Many applications of quantum communication crucially depend on reversible transfer of quantum states between light and matter. Motivated by rapid recent developments in theory and experiment, we review research related to quantum memory based on a photon-echo approach in solid state material with emphasis on use in a quantum repeater. After introducing quantum communication, the quantum repeater concept, and properties of a quantum memory required to be useful in a quantum repeater, we describe the historical development from spin echoes, discovered in 1950, to photon-echo quantum memory. We present a simple theoretical description of the ideal protocol, and comment on the impact of a non-ideal realization on its quantum nature. We extensively discuss rare-earth-ion doped crystals and glasses as material candidates, elaborate on traditional photon-echo experiments as a test-bed for quantum state storage, and describe the current state-of-the-art of photon-echo quantum memory. Finally, we give a brief outlook on current research.The picture shows a Europium doped Y2SiO5 crystal surrounded by electrodes in the setup used for the first proof-of-principle demonstration of the novel, photon-echo based quantum memory protocol.
Effective multi-mode photon echo based quantum memory on multi-atomic ensemble in the QED cavity is proposed. Analytical solution is obtained for the quantum memory efficiency that can be equal unity when optimal relations for the cavity and atomic parameters are held. Numerical estimation for realistic atomic and cavity parameters demonstrates the high efficiency of the quantum memory for optically thin resonant atomic system. 42.50.Ct, 42.50.Md Quantum communications and quantum computation require an effective quantum memory (QM) that should possess a multi-mode and high fidelity character [1][2][3][4][5]. Most well-known QM based on electromagnetically induced transparency effect [6] demonstrates an efficient storage and retrieval only for a specific single temporal mode regime [7][8][9]. Photon echo QM [10-14] offers most promising properties for realization of the multi-mode QM [15][16][17]. However, the quantum efficiency of all discussed multi-mode variants of the photon echo QM tends to unity for infinite optical depth αL as [1 − exp(−αL)] 2 , where α and L are resonant absorption coefficient and length of the medium along the light field propagation [18,19]. It imposes a fundamental limit for the QM efficiency so it is necessary to increase either the atomic concentration or the medium length. However, the QM device should be compact and the large increase of the atomic concentration gives rise to atomic decoherence due to the dipole-dipole interactions limiting thereby a storage time. So, using the free space QM scheme is quite problematic for practical devices. Efficient photon echo QM with controlled reverse of inhomogeneous broadening (CRIB) have been studied recently in ideal cavity [20] and in bad cavity [21] where high QM efficiency has been demonstrated only for a specific optimal single mode regime. Here, we propose a general approach for multi-mode photon echo type of QM in QED cavity (single mode resonator). We demonstrate a high efficiency of the QM for the optimized system of atoms and QED cavity at arbitrary temporal shape of the stored field modes. We find a simple analytical solution for QM efficiency and the optimal conditions for matching of the atomic and cavity parameters where the QM efficiency can reach unity even for small optical depth of the medium loaded in the cavity. Basic equations:We analyze resonant multi-atomic system in a single mode QED cavity coupled with signal and bath fields. By following to the cavity mode formalism [22], we use a Tavis-Cumming Hamiltonian [23]Ĥ =Ĥ o +Ĥ 1 , for N atoms, field modes and their interactions taking into account the inhomogeneous and homogeneous broadenings of the atomic frequencies and continuous spectral distribution of the field modes wherêare main energies of atoms (S j z is a z-projection of the spin 1/2 operator), energy of cavity field (â + andâ are arising and decreasing operators), energies of signal (l=1) and bath (l=2) fields (b + l and b l are arising and decreasing operators of the field modesThe first term in (2) compr...
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