Abstract We report on the adaptation of a commercially available fiber-based quantum key distribution (QKD) system with phase-time encoding to free-space operation. The proposed approach minimizes architectural modifications by replacing the single-mode receiving interferometer with a free-space multi-mode delay interferometer, enabling efficient detection of spatially distorted quantum states typical for atmospheric channels. Several interferometer configurations are experimentally investigated, and a multi-mode Michelson interferometer with polarization routing is identified as the optimal solution in terms of interference visibility, loss, and alignment robustness. The impact of chromatic and modal dispersion on phase-time interference is analyzed and shown to be manageable under realistic system parameters. A laboratory QKD experiment demonstrates stable operation with a quantum bit error rate of about 4.5% and a secret key rate of hundreds of bits per second, confirming the feasibility of extending fiber-based QKD platforms to free-space links.
Abstract We report on the adaptation of a commercially available fiber-based quantum key distribution (QKD) system with phase-time encoding to free-space operation. The proposed approach minimizes architectural modifications by replacing the single-mode receiving interferometer with a free-space multi-mode delay interferometer, enabling efficient detection of spatially distorted quantum states typical for atmospheric channels. Several interferometer configurations are experimentally investigated, and a multi-mode Michelson interferometer with polarization routing is identified as the optimal solution in terms of interference visibility, loss, and alignment robustness. The impact of chromatic and modal dispersion on phase-time interference is analyzed and shown to be manageable under realistic system parameters. A laboratory QKD experiment demonstrates stable operation with a quantum bit error rate of about 4.5% and a secret key rate of hundreds of bits per second, confirming the feasibility of extending fiber-based QKD platforms to free-space links.
Abstract We demonstrate a free-space quantum key distribution (QKD) system based on phase-time encoding by reconfiguring a commercially available fiber-based QKD platform. The main goal of this work is to develop a free-space QKD system with minimal modifications, primarily through the implementation of a free-space multi-mode (MM) delay interferometer at the receiver. The use of a MM interferometer is crucial for enabling phase encoding over atmospheric channels, where direct coupling into single-mode fibers suffers from prohibitive losses due to atmospheric turbulence and beam distortions. We review existing methods for detecting spatially distorted quantum states, propose technological improvements, and experimentally evaluate several interferometer designs. A free-space MM Michelson interferometer with polarization routing is identified as the optimal solution, offering high interference visibility, low loss, and ease of integration into the system. A laboratory QKD experiment conducted with the modified system achieved a quantum bit error rate of 4.5% and a stable secret key rate of hundreds bit s−1, demonstrating the feasibility of secure key distribution over free-space links using adapted fiber-based technology. Detailed theoretical and experimental studies of chromatic and modal dispersion effects confirm that their impact can be effectively managed. This approach provides a practical and scalable route for extending quantum-secure communications into scenarios where optical fiber deployment is impractical or impossible.
Abstract We demonstrate a compensation-free approach to the realization of multi-mode delay interferometers, mainly for use in phase-encoded quantum key distribution (QKD). High interference visibility of spatially multi-mode beams in unbalanced Michelson or Mach–Zehnder interferometers with a relatively wide range of delays is achieved by the appropriate choice of the transverse size of the beam. We provide a simple theoretical model that gives a direct connection between the visibility of interference, the delay and the beam parameters. The performed experimental study confirms our theoretical findings and demonstrates measured visibility of up to 0.95 for a delay of 2 ns. Our approach’s simplicity and robust performance make it a practical choice for the implementation of QKD systems, where a quantum signal is received over a multi-mode fiber. The important application of such a configuration is an intermodal QKD system, where the free-space atmospheric communication channel is coupled into a span of the multi-mode fiber, delivering the spatially distorted beam to the remote receiver with minimal coupling loss.
Abstract We report the implementation and field testing of an atmospheric quantum key distribution (QKD) system obtained by adapting the commercially available fiber-based ViPNet-QTS platform. The phase-time state-preparation module, detector electronics, and post-processing procedures were retained, whereas the fiber channel was replaced by external optical devices forming a 400 m urban free-space link. The 1550 nm quantum-channel signal was coupled into a multimode fiber (MMF) at the receiver and analyzed by a free-space multimode delay interferometer, allowing the quantum-cryptographic equipment to remain inside trusted zones. Synchronization and bidirectional pointing were implemented using spectrally separated 850 and 780 nm auxiliary channels. We characterized the channel loss, background-count probability, atmospheric turbulence, and performance of the fast and low-bandwidth tracking loops. Although feedback based on fast steering mirrors strongly reduced the fluctuations of the tracking-beam position at the CMOS sensor, it increased the mean power coupled into the MMF by only 4.07% under the tested conditions. Long-term alignment was instead maintained by low-bandwidth feedback to the motorized mounts, demonstrating that continuous high-bandwidth fine tracking was not required for the investigated 400 m link. During an autonomous four-day QKD experiment, the system operated fully unattended, with no manual intervention in alignment, system control, or post-processing. Secret keys were generated over most of the measurement interval under changing day–night and weather conditions. Temporary interruptions in key generation coincided with strong daylight background, whereas the measured turbulence variations had no similarly direct effect on system operation. These results demonstrate a practical approach to extending existing fiber-based QKD platforms to atmospheric links using an MMF-coupled receiver and a comparatively simple tracking architecture.
Abstract We demonstrate the experimental feasibility of a commercially available fiber-based phase-encoding quantum key distribution system ViPNet QTS, operating over a communication link incorporating a multi-mode fiber (MMF) segment of up to 20 km. Since ViPNet QTS is designed for deployment in single-mode fiber (SMF) communication links, all experiments were conducted in an SMF-MMF-SMF configuration. We report systematic measurements of transmission losses, as well as secret key generation performance for different MMF link lengths. As a result, stable secret key generation was achieved at a rate of 1.75 kbit min −1 over MMF links of up to 20 km.
Quantum key distribution is a quantum communication protocol which seeks to address potential vulnerabilities in data transmission and storage. One of the main challenges in the field is achieving high rates of secret key in lossy and turbulent free-space channels. In this scenario, most experimental demonstrations have used the polarization of photons as their qubit carriers, due to the relative robustness of polarization in free-space propagation. Time-bin or phase-based protocols are considered less practical due to the wave-front distortion caused by atmospheric turbulence. However, demonstrations of novel free-space interferometer designs are enabling interferometers to measure multimodal signals with high visibility. That means it is now viable to consider the prospects of implementing time-bin or phase-based protocols, which have demonstrated high key rates and long transmission distances in optical fiber. In this work, we present the possibilities of implementing time-bin protocols in turbulent free-space channels, using the coherent one-way protocol as the example. We present an analysis of the secret key rate and quantum bit error rate of the system, providing the errors due to noise counts, and the extinction ratio of the pulses. Finally, we developed a model to quantify the expected losses for a turbulence free-space channel, specifically for a free-space satellite-to-ground station channel.
This paper evaluates the feasibility of off-the-shelf quantum key distribution (QKD) systems over multimode fiber (MMF) links. QKD systems with two types of encoding, polarization-based and phase/time-bin-based, are tested using OM1, OM3, and OM4 fibers, with single mode fiber (SMF) as a reference. The connection between single mode QKD components and MMF forms an interferometric single modemulti mode-single mode (SMS) structure that exhibits intensity fluctuations. These, together with modal dispersion enabled by mode coupling, lead to decoherence in phase-encoded systems and significant depolarization in polarization-encoded systems. To minimize mode excitation, the fibers are operated in an underfilled launch regime, enabling near-single mode transmission over short spans.