https://doi.org/10.1109/LPT.2024.3424432.Quantum key distribution (QKD) has been studied, which is basically a point-to-point system. Therefore, when constructing a QKD network in which any pair of multiple users shares a secret key, full-mesh connections equipping each user with a transmitter and a receiver must be installed. This study presents a QKD system based on the differential-phase-shift QKD protocol, which accommodates multiusers on one transmission line. The proposed system uses a ring network connecting multiple nodes, in which one node is equipped with a transmitter and a receiver and the other nodes are with phase-modulation circuits. The system performance is evaluated and the results indicate that a 100-km ring network can support up to seven nodes

Scalable implementation of (d+1)(d+1) mutually unbiased bases for dd-dimensional quantum key distribution

A high-dimensional quantum key distribution (QKD) can improve error rate tolerance and the secret key rate. Many $d$-dimensional QKDs have used two mutually unbiased bases (MUBs), while $(d+1)$ MUBs enable a more robust QKD. However, a scalable implementation has not been achieved because the setups have required $d$ devices even for two MUBs or a flexible convertor for a specific optical mode. Here, we propose a scalable and general implementation of $(d+1)$ MUBs using $\log_p d$ interferometers in prime power dimensions $d=p^N$. We implemented the setup for time-bin states and observed an average error rate of 3.8% for phase bases, which is lower than the 23.17% required for a secure QKD against collective attack in $d=4$.Comment: 6 pages, 3 figures, followed by Supplemental Material of 8 pages, 1 figure, 1 tabl

Generation of a time-bin Greenberger--Horne--Zeilinger state with an optical switch

Multipartite entanglement is a critical resource in quantum information processing that exhibits much richer phenomenon and stronger correlations than in bipartite systems. This advantage is also reflected in its multi-user applications. Although many demonstrations have used photonic polarization qubits, polarization-mode dispersion confines the transmission of photonic polarization qubits through an optical fiber. Consequently, time-bin qubits have a particularly important role to play in quantum communication systems. Here, we generate a three-photon time-bin Greenberger-Horne-Zeilinger (GHZ) state using a 2 x 2 optical switch as a time-dependent beam splitter to entangle time-bin Bell states from a spontaneous parametric down-conversion source and a weak coherent pulse. To characterize the three-photon time-bin GHZ state, we performed measurement estimation, showed a violation of the Mermin inequality, and used quantum state tomography to fully reconstruct a density matrix, which shows a state fidelity exceeding 70%. We expect that our three-photon time-bin GHZ state can be used for long-distance multi-user quantum communication.Comment: 8 pages, 4 figures, 1 tabl

10-GHz-clock time-multiplexed non-degenerate optical parametric oscillator network with a variable planar lightwave circuit interferometer

A coherent XY machine (CXYM) is a physical spin simulator that can simulate the XY model by mapping XY spins onto the continuous phases of non-degenerate optical parametric oscillators (NOPOs). Here, we demonstrated a large-scale CXYM with >47,000 spins by generating 10-GHz-clock time-multiplexed NOPO pulses via four-wave mixing in a highly nonlinear fiber inside a fiber ring cavity. By implementing a unidirectional coupling from the i-th pulse to the (i+1)-th pulse with a variable 1-pulse delay planar lightwave circuit interferometer, we successfully controlled the effective temperature of a one-dimensional XY spin network within two orders of magnitude.Comment: 5 pages, 4 figure