Magnetic Resonance with Squeezed Microwaves

Abstract

Vacuum fluctuations of the electromagnetic field set a fundamental limit to the sensitivity of a variety of measurements, including magnetic resonance spectroscopy. We report the use of squeezed microwave fields, which are engineered quantum states of light for which fluctuations in one field quadrature are reduced below the vacuum level, to enhance the detection sensitivity of an ensemble of electronic spins at millikelvin temperatures. By shining a squeezed vacuum state on the input port of a microwave resonator containing the spins, we obtain a 1.2-dB noise reduction at the spectrometer output compared to the case of a vacuum input. This result constitutes a proof of principle of the application of quantum metrology to magnetic resonance spectroscopy.

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Plain Language Summary

Magnetic resonance spectroscopy has a significant impact on our everyday lives, from medical imaging in hospitals to quality control in beer production. The technique uses powerful magnets and radio waves to reveal concentrations of molecules in a substance. Improving the sensitivity—achieved by both boosting the signal measured and reducing the noise—allows for smaller quantities of materials to be measured. The thermal contribution to this noise can be reduced by cooling to low temperatures (less than 1 K), but even in this limit, fluctuations of the electromagnetic field remain (as required by Heisenberg’s uncertainty principle). Recently, the noise in magnetic resonance spectroscopy of electron spins was reduced to the level of these vacuum quantum fluctuations by using a superconducting amplifier operating at 10 mK. One might expect that the noise cannot be reduced below this limit. We have found a way, however, to improve on this performance.

While the Heisenberg uncertainty principle limits the combined uncertainty of two variables, it does not forbid us from engineering states where the variance in one quantity is reduced at the expense of the other. Such states are called squeezed states, and they provide a way to measure data with a signal-to-noise ratio beyond the vacuum fluctuation limit. We use a Josephson parametric amplifier (a type of driven harmonic oscillator that can amplify a signal) to prepare a squeezed state of the electromagnetic microwave field. In this state, fluctuations that are in phase with the signal from the electron spins are reduced, while noise that was out of phase is amplified. We find a 1.2-dB improvement in the signal-to-noise ratio of the spin echo.

Future work will target larger degrees of squeezing and applications to real-world magnetic resonance measurements.