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The Rochester Conferences on Coherence and Quantum Optics and the Quantum Information and Measurement meeting

Measurement back-action in a two-component Bose-Einstein condensate

Ebubechukwu O. Ilo-Okeke, Tim Byrnes

Published Jan 1, 2013• 0 citations
Publisher

Abstract

We investigate a non-destructive homodyne measurement of atomic condensates using off-resonant optical laser beam that interacts with atomic Bose-Einstein condensate (BEC). The interaction between laser beam and the atomic BEC creates entangled states of atom and light that are destroyed in a homodyne detection of light. The destruction of the entangled states causes back-action on the atomic states. We derive expression for the measurement operator on the atomic states due to the photon measurement. We characterize the back-action by calculating quantities such as the Q-function and observe the variation of the measured state with the strength of the light-BEC interaction.

DOI
10.1364/qim.2013.w6.55

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Abstract A fundamental tenet of quantum mechanics is that measurements change a system’s wavefunction to that most consistent with the measurement outcome, even if no observer is present. Weak measurements produce only limited information about the system, and as a result only minimally change the system’s state. Here, we theoretically and experimentally characterize quantum back-action in atomic Bose-Einstein condensates interacting with a far-from resonant laser beam. We theoretically describe this process using a quantum trajectories approach where the environment measures the scattered light and present a measurement model based on an ideal photodetection mechanism. We experimentally quantify the resulting wavefunction change in terms of the contrast of a Ramsey interferometer and control parasitic effects associated with the measurement process. The observed back-action is in good agreement with our measurement model; this result is a necessary precursor for achieving true quantum back-action limited measurements of quantum gases.
In this paper, we investigate theoretically the back-action evading measurement of the collective mode of an interacting atomic Bose–Einstein condensate (BEC) trapped in an optical cavity that is driven coherently by a pump laser with a modulated amplitude. It is shown that, for a specified kind of amplitude modulation of the driving laser, one can measure a generalized quadrature of the collective mode of the BEC indirectly through the output cavity field with a negligible back-action noise in the good-cavity limit. Nevertheless, the on-resonance added noise of measurement is suppressed below the standard quantum limit (SQL), even in the bad cavity limit. Moreover, the measurement precision can be controlled through the s -wave scattering frequency of atomic collisions.
We study the robustness of quantum nondemolition (QND) measurement induced entanglement between Bose-Einstein Condensates (BECs). We consider an experimental scheme where two BECs are placed in the paths of a Mach-Zehnder interferometer, and a QND interaction creates entanglement between coherent light and the atoms. We analyze the two dominant channels of decoherence, atomic dephasing and photon loss on the entangled states produced by this scheme. We calculate the effect of dephasing on the variance and expectation values of the spin operators, entanglement, and correlation criteria. Our analysis does not use the Holstein-Primakoff approximation and is capable of modeling long light-atom interaction times, producing non-Gaussian states beyond the two-mode squeezed states. In the presence of dephasing, the entangled states are robust in the macroscopic limit as long as the dimensionless interaction time is less than $ 1/\sqrt{N}$, where $ N $ is the number of atoms in the BEC. For photon loss, the entangled states generated by long interaction times show a remarkable robustness that makes the scheme promising for various quantum information applications.