Electron spin resonance of organic radicals with squeezed microwaves

The generation of quantum squeezed light states with vacuum fluctuations below the quantum-limit in the microwave regime has proved a useful tool to enhance the sensitivity of electron spin resonance (ESR) at millikelvin temperatures [1]. However, microwave component losses and inherent high-order non-linearities present in the degenerate parametric amplifiers used to generate these states have to date restricted the sensitivity enhancement. Here we present the use of a squeezing apparatus incorporating two superconducting kinetic inductance parametric amplifiers [2] to enhance the ESR detection sensitivity of a triarylmethyl (trityl) spin ensemble deposited on a planar superconducting microresonator at 10 mK. The exceptionally narrow line width and stability of trityl spin centres make them attractive spin labels for biological ESR applications. We first demonstrate the near quantum-limited amplification of the spin echo: achieving an order of magnitude SNR enhancement compared to when using only a HEMT. We then achieve a vacuum state squeezing level of ~ 7.8 dB [2], which when introduced into the spectrometer leads us to expect a squeezing-induced spin echo SNR gain. These results highlight the potential to extend high-sensitivity ESR spectroscopy using squeezed microwaves to biological applications.

[1] Bienfait et al. Phys Rev X 7.4 (2017): 041011.

[2] Vaartjes et al. Nat Commun 15.1 (2024): 4229.