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arXiv:2310.02115

Daytime and Nighttime QKD over an atmospheric free space channel with passive polarisation bases compensation

Saumya Ranjan Behera, Melvee George, Urbasi Sinha

Published Oct 3, 2023• 1 citation
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Abstract

Quantum Communication (QC) represents a promising futuristic technology, revolutionizing secure communication. Photon-based Quantum Key Distribution (QKD) is the most widely explored area in QC research, utilizing the polarisation degree of freedom of photons for both fibre and free-space communication. In this work, we investigate and mitigate the challenges posed by fibre birefringence and atmospheric effects on QKD, using a $50$-meter free-space optical link and entanglement-based BBM92 QKD protocol. We implement a passive polarisation correction scheme to address the critical issue of polarisation scrambling induced by fibre birefringence and the difference in the frame of reference between Alice and Bob. This scheme effectively mitigates these adverse effects, ensuring reliable polarisation control over the quantum channel. Furthermore, we conduct QKD experiments in both day and night conditions, encountering challenges such as high background noise levels and dynamic environmental changes. To overcome these issues, we employ various filtering techniques to enhance signal quality and security. Our results demonstrate the successful implementation of QKD over a free-space optical link by producing information-theoretic secure QBER of $<11\%$ on an average and high keyrate, even under varying lighting and weather conditions. Over one 24 hour cycle of data acquisition, we measured an average daylight keyrate and QBER of ($3.9118\pm0.7339 KHz$ and $10.5518\pm1.3428\%$) respectively and night time keyrate and QBER of ($4.6118\pm0.8088 KHz$ and $10.3545\pm1.2501\%$) respectively.

DOI
10.48550/arxiv.2310.02115

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Quantum key distribution (QKD) enables information-theoretically secure communication by exploiting the quantum properties of single photons. Real-world deployment of QKD, however, requires reliable operation under uncontrolled environmental conditions where background illumination, atmospheric loss, coupling fluctuations, and polarization drift can significantly degrade performance. In this work, we experimentally demonstrate entanglement-based free-space QKD over an atmospheric channel with a passive, low-bandwidth polarization-correction strategy that avoids high-bandwidth real-time polarization tracking during the QKD acquisition. Using a 50-m free-space optical link implementing the BBM92 protocol, we compensate polarization scrambling arising from fiber birefringence and reference-frame misalignment by optimizing Bob’s measurement bases through quantum state tomography. QKD performance is evaluated across three 24-h acquisition campaigns spanning daylight and nighttime conditions, where high background noise, atmospheric fluctuations, and polarization drift pose significant challenges for secure key generation. By combining spectral, temporal, and spatial filtering with optimized coincidence-window selection, threshold-compatible average QKD performance is maintained across these acquisition campaigns. The optimized daylight key rate and QBER are ( 6.39 ± 0.81 k H z , 8.05 ± 1.48 % ) , while nighttime operation yields ( 6.98 ± 0.81 k H z , 8.25 ± 0.85 % ) . Under representative operating conditions with 8.3 % QBER, we obtain a final secure key rate of 351 H z after reconciliation and privacy amplification. To assess loss tolerance, we further emulate attenuation equivalent to a 1 k m free-space link using controlled attenuation, demonstrating approximately 6 k H z raw key rate under 30 % additional link loss. This attenuation study reproduces the additional loss budget only and does not constitute a physical kilometer-scale free-space demonstration. These results demonstrate that tomography-assisted measurement-basis correction can enable stable free-space entanglement-based QKD without high-bandwidth polarization tracking during key acquisition, providing a basis for future studies of more dynamic terrestrial and satellite-relevant quantum communication links.
Free-space quantum key distribution (QKD) systems are often designed to implement polarization-encoding protocols. Alternatively, time-bin/phase-encoding protocols are considerably more challenging to perform over a channel experiencing atmospheric turbulence. However, over the last decade, new and improved optical platforms have revived the interest in them. In this paper, we present a free-space multi-protocol receiver designed to work with three different time-bin/phase-encoding protocols highlighting its interoperability with different systems and architectures for potential satellite-based communications. We also present a detailed analysis of different experimental configurations when implementing the coherent one-way (COW) protocol in a free-space channel, as well as a polarization filtering technique showing how time-bin/phase-encoding protocols could be used for QKD applications in daylight conditions. We demonstrate secret key rates of several kbps for channels with a total 30 dB attenuation even with moderately high QBERs of ≈3.5%. Moreover, a 2.6 dB improvement in the signal to noise ratio is achieved by filtering background light in the polarization degree of freedom, a technique that could be used in daylight QKD.
Quantum Key Distribution (QKD) employs the laws of quantum mechanics for the purpose of cryptography. Two parties, commonly called Alice and Bob, are able to share a random key which is used to encrypt a message. Any eavesdropper trying to intercept their key will have to make measurements, thereby disturbing the system. This can be detected by Alice and Bob and they will then discard their key. Polarisation encoded QKD protocols use the polarisation of single photons as qubits to generate a cryptographic key. This can be implemented using a fibre optic link between Alice and Bob but the polarisation of light is altered when passed through a fibre due to birefringence caused by asymmetries in the fibre. This causes refractive differences for orthogonal components of the state of polarisation of light, so the polarisation is rotated as the photon is transmitted through the fibre. If the fibre is fixed, the change of polarisation will be unique and constant. This can be compensated by rotating each photon appropriately to its original state. Under typical environmental conditions, such as temperature changes and vibrations, the birefringence effects vary and should be compensated in real time. Therefore, an active polarisation controller is needed in order to maintain the state of polarisation of each qubit. An investigation was done to first track how the state of polarisation changes over time in a natural environment. Both wavelength-division multiplexing and time-division multiplexing were investigated as testing methods for the compensation system. A timedivision multiplexed system was developed to compensate the changes in polarisation. Since QKD protocols such as BB84 and B92 utilise two non-orthogonal bases, two polarisation controllers are usually used for compensation. However, by using a search algorithm, one polarisation controller was able to isolate the plane on the Poincare sphere that passes through both bases, thus compensating non-orthogonal states with one device.
Quantum key distribution (QKD) promises information-theoretically secure communication, but future networks must bridge fiber and free-space links that naturally employ different photonic encodings, namely time-bin in fiber and polarization in free space. Here we demonstrate a complete hybrid fiber and free-space QKD link that bridges both media within a single end-to-end protocol, converting between the two encodings entirely in the optical domain. Using the decoy-state BB84 protocol operating at 1550 nm, we demonstrate continuous secure-key generation over a 90 m outdoor free-space link. The system operates across atmospheric conditions spanning more than two orders of magnitude in the refractive-index structure parameter Cn^2, from strong daytime turbulence to quiescent nighttime conditions, and we further validate photon-level operation over a 750 m free-space extension. Throughout, the link maintains a session-mean quantum bit error rate (QBER) of 5.6-6.8%, well below the 11% BB84 security threshold. The encoding conversion is performed entirely in the optical domain without measurement or state reconstruction, preserving the security assumptions of the BB84 protocol. Consequently, the time-bin-to-polarization (T2P) and polarization-to-time-bin (P2T) converters remain part of the untrusted quantum channel rather than trusted intermediate nodes. These results establish secure photonic encoding conversion as a practical interface between fiber and free-space quantum communication platforms, providing a building block for future quantum networks applications.
Quantum key distribution – a process that encodes digital information – often utilises fibre optic technologies for commercial applications. Fibre provides the benefit of a dark channel as well as the convenience of independence of a line-of-sight connection between the sender and receiver. In order to implement quantum key distribution protocols utilising polarisation encoding, the birefringence effects of fibre must be compensated for. Birefringence is caused by manufacturing impurities in the fibre or a change in environmental conditions and results in a rotation of the state of polarisation of light as it is propagated through the fibre. With dynamic environmental conditions, the birefringence effects should be monitored with a test signal at regular time intervals so that the polarisation of each photon can be appropriately compensated to its original state. Orthogonal states are compensated simultaneously, but most protocols, such as BB84 and B92, require non-orthogonal basis sets. Instead of using a compensator for each basis, the presented scheme fixes the polarisation controller onto the plane on the Poincaré that passes through both bases, compensating both non-orthogonal bases simultaneously.
Quantum key distribution (QKD) is a promising technology for secure communication, using quantum physics concepts to encrypt information.The present study examines the effects of modulation schemes and adaptive optics (AO) on the QKD system performance over a variable link distance upto 100 km.AO is implemented to mitigate turbulence effects, while various modulation schemes are tested to improve the system performance for varying link ranges and atmospheric conditions.To check the system performance: transmittance, signal-to-noise ratio (SNR), and quantum bit error rate (QBER) are evaluated.The study found that AO significantly improved transmittance and SNR, particularly over longer distances.Binary Phase Shift Keying (BPSK) modulation demonstrated superior performance in terms of QBER compared to other digital modulation schemes, indicating its efficacy in enhancing data security and transmission reliability.