Putting the Squeeze on Magnetic Resonance
Magnetic-resonance measurements of electron spins are starting to bump up against quantum limits, in which the noise level is dominated by quantum fluctuations in the microwave probe pulses. To go further in sensitivity, Audrey Bienfait from the University of Paris-Saclay and her colleagues have “squeezed” the incoming microwave light. This form of quantum-state engineering provided as much as a 25% reduction in the noise as compared to the unsqueezed case.
Quantum squeezing is a way of increasing the sensitivity of a measurement by rigging Heisenberg’s uncertainty principle, which sets the lower noise limit for measurements of two complementary variables, such as position and momentum. In a squeezed state, the noise is lowered in the variable of interest, while the balance is made up with increased noise in the other variable. Researchers have used squeezed optical states to improve imaging and gravitational-wave measurements.
Bienfait and her colleagues have now shown that electron-spin-resonance (ESR) techniques can also benefit from squeezing at microwave frequencies. Like their nuclear counterpart, ESR techniques can image biological and material samples by recording the microwave-induced precession of tiny magnetic spins, which in the ESR case are unpaired electrons. In its experiment, the team placed a bismuth-doped silicon sample inside a microwave cavity and applied a magnetic field. With a pair of microwave pulses, the researchers recorded the so-called spin echo, which is proportional to the number of spins whose precession rate matches the cavity resonance. Squeezing one phase-based variable for the pulses reduced the noise in the spin-echo signal. With further improvements, this squeezing technique could lower the noise by a factor of 5, which would speed up ESR measurement times.
This research is published in Physical Review X.
Michael Schirber is a Corresponding Editor for Physics based in Lyon, France.