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Optics and Spectroscopy

Entangled states of signal pulses in multimode quantum memory

K. S. Tikhonov, T. Yu. Golubeva, Yu. M. Golubev

Published May 1, 2015• 5 citations
Publisher

Abstract

The possibility of preserving an entangled state of two signal pulses during writing and retrieval processes in a multimode quantum memory is investigated. Two stationary laser sources of light squeezed in two orthogonal quadratures are used for generation of the entangled state. “Carved out” pulses of appropriate duration are mixed on a symmetric beamsplitter, which allows obtaining the desired entanglement. It is assumed that each of the obtained entangled pulses is stored in a separate memory cell and subsequently retrieved from it. The obtained analytical expressions for Duan entanglement criterion are analyzed numerically for the case of broadband memory. It is demonstrated that the entanglement of pulses can be effectively preserved not only in the case of symmetric geometry of the experiment, wherein both pulses are written in two identical independent memory cells, but also in the case of different memory cells and even when one of the cells is absent completely. An important distinguishing feature of the problem under consideration, compared to the single mode case where high memory efficiency ensures good preservation of quantum properties of light, is the possibility of obtaining good preservation of entanglement at low memory efficiency (under shortened readout times).

DOI
10.1134/s0030400x15050264

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The possibility of preserving an entangled state of two signal pulses during writing and retrieval processes in a multimode quantum memory is investigated. Two stationary laser sources of light squeezed in two orthogonal quadratures are used for generation of the entangled state. “Carved out” pulses of appropriate duration are mixed on a symmetric beamsplitter, which allows obtaining the desired entanglement. It is assumed that each of the obtained entangled pulses is stored in a separate memory cell and subsequently retrieved from it. The obtained analytical expressions for Duan entanglement criterion are analyzed numerically for the case of broadband memory. It is demonstrated that the entanglement of pulses can be effectively preserved not only in the case of symmetric geometry of the experiment, wherein both pulses are written in two identical independent memory cells, but also in the case of different memory cells and even when one of the cells is absent completely. An important distinguishing feature of the problem under consideration, compared to the single mode case where high memory efficiency ensures good preservation of quantum properties of light, is the possibility of obtaining good preservation of entanglement at low memory efficiency (under shortened readout times).
In this paper, we examined a possibility of preservation of a substantially multimode radiation in a single cell of quantum memory. As a light source we considered a synchronously pumped optical parametric oscillator (SPOPO). As was shown in several studies, SPOPO radiation has a large number of the correlated modes making it attractive for the purposes of quantum communication and computing. We showed that these correlations can be mapped on the longitudinal spin waves of the memory cell and be restored later in the readout light. The efficiencies of the writing and readout depend on the mode structure of the memory determined by a mechanism of the light-matter interaction under consideration (the nonresonance Raman interaction) and by the profile of the driving light field. We showed that like the initial light pulse train, the restored one can be represented by a set of squeezed supermodes. The mapping of the quantum multimode correlations on the material medium offers opportunities to manipulate the quantum states within the memory cell followed by the reading of the transformed state.
We present a new scheme of quantum memory for optical images (spatially multimode light fields) that allows mapping the quantum state of the signal onto the long-lived coherence of the ground state of an ensemble of stationary atoms or impurity centers. The memory medium is embedded in an optical cavity with degenerate transverse modes, which increases the effective optical thickness of the medium and allows one, in principle, to store information in optically thin atomic layers. Since, in reality, storage and retrieval of limited-duration signals, including signals shorter than the lifetime of the field in the cavity, is of interest, we do not use the low- Q cavity approximation. The influence of losses due to partial reflection of the nonstationary signal field incident on a coupling mirror on the storage efficiency is considered. We used the method of approximate impedance matching, wherein losses due to reflection can be minimized by controlling the coupling parameter of the light field with memory medium in time, thus creating conditions for destructive interference of the signal and local fields on the coupling mirror. The influence of diffraction on the transverse resolution of memory at the writing and readout stages is investigated, and the number of effectively stored transverse spatial modes of the signal is estimated.
Abstract We have considered theoretically the feasibility of broadband quantum memory based on the resonant tripod-type atomic configuration. In this case, the writing of a signal field is carried out simultaneously into two channels, and characterized by an excitation of two spin waves of the atomic ensemble. With simultaneous read out from both channels, quantum properties of the original signal are mapped onto the retrieval pulse no worse than in the case of memory based on a Λ-type atomic configuration. At the same time new possibilities are opened up for the manipulation of quantum states associated with sequential reading out (and/or sequential writing) of signal pulses. For example, a pulse in the squeezed state is converted into two partially entangled pulses with partially squeezed quadratures. Alternatively, two independent signal pulses with orthogonally squeezed quadratures can be converted into two entangled pulses.
Abstract In this article, we consider the possibility of manipulation of quantum signals, ensured by the use of the tripod-type atomic memory cell. We show that depending on a configuration of driving fields at the writing and reading, such a cell allows the signal to both be stored and transformed. It is possible to provide the operation of the memory cell in a Mach–Zehnder interferometer mode passing two successive pulses at the input. We proposed a procedure of partial signal readout that provides entanglement between the retrieved light and the atomic ensemble. Thus, we have shown that a tripod atomic cell is a promising candidate to implement quantum logical operations, including two-qubit ones, which can be performed on the basis of only one cell.
In this article the ability to record, store, and read out the quantum properties of light is studied. The discussion is based on high-speed and adiabatic models of quantum memory in $\ensuremath{\lambda}$ configuration and in the limit of strong resonance. We show that in this case the equality between efficiency and squeezing ratio, predicted by the simple beam-splitter model, is broken. The requirement of the maximum squeezing in the output pulse should not be accompanied by the requirement of maximum efficiency of memory, as in the beam-splitter model. We have demonstrated that for the same optical depth a high output pulse squeezing can be reached earlier than the high efficiency. Comprehension of this ``paradox'' is achieved on the basis of mode analysis. The memories eigenmodes, which have an impact on the memory process, are found numerically. Also, the spectral analysis of modes was performed to match the spectral width of the input signal to the capacities of the memories.
The ability of storing and retrieving quantum states of light is an important experimental challenge in quantum information science. A powerful quantum memory for light is required in a quantum repeater, which would allow long distance (>500km) quantum communications. To be implemented in such application, the quantum memory must allow on-demand readout, with high fidelity and efficiency, and a long storage time. Additionally a multimode capacity (for temporal or spatial modes) would allow multiplexing. Our approaches focus on rare-earth doped crystals, i.e. solid state quantum memory. I present, in this work, our contributions for a solid-state quantum storage, with good performances in every criteria. In particular, I present the preservation of quantum entanglement during the storage, which paves the way for the implementation of quantum memories in quantum repeaters.