A Hybrid Measurement Scheme for Generating nonGaussian Spin States

TL;DR

This paper introduces a hybrid measurement protocol combining homodyne and single photon detection to generate non-Gaussian spin states with quantum advantage for sensing, even under decoherence.

Contribution

The paper proposes a novel hybrid measurement scheme for creating non-Gaussian spin states, enhancing quantum sensing capabilities in atomic ensembles.

Findings

Successfully generates non-Gaussian spin states with quantum advantage.

Demonstrates robustness of the protocol under decoherence.

Identifies near-optimal measurement basis for quantum Cramér-Rao bound.

Abstract

We present a protocol for generating nonclassical states of atomic spin ensembles through the backaction induced by a hybrid measurement of light that is entangled with atoms, combining both homodyne and single photon detection. In phase-I of the protocol we create a spin squeezed state by measuring the light's polarization rotation due to the Faraday effect in a balanced polarimeter, equivalent to a homodyne measurement. In phase-II we send a second probe beam through the sample and detect single photons scattered into the signal mode. Before doing so, we rotate the uncertainty bubble to increase the projection fluctuations of the measured spin component. This increases the coupling strength between the atoms and photons and thus the rate of scattering of single photons into the signal mode. In the ideal case, the result is a squeezed Dicke state, with substantial quantum advantage for sensing spin rotations. We benchmark the protocol's utility in the presence of inevitable decoherence due to optical pumping using the Fisher information as a measure of quantum advantage. We show that in the presence of decoherence, the quantum Fisher information associated with the nonGaussian mixed state we prepare is substantially larger than the classical Fisher information obtained from the standard measurement of spin rotations. We deduce a measurement basis that is close to optimal for achieving the quantum Cram\'{e}r Rao bound in the presence of decoherence.