Improve photon number discrimination for a superconducting series nanowire detector by applying a digital matched filter

TL;DR

This paper demonstrates that applying a digital matched filter to a superconducting nanowire detector significantly enhances photon number resolution by improving signal-to-noise ratio and pulse amplitude spacing, enabling detection of higher photon numbers.

Contribution

The study introduces a matched filter technique for series PNR-SNSPDs, significantly improving photon number discrimination and maximum resolved photon number compared to conventional methods.

Findings

Normalized pulse amplitude spacing increased by up to 5.3 times with the matched filter.

Achieved a minimum timing jitter of 55 ps.

Potential to resolve up to 65 or more photons.

Abstract

Photon number resolving (PNR) is an important capacity for detectors working in quantum and classical applications. Although a conventional superconducting nanowire single-photon detector (SNSPD) is not a PNR detector, by arranging nanowires in a series array and multiplexing photons over space, such series PNR-SNSPD can gain quasi-PNR capacity. However, the accuracy and maximum resolved photon number are both limited by the signal-to-noise (SNR) ratio of the output pulses. Here, we introduce a matched filter, which is an optimal filter in terms of SNR for SNSPD pulses. Experimentally, compared to conventional readout using a room-temperature amplifier, the normalized spacing between pulse amplitudes from adjacent photon number detections increased by a maximum factor of 2.1 after the matched filter. Combining with a cryogenic amplifier to increase SNR further, such spacing increased by a maximum factor of 5.3. In contrast to a low pass filter, the matched filter gave better SNRs while maintaining good timing jitters. Minimum timing jitter of 55 ps was obtained experimentally. Our results suggest that the matched filter is a useful tool for improving the performance of the series PNR-SNSPD and the maximum resolved photon number can be expected to reach 65 or even large.