Fidelity of an Optical Memory Based on Stimulated Photon Echoes

Abstract

We investigated the preservation of information encoded into the relative phase and amplitudes of optical pulses during storage and retrieval in an optical memory based on stimulated photon echo. By interfering photon echoes produced in a Ti-indiffused single-mode Er-doped LiNbO3 waveguiding structure at telecom wavelength, we found that decoherence in the atomic medium translates only as losses (and not as degradation) of information, as long as the data pulse series is short compared to the atomic decoherence time. The experimentally measured value of the visibility for interfering echoes is close to 100 %. In addition to the expected three-pulse photon-echo interferences we also observed interference due to a four-pulse photon echo. Our findings are of particular interest for future long-distance quantum communication protocols, which rely on the reversible transfer of quantum states between light and atoms with high fidelity. Transfer of coherence properties between light and atoms can be investigated through interferometric and spectroscopic techniques. These studies are of fundamental interest, but also deliver important information for future applications in the field of quantum information science. In quantum communication schemes, such as quantum cryptography, non-orthogonal states of light are used as information carrier. The encoding of information into the relative phase and amplitudes of a time-bin qubit has proven to be well suited for transmission over long distances, because this coding is robust to the decoherence mechanism in optical fibers [1,2]. However, the extension of quantum communication to arbitrary distances relies on the availability of quantum memories, which are key to the building of a quantum repeater [3]. Although significant progress has recently been reported [4,5,6], coherent, reversible transfer of quantum information from photons to atoms with high fidelity and efficiency remains an important and open challenge. From this perspective, it is important to understand how the fidelity of a time-bin qubit evolves when the information is stored in a quantum memory. It is the primary objectives of this Letter to address this issue. In particular, we show a case in which the decoherence in the atomic medium is a state-independent coupling with the environment: its effect on the retrieved signal is therefore only losses, i.e. a decrease of the retrieval probability. By post-selecting only cases when photons are actually emitted, one retrieves uncorrupted information, which does not require complicated classical or quantum error correction. We work in the framework of a recent proposal [7], and of first experimental studies [8], for storage of time-bin qubits based on controlled, reversible, inhomogeneous broadening. This is a photon-echo type approach to quantum memories with a theoretical efficiency of 100 %. Photon-echoes are well known for storage of classical optical pulses [9,10] as well as for being a phase-preserving process. However, the storage of information encoded in the ...

Quantum repeaters based on atomic ensembles and linear optics

“…None of the quantum memories that have been demonstrated so far work at this wavelength. Possible technological solutions include the use of wavelength conversion, which however so far is not very efficient at the single photon level (Tanzilli et al, 2005) (primarily due to coupling losses), or the use of Erbium-doped crystals as quantum memories, where first experimental (Staudt et al, 2007a(Staudt et al, , 2006(Staudt et al, , 2007b and theoretical (Ottaviani et al, 2009) investigations have been performed, but implementing the DLCZ protocol is still a distant and uncertain prospect.…”

Photon‐echo quantum memory in solid state systems

“…Interestingly, it is possible to examine many features of CRIB using much simpler traditional photonechoes [188,189], as both approaches to storage are based on re-and dephasing of coherences in an inhomogeneously broadened medium. The performance of a quantum memory can be qualified by measures such as memory time, efficiency, and fidelity (see discussion in Sect.…”

Towards an efficient atomic frequency comb quantum memory

“…The fidelity of the storage was measured by storing a timebin qubit and performing an interference measurement on the recalled qubit. The resulting interference fringe visibility was V=95%, which corresponds to a fidelity F = (1+V)/2 [19] of 97.5%. This shows that light at the single photon level can be stored and retrieved without introducing noise, and future experiments are likely to improve the fidelity further.…”