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arXiv:1305.6614 · New Journal of Physics

Advanced quantum noise correlations

Ulrich Vogl, Ryan T. Glasser, Jeremy B. Clark, Quentin C. Glorieux, Tian Li, Neil Corzo, Paul D. Lett

Published Jan 10, 2014• 29 citations
arXivHTMLPDF

Abstract

We use the quantum correlations of twin-beams of light to probe the added noise when one of the beams propagates through a medium with anomalous dispersion. The experiment is based on two successive four-wave mixing processes in rubidium vapor, which allow for the generation of bright two-mode-squeezed twin-beams followed by a controlled advancement while maintaining the shared quantum-correlations between the beams. The demonstrated effect allows the study of irreversible decoherence in a medium exhibiting anomalous dispersion, and for the first time shows the advancement of a bright nonclassical state of light. The advancement and corresponding degradation of the quantum correlations are found to be operating near the fundamental quantum limit imposed by using a phase-insensitive amplifier.

DOI
10.1088/1367-2630/16/1/013011

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We use the quantum correlations of twin-beams of light to probe the added noise when one of the beams propagates through a medium with anomalous dispersion. The experiment is based on two successive four-wave mixing processes in rubidium vapor, which allow for the generation of bright two-mode-squeezed twin-beams followed by a controlled advancement while maintaining the shared quantum-correlations between the beams. The demonstrated effect allows the study of irreversible decoherence in a medium exhibiting anomalous dispersion, and for the first time shows the advancement of a bright nonclassical state of light. The advancement and corresponding degradation of the quantum correlations are found to be operating near the fundamental quantum limit imposed by using a phase-insensitive amplifier.
This dissertation is a summary of investigations on the optical properties of a quantum-noise-limited phase-sensitive amplifier (PSA). The PSA is implemented using four-wave mixing in hot Rb 85 vapor based on a double-Lambda atomic scheme. We experimentally demonstrate the ability of a PSA to pre-amplify quantum correlations in twin light beams produced by a phase-insensitive amplifier (PIA) before degradation due to loss and detector inefficiency. By including a PSA before loss, one is able to preserve the correlations as well as the two-mode squeezing level. We compare the results to simulations employing a simple quantum-mechanical model and find a good agreement. We have demonstrated that the cross-correlation between the two modes of a bipartite entangled state can be advanced by propagation through a PIA acting as a fast-light medium. The extra noise added by the PIA has been speculated to be the mechanism that limits the advance of entanglement, preventing the mutual information from traveling superluminally. As an extension of this phase-insensitive, gain-assisted, anomalous dispersion investigation, we explore the advance and delay of information transmitted through the PSA. We start with a two-mode squeezed state created by the PIA and measure the mutual information shared by the correlated quadratures. We then pass one of these two modes through a PSA and investigate the shift of the mutual information as a function of the PSA phase. In the case of a PSA, it is well known that no extra noise will be added to the quadrature with the correct input phase (e.g., the quadrature with the maximal amplification or the maximal deamplification). We find that there is no dispersion-like behavior at these two phases, however, the peak of mutual information could either be delayed or advanced at any other phase. We also observe an almost identical behavior when we input an amplitude modulated signal to the PSA. We are able to explain the physics of this ``fast-and-slow-light'' type of behavior utilizing a model assuming imbalanced gain on the positive and negative side bands. We obtain a good agreement between the experimental results and the theoretical simulations.
We describe our efforts to observe squeezing, and related nonclassical effects, in three forward four-wave mixing experiments using sodium vapor as a nonlinear medium. In a single-beam experiment with homodyne detection, we found optically phase-sensitive noise, which was radio-frequency phase insensitive, in a cw dye-laser beam that had propagated through sodium vapor. The minima of the phase-sensitive noise were always at or above the coherent-state value. In our forward four-wave mixing experiment with probe–conjugate-beam direct detection and correlation, we found that the sodium-vapor interaction produced positive noise correlation between the probe and conjugate beams. The correlation, however, did not exceed the excess noises on these beams, so that a nonclassical behavior was not demonstrated. Finally, our forward four-wave mixing experiment with probe–conjugate-beam combination and homodyne detection did demonstrate the generation of squeezed-state light in our setup. Optically phase-sensitive noise with a minimum falling 4% below the coherent-state level was observed.
We have built a compact light source for bright squeezed twin-beams at 795nm based on four-wave-mixing in atomic 85Rb vapor. With a total optical power of 400mW derived from a free running diode laser and a tapered amplifier to pump the four-wave-mixing process, we achieve 2.1 dB intensity difference squeezing of the twin beams below the standard quantum limit, without accounting for losses. Squeezed twin beams generated by the type of source presented here could be used as reference for the precise calibration of photodetectors. Transferring the quantum correlations from the light to atoms in order to generate correlated atom beams is another interesting prospect. In this work we investigate the dispersion that is generated by the employed fourwave- mixing process with respect to bandwidth and dependence on probe detuning. We are currently using this squeezed light source to test the transfer of spatial information and quantum correlations through media of anomalous dispersion.
We have built a compact light source for bright squeezed twin-beams at 795\,nm based on four-wave-mixing in atomic 85^{85}85Rb vapor. With a total optical power of 400\,mW derived from a free running diode laser and a tapered amplifier to pump the four-wave-mixing process, we achieve 2.1\,dB intensity difference squeezing of the twin beams below the standard quantum limit, without accounting for losses. Squeezed twin beams generated by the type of source presented here could be used as reference for the precise calibration of photodetectors. Transferring the quantum correlations from the light to atoms in order to generate correlated atom beams is another interesting prospect. In this work we investigate the dispersion that is generated by the employed four-wave-mixing process with respect to bandwidth and dependence on probe detuning. We are currently using this squeezed light source to test the transfer of spatial information and quantum correlations through media of anomalous dispersion.
We recently demonstrated optical pulses with large negative group velocities using a scheme based on four-wave-mixing in rubidium vapor. Both the probe and the generated conjugate pulses can experience anomalous dispersion, and we have demonstrated a maximum group advancement of 64% relative to the input pulse width. The four-wave-mixing scheme allows us to send whole images through a fast light medium, and we have analyzed the arrival of spatially-encoded information in the system. We are currently investigating the transport of quantum correlations that are shared by the probe and conjugate modes when sent through fast light media.