Simulation and realization of a second-order quantum-interference-based quantum clock synchronization at the femtosecond level.

Quantum clock synchronization schemes utilizing frequency-entangled pulses have flourished for their potentially superior precision to the classical protocols. In this Letter, a new experimental record based on the second-order quantum interference algorithm is reported, to the best of our knowledge. The synchronization accuracy between two parties separated by a 6 km fiber coiling link, which is evaluated by the time offset shift relative to that with the fibers removed, has been measured to be 13±1 ps. The stability in terms of time deviation (TDEV) of 0.81 ps at an averaging time of 100 s has been achieved. The long-term synchronization stability is seen determined by the measurement device, and a minimum stability of 60 fs has been reached at 25,600 s. Furthermore, for the first time to the best of our knowledge, we quantify the performance of this quantum synchronization scheme, and very good agreements with the experimental results have been achieved. According to the quantum simulation, further improvements for both the synchronizing stability and accuracy can be expected.

Improving the timing jitter of a superconducting nanowire single-photon detection system.

Low timing jitter is a unique merit of superconducting nanowire single-photon detectors (SNSPDs) for time-correlated applications. Quantitative analysis was performed for the SNSPD system. Aided by an oscilloscope with an optimal signal amplitude, we were able to measure a full width at half-maximum system timing jitter as low as 14.2 ps for a high-switching-current SNSPD using a room-temperature low-noise amplifier. When using a time-correlated single-photon counting module, the system timing jitter was 17.3 ps. The detector's intrinsic timing jitter was estimated at ∼12.0 ps.

Hotspot relaxation time of NbN superconducting nanowire single-photon detectors on various substrates.

Hotspot relaxation time (τ th ) is one of the essential parameter which defines the maximum count rate of superconducting nanowire single-photon detectors (SNSPDs). We studied the τ th for NbN-based SNSPDs on various substrates using the two-photon detection method based on the pump-probe spectroscopy technique. We observed that τ th strongly increased with increasing bias current in the two-photon detection regime. In addition, the minimum hotspot relaxation time (τ th )min was not significantly affected by the bath temperature; this is different from the previous observations reported for WSi SNSPDs. In addition, a strong dependency of (τ th )min on the substrate was found. The minimum (τ th )min was 11.6 ps for SNSPDs made of 5.5-nm-thick NbN on MgO (100), whereas the maximum (τ th )min was 34.5 ps for SNSPDs made of 7.5-nm-thick NbN on Si (100). We presented a direct correlation between the values of τ th and degrees of disorder of NbN films grown on different substrates.