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Volume Bragg gratings for wavelength division multiplexing of entanglement-based quantum key distribution in free space

Riza Fazili, Fabian Steinlechner

Published Feb 28, 2020• 2 citations
Publisher

Abstract

Wavelength division multiplexing (WDM) improves key generation rates in quantum key distribution (QKD) and enables multi-user networks using shared quantum resources. The last years have seen a flurry of research on the implementation of WDM in fiber-based QKD where low-loss telecom components are widely available. However, WDM has not yet been demonstrated in free-space and satellite-based quantum communication in the wavelength range detectable by spacesuitable silicon single photon avalanche diodes. Here, we report a low-loss WDM at around 810 nm based on volume Bragg gratings. The total insertion loss for four wavelength channels matches state-of-the-art telecommunication WDM in the C-band. We characterize the performance in conjunction with a polarization-entangled photon source and report on preliminary assessments of background noise for a terrestrial free-space link in broad daylight.

DOI
10.1117/12.2545671

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Wavelength division multiplexing (WDM) improves key generation rates in quantum key distribution (QKD) and enables multi-user networks using shared quantum resources. The last years have seen a flurry of research on the implementation of WDM in fiber-based QKD where low-loss telecom components are widely available. However, WDM has not yet been demonstrated in free-space and satellite-based quantum communication in the wavelength range detectable by spacesuitable silicon single photon avalanche diodes. Here, we report a low-loss WDM at around 810 nm based on volume Bragg gratings. The total insertion loss for four wavelength channels matches state-of-the-art telecommunication WDM in the C-band. We characterize the performance in conjunction with a polarization-entangled photon source and report on preliminary assessments of background noise for a terrestrial free-space link in broad daylight.
Quantum Key Distribution (QKD) exploits the rules of quantum mechanics to generate and securely distribute a random sequence of bits to two spatially separated clients. Typically a QKD system can support only a single pair of clients at a time, and so a separate quantum link is required for every pair of users. We overcome this limitation with the design and characterization of a multi-client entangled-photon QKD system with the capacity for up to 100 clients simultaneously. The time-bin entangled QKD system includes a broadband down-conversion source with two unique features that enable the multi-user capability. First, the photons are emitted across a very large portion of the telecom spectrum. Second, and more importantly, the photons are strongly correlated in their energy degree of freedom. Using standard wavelength division multiplexing (WDM) hardware, the photons can be routed to different parties on a quantum communication network, while the strong spectral correlations ensure that each client is linked only to the client receiving the conjugate wavelength. In this way, a single down-conversion source can support dozens of channels simultaneously--and to the extent that the WDM hardware can send different spectral channels to different clients, the system can support multiple client pairings. We will describe the design and characterization of the down-conversion source, as well as the client stations, which must be tunable across the emission spectrum.
Quantum key distribution (QKD) is a new technique for secure key distribution based on the laws of physics rather than mathematical or algorithmic computational complexity used by current systems. Understanding the compatibility of QKD at 1310 nm with the existing commercial optical networks bearing classical wavelength-division-multiplexed (WDM) channels at 1550 nm is important to advance the deployment of QKD systems in such networks. The minimum wavelength separation for multiplexing QKD and WDM channels on a shared fiber is experimentally determined for impairment-free QKD+WDM transmission.
Top-performance sources of photonic entanglement are an indispensable resource for many applications in quantum communication, most notably quantum key distribution. However, up to now, no source has been shown to simultaneously exhibit the high pair-creation rate, broad bandwidth, excellent state fidelity, and low intrinsic loss necessary for gigabit secure key rates. In this work, we present for the first time a source of polarization-entangled photon pairs at telecommunication wavelengths that covers all these needs of real-world quantum-cryptographic applications, thus enabling unprecedented quantum-secure key rates of more than 1 Gbit/s. Our source is designed to optimally exploit state-of-the-art telecommunication equipment and detection systems. Any technological improvement of the latter would result in an even higher rate without modification of the source. We discuss the used wavelength-multiplexing approach, including its potential for multi-user quantum networks and its fundamental limitations. Our source paves the way for high-speed quantum encryption approaching present-day internet bandwidth.
A high-speed quantum key distribution system was developed with the wavelength-division multiplexing (WDM) technique and dedicated key distillation hardware engines. Two interferometers for encoding and decoding are shared over eight wavelengths to reduce the system's size, cost, and control complexity. The key distillation engines can process a huge amount of data from the WDM channels by using a 1 Mbit block in real time. We demonstrated a three-channel WDM system that simultaneously uses avalanche photodiodes and superconducting single-photon detectors. We achieved 12 h continuous key generation with a secure key rate of 208 kilobits per second through a 45 km field fiber with 14.5 dB loss.
Future quantum communication infrastructures will need to serve heterogeneous users on shared physical channels: short-range, high-throughput links favor Continuous-Variable Quantum Key Distribution (CV-QKD), while long-reach, high-loss links remain the domain of Discrete-Variable QKD (DV-QKD). Wavelength-division multiplexing (WDM) of the two protocols on a common channel would address both regimes simultaneously, but their markedly different noise sensitivities make coexistence non-trivial and, to date, experimentally untested. Here we report the first simultaneous operation of two independent CV- and DV-QKD systems on a common optical channel, using standard C-band DWDM filters at 1550.12 nm (CV) and 1545.32 nm (DV). We demonstrate joint operation on both optical fiber and a 620 m urban daylight free-space link. On fiber, the two systems exhibit the expected complementarity, crossing over at 7.56 dB of channel loss where both deliver $\sim$1.43 Mbit/s; in daylight free-space, both sustain Mbit/s key rates under time-varying atmospheric attenuation. Across all configurations we observe no measurable multiplexing-induced penalty in QBER or excess noise. These results establish hybrid CV-DV WDM as a practical building block for heterogeneous quantum communication networks, where metropolitan high-throughput users and long-reach backbone links can be served on a single physical infrastructure.
In view of real world applications of quantum information technologies, the combination of miniature quantum resources with existing fibre networks is a crucial issue. Among such resources, on-chip entangled photon sources play a central role for applications spanning quantum communications, computing and metrology. Here, we use a semiconductor source of entangled photons operating at room temperature in conjunction with standard telecom components to demonstrate multi-user quantum key distribution, a core protocol for securing communications in quantum networks. The source consists of an AlGaAs chip emitting polarization entangled photon pairs over a large bandwidth in the main telecom band around 1550 nm without the use of any off-chip compensation or interferometric scheme; the photon pairs are directly launched into a dense wavelength division multiplexer (DWDM) and secret keys are distributed between several pairs of users communicating through different channels. We achieve a visibility measured after the DWDM of 87% and show long-distance key distribution using a 50-km standard telecom fibre link between two network users. These results illustrate a promising route to practical, resource-efficient implementations adapted to quantum network infrastructures.