Reaching the quantum limit of sensitivity in electron spin resonance

TL;DR

This paper demonstrates a groundbreaking increase in electron spin resonance sensitivity, reaching the quantum limit by combining advanced microwave amplification and superconducting resonators at millikelvin temperatures, enabling detection of extremely small spin ensembles.

Contribution

It introduces a novel ESR detection method that achieves sensitivity at the quantum fluctuation limit, surpassing previous techniques by nearly four orders of magnitude.

Findings

Detected 1700 bismuth donor spins with unit SNR in a single echo

Reduced detectable spins to 150 with a single pulse sequence

Achieved sensitivity limited by quantum electromagnetic fluctuations

Abstract

We report pulsed electron-spin resonance (ESR) measurements on an ensemble of Bismuth donors in Silicon cooled at 10mK in a dilution refrigerator. Using a Josephson parametric microwave amplifier combined with high-quality factor superconducting micro-resonators cooled at millikelvin temperatures, we improve the state-of-the-art sensitivity of inductive ESR detection by nearly 4 orders of magnitude. We demonstrate the detection of 1700 bismuth donor spins in silicon within a single Hahn echo with unit signal-to-noise (SNR) ratio, reduced to just 150 spins by averaging a single Carr-Purcell-Meiboom-Gill sequence. This unprecedented sensitivity reaches the limit set by quantum fluctuations of the electromagnetic field instead of thermal or technical noise, which constitutes a novel regime for magnetic resonance.