Experimental demonstration of scalable quantum cryptographic conferencing

TL;DR

The paper addresses the challenge of scalable multi-user quantum cryptographic conferencing (QCC) by eliminating the need for multi-user coincidence detection. It introduces mode-pairing QCC with three-party phase compensation, time-bin phase encoding, and an efficient port-index pairing strategy to construct GHZ-state correlations from coherence-time correlated events. Experimentally, it demonstrates three-user QCC over a total loss of $66.3$ dB (≈$331.5$ km) achieving a secure key rate of $5.4$ bit/s, surpassing the repeaterless bound $R \le -\log_{2}(1-\eta^{2})$ and showing near-linear scaling with transmittance $\eta$. The results establish a practical pathway to scalable metropolitan quantum networks and generalize to more users, enabling secure multi-user communication without entanglement distribution or quantum repeaters.

Abstract

Quantum network enables a variety of quantum information processing tasks, where multi-user quantum communication is one of the important objectives. Quantum cryptographic conferencing serves as an essential solution to establish secure keys to realize secure multi-user communications. However, existing QCC implementations have been fundamentally limited by the low probability of multi-user coincidence detection to measure or construct the Greenberger-Horne-Zeilinger (GHZ) entangled state. In this work, we report the experimental realization of QCC eliminating the need for coincidence detection, where the GHZ state is constructed by correlating detection events occurring within the coherence time, thereby greatly enhancing the success probability of GHZ-state measurement. Meanwhile, to establish and maintain high-visibility GHZ measurement among three independent users, we developed a three-party phase compensation scheme combined with precise temporal and polarization alignment within a time-bin-phase encoding framework. Furthermore, we designed an efficient pairing strategy to simplify subsequent data processing and enhance processing efficiency. Based on these techniques, we successfully performed QCC experiments over total channel losses of 66.3 dB, corresponding to 331.5 km of commercial fiber (0.2 dB/km), achieving secure key rates of 5.4 bit/s, whereas previous QCC experiments have been limited to 100 km. The results surpass the multi-user repeaterless bound in quantum networks, establishing a new regime of scalable, multi-user quantum communication and paving the way for metropolitan quantum networks.

Figures (4)

• Figure 1: (a) Multiple users send light pulses to a central measurement node, where a ring-structured detection architecture enables pairwise single-photon interference among all users through multiple detectors ports. (b) At each detection port, the pulse sequence indicates the responses details of single-photon detectors (SPD). Green pulses indicate time slots where at least one single-photon detector registers a click, whereas gray pulses correspond to no detector response. In the coincidence-based method, an effective event occurs only when all ports produce detections at the same pulse index. In contrast, the proposed pairing method identifies valid events by finding detector responses from each port within the coherence time window, without requiring detections to occur at identical pulse positions. (c) We present the experimental results showing the variation in the number of effective events as a function of the maximum pairing interval under two different pulse detection probabilities, $P_{\mathrm{click}}$. For comparison, the number of effective events obtained from coincidence detection is also plotted, while Total denotes the total number of detected pulses. The Pairing results indicate that most detection clicks can be successfully paired, whereas the Coincidence events correspond to the expected $P_{\mathrm{click}}^2$ fraction of the total detected clicks.
• Figure 2: Experimental Setup. A single laser is split into three beams serving as sources for Alice, Bob, and Charlie. Each beam is modulated by an intensity modulator and a Sagnac loop comprising a circulator, a beam splitter, and a phase modulator, followed by discrete 16-level phase modulation for randomization. The pulses are attenuated to the single-photon level using an electronically variable optical attenuator before transmission through either fiber or free-space attenuation channels. At the receiver, three BSs enable pairwise interferences between Alice–Bob, Bob–Charlie, and Charlie–Alice. Polarization feedback is applied using electronic polarization controllers and polarization beam splitters, and the outputs are detected by single-photon detectors. Abbreviations: IM, intensity modulator; Cir, circulator; BS, beam splitter; Att, attenuator; PM, phase modulator; EVOA, electronically variable optical attenuator; EPC, electronic polarization controller; SPD, single-photon detector.
• Figure 3: Single-photon interference and collective phase dependence among users. (a–c) Error rate of single-photon interference as a function of the relative delay $\Delta t$ between interfering pulses for each user pair: (a) Alice–Bob, (b) Bob–Charlie, and (c) Charlie–Alice. The error rate reaches a minimum at $\Delta t \approx 0$, indicating optimal temporal overlap and high-visibility interference. (d,e) Measured proportions of $\Phi^+$ and $\Phi^-$ components as functions of the relative phase-difference signs $(\theta_B - \theta_A)$, $(\theta_C - \theta_B)$, and $(\theta_A - \theta_C)$. Each cube vertex represents one combination of sign conventions (“$+$” or “$-$”) corresponding to the measured proportion. “No flip” denotes the physically correct choice of phase-difference sign, while “flip” indicates an incorrect sign assignment. The results highlight the collective phase dependence among the three parties in GHZ-state measurement.
• Figure 4: Key-rate performance and phase-compensation results. a. Secret key rate versus total channel loss, compared with the repeaterless the repeaterless multi-end communication bound and previous experimental results du2025experimentalyang2024experimentalproietti2021experimental. b. Stability of the X-basis error rate over time, with the theoretical lower bound (37.5%) indicated. c. Error-rate distribution with and without phase compensation at 51.8 dB and 66.3 dB total loss, where W/O PC denotes the case without phase compensation and With PC denotes the case with phase compensation.