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

Gradient echo quantum memory for light using two-level atoms

Jevon J. Longdell, Gabriel Hetet, A. L. Alexander, Ping Koy Lam, Matthew J. Sellars

Published Sep 11, 2007• 3 citations
Publisher

Abstract

Here we show that the photon echo equivalent of an NMR gradient echo is completely efficient if the sample is optically thick, the detunings of the atoms vary linearly along the direction of propagation and the storage time is short compared to the decay rate of the atoms. In this process the only light that interacts with the sample of atoms during the storage and retrieval process is the light that is to be stored and then retrieved, their are no auxiliary beams. The stored and recalled light travel in the same direction and their is no need for the phase matching operation that is present in previous quantum memory proposals using controlled inhomogeneous broadening. This greatly simplifies various possible implementations. We present analytical, numerical and experimental results of this scheme. We report experimental efficiencies of up to 15% and suggest simple realizable improvements to significantly increase the efficiency.

DOI
10.1117/12.735471

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Here we show that the photon echo equivalent of an NMR gradient echo is completely efficient if the sample is optically thick, the detunings of the atoms vary linearly along the direction of propagation and the storage time is short compared to the decay rate of the atoms. In this process the only light that interacts with the sample of atoms during the storage and retrieval process is the light that is to be stored and then retrieved, their are no auxiliary beams. The stored and recalled light travel in the same direction and their is no need for the phase matching operation that is present in previous quantum memory proposals using controlled inhomogeneous broadening. This greatly simplifies various possible implementations. We present analytical, numerical and experimental results of this scheme. We report experimental efficiencies of up to 15% and suggest simple realizable improvements to significantly increase the efficiency.
It has recently been discovered that the optical analog of a gradient echo, in an optically thick material, could form the basis of an optical memory that is both completely efficient and noise-free. Here we present analytical calculations showing that this is the case. There is close analogy between the operation of the memory and an optical system with two beam splitters. We can use this analogy to calculate efficiencies as a function of optical depth for a number of quantum memory schemes based on controlled inhomogeneous broadening. In particular, we show that multiple switching leads to a net 100% retrieval efficiency for the optical gradient echo even in the optically thin case.
We present a simple quantum memory scheme that allows for the storage of a light field in an ensemble of two-level atoms. The technique is analogous to the NMR gradient echo for which the imprinting and recalling of the input field are performed by controlling a linearly varying broadening. Our protocol is perfectly efficient in the limit of high optical depths and the output pulse is emitted in the forward direction. We provide a numerical analysis of the protocol together with an experiment performed in a solid state system. In close agreement with our model, the experiment shows a total efficiency of up to 15%, and a recall efficiency of 26%. We suggest simple realizable improvements for the experiment to surpass the no-cloning limit.
The non-interacting and high-speed nature of light makes it an ideal carrier of information that is essential for transmission of quantum information. Indeed, many proposals and demonstrations of quantum cryptography rely on the use of fibre-optic networks. Construction of a memory that can store light and preserve its quantum properties will be useful in a range of quantum information systems such as secure quantum communication and quantum computation. This is why a quantum memory for light is a remarkable objective. The key to quantum memory is to store the probability amplitude of the possible outcomes of measurement but without measurement. An important criterion for a quantum memory is that the efficiency of the recall must exceed 50%. This is the crucial no-cloning limit for security of information, since it guarantees that nobody can access the information by copying it. This benchmark is important because any kind of deterministic amplification of quantum information is fundamentally impossible. On-demand retrieval of information and ability to controllably manipulate the quantum information are also important for quantum applications. When light is absorbed by atoms, it is actually possible to reverse the absorption process. In our memory system: light is absorbed by an ensemble of atoms and, using careful conditioning and control, we can cause the stored light to be regenerated and released at a later time. This is done by applying a gradient of magnetic field along the atomic ensemble that is the basis for our optical memory. To recall the light we flip the sign of the gradient field. This kind of reversible absorption is called photon echo, hence the name of our scheme: The Gradient Echo Memory (GEM). This simple protocol is used in our experiment and can be applied to a range of different atomic systems. We have extended the GEM protocol and experimentally implemented a memory using three-level atoms. We used an off-the-shelf Rb vapour cell operating above room temperature as the memory medium. In this realisation, we broke the efficiency record with 87% recall of the input light pulse. Moreover, through complete state tomography of coherent states we have demonstrated the ability of our memory to noiselessly store quantum states of light. We have also demonstrated that the memory can store a string of pulses and then recall the pulses ondemand in arbitrary order allowing re-sequencing of the stored information. Furthermore, we have shown that pulses could be time-compressed, time-stretched or split into multiple smaller pulses and selectively recalled in several pieces. This technique enables the construction of an optical random-access memory for quantum information. Moreover, the scheme to manipulate the spectral properties of optical data, stored inside the memory, has been introduced. We have also investigated the possibility of obtaining large nonlinear phase shifts between single photons inside the memory. Such strong interactions can be used for the implementation of universal quantum gates.
This paper presents a simple quantum memory method for efficient storage and retrieve of light. The technique is based on the principle of controlled reversible inhomogeneous broadening for which the information of the quantum state light is imprinted in a two-level atoms ensemble and recalled by flipping the external nonuniform electric field. In present work, the induced Stark shift varied linearly with position, and a numerical analysis for this protocol has been studied. It shows that the storage efficiency can nearly reach 100% with a large enough optical depth, and the optimal broadening for a given pulse width is also analyzed.
The photon-echo quantum memory is based on a controlled rephasing of the atomic coherence excited by a signal light field in the inhomogeneously broadened resonant line. Here, we demonstrate an active mechanism of the atomic rephasing that provides a perfect retrieval of the stored light field in the photon-echo quantum memory based on the use of arbitrary initial inhomogeneous broadening of the resonant line. We show that the rephasing mechanism can exploit all resonant atoms, thereby maximally increasing an optical depth of the resonant transition, which is one of the critical parameters for the realization of highly efficient quantum memory. We also demonstrate that the rephasing mechanism can be used for various realizations of the photon-echo quantum memory, thereby creating many possibilities for its practical realization.