Quantum key distribution (QKD) has reached a commercially viable stage, with several companies offering hardware systems designed for operational deployment. Evaluating these systems under real-world conditions provides researchers and users with experimental references for their performance and operational variability. In this paper, we present a long-term performance analysis of ID Quantique's Clavis XGR deployed within the Hermes Quantum Network, in Brazil. We characterize the secret key rate, quantum bit error rate (QBER), visibility, and detection count rate across two distinct optical links: a 40.3 km indoor spooled fiber and a 3.5 km outdoor underground fiber. The experimental campaign combines multiweek monitoring under routine laboratory conditions with controlled variations of laboratory temperature. The system exhibited mean visibility above $97\%$ and mean QBER below $1\%$, with brief anomalous values of the reported QBER and secret key rate observed during rapid temperature changes in the spooled-fiber configuration. Visibility and secret key rate were positively associated in both configurations. The resulting operational record and corresponding raw dataset, publicly available on IEEE DataPort, support reproduction of the analyses, examination of alternative averaging intervals, and comparisons with other QKD experiments, accounting for differences in configuration and operating conditions. These findings provide practical insights into the expected behavior of commercial QKD systems in field deployments.
Silicon-based single-photon avalanche photodiodes (SPADs) widely used in space-based quantum communication payloads exhibit an increasing dark count rate (DCR) due to proton radiation induced displacement damage accumulating in their active areas, which could make the device unusable for quantum communication protocols. One known mitigation strategy is laser annealing, and previous studies have demonstrated that laser illumination with about 1 W optical power can significantly reduce DCR of SPADs irradiated at ground facilities. This method is very effective however it has yet to be implemented in space. Here we present the design of a detector module in a CubeSat form factor for testing on-orbit laser and thermal annealing of four Si SPAD samples, in order to assess the viability of this technique under a constant radiation exposure in low-Earth orbit. We report the detailed design and ground performance of this module in thermal vacuum testing. This modular design enables future on-orbit demonstrations and operational use in quantum communication payloads.
Quantum datacenter (QDC) architectures must support communication among quantum processing units (QPUs) under limited communication qubits, optical switching and measurement resources, probabilistic entanglement generation, and physical-layer noise. This paper presents Q-REFINE, a framework for Quantum Resource Exploration via Factorial & Iterative N-factor Experiments. Its measure is the completion rate, the rate at which requests are served with at least a threshold fidelity. For a given QDC topology, Q-REFINE asks which physical-layer factors and their combinations change this rate most, and over what range improving each factor still pays off. Q-REFINE combines a discrete event simulator, controllable inter-QPU request workloads, topology instances with explicit switches and measurement units, and a design of experiments (DOE). We apply it to Fat-tree, Clos, QFly, BCube and DCell. Load selection fixes, with every factor at its best level, the smallest concurrency limit whose completion rate is within 5% of the best while at most 5% of requests go unserved. At that limit, a full-factorial DOE finds which factor combinations serve any request and how much each factor raises the rate, and one-factor-at-a-time sweeps show over what range each improvement pays off. Emitted-pair fidelity, Hong–Ou–Mandel visibility and memory coherence time jointly decide whether any request is served. The emission efficiency raises the rate most, 8.4–9.5 times between its two DOE levels, and a higher attempt rate pays off only with a short communication-qubit cooldown time. Although we use a particular workload, performance metric, and configurations, the framework applies to any workload, metric(s), or configurations.
Optimal bi- or multi-phase code sequences are widely used in applications such as pulse compression in radar signals, frame synchronization in communications, and error detection in materials science. However, only a very limited number of optimal short code sequences are currently known, and existing approaches for finding new codes are computationally prohibitive due to the complexity of the underlying optimization problem. Quantum algorithms have been shown to provide speedups for certain optimization problems and, due to recent advances in quantum computing technology, have become a key research area. In this paper, we explore the possibility of using the Grover search algorithm to find optimal phase codes, which provides a generic quadratic speedup when given access to an oracle that detects solutions. We therefore provide a concrete implementation of such Grover oracles for optimal binary peak-sidelobe and integrated sidelobe as well as multi-phase Barker codes and evaluate the respective cost in terms of the number of qubits, gates, T-gates, and depth.
Rydberg-atom electrometry promises a radiofrequency (RF) receiver front end with intrinsic self-calibration, all-optical readout, and freedom from conventional antenna constraints. However, existing studies largely address isolated physical effects—local susceptibility, spectral fitting, or a single observable–so a unified engineering description that links device physics to communication performance is still lacking. This work develops a system-level design framework that treats the Rydberg vapor cell as an RF receiver and connects the incident field to the electrical output along a single analytic chain. A closed-form static input–output map is obtained by combining a four-level continued-fraction susceptibility, Beer–Lambert propagation, and direct photodetection. Small-signal linearization yields a thirdorder complex-valued state-space model whose physical transfer function, because the photocurrent is a real projection of a complex coherence, must be assembled from both positive- and negative-frequency resolvent responses—a construction that has no counterpart in real-valued linear systems and is material for bandwidth prediction, which we show can otherwise be misestimated by up to a factor of two to three. A multisource noise model then defines a frequency-dependent minimum detectable field, exposing how static gain, spectral roll-off, and technical noise jointly bound sensitivity. The framework yields explicit reception limits for amplitude, frequency, and on–offkeyed modulation, unifies biased direct detection with superheterodyne operation, provides quantitative engineering design maps for bandwidth, sensitivity, and dynamic range, and is benchmarked against published 87Rb and 133Cs experiments, reproducing the measured calibration slope to within 2% and the reported bandwidth scale to order of magnitude.
