Enhanced cooperativity for quantum-nondemolition-measurement--induced spin squeezing of atoms coupled to a nanophotonic waveguide

TL;DR

This paper demonstrates how strategic placement of atoms near nanophotonic waveguides enhances cooperativity, leading to significant spin squeezing via quantum nondemolition measurements, with potential for practical quantum information applications.

Contribution

It reveals a counterintuitive method to increase cooperativity by positioning atoms at low-intensity points, improving spin squeezing in nanophotonic systems.

Findings

Achieved 6.3 dB spin squeezing on nanofiber

Achieved 13 dB spin squeezing on square waveguide

Enhanced cooperativity by geometric optimization

Abstract

We study the enhancement of cooperativity in the atom-light interface near a nanophotonic waveguide for application to quantum nondemolition (QND) measurement of atomic spins. Here the cooperativity per atom is determined by the ratio between the measurement strength and the decoherence rate. Counterintuitively, we find that by placing the atoms at an azimuthal position where the guided probe mode has the lowest intensity, we increase the cooperativity. This arises because the QND measurement strength depends on the interference between the probe and scattered light guided into an orthogonal polarization mode, while the decoherence rate depends on the local intensity of the probe. Thus, by proper choice of geometry, the ratio of good to bad scattering can be strongly enhanced for highly anisotropic modes. We apply this to study spin squeezing resulting from QND measurement of spin projection noise via the Faraday effect in two nanophotonic geometries, a cylindrical nanofiber and a square waveguide. We find that, with about 2500 atoms and using realistic experimental parameters, $ \sim 6.3 $ and $ \sim 13 $ dB of squeezing can be achieved on the nanofiber and square waveguide, respectively.