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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Impact of non-adiabatic fields and dissipation on quantum storage and retrieval

Joyee Ghosh, Rupamanjari Ghosh, Deepak Kumar

Published Dec 29, 2008• 0 citations
Publisher

Abstract

In a Λ-type atomic system with electromagnetically-induced transparency, we probe the fidelity of the storage and retrieval of an electromagnetic signal, as the control laser field is varied with time. We study numerically the adiabatic transfer problem for an isolated atom, and show that for a weak signal, even the slowest variations of the control field take the system out of the dark state, which is a coherent superposition of the two lower levels of the Λ system. Following this, we incorporate the effect of dissipation on system dynamics by allowing for spontaneous decay of the system to the lower levels in a wavefunction approach. We conclude that dissipation definitely aids the retrieval process but not so much the storage. Further, for storing the signal, the control field should be switched off as slowly as possible, but while retrieving it, the faster we switch on the control field, the better the signal is retrieved. Also, for a given system there is an optimal control power for the best retrieval. Our results find partial support in the reported experimental observations.

DOI
10.1117/12.816345

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In a Λ-type atomic system with electromagnetically-induced transparency, we probe the fidelity of the storage and retrieval of an electromagnetic signal, as the control laser field is varied with time. We study numerically the adiabatic transfer problem for an isolated atom, and show that for a weak signal, even the slowest variations of the control field take the system out of the dark state, which is a coherent superposition of the two lower levels of the Λ system. Following this, we incorporate the effect of dissipation on system dynamics by allowing for spontaneous decay of the system to the lower levels in a wavefunction approach. We conclude that dissipation definitely aids the retrieval process but not so much the storage. Further, for storing the signal, the control field should be switched off as slowly as possible, but while retrieving it, the faster we switch on the control field, the better the signal is retrieved. Also, for a given system there is an optimal control power for the best retrieval. Our results find partial support in the reported experimental observations.
A three-level atom in a ΛΛΛ configuration trapped in an optical cavity forms a basic unit in a number of proposed protocols for quantum information processing. Through control with an appropriate laser, this system allows for efficient storage of cavity photons into long-lived atomic excitations, and their retrieval with high fidelity. This process presumes an adiabatic transfer through the `dark state', a coherent superposition of the two lower levels of the ΛΛΛ system, by a slow variation of the intensity of the control laser. We study the full quantum mechanics of this transfer process with a view to examine the nonadiabatic effects, as the control laser is varied in time. The nonadiabatic effects arise due to inevitable excitations of the system to states involving the highest level of the ΛΛΛ configuration, which is radiative. We formulate the problem in terms of the instantaneous eigenstates and solve it numerically, allowing for spontaneous decays from the excited level. We present detailed results for both storage and retrieval for a suitable range of variation rates and intensity of the control laser, and examine optimum conditions for the transfer process.
We theoretically explore coherent information transfer between ultraslow light pulses and Bose-Einstein condensates (BEC's) and find that storing light pulses in BEC's allows the coherent condensate dynamics to process optical information. We consider BEC's of alkali atoms with a $\ensuremath{\Lambda}$ energy level configuration. In this configuration, one laser (the coupling field) can cause a pulse of a second pulsed laser (the probe field) to propagate with little attenuation (electromagnetically induced transparency) at a very slow group velocity $(\ensuremath{\sim}10\phantom{\rule{0.3em}{0ex}}\mathrm{m}∕\mathrm{s})$ and be spatially compressed to lengths smaller than the BEC. These pulses can be fully stopped and later revived by switching the coupling field off and on. Here we develop a formalism, applicable in both the weak- and strong-probe regimes, to analyze such experiments and establish several results: (1) We show that the switching can be performed on time scales much faster than the adiabatic time scale for electromagnetically induced transparancy even in the strong-probe regime. We also study the behavior of the system changes when this time scale is faster than the excited state lifetime. (2) Stopped light pulses write their phase and amplitude information onto spatially dependent atomic wave functions, resulting in coherent two-component BEC dynamics during long storage times. We investigate examples relevant to 87Rb^{87}\mathrm{Rb}87Rb experimental parameters and see a variety of novel dynamics occur, including interference fringes, gentle breathing excitations, and two-component solitons, depending on the relative scattering lengths of the atomic states used and the probe to coupling intensity ratio. We find that the dynamics when the levels $\ensuremath{\mid}F=1,{M}_{F}=\ensuremath{-}1⟩$ and $\ensuremath{\mid}F=2,{M}_{F}=+1⟩$ are used could be well suited to designing controlled processing of the information input on the probe. (3) Switching the coupling field on after the dynamics writes the evolved BEC wave functions density and phase features onto a revived probe pulse, which then propagates out. We establish equations linking the BEC wave function to the resulting output probe pulses in both the strong- and weak-probe regimes. We then identify sources of deviations from these equations due to absorption and distortion of the pulses. These deviations result in imperfect fidelity of the information transfer from the atoms to the light fields and we calculate this fidelity for Gaussian-shaped features in the BEC wave functions. In the weak-probe case, we find that the fidelity is affected both by absorption of very-small-length-scale features and absorption of features occupying regions near the condensate edge. We discuss how to optimize the fidelity using these considerations. In the strong-probe case, we find that when the oscillator strengths for the two transitions are equal the fidelity is not strongly sensitive to the probe strength, while when they are unequal the fidelity is worse for stronger probes. Applications to distant communication between BEC's, squeezed light generation, and quantum information are anticipated.
The recent prospect of efficient, reliable, and secure quantum communication relies on the ability to coherently and reversibly map nonclassical states of light onto long-lived atomic states. A promising technique that accomplishes this employs Electromagnetically Induced Transparency (EIT), in which a strong classical control field modifies the optical properties of a weak signal field in such a way that a previously opaque medium becomes transparent to the signal field. The accompanying steep dispersion in the index of refraction allows for pulses of light to be decelerated, then stored as an atomic excitation, and later retrieved as a photonic mode. This dissertation presents the results of investigations into methods for optimizing the memory efficiency of this process in an ensemble of hot Rb atoms. We have experimentally demonstrated the effectiveness of two protocols for yielding the best memory efficiency possible at a given atomic density. Improving memory efficiency requires operation at higher optical depths, where undesired effects such as four-wave mixing (FWM) become enhanced and can spontaneously produce a new optical mode (Stokes field). We present the results of experimental and theoretical investigations of the FWM-EIT interaction under continuous-wave (cw), slow light, and stored light conditions. In particular, we provide evidence that indicates that while a Stokes field is generated upon retrieval of the signal field, any information originally encoded in a seeded Stokes field is not independently preserved during the storage process. We present a simple model that describes the propagation dynamics and provides an intuitive description of the EIT-FWM process.
Electromagnetically induced transparency is a promising approach to implement quantumcommunication in quantum system. In this study, quantum coherence and interference in theexcitation pathways of a three-level Λ atomic system leads to the formation of new states of matter(i.e. the dark state). This state decouples itself from the excitation beams leading to phenomenasuch as coherent population trapping (CPT), electromagnetically induced transparency (EIT).Taking into account quantum coherence and interference in an optically dense electromagneticallyinducedtransparent medium, we investigate the non-destructive storage and retrieval of singlephotonquantum states in the dark state of this dense ensemble of gas phase medium. The storageand retrieval process is based on the intracavity electromagnetically induced transparency by whichproperties of a cavity filled with three-level Λ-type atoms are manipulated by an external control field.We demonstrate the peculiar case for the storage and retrieval of time-entangled periodic input pulses(a soliton train) in the dark state by adiabatic photon transfer under conditions of dynamicalimpedance matching. These multiplexed photon states are expected to allow sharing quantuminformation among many users, and are currently of very high demand for applications in longdistanceand multiplexed quantum communication.
Electromagnetically induced transparency is a promising approach to implement quantumcommunication in quantum system. In this study, quantum coherence and interference in theexcitation pathways of a three-level Λ atomic system leads to the formation of new states of matter(i.e. the dark state). This state decouples itself from the excitation beams leading to phenomenasuch as coherent population trapping (CPT), electromagnetically induced transparency (EIT).Taking into account quantum coherence and interference in an optically dense electromagneticallyinducedtransparent medium, we investigate the non-destructive storage and retrieval of singlephotonquantum states in the dark state of this dense ensemble of gas phase medium. The storageand retrieval process is based on the intracavity electromagnetically induced transparency by whichproperties of a cavity filled with three-level Λ-type atoms are manipulated by an external control field.We demonstrate the peculiar case for the storage and retrieval of time-entangled periodic input pulses(a soliton train) in the dark state by adiabatic photon transfer under conditions of dynamicalimpedance matching. These multiplexed photon states are expected to allow sharing quantuminformation among many users, and are currently of very high demand for applications in longdistanceand multiplexed quantum communication.