A Quantum Optical Microphone in the Audio Band

TL;DR

This paper introduces a simple quantum optical microphone operating in the audio band that surpasses classical limits, demonstrating improved speech recognition and making quantum effects perceptible to humans.

Contribution

It presents a novel, robust method for quantum-enhanced phase measurement using standard intensity detection, enabling practical quantum microphones in the audio frequency range.

Findings

Quantum microphone improves speech recognition threshold by -0.57 dB SPL.

The device maintains high sampling rates and robustness of classical sensors.

Quantum advantage is perceptible in real-world speech tests.

Abstract

The ability to perform high-precision optical measurements is paramount to science and engineering. Laser interferometry enables interaction-free sensing with a precision ultimately limited by shot noise. Quantum optical sensors can surpass this limit, but single- or multi-photon schemes are challenged by low experimental sampling rates, while squeezed-light approaches require complex optical setups and sophisticated time gating. Here, we introduce a simple method that infers optical phase shifts through standard intensity measurements while still maintaining the quantum advantage in the measurement precision. Capitalising on the robustness and high sampling rates of our device, we implement a quantum optical microphone in the audio band. Its performance is benchmarked against a classical laser microphone in a standardised medically-approved speech recognition test on 45 subjects. We find that quantum-recorded words improve the speech recognition threshold by $-0.57\, \text{dB}_{\text{SPL}}$, thus making the quantum advantage audible. Not only do these results open the door towards applications in quantum nonlinear interferometry, but they also show that quantum phenomena can be experienced by humans.