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arXiv:2011.03843 · IET Quantum Communication

Increasing the Link‐distance of Free‐space Quantum Coherent Communication with Large Area Detectors

Rupesh Kumar, Igor Konieczniak, Gerald M. Bonner, Timothy P. Spiller

Published Mar 1, 2021• 0 citations
arXivHTMLPDF

Abstract

Abstract A large area photo‐diode based homodyne detector for free‐space quantum coherent communication is reported. The detector's performance is studied in terms of the detection bandwidth and electronic noise for shot‐noise limited quantum signal detection. Using large area photo‐diodes increases the signal collection efficiency from turbulent atmospheric channels, in comparison with thetypical fibre‐based free‐space homodyne detectors. Under identical atmospheric turbulence and receiver aperture conditions, over a 700‐km free‐space link at 90° elevation angle, our homodyne detector based on a diameter of 1mm photo‐diode experiences 0dB loss due to turbulence, while a 10‐um fibre‐based detector experiences 13.5dB of signal loss.

DOI
10.1049/qtc2.12007

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Abstract A large area photo‐diode based homodyne detector for free‐space quantum coherent communication is reported. The detector's performance is studied in terms of the detection bandwidth and electronic noise for shot‐noise limited quantum signal detection. Using large area photo‐diodes increases the signal collection efficiency from turbulent atmospheric channels, in comparison with thetypical fibre‐based free‐space homodyne detectors. Under identical atmospheric turbulence and receiver aperture conditions, over a 700‐km free‐space link at 90° elevation angle, our homodyne detector based on a diameter of 1mm photo‐diode experiences 0dB loss due to turbulence, while a 10‐um fibre‐based detector experiences 13.5dB of signal loss.
ABSTRACT In free‐space quantum key distribution (QKD), atmospheric turbulence affects quantum signals by causing intensity fluctuations, phase distortions, and beam wander, which may be exploited by an eavesdropper. Accurate characterization of the quantum channel and receiver is, therefore, essential. Detector efficiency mismatch has previously been identified as a possible side‐channel vulnerability. This study examines the impact of atmospheric turbulence on the security of a BB84 QKD receiver under a spatial‐mode mismatch attack. Turbulence is emulated in the laboratory using phase screens implemented on a spatial light modulator. Results show that turbulence exacerbates inherent detection probability mismatches, increasing the receiver's vulnerability. Information leakage to an eavesdropper is characterized and quantified based on both detection mismatches and turbulence strengths. A comparative analysis between Gaussian and Laguerre‐Gaussian (LG) modes shows that LG modes, particularly those of higher order, exhibit enhanced resilience against turbulence‐induced information leakage. These findings highlight the importance of receiver characterization and spatial mode selection for the secure implementation of QKD in long‐distance and satellite‐based systems under realistic atmospheric conditions.
Optimising the interconnection between free-space and fibre links will be necessary for future quantum communication networks. In daylight free-space quantum communication based on direct detection, the required Field Of View (FoV) of the receiver system needs to be minimised to reduce solar background noise coupling into the detectors. Reducing the FoV requires minimising beam wander effects caused by atmospheric turbulence through active optics. We implement a fine tracking system designed to correct tip and tilt wavefront aberrations, using two feedback loops; each of them consisting of a quadrant detector and a fast steering mirror for stabilising the beam in the whole optical axis of the receiver. We test the performance of the tracking system with different optical fibres in order to evaluate the reduction in the quantum bit error rate (QBER) caused by solar background noise. A reduction of 75% for single mode fibre was obtained, and 45% reduction for a 25 µm core diameter fibre, both cases for strong turbulence (Cn2~10-12 – 10-13 m-2/3) and 100 m propagating channel. These results look promising for enabling free-space Quantum Key Distribution (QKD) in wireless networks for realistic/adverse conditions such as daylight and strong turbulent regimes.
Summary form only given. Quantum key distribution (QKD) has raised increased attention over the past years as one of the most attractive quantum technologies for practical implementation. QKD has already been implemented in intra-city networks all around the world. But up to now, bridging global distances with quantum communication remains an outstanding challenge. A promising candidate to provide this link is via optical satellite communication. As space-to-ground communication is already well developed for classical applications, one can make use of the already existing technology for QKD, i.e. modern Laser Communication Terminals (LCTs) may be adapted for quantum communication. An important first step to achieve this goal is a precise characterization of the system and the channel with regard to their quantum noise behaviour. We present quantum-limited measurements of signals from an LCT located on the satellite Alphasat in geostationary Earth orbit. The LCT generates binary phase shift keyed (BPSK) coherent states originally used for classical data communication. Our receiver is a Transportable Adaptive Optical Ground Station (T-AOGS) located at the Teide observatory on Tenerife. It corrects for phase front distortions in order to interfere the incoming signal with a local oscillator. This allows for heterodyne detection or, by phase-locking the local oscillator, for homodyne detection. The detector itself is quantum noise limited such that it can resolve the quantum uncertainty of the detected states (see Fig. 1). Evaluating the signal variance for homodyne detection, we are able to show that the detected states are quantum limited coherent states. Thus, we conclude that the coherence of the quantum states is preserved after propagation over a distance of 38600 km including the passage through turbulent atmosphere and we give an upper bound for any excess noise that the states might have acquired [1]. Moreover, we extent our analysis to heterodyne detection, characterizing arbitrary field quadratures. Our results indicate that satellite quantum communication is feasible in principle in such a scenario.
Entanglement is key quantum information processing feature enabling quantum communication, networking, and sensing [1]. Unfortunately, long-distance quantum communication is affected by the photon loss, given that quantum signals cannot be amplified. As an alternative, free-space optical (FSO) links can be used instead for entanglement distribution. However, the FSO links are severely affected by atmospheric turbulence effects, in addition to scattering effects. In this paper, we demonstrate that adaptive optics [2] is a useful tool to improve fidelity of entanglement when entangled states are distributed over turbulent FSO links. We present results from our FSO communication experiment where we generated entangled photons pairs, launched them into free-space, and later collected them for coincidence count measurement. We demonstrate that adaptive optics can be used to make improvements in coincidence counts.
Abstract In Discrete Variable Quantum Key Distribution (DV-QKD), the homodyne detection method is frequently employed for its simplicity in use, effectiveness in terms of error correction, and suitability with contemporary optical communication systems. Being a coherent detection method, it relies on a local oscillator whose frequency is matched to that of the transmitted carrier's signal. In this paper, we evaluate a Free Space Optical (FSO) DV-QKD system based on the KMB09 protocol using Homodyne detection under random phase fluctuation and depolarizing noise error. We present simulation results for System Efficiency and Quantum Bit Error Rate (QBER) for the proposed model. An obtained efficiency ({25%25\%25%}) for our proposed DV-QKD system model shows that under atmospheric turbulence and noise effect and it is in line with the available analytical results. However, the inclusion of random phase fluctuation and noise led to higher-than-normal QBER which is anticipated in a real-world scenario.
Quantum Key Distribution (QKD) promises a provably secure method to transmit information from one party to another. Free-space QKD allows for this information to be sent over great distances and in places where fibre-based communications cannot be implemented, such as ground-satellite. The primary limiting factor for free-space links is the effect of atmospheric turbulence, which can result in significant error rates and increased losses in QKD channels. Here, we employ the use of a high-speed Adaptive Optics (AO) system to make real-time corrections to the wavefront distortions on spatial modes that are used for high-dimensional QKD in our turbulent channel. First, we demonstrate the effectiveness of the AO system in improving the coupling efficiency of a Gaussian mode that has propagated through turbulence. Through process tomography, we show that our system is capable of significantly reducing the crosstalk of spatial modes in the channel. Finally, we show that employing AO reduces the quantum dit error rate for a high-dimensional orbital angular momentum-based QKD protocol, allowing for secure communication in a channel where it would otherwise be impossible. These results are promising for establishing long-distance free-space QKD systems. High-dimensional quantum key distribution will allow for higher information density and greater error tolerance in future quantum networks. This work experimentally demonstrates how implementing an adaptive optics system in a spatial-mode free-space optical link can allow for quantum communications where it would otherwise be impossible.