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Multi-Field Quantum Conference Key Agreement Using an Integrated Photonic Green Machine

Fisher, Benjamin J

Published Jun 13, 2025• 0 citations
OpenAlex

Abstract

Quantum Conference Key Agreement (QCKA) enables a group of users to generate a shared secret key, which reduces the latency and equipment overhead of establishing multiple Quantum Key Distribution (QKD) links. Conventional QCKA protocols send a multi-photon entangled state from one user to others and a key is generated only when all photons are detected, which is inefficient at high loss and opens side-channels at the receiver side. Here, we overcome this barrier through a multi-field QCKA protocol enabled by designing, fabricating, and testing an integrated photonic Green Machine that implements an eight-mode Hadamard unitary transformation through a beam-splitter network. With this integrated photonic circuit as a central untrusted relay, up to eight users can generate the same secure key upon measuring a single arriving photon. In addition to the first-ever realization and packaging of this integrated photonic Green Machine chip, we characterize the Green Machine and create a digital twin simulator which predicts that this chip enables multi-field QCKA among five users up to a range of 154 km between each party and a central relay.

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Quantum Conference Key Agreement (QCKA) enables a group of users to generate a shared secret key, which reduces the latency and equipment overhead of establishing multiple Quantum Key Distribution (QKD) links. Conventional QCKA protocols send a multi-photon entangled state from one user to others and a key is generated only when all photons are detected, which is inefficient at high loss and opens side-channels at the receiver side. Here, we overcome this barrier through a multi-field QCKA protocol enabled by designing, fabricating, and testing an integrated photonic Green Machine that implements an eight-mode Hadamard unitary transformation through a beam-splitter network. With this integrated photonic circuit as a central untrusted relay, up to eight users can generate the same secure key upon measuring a single arriving photon. In addition to the first-ever realization and packaging of this integrated photonic Green Machine chip, we characterize the Green Machine and create a digital twin simulator which predicts that this chip enables multi-field QCKA among five users up to a range of 154 km between each party and a central relay.
Future quantum networks will provide multi-node entanglement enabling secure quantum communication on a global scale. Traditional two-party quantum key distribution (2QKD) consumes pairwise entanglement which is costly in constrained networks. Quantum conference key agreement (QCKA) leverages multipartite entanglement within networks to directly produce identical keys among N users, providing up to N-1 rate advantage over 2QKD. In this contribution I will present work on the implementation of QCKA using photonic GHZ states distributed over telecom fibre of up to 50 km combined length. Furthermore, we implemented QCKA on a constrained network consisting of a 6-qubit photonic graph state on which we apply network coding routines to demonstrate the multi-partite advantage over the two-party paradigm.
Future quantum networks will provide multi-node entanglement enabling secure quantum communication on a global scale. Traditional two-party quantum key distribution (2QKD) consumes pairwise entanglement which is costly in constrained networks. Quantum conference key agreement (QCKA) leverages multipartite entanglement within networks to directly produce identical keys among N users, providing up to N-1 rate advantage over 2QKD. Here, we present a four-user QCKA protocol using photonic GHZ states distributed over fibre with combined lengths up to 50 km. Furthermore, we investigate a constrained network consisting of a 6-qubit photonic graph state which we apply network coding routines to distil suitable resource states.
Multipartite entanglement enables secure group key distribution among multiple users while providing immunity against hacking attacks targeting source devices, thereby realizing source-independent quantum conference key agreement (SI-QCKA). However, previous experimental demonstrations of SI-QCKA have encountered substantial technical challenges, primarily due to the low efficiency and scalability limitations inherent in the generation and distribution of multipartite entanglement. Here, we experimentally demonstrate a scalable and efficient SI-QCKA protocol using polarization-entangled photon pairs in a three-user star network, where Greenberger-Horne-Zeilinger correlations are realized via a post-matching method. We achieve a secure group key rate of 2.11×1042.11 \times 10^{4}2.11×104 bits/s under the single-user channel transmission of 1.64 ×\times× 10−110^{-1}10−1 in a symmetric channel loss network. Additionally, we conduct six sets of experiments to investigate the impact of varying channel transmission and random basis selection probabilities on secure key rates. Our work establishes an efficient pathway for SI-QCKA and demonstrates potential scalability for future large-scale multi-user quantum networks.
Quantum networks aim to enable quantum information tasks among multiple parties. Quantum conference key agreement (QCKA) is a typical task in quantum networks, which distributes information-theoretically secure keys among multiple users. However, QCKA relying on directly distributing Greenberger-Horne-Zeilinger (GHZ) states over long distances faces significant challenges due to the fragility of these states. Measurement-device-independent QCKA (MDI-QCKA) based on distributing the postselected GHZ entanglement can address this issue and eliminate all loopholes in detection side channels. Here, by developing three-photon GHZ interference technology with high visibility among three independent coherent sources, we realized the first MDI-QCKA experiment over a 60 km fiber link, achieving a secret key rate of 45.5 bits/s. Our result represents a significant step towards practical long-distance QCKA using realistic devices. Moreover, the technology we developed opens the way to future multiparty quantum communications in quantum networks.
Abstract 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.