Noise Mitigation in Single Microwave Photon Counting by Cascaded Quantum Measurements

TL;DR

This paper introduces a cascaded quantum measurement approach for microwave photon detectors that significantly reduces intrinsic noise, enhancing sensitivity for quantum sensing applications.

Contribution

The authors develop a cascaded quantum measurement scheme using Four-Wave-Mixing to mitigate noise in microwave photon detection, achieving two orders of magnitude noise reduction.

Findings

Achieved two-order-of-magnitude reduction in intrinsic detector noise.

Reported an intrinsic sensitivity of approximately 8×10^{-24} W/√Hz.

Operational sensitivity limited by thermal photons in the input line.

Abstract

While single-photon counting is routinely achieved in the optical domain, operational single microwave photon detectors (SMPDs) have only recently been demonstrated. SMPDs are critical for sensing weak signals from incoherent emitters, with applications ranging from the detection of individual electron spins and dark-matter candidates to advancements in hybrid quantum devices and superconducting quantum computing. These detectors offer a substantial advantage over quantum-limited amplification schemes by bypassing the standard quantum limit for power detection, therefore further reductions in their intrinsic noise are essential for advancing quantum sensing at microwave frequencies. Several SMPD designs utilize the state of a superconducting qubit to encode the detection of an itinerant photon, and rely on a non-destructive photon-qubit interaction. Here, we leverage this Quantum-Non-Demolition feature by repeatedly measuring the impinging photon with cascaded Four-Wave-Mixing processes and encoding the detection on several qubits. This cascaded detector mitigates the intrinsic local noise of individual qubits, achieving a two-order-of-magnitude reduction in intrinsic detector noise at the cost of halving the efficiency. We report an intrinsic sensitivity of $8(1)\times10^{-24}\text{W}/\sqrt{\text{Hz}}$, with an operational sensitivity of $5.9(6)\times 10^{-23}\text{W}/\sqrt{\text{Hz}}$ limited by thermal photons in the input line.