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Physical Review A

Quantum noise reduction close to an optically bistable dark resonance

Klaus M. Gheri, Daniel F. Walls, Monika A. M. Marte

Published Aug 1, 1994• 12 citations
Publisher

Abstract

We show that \ensuremath{\Lambda} three-level atoms interacting with two coherent light fields tuned close to two-photon or dark resonance behave as a saturable dispersive nonlinearity. If one of the light fields is allowed to resonate within an optical cavity the quantum noise in a quadrature of the output light can be almost completely suppressed. As no dynamic retuning of the cavity is required to achieve noise reduction and cold atomic media are readily available in traps which conform in essence to the proposed configuration, an experimental realization would seem within reach.

DOI
10.1103/physreva.50.1871

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The interaction of light with atomic media, i.e., media describable as an ensemble of free particles, can be used to alter the quantum statistics of the interacting light field. Nonlinear effects present in such an interaction are rather weak unless the light is tuned dose to resonance with a transition between two atomic energy eigenstates. Although semiclassical optical effects are now enhanced, the desired changes to the quantum statistics are normally washed out by atomic noise from absorption and spontaneous emission. Tuning away from resonance turns off nonlinearities faster than it turns off absorption.

•May 8, 1994•0 citations
We show that \ensuremath{\Lambda} three-level atoms interacting with two coherent light fields tuned close to two-photon or dark resonance behave as a saturable dispersive nonlinearity. If one of the light fields is allowed to resonate within an optical cavity the quantum noise in a quadrature of the output light can be almost completely suppressed. As no dynamic retuning of the cavity is required to achieve noise reduction and cold atomic media are readily available in traps which conform in essence to the proposed configuration, an experimental realization would seem within reach.
The squeezing spectrum for a cavity field mode interacting with an ensemble of three-level ``\ensuremath{\Lambda}-configuration'' atoms by an effective two-photon transition is calculated. The advantage of the three-level \ensuremath{\Lambda} system as a squeezing medium, that is, optical nonlinearity without atomic saturation, has recently been pointed out by Reid, Walls, and Dalton. We predict perfect squeezing at the turning points for dispersive optical bistability and good squeezing for a range of other cases. Three-level ladder atoms interacting by an effective two-photon transition are also shown to give perfect squeezing in the dispersive limit.
We investigate two schemes for the efficient conversion of coherent input light into bright-squeezed output light. Both schemes utilize strong signal and weak probe fields, interacting with three-level ladder-configuration atoms inside optical cavities. The schemes differ in the resonance requirements of the cavities and produce noise suppression for quite different tuning regimes. Quantum-noise reduction is a consequence of the dressing of the atoms with two coherent fields. By tuning the probe light in the right fashion, spontaneous emission from the excited state can be made to counteract signal-light intensity fluctuations.
We examine the field-field and field-atom entanglements generated by \ensuremath{\Lambda}-type three-level atoms placed in an optical cavity driven above threshold by two coherent input optical fields. It is shown that under realistic experimental conditions entanglement between the two output cavity fields as well as between the fields and the atoms can be achieved when the two cavity fields are both one- and two-photon (Raman) resonant with the atomic transitions, as in the configuration for electromagnetically induced transparency (EIT). The entanglement has similar features to pump-signal-idler three-color entanglement in an above-threshold optical parametric oscillator [Villar et al., Phys. Rev. Lett. 97, 140504 (2006)]. This cavity-enhanced atom-field cooperative coupling could enable convenient and efficient generation of bright, nondegenerate, narrow-band entangled fields, which may find potential applications in realistic quantum communications and networking protocols.
A cavity QED system is analysed which duplicates the dynamics of a two-level atom in free space interacting exclusively with broadband squeezed light. We consider atoms in a three-or four-level Λ configuration coupled to a high-finesse optical cavity which is driven by a squeezed light field. Raman transitions are induced between a pair of stable atomic ground states via the squeezed cavity mode and coherent driving fields. An analysis of the reduced master equation for the atomic ground states shows that a three-level atomic system has insufficient parameter flexibility to act as an effective two-level atom interacting exclusively with a squeezed reservoir. However, the inclusion of a fourth atomic level, coupled dispersively to one of the two ground states by an auxiliary laser field, introduces an extra degree of freedom and enables the desired interaction to be realized. As a means of detecting the reduced quadrature decay rate of the effective two-level system, we examine the transmission spectrum of a weak coherent probe field incident upon the cavity. The extension to multiple atoms is also considered and we show that such a cavity QED system may also duplicate the dynamics of an ensemble of two-level atoms collectively coupled to broadband squeezed light.
The interaction of light with atomic media, i.e., media describable as an ensemble of free particles, can be used to alter the quantum statistics of the interacting light field. Nonlinear effects present in such an interaction are rather weak unless the light is tuned dose to resonance with a transition between two atomic energy eigenstates. Although semiclassical optical effects are now enhanced, the desired changes to the quantum statistics are normally washed out by atomic noise from absorption and spontaneous emission. Tuning away from resonance turns off nonlinearities faster than it turns off absorption.