Kavli Affiliate: Lee McCuller
| First 5 Authors: James W. Gardner, James W. Gardner, , ,
| Summary:
Searching for a weak signal at an unknown frequency is a canonical task in
experiments probing fundamental physics such as gravitational-wave
observatories and ultra-light dark matter haloscopes. These state-of-the-art
sensors are limited by quantum noise arising from the fundamental uncertainty
about the state of the device. Classically, frequency estimation suffers from a
threshold effect in the signal-to-noise ratio such that weak signals are
extremely hard to localise in frequency. We show that this phenomenon persists
at the fundamental quantum limit but that the classical approach, a quadrature
measurement, can nevertheless be beaten by a coherent protocol of projecting
onto the "quantum whitened" possible quantum states. Quantum whitening is a
covariant measurement, and we examine it analytically in the wide-prior limit
and numerically for finite-width priors. Beyond accelerating searches for
unknown frequencies, quantum whitening may be used generally to sense the
parameter of a unitary encoding given no prior information about the parameter.
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