Phonon-trapping enhanced energy resolution in superconducting single photon detectors

TL;DR

This paper demonstrates that phonon trapping significantly enhances the energy resolution of superconducting MKID photon detectors, with experimental evidence showing improvements when the detector is suspended on a membrane, paving the way for near-Fano limit performance.

Contribution

The study provides experimental validation that phonon trapping improves energy resolution in superconducting MKIDs and introduces a geometrical phonon propagation model for hot phonons.

Findings

Suspending the detector on a SiN membrane improves resolving power by a factor of 8.

Measured resolving power ranges from 10-21 on bulk substrates to 19-52 on membranes.

Phonon trapping efficiency correlates with geometrical design, enabling near-Fano limit performance.

Abstract

A noiseless, photon counting detector, which resolves the energy of each photon, could radically change astronomy, biophysics and quantum optics. Superconducting detectors promise an intrinsic resolving power at visible wavelengths of $R=E/\delta E\approx100$ due to their low excitation energy. We study superconducting energy-resolving Microwave Kinetic Inductance Detectors (MKIDs), which hold particular promise for larger cameras. A visible/near-infrared photon absorbed in the superconductor creates a few thousand quasiparticles through several stages of electron-phonon interaction. Here we demonstrate experimentally that the resolving power of MKIDs at visible to near-infrared wavelengths is limited by the loss of hot phonons during this process. We measure the resolving power of our aluminum-based detector as a function of photon energy using four lasers with wavelengths between $1545-402$ nm. For detectors on thick SiN/Si and sapphire substrates the resolving power is limited to $10-21$ for the respective wavelengths, consistent with the loss of hot phonons. When we suspend the sensitive part of the detector on a 110 nm thick SiN membrane, the measured resolving power improves to $19-52$ respectively. The improvement is equivalent to a factor $8\pm2$ stronger phonon trapping on the membrane, which is consistent with a geometrical phonon propagation model for these hot phonons. We discuss a route towards the Fano limit by phonon engineering.