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Canadian Workshop on Information Theory (CWIT)

Relay-assisted free-space quantum-key distribution

Majid Safari, Murat Uysal

Published May 1, 2009• 1 citation
Publisher

Abstract

In this paper, we investigate a terrestrial relay-assisted scheme for a free-space quantum key distribution (QKD) system based on BB84 protocol. We consider the deployment of passive relays which simply redirect (i.e., collect and point) the qubits to the next relay node or to the receiver without performing any measurement or detection process. Based on a near-field analysis, we derive an upper bound on quantum bit error rate (QBER) of the relay-assisted QKD system. Our results demonstrate that the relay-assisted scheme is able to outperform point-to-point direct transmission for long link ranges in which turbulence effects are particularly degrading.

DOI
10.1109/cwit.2009.5069534

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In this paper, we investigate a terrestrial relay-assisted scheme for a free-space quantum key distribution (QKD) system based on BB84 protocol. We consider the deployment of passive relays which simply redirect (i.e., collect and point) the qubits to the next relay node or to the receiver without performing any measurement or detection process. Based on a near-field analysis, we derive an upper bound on quantum bit error rate (QBER) of the relay-assisted QKD system. Our results demonstrate that the relay-assisted scheme is able to outperform point-to-point direct transmission for long link ranges in which turbulence effects are particularly degrading.
In this paper, we investigate a terrestrial relay-assisted scheme for a free-space quantum key distribution (QKD) system based on BB84 protocol. We consider the deployment of passive relays which simply redirect the qubits to the next relay node or to the receiver without performing any measurement or detection process. Based on a near-field analysis, we derive an upper bound on quantum bit error rate (QBER) of the relay-assisted QKD system. Our results demonstrate that the relay-assisted scheme is able to outperform point-to-point direct transmission for long link ranges in which turbulence effects are particularly degrading.
Absorption, scattering, and turbulence experienced in underwater channels severely limit the range of quantum communication links. In this paper, as a potential solution to overcome range limitations, we investigate a multi-hop underwater quantum key distribution (QKD) where intermediate nodes between the source and destination nodes help the key distribution. We consider the deployment of passive relays that simply redirect the qubits to the next relay node or the receiver without any measurement. Based on the near-field analysis, we present the performance of a relay-assisted QKD scheme in terms of quantum bit error rate and secret key rate in different water types and turbulence conditions. We further investigate the effect of system parameters such as aperture size and detector field of view on the performance. Our results demonstrate under what conditions relay-assisted QKD can be beneficial and what end-to-end transmission distances can be supported with a multi-hop underwater QKD system.
Absorption, scattering, and turbulence experienced in underwater channels severely limit the range of quantum communications. In this paper, to overcome range limitations, we investigate a multi-hop underwater quantum key distribution (QKD) where intermediate nodes help the key distribution between the source and destination nodes. We consider deployment of passive-relays which simply redirect the qubits to the next relay node or receiver without any measurement. Based on nearfield analysis, we present the performance of relay-assisted QKD scheme in clear ocean under different atmospheric conditions. We further investigate the effect of system parameters (aperture size and detector field-of-view) on the achievable distance.
The current literature on quantum key distribution (QKD) is mainly limited to the transmissions over fiber optic, atmospheric or satellite links and are not directly applicable to underwater environments with different channel characteristics. Absorption, scattering, and turbulence experienced in underwater channels severely limit the range of quantum communication links. In the first part of this thesis, we analyze the quantum bit error rate (QBER) and secret key rate (SKR) performance of the well-known BB84 protocol in underwater channels. As path loss model, we consider a modified version of Beer-Lambert formula which takes into account the effect of scattering. We derive a closed-form expression for the wave structure function to determine the average power transfer over turbulent underwater path and use this to obtain an upper bound on QBER as well as a lower bound on SKR. In the second part of this thesis, as a potential solution to overcome range limitations, we investigate a multi-hop underwater QKD where intermediate nodes between the source and destination nodes help the key distribution. We consider the deployment of passive relays which simply redirect the qubits to the next relay node or the receiver without any measurement. Based on the near-field analysis, we present the performance of relay-assisted QKD scheme in terms of QBER and SKR in different water types and turbulence conditions. In the last part of this thesis, we investigate the fundamental performance limits of decoy BB84 protocol over turbulent underwater channels and provide a comprehensive performance characterization. Based on near field analysis, we utilize the wave structure function to determine the average power transfer over turbulent underwater path and use this to obtain a lower bound on key generation rate. Based on this bound, we present the performance of decoy BB84 protocol in different water type.
A relay-assisted scheme for quantum key distribution (QKD) over long atmospheric channels is proposed. In this scheme, a polarization orbital-angular-moment with measurement-device-independent (POAM-MDI) QKD for quantum relay is designed to both reconstruct the turbulence-degraded qubits and redirect the qubits to the next relay or to Bob who performs detection randomization. The proposed scheme will neither be time-consuming nor lead to unstable or even intermittent in volatile environments. Theoretical and numerical results show that the proposed relay-assisted scheme outperforms the conventional direct transmission over long atmospheric channels.
Free-space quantum key distribution (QKD) provides crucial advantages, including mobility and deployment exibility, for securing next-generation communication networks. However, atmospheric turbulence and beam wander severely degrade single-photon coupling effciency and link stability over outdoor channels. In this work, we experimentally demonstrate a free-space decoy-state BB84 QKD system operating at 803 nm with a 100 MHz repetition rate over a 2 km outdoor link. By employing an active beam-wander correction based on fast-steering mirrors (FSMs) and position sensitive detectors (PSDs) with a 500 Hz control loop, we enhance the ?ber coupling effciency and achieve an average ?nite-key secure key rate of 110 kbps with an average quantum bit error rate (QBER) of 3.28 %, representing a 2.5 times secure key rate enhancement under an active beam stabilization. This work demonstrates the indispensability of active beam stabilization and provides an operational reference for robust, long-range free-space quantum communication.