Lixing You

Dissipative Kerr solitons in optical microresonators underpin chip-scale frequency combs with applications ranging from coherent telecommunications to precision spectroscopy. Yet the characterization of their intrinsic femtosecond temporal structure remains challenging, as the low pulse energy and broad spectral bandwidth necessitate optical amplification and careful dispersion compensation in conventional ultrafast diagnostics, both of which can substantial distort the waveform. Here, we demonstrate a quantum-interference metrology of microcomb solitons based on Hong-Ou-Mandel interference. By attenuating the soliton stream to the single-photon level and measuring fourth-order interference, we directly retrieve near transform-limited pulse durations without amplification or dispersion management, remaining accurate even after propagation through 25 kilometers of standard fiber. The same interferogram also provides direct access to the temporal separations in multisoliton states by converting intersoliton separations into additional interference dips at corresponding delays, enabling subpicosecond characterization of their intracavity temporal structure. This quantum-inspired paradigm introduces a metrological approach that is immune to amplification and dispersion distortions, offering a powerful tool for the characterization of complex soliton physics.

We report a 16-pixel (4×4) superconducting nanowire single-photon detector (SNSPD) array featuring pixel-level photon-number-resolving capability. This multidimensional sensing is enabled by integrating a compact four-element series nanowire architecture into each spatial pixel, allowing direct discrimination of one to four photons. Under single-mode fiber illumination, the device achieves a total system detection efficiency of 41.5%, given the array geometrical fill factor of 53%. Across all channels, the device exhibits reliable multiphoton discrimination, maintaining minimum readout fidelities of 96%, 89%, 73%, and 77% for one- to four-photon states, respectively. Furthermore, independent channel evaluations reveal low timing jitter and negligible inter-channel crosstalk (% ). By implementing differentiating pulse shaping for high-flux operation, a maximum single-channel −3 dB count rate of 984 MHz is achieved, establishing an aggregated array bandwidth potential of 8.9 GHz. This sensing architecture may provide a viable pathway toward high-flux single-photon imaging, offering potential applications in quantum optics and light detection and ranging (LiDAR) systems.

The scaling of photonic quantum technologies, including quantum networks and optical quantum computing, necessitates high‐fidelity and massively parallel single‐photon detection. While superconducting nanowire single‐photon detectors (SNSPDs) provide state‐of‐the‐art performance, their large‐scale deployment is often hindered by stringent cryogenic requirements and integration complexity. In this work, we present a parallel architecture designed to overcome the scalability bottlenecks. Experimentally, we demonstrate the largest SNSPD system to date, comprising 100 channels operating stably below 2.1 K within a compact Gifford‐McMahon cryocooler. All channels exhibit exceptional performance, achieving system detection efficiencies (SDE) between 80.1% and 98.3%, with dark count rates (DCR) maintained below 300 cps. Furthermore, leveraging this parallel capability, we demonstrate a fully connected multi‐user quantum entanglement distribution network. This work establishes a robust and scalable infrastructure, paving the way for large‐scale quantum photonic applications.

The nanoscale charge environment critically influences the performance of semiconductor devices, yet traditional characterization techniques usually lack the spatial resolution to resolve nanoscale charge heterogeneity and identify microscopic noise sources. Here, we address this critical gap using individual PL5 color centers in commercial silicon carbide as room-temperature, broadband (near-dc to gigahertz) quantum spectrometers. We report real-time, nanoscale observation of single-charge tunneling dynamics at room temperature in 4H-SiC by monitoring the random telegraph noise using optically detected magnetic resonance. This capability enables an electrical noise imaging technique that shows distinct noise variations on different wafers. Using dynamical decoupling and T 1 relaxation spectroscopy, we observe correlations of noise spectral density across frequency bands and obtain the nanoscale electron paramagnetic resonance fingerprint of spin defects in silicon carbide. Ultimately, through surface chemical engineering, we identify the native oxide layer as the main source of resolved electromagnetic fluctuations. This work establishes a versatile atomic-scale metrology platform bridging quantum sensing and semiconductor engineering, offering critical insights for optimizing third-generation semiconductor interfaces and advancing solid-state quantum technologies.

Accessing the intrinsic critical current density in type-II superconductors can provide a means to probe microscopic superconducting properties and increase current limits in high-magnetic-field devices and electrical power systems. However, the critical current density measured using direct currents generally lies below this intrinsic limit, mainly because of vortex motion and self-heating. Here we show that picosecond electrical pulses, which act on timescales too short for vortices to move, drive supercurrents up to the intrinsic depairing limit. We probe picosecond critical currents in NbN and YBa ₂ Cu ₃ O ₇ , representative s -wave and d -wave superconductors, respectively. In NbN, we find a sharp onset of the picosecond depairing at a current density of about 2.2 times the conventional critical current density, consistent with microscopic dynamics based on Bardeen–Cooper–Schrieffer theory. By contrast, YBa ₂ Cu ₃ O ₇ exhibits a gradual suppression of superconductivity with increasing current, reflecting its d -wave symmetry. These results provide a probe of superconductors beyond the reach of conventional transport measurements. Attaining the depairing current could also support superconducting electronics operating closer to intrinsic current limits.

Quantum Key Distribution (QKD) is an optical communication technique that allows two authenticated parties to exchange an encryption key using protocols grounded in the laws of quantum mechanics, making the communication intrinsically secure. A fundamental requirement for key exchange through such protocols is the use of high-performance single-photon detectors and infrared single-photon sources with low multi-photon components. In addition to the rapid progress in single-photon sources (SPSs), achieving the ideal scenario where a source emits only single photons at telecom wavelength with high efficiency remains a theoretical concept. Multi-photon components emitted by non-ideal light sources such as lasers or Spontaneous Parametric Down Conversion (SPDC) sources can open the door to successful attacks from non-legitimate parties. In this work, we demonstrate, by using a Photon-Number-Resolving Detector (PNRD), based on parallel superconducting strips, the possibility of obtaining a single-photon source with extremely multi-photon components starting from a non-single-photon light source, namely an SPDC one. To estimate the reduction of the multi-photon component, we analyzed the second-order autocorrelation function, g(2)(0), of the light source using a PNRD modelled as 8 independent pixels in parallel, in a heralding protocol. We managed to obtain a reduction of the g(2)(0) using a CW SPDC with an emission rate of about 10 MHz using several sampling bin-widths, spanning from less than one nanoseconds to hundreds of nanoseconds. These results align with the goals of the Quantum Italy Deployment project, as sources with suppressed multi-photon components could play a key role in improving the performance of QKD systems.

Multimode fiber (MMF)-coupled superconducting nanowire single-photon detectors (SNSPDs) are critical for free-space quantum communication and sensing, yet their performance is severely limited by polarization-dependent detection efficiency and high dark count rate (DCR) arising from thermal photons. Here, we present a transformative solution that addresses both challenges simultaneously. We demonstrate a polarization-insensitive SNSPD based on a dielectric-compensated bilayer nanowire structure, achieving a system detection efficiency (SDE) of 88% at 1.5 μm with a DCR of ∼4.8×10 ⁴ cps for MMF-coupled signals. To mitigate the elevated thermal noise inherent to multimode coupling, we integrate a compact, dual-stage on-fiber filter with a suppression ratio exceeding 60 dB. The packaged device delivers an SDE of 81% with a DCR of only 4 cps, corresponding to an over 11,000-fold SNR enhancement relative to the unfiltered MMF-coupled configuration.