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Quantum-Atom Optics Downunder (QAO)

Information Storage in Rubidium via Gradient Echo and EIT

Gabriel Hetet, B. C. Buchler, Magnus T. L. Hsu, Jevon J. Longdell, Matthew J. Sellars, Hans‐A. Bachor, Ping Koy Lam

Published Jan 1, 2007• 0 citations
Publisher

Abstract

The aim of our work is to realize the atomic storage of continuous variable quantum information encoded in light fields. Following recent ideas, we propose a new scheme based on both Raman absorption and gradient echo and compare its efficacy with electromagnetically induced transparency (EIT)-based memories. A tuneable source of low frequency squeezing at rubidium wavelengths was built to study and compare the efficacy of the memory schemes. We will present our latest theoretical investigations and experimental observations of the coupling of squeezing to Rubidium atoms using these protocols.

DOI
10.1364/qao.2007.qtuc3

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The aim of our work is to realize the atomic storage of continuous variable quantum information encoded in light fields. Following recent ideas, we propose a new scheme based on both Raman absorption and gradient echo and compare its efficacy with electromagnetically induced transparency (EIT)-based memories. A tuneable source of low frequency squeezing at rubidium wavelengths was built to study and compare the efficacy of the memory schemes. We will present our latest theoretical investigations and experimental observations of the coupling of squeezing to Rubidium atoms using these protocols.
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Recent years have observed fast developments in neutral atomic vapor based quantum information storage technology. The technique utilizes light fields in the optical wavelength region as signal carrier and retriever and neutral atomic systems (e.g. single atoms, ensembles of atoms, atom-like defects in solids) as storage media. Photons are robust carriers of information due to their high velocity (c = 2.998 x 108 m/s) and ease of transportation (they propagate rectilinearly with low loss). Their high mobility, however, makes it a challenging task to spatially localize and therefore store them. Atoms (or atom-like systems), on the other hand, can be easily localized in space while providing quantum states accessible to photons and therefore are ideal candidates for storage media. Electromagnetically-induced transparency (EIT) is a light-induced atomic coherence phenomenon that dramatically modifies the optical properties of an atomic medium to a weak probe field by the application of a strong coupling field. As a probe pulse propagates in an EIT medium it experiences ultraslow group velocity and generates a copropagating spin wave. These consequences of EIT can be exploited for light pulse storage and retrieval in an atomic medium.
Quantum information systems require an interface between photons as a physical system to transport information and atoms which are better suited to store and process it. In this thesis the construction of a novel pulsed, bright and narrow-band light source is presented. It consists of an optical parametric oscillator featuring a periodically poled KTP crystal and a mechanical chopper and produces ?s-pulses of squeezed vacuum. The generated states are resonant to the Rubidium-DI-transition, are almost transform-limited and show more than 3 dB of squeezing. The physical effect of electromagnetically induced transparency (EIT) in hot rubidium vapor can be used to generate ultra-low group velocities and allows for a reversible adiabatic conversion of optical quantum states into collective spin excitations which can be used to stop pulses of light. I report about recent progress in slowing down a pulse of squeezed vacuum by one third of its width while still preserving 0.36 dB of squeezing in the process. I analyze electromagnetically induced transparency and light storage in an ensemble of atoms with multiple excited levels (multi-A configuration) which are coupled to one of the ground states by quantized signal fields and to the other one via classical control fields. A basis transformation of atomic and optical states which reduces the analysis of the system to that of electromagnetically induced transparency in a regular three-level configuration is presented. We demonstrate the existence of dark state polaritons and propose a protocol to transfer quantum information from one optical mode to another by adiabatic control of the control fields and present a proof-of principle experiment to adiabatically transfer light between modes of different frequencies in Rb-87 vapor. To that end a digital phase lock circuit has been designed that allows to electronically phase lock an external-cavity diode laser to a reference laser over a 7 GHz frequency range with sub-Hertz precision.
We demonstrate a hybrid quantum memory that combines Gradient Echo Memory (GEM) and Electromagnetically Induced Transparency (EIT) protocols for reversible mapping between light and atomic coherence. By leveraging GEM and EIT complementarity, we realize time-to-frequency and frequency-to-time conversion mechanisms for spectro-temporal modes. This capability provides a versatile tool for quantum communication, where coherent frequency-time conversion enhances network interoperability. In addition, the protocol may enable fundamental studies of atomic coherence, including investigations of Rydberg polaritons and mapping of single Rydberg excitations and ionic impurities.
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.