Abstract
We present a scalable star-shaped quantum-key-distribution (QKD) optical-fiber network. We use wavelength-division demultiplexing (WDM) of broadband photon pairs to establish key exchange between multiple pairs of participants simultaneously. Our QKD system is the first entanglement-based network of four participants using BBM92 time-bin coding and the first network to achieve timing synchronization solely by clock recovery based on the photon arrival times. We demonstrate simultaneous bipartite key exchange between any possible combination of participants and show that the quantum bit error rate (QBER) itself can be used to stabilize the phase in the interferometers by small temperature adjustments. The key distribution is insensitive to polarization fluctuations in the network, enabling key distribution using deployed fibers even under challenging environmental conditions. We show that our network can be readily extended to 34 participants by using a standard arrayed-waveguide grating for WDM with 100 GHz channel spacing and that reconfigurable network connections are possible with a wavelength-selective switch. In a field test, we demonstrate secure key rates of 6.3 bits/s with a QBER of 4.5% over a total fiber length of 108 km with 26.8 km of deployed fiber between two participants with high stability. Our system features a relatively simple design of the receiver modules and enables scaling QKD networks without trusted nodes to distances up to more than 100 km and to more than 100 users. With such a network, a secure communication infrastructure on a metropolitan scale can be established.
2 More- Received 21 December 2021
- Accepted 25 April 2022
DOI:https://doi.org/10.1103/PRXQuantum.3.020341
Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.
Published by the American Physical Society
Physics Subject Headings (PhySH)
Popular Summary
The public key infrastructure on which the security of the Internet is based will be rendered insecure by quantum computers. However, physics also provides a solution, in the form of quantum key distribution (QKD): the laws of quantum physics enable provably secure cryptographic key exchange. The main focus of QKD research has been the advancement of key rates and distances for connections between two users but QKD networks are essential for a variety of applications.
So far, QKD networks have relied on daisy-chained connections requiring trusted nodes or have been very sensitive to environmental fluctuations between the network participants. In this work, we present the first robust star-shaped optical-fiber network for simultaneous QKD between any two parties based on time-bin entanglement. It is implemented with imbalanced interferometers in the photon-pair source and the receiver modules. In contrast to polarization-based protocols, it is inherently robust against environmental influences causing variations of the birefringence in the fiber link. But the interferometers must show a high degree of phase stability. We achieve superb stability and are thus able to employ the quantum bit error rate itself as the feedback signal for stabilization. As a proof of concept, we demonstrate a network with four participants and simultaneous key exchange between any two parties.
Our setup is scalable to over 100 participants, providing the first practical approach to networks in real-world applications without a trusted node. This enables the securing of communication infrastructure and the establishment of robust metropolitan-scale QKD networks.