PhD Defense: Jun Jia

Conditional broadband quantum noise reduction with negative mass spin oscillators

Quantum sensing represents a significant research direction within Quantum Technologies, particularly promising in the acoustic frequency regime. This potential offers a wide range of scientific applications, including the detection of magnetic fields generated by the brain's activity, heartbeat, and the measurement of weak forces such as gravitational wave signals emitted by extreme astronomical events.

A principal challenge in enhancing the sensitivity of current gravitational wave detectors is managing two competing types of quantum noise; shot noise: which arises from the uncertainty associated with the arrival of photons, and quantum backaction noise: which results from the transfer of photon momentum to the probe sensor as radiation pressure during the interaction. These noises, arise from the quantum nature of light, scale differently with the light power, and dominate at different frequencies. Their broadband reduction requires the injection of a squeezed vacuum source, with frequency-dependent rotation of the squeezed quadrature presently accomplished via a 300 m long filter cavity.

Here, we introduce an alternative theoretical proposal by E.S.Polzik and F.Ya.Khalili , in principle, we can simultaneously achieve quantum backaction and shot noise reduction using polarized cesium atoms prepared in an effective negative mass reference frame and operating in an acoustic frequency range. The conditional measurement of the atomic spin oscillator, utilizing the cross-correlation inherent in the EPR entanglement, provides an opportunity to realize broadband sensitivity improvement for gravitational wave detectors. We will present the construction and characterization of a quantum backaction dominant atomic system with calibrated quantum cooperativity of approximately 3, evidenced by the observed ponderomotive squeezing across a wide range of Larmor frequencies.

We then demonstrate the nondegenerate EPR entangled sources, which bridge the gap between the atomic system and gravitational wave detection, alongside theoretical predictions using the calibrated experimental parameters for the broadband quantum noise reduction. Additionally, we report the preliminary experimental achievement at 50 kHz with joint measurements of two hybrid systems. This result sets stage for our ongoing proof-of-principle frequency-dependent entangled source, aimed at broadband quantum noise reduction in the acoustic frequency regime.           

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