We characterize the indistinguishability between signal and idler photons that originate from independent down-conversion events in a network-compatible single photon source. We also present a resource-efficient method to obtain a benchmark for their indistinguishability.

We experimentally demonstrate the distribution of polarization-entangled photons across a 62-km, partially-aerial fiber. With polarization stabilization applied to the fiber link, we achieve a photon pair rate of approximately 1500 per second and observe a CHSH inequality violation with S=2.34.

Quantum networking, distributed quantum sensing and computing, financial markets, and position navigation and timing of satellites, require high precision clock synchronization. Existing quantum two-way time transfer protocols rely on identical wavelengths at each location. In this letter we derive and experimentally validate a heterogeneous wavelength quantum two-way time transfer protocol that eliminates the need for matched entangled-photon sources. The difference in propagation time is explicitly accounted for by incorporating the group refractive index to the calculations. Using a custom-built spontaneous parametric down-conversion photon-pair source, we separate signal and idler with wavelength division multiplexing, while maintaining strong frequency anti-correlation between signal and idler. We show that the dispersion effects on the second-order correlation function can be removed with the addition of dispersion compensation fiber. We performed quantum time transfer between two independent atomic clocks separated by a 15 km fiber spool with a time deviation of 658 fs at 0.2 s averaging time and 3.9 ps at 1 s averaging time. These results demonstrate a pathway for high precision time transfer in heterogeneous networks.

Quantum networking testbeds lack a distinct plane for coordinating distributed measurements and collecting experimental data across heterogeneous devices. To address this gap, we present the Measurement Plane, a dedicated plane that complements the data, control, and management planes rather than replacing or extending their pipelines. The contribution is presented as a distributed framework that organizes measurement functions into four layers: application, experiment coordination, capability, and resource agents. Our design separates user workflows from device-specific control. We implemented the framework as containerized microservices connected through publish–subscribe messaging, and validated it on a two-node quantum networking setup connected by an optical network. The framework successfully coordinated remote nodes to execute coincidence measurement and polarization entanglement distribution experiments with visibility interference of up to 98 percent. This evaluation demonstrated the effectiveness of the framework for supporting complex, distributed quantum experiments, enabling online measurement and feedback, and significantly reducing manual configuration and execution effort.

We present a scheme for improving polarization-entanglement visibility for non-ideal entangled-photon states, such as those with amplitude imbalance. Quarter-wave plates can be used to improve the visibility of the polarization-entanglement fringes.

We present the characterization of low-loss, high-repeatability optical fiber-to-fiber connectors and show an average insertion loss of 0.39 ± 0.21% at 1550 nm and 0.94 ± 0.46% at 800 nm with excellent agreement between two independent laboratories.

Atomic vapor-cell quantum memories are key devices for quantum information science. Anti-relaxation coatings on the inner surface of the cell suppress ground-state dephasing caused by atom-wall collisions, thereby improving memory performance. Nevertheless, systematic performance comparisons of vapor-cell quantum memories with different anti-relaxation coatings under identical conditions remain limited. Here, we experimentally compare quantum-memory performance in cesium vapor cells with and without anti-relaxation coatings-uncoated, paraffin-coated, and alkene-coated-under otherwise identical conditions to assess coating-dependent effects. Using electromagnetically induced transparency (EIT) as the light storage protocol, we characterize transmission, memory efficiency, and storage lifetime. The alkene-coated cell yields the largest enhancement in retrieval efficiency-more than fourfold relative to the uncoated cell and more than twofold relative to the paraffin-coated cell-while exhibiting a modest (approximately 16%) increase in storage lifetime. These results provide a systematic benchmark for evaluating anti-relaxation coatings and for this instance, identify alkene coatings as an effective choice for improving retrieval efficiency in warm-vapor-cell memories.

Distributing quantum entanglement over telecommunication fiber is a fundamental task in metropolitan-scale quantum networks, enabling advanced applications such as quantum-secured communication, quantum sensing, and distributed quantum computing. Although polarization entanglement is relatively easy to generate and manipulate, its distribution across deployed fibers is challenging because of the time-varying polarization transformations imposed by the fiber, which must be actively stabilized. In this work, we experimentally demonstrate the distribution of polarization-entangled photons over a 62 km, partially aerial fiber between the National Institute of Standards and Technology and the University of Maryland. With polarization stabilization applied to the fiber, we achieved a photon pair rate of approximately $1500\;\text{s}^{- 1}$ and observed a time-averaged CHSH inequality parameter of $S=2.34\pm 0.37$. During a continuous 24 h experiment, time-multiplexed polarization compensation sessions only cost 7.2% of the total link operation time, leaving an entanglement distribution uptime of 92.8%. Our results demonstrate the feasibility of distributing polarization-entangled photons over challenging fiber conditions, which is an important step toward the practical deployment of quantum networks.

Quantum metrology plays a crucial role in characterizing and stabilizing optical channels in a quantum network, where precise estimation of loss is critical for loss-sensitive quantum protocols. Optical fiber channels are the primary medium for linking quantum nodes, where loss estimation with optical quantum states may enhance the precision compared to classical probes. In this study, we investigate the use of two-mode squeezed vacuum (TMSV) states, an optical quantum state generated via parametric down-conversion, and their detection with photon-number-resolving detectors for characterization of transmission loss. Theoretical analysis indicates that TMSV states exhibit higher Fisher information for transmission parameter estimation than coherent states, suggesting they are superior probes that can significantly enhance measurement precision. Our experimental results confirm that these TMSV states outperform coherent states in the context of loss estimation in an optical channel, achieving up to 3.9 dB higher Fisher information for transmissivity estimation without relying on post-selection measurements. This source can effectively probe transmission losses in optical fibers up to 35 km, maintaining a measurement precision advantage over coherent probe states. These findings underscore the potential of TMSV states to advance quantum metrology and improve the robustness of quantum network applications. (c) 2025 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement

We propose a single-photon time-of-flight (ToF) measurement method to benchmark fiber-path delay estimation in optical two-way time and frequency transfer (OTWTFT) protocols. The single-photon ToF measurement yields uncertainties better than 2 ps (0.5 mm fiber-path-length uncertainty) at 1 s integration times for a deployed 120 km (loopback) fiber. Differences between the ToF and the roundtrip time measurements from a White Rabbit precision time protocol appear to correlate with the clock phase error between two White Rabbit switches. The results suggest that augmenting single-photon ToF measurements with existing OTWTFT protocols could enhance the precision to sub-10-ps levels at metropolitan distances. Such a level of precision will be critical for synchronization in quantum networks.

We developed a source generating TMSV states for precision metrology. Experimentally, with no post section measurements, we demonstrate TMSV states offer greater quantum advantage compared to coherent states determining optical fiber loss on metropolitan scales.

We demonstrate picosecond-level synchronization of two actively mode-locked Ti:Sapphire lasers via the White Rabbit Precision Time Protocol (WR-PTP), tested over 120 km of deployed optical fiber. This synchronization capability, in combination with the highly single-mode, transform-limited pulses produced by each laser, enables their use as pump lasers for indistinguishable photon sources at remote locations in a quantum network. Here, the WR-PTP serves as a scalable network synchronization protocol, and its performance is compared to traditional methods of local synchronization. We demonstrate pulse-to-pulse synchronization better than 3 ps and time deviation (TDEV) values below 4 ps for all averaging times up to 10 s. With a designed coherence time of 35 ps for single photon sources utilizing these lasers, the achievable temporal overlap corresponds to 98 % Hong-Ou-Mandel (HOM) interference visibility between independent sources.

Trap detectors are effective as transfer or secondary standards in radiometry, with silicon photodiodes commonly used for visible and germanium for near-infrared (NIR) light detection due to their cost-effectiveness and simple fabrication. However, indium gallium arsenide (InGaAs) photodiodes offer superior performance in the short-wave infrared, 1-1.65 µm range, featuring high internal quantum efficiency, excellent temperature stability, and enhanced durability. In this work, we developed and evaluated a small-footprint InGaAs-based trap detector using an optical trap configuration with two off-the-shelf InGaAs photodiodes and a high-reflectivity mirror. The prototype trap demonstrated excellent stability, low polarization dependence, and good responsivity stability, achieving system detection efficiency exceeding 98% for wavelengths between 1 and 1.3 µm. This versatile detector is suitable for both free-space and optical fiber measurements. The near-unity efficiency of the optical trap configuration shows potential as a primary standard with 1% uncertainty for short-wave infrared standard (SWIR) detection. While the 1% uncertainty of the trap detector is 2 orders of magnitude higher than the 0.01% possible with cryogenic radiometry, the latter requires an expensive cryostat and very specialized expertise. The trap detector, with its robust design and its 1% uncertainty, provides a practical and cost-effective alternative as a standard detector for a wide range of applications, offering a viable solution to reduce reliance on cryogenic radiometry for photodiode calibration.

Optical crosstalk from sub-milliwatt classical-channel power into quantum channels presents a significant challenge in quantum network development, introducing substantial noise that limits the network's performance, scalability, and fidelity. Here we report the first demonstration using photon counting optical time-domain reflectometry (ν-OTDR) to precisely identify and localize crosstalk between separate channels within the same fiber and between separate fibers. The coexistence of classical and quantum signals in the same network necessitates the use of optical switches for efficient routing and control. Crosstalk characterization of an optical switch reveals that crosstalk depends strongly on cross connect configuration, with higher levels observed when connections are presumed to be physically closer and lower levels when further apart. Additionally, we found that crosstalk exhibits a pronounced wavelength dependence, increasing over tenfold at longer wavelengths. These findings demonstrate the value of ν-OTDR in diagnosing and mitigating crosstalk in quantum networks. They highlight the importance of optimizing optical switch configurations and wavelength management to minimize noise, ultimately enhancing the scalability, fidelity, and overall performance of quantum networks. This work establishes a foundational approach to addressing crosstalk, paving the way for more robust and efficient quantum network designs.

In distributed quantum applications such as entanglement distribution, precise time synchronization and efficient time-tagged data handling are essential. Traditional systems often suffer from overflow, synchronization drift, and storage inefficiencies. We propose a modular Time Tagging (TT) agent that uses a 1 pulse per second (PPS) signal from White Rabbit (WR) devices to achieve network-wide synchronization, while applying real-time calibration, overflow mitigation, and compression. A live two-lab entanglement distribution experiment validated the system's performance, achieving synchronized coincidence detection at 25,000 counts/sec.