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.

Using a network-compatible source of single photons, we study the interference of heralded signal and idler photons that are not part of the same biphoton pair. We characterize the indistinguishability of such arbitrary single photons that are generated in independent parametric down-conversion events using a Hong-Ou-Mandel interferometer, and show non-classical coalescence, C = 0.85(9). Separately, we measure coalescence of multi-mode thermal fields corresponding to the signal and idler photons and characterize their indistinguishability. This source exhibits ubiquitous indistinguishability between any photons that it generates and thus enables the interchangeable use of signal and idler photons in multi-node, photon-interference-based protocols for scalable quantum networks. The indistinguishability between arbitrary photons from such sources enables requirement-driven entanglement to be generated in nodes within a quantum network.

Multi-institutional quantum networks that connect nodes at a metropolitan scale are being developed. Those networks face limitations due to the need for synchronous, real-time communication of classical information alongside with quantum channels. Using the same fiber simultaneously for classical and quantum traffic is beneficial but requires additional consideration. Unexpectedly, blending classical and quantum traffic can enhance classical data transmission with quantum features, such as unambiguous security. Additionally, classical information capacity can be dramatically improved by reusing quantum-networking hardware. I will discuss communicating with faint light, significantly weaker than that used for conventional classical communication, yet significantly stronger than that used for quantum communication and quantum key distribution.

We theoretically propose a quantum plasmonic sensor using Hong-Ou-Mandel interferometry to measure a refractive index embedded in the plasmonic beamsplitter. There is quantum advantage in terms of the Fisher information.

We propose a quantum plasmonic sensor using Hong-Ou-Mandel (HOM) interferometry that measures the refractive index of an analyte, embedded in a plasmonic beam splitter composed of a dual-Kretschmann configuration, which serves as a frustrated total internal reflection beamsplitter. The sensing performance of the HOM interferometry, combined with single-photon detectors, is evaluated through Fisher information for estimation of the refractive index of the analyte. This is subsequently compared with the classical benchmark that considers the injection of a coherent state of light into the plasmonic beamsplitter. By varying the wavelength of the single photons and the refractive index of the analyte, we identify a wide range where a 50 % quantum enhancement is achieved and discuss the observed behaviors in comparison with the classical benchmark. We expect this study to provide a useful insight into the advancement of quantum-enhanced sensing technologies, with direct implications for a wide range of nanophotonic beamsplitter structures.

We demonstrate a source of transform-limited indistinguishable photons in telecom band synchronized to the external clock. We observe near-perfect indistinguisha-bility and sub-picosecond synchronization timing jitter. This network-compatible source enables large-scale, multi-node practical quantum networks.

The development of prototype metropolitan-scale quantum networks is underway and entails transmitting quantum information via single photons through deployed optical fibers spanning several tens of kilometers. The major challenges in building metropolitan-scale quantum networks are compensation of polarization mode dispersion, high-precision clock synchronization, and compensation for cumulative transmission time fluctuations. One approach addressing these challenges is to co-propagate classical probe signals in the same fiber as the quantum signal. Thus, both signals experience the same conditions, and the changes of the fiber can therefore be monitored and compensated. Here, we demonstrate the distribution of polarization entangled quantum signals co-propagating with the White Rabbit Precision Time Protocol (WR-PTP) classical signals in the same single-core fiber strand at metropolitan-scale distances. Our results demonstrate the feasibility of this quantum-classical coexistence by achieving high-fidelity entanglement distribution between nodes separated by 100 km of optical fiber. This advancement is a significant step towards the practical implementation of robust and efficient metropolitan-scale quantum networks.

We achieved successful polarization entanglement distribution in metropolitan-scale quantum networks, coexisting with classical systems. We utilized deployed fiber infrastructure across the NIST campus and extended the distance to over 100 km using fiber spools.

We demonstrate an in-situ scheme for monitoring conditions of a classical communications channel via regular data transmission, and with no auxiliary communication. Employing quantum state discrimination confidence, we experimentally observe amplitude and phase noise detection.

We demonstrate phase stabilization of a 3.2 km quantum network link with faint light. Our stabilization signal can empower phase-reliant quantum communication protocols and supports coexistent multiplexing of classical/quantum channels in a scaleable quantum network.

We present the first unambiguous experimental method enabling single-fluorophore sensitivity in a flow cytometer using quantum properties of single-photon emitters. We use a quantum measurement based on the second-order coherence function to prove that the optical signal is produced by individual biomarkers traversing the interrogation volume of the flow cytometer from the first principles. This observation enables the use of the quantum toolbox for rapid detection, enumeration, and sorting of single fluorophores in large cell populations as well as a ‘photons-to-moles’ calibration of this measurement modality.