An efficient multimode quantum memory is a crucial resource for long-distance quantum communication based on quantum repeaters. We propose a quantum memory based on spectral shaping of an inhomogeneously broadened optical transition into an atomic frequency comb ͑AFC͒. The spectral width of the AFC allows efficient storage of multiple temporal modes without the need to increase the absorption depth of the storage material, in contrast to previously known quantum memories. Efficient readout is possible thanks to rephasing of the atomic dipoles due to the AFC structure. Long-time storage and on-demand readout is achieved by use of spin states in a lambda-type configuration. We show that an AFC quantum memory realized in solids doped with rare-earth-metal ions could store hundreds of modes or more with close to unit efficiency, for material parameters achievable today.

Coherent and reversible mapping of quantum information between light and matter is an important experimental challenge in quantum information science. In particular, it is a decisive milestone for the implementation of quantum networks and quantum repeaters [1,2,3]. So far, quantum interfaces between light and atoms have been demonstrated with atomic gases [4,5,6,7,8,9], and with single trapped atoms in cavities [10]. Here we demonstrate the coherent and reversible mapping of a light field with less than one photon per pulse onto an ensemble of ∼ 10 7 atoms naturally trapped in a solid. This is achieved by coherently absorbing the light field in a suitably prepared solid state atomic medium [11]. The state of the light is mapped onto collective atomic excitations on an optical transition and stored for a pre-programmed time up of to 1µs before being released in a well defined spatio-temporal mode as a result of a collective interference. The coherence of the process is verified by performing an interference experiment with two stored weak pulses with a variable phase relation. Visibilities of more than 95% are obtained, which demonstrates the high coherence of the mapping process at the single photon level. In addition, we show experimentally that our interface allows one to store and retrieve light fields in multiple temporal modes. Our results represent the first observation of collective enhancement at the single photon level in a solid and open the way to multimode solid state quantum memories as a promising alternative to atomic gases.Efficient and reversible mapping of quantum states between light and matter requires strong interactions between photons and atoms. With single quantum systems, this regime can be reached with high finesse optical cavities, which is technically highly demanding [10]. In contrast, light can be efficiently absorbed in ensembles of atoms in free space. Moreover, it is possible to engineer the atomic systems such that the stored light can be retrieved in a well defined spatio-temporal mode due to a collective constructive interference between all the emitters. This collective enhancement is at the heart of protocols for storing photonic quantum states in atomic ensembles, such as schemes based on Electromagnetically-Induced Transparency (EIT) [12], off-resonant Raman interactions [2,13] and modified photon echoes using Controlled Reversible Inhomogeneous Broadening (CRIB) [14,15,16] and Atomic Frequency Combs (AFC) [11].All previous quantum storage experiments with ensembles have been performed using atomic gases as the storage material [4,5,6,7,8,9]. However, some solid state systems have properties that make them very attractive for applications in quantum storage. In particular, rare-earth ion doped solids provide a unique physical system where large ensembles of atoms are naturally trapped in a solid state matrix, which prevents decoherence due to the motion of the atoms and allows the use of trapping free protocols. Moreover, these systems also exhibit excellent coherence pro...

We propose a quantum repeater protocol which builds on the well-known DLCZ protocol [L.M. Duan, M.D. Lukin, J.I. Cirac, and P. Zoller, Nature 414, 413 (2001)], but which uses photon pair sources in combination with memories that allow to store a large number of temporal modes. We suggest to realize such multi-mode memories based on the principle of photon echo, using solids doped with rare-earth ions. The use of multimode memories promises a speedup in entanglement generation by several orders of magnitude and a significant reduction in stability requirements compared to the DLCZ protocol.The distribution of entanglement over long distances is an important challenge in quantum information. It would extend the range for tests of Bell's inequalities, quantum key distribution and quantum networks. [5,6]. A basic element of all protocols is the creation of entanglement between neighboring nodes A and B, typically conditional on the outcome of a measurement, e.g. the detection of one or more photons at a station between two nodes. In order to profit from a nested repeater protocol [1], the entanglement connection operations creating entanglement between non-neighboring nodes can only be performed once one knows the relevant measurement outcomes. This requires a communication time of order L 0 /c, where L 0 is the distance between A and B. Conventional repeater protocols are limited to a single entanglement generation attempt per elementary link per time interval L 0 /c. Here we propose to overcome this limitation using a scheme that combines photon pair sources and memories that can store a large number of distinguishable temporal modes. We also show that such memories could be realized based on the principle of photon echo, using solids doped with rare-earth ions.Our scheme is inspired by the DLCZ protocol [2], which uses Raman transitions in atomic ensembles that lead to nonclassical correlations between atomic excitations and emitted photons [7]. The basic procedure for entanglement creation between two remote locations A and B in our protocol requires one memory and one source of photon pairs at each location, denoted M A(B) and S A(B) respectively. The two sources are coherently excited such that each has a small probability p/2 of creating a pair, corresponding to a stateHere a and a ′ (b and b ′ ) are the two modes corresponding to S A (S B ), φ A (φ B ) is the phase of the pump laser at location A (B), and |0 is the vacuum state. The O(p) term introduces errors in the protocol, leading to the requirement that p has to be kept small, cf. below. The photons in modes a and b are stored in the local memories M A and M B . The modes a ′ and b ′ are coupled into fibers and combined on a beam splitter at a station between A and B. The modes after the beam splitter, where χ A,B are the phases acquired by the photons on their way to the central station. Detection of a single photon inã, for example, creates a state, where a and b are now stored in the memories. This can be rewritten as an entangled state of the two m...

Entanglement is the fundamental characteristic of quantum physics. Large experimental efforts are devoted to harness entanglement between various physical systems. In particular, entanglement between light and material systems is interesting due to their prospective roles as "flying" and stationary qubits in future quantum information technologies, such as quantum repeaters [1][2][3] and quantum networks [4]. Here we report the first demonstration of entanglement between a photon at telecommunication wavelength and a single collective atomic excitation stored in a crystal. One photon from an energytime entangled pair [5] is mapped onto a crystal and then released into a well-defined spatial mode after a predetermined storage time. The other photon is at telecommunication wavelength and is sent directly through a 50 m fiber link to an analyzer. Successful transfer of entanglement to the crystal and back is proven by a violation of the Clauser-Horne-Shimony-Holt (CHSH) inequality [6] by almost three standard deviations (S = 2.64 ± 0.23). These results represent an important step towards quantum communication technologies based on solid-state devices. In particular, our resources pave the way for building efficient multiplexed quantum repeaters [7,8] for long-distance quantum networks.While single atoms [9,10] and cold atomic gases [11][12][13][14][15][16] are currently some of the most advanced light-matter quantum interfaces, there is a strong motivation to control light-matter entanglement with more practical systems, such as solid-state devices [17]. Solid-state quantum memories for photons can be implemented with cryogenically cooled crystals doped with rare-earth-metal (RE) ions [18], which have impressive coherence properties at temperatures below 4 K. These solid-state systems have the advantage of simple implementation since RE-doped crystals are widely produced for solid-state lasers, and closed-cycle cryogenic coolers are commercially available. Important progress has been made over the last years in the context of light storage into solidstate memories, including long storage times [19], high efficiency [20] and storage of light at the single photon level with high coherence and negligible noise [8,[20][21][22][23][24]. Yet, these experiments were realized with classical bright or weak coherent states of light. While this is sufficient to characterize the performances of the memory, and even to infer the quantum characteristics of the device [20,21], it is not sufficient for the implementation of more sophisticated experiments involving entanglement, as required for most applications in quantum information science. For this purpose, it is necessary to store non-classical light, in particular individual photons that are part of an entangled state. In addition, for quantum communication applications, the other part of the entangled state should be a photon at telecommunication wavelength in order to minimize loss during transmission in optical fibers.In this Letter, we report on an experiment where a phot...

Quantum memories : a review based on the European integrated project "Qubit Applications (QAP)"

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.

We present a light-storage experiment in a praseodymium-doped crystal where the light is mapped onto an inhomogeneously broadened optical transition shaped into an atomic frequency comb. After absorption of the light the optical excitation is converted into a spin-wave excitation by a control pulse. A second control pulse reads the memory (on-demand) by reconverting the spin-wave excitation to an optical one, where the comb structure causes a photon-echo type rephasing of the dipole moments and directional retrieval of the light. This combination of photon echo and spin-wave storage allows us to store sub-microsecond (450ns) pulses for up to 20 µs. The scheme has a high potential for storing multiple temporal modes in the single photon regime, which is an important resource for future long-distance quantum communication based on quantum repeaters.A quantum memory (QM) for photons is a light-matter interface that can achieve a coherent and reversible transfer of quantum information between a light field and a material system [1]. A QM should enable efficient, highfidelity storage of non-classical states of light, which is a key resource for future quantum networks, particularly in quantum repeaters [2][3][4][5][6] that have the potential for distributing entangled states over long distances for quantum communication tasks. In order to achieve reasonable entanglement distribution rates, it has been shown that some type of multiplexing is required [4,5], using for instance independent frequency, spatial or temporal modes (multimode QM).Several types of light-matter interactions have been proposed for building a QM, for instance electromagnetically induced transparency [7][8][9][10], Raman interactions [11][12][13][14], or photon-echo techniques [15][16][17][18][19][20][21][22]. Photon echo techniques in rare-earth-ion doped crystals have an especially high multimode capacity for storing classical light [23]. Classical photon echoes are not useful, however, for single-photon storage due to inherent noise problems due to unwanted spontaneous and stimulated emission processes when storing light on a single photon level [24]. The photon-echo QM based on controlled reversible inhomogeneous broadening [15][16][17][18][19] is free of these noise problems. But this technique has a lower time-multiplexing capacity than classical photon echoes, for a given optical depth, due to loss of storage efficiency as the controlled frequency bandwidth is increased [20,25]. Some of us recently proposed a photon-echo type QM based on an atomic frequency comb (AFC) [20] that has a storage efficiency independent of the bandwidth, allowing optimal use of the inhomogeneous broadening of rare-earthdoped crystals. An AFC memory has the potential for providing multimode storage capacity [20,25] crucial to quantum repeaters. In a first experiment [21] based on this scheme we performed a light-matter interface at the single-photon level. However, the light was retrieved after a predetermined storage time, while for quantum repeaters it is crucia...

An optical quantum memory can be broadly defined as a system capable of storing a useful quantum state through interaction with light at optical frequencies. During the last decade, intense research was devoted to their development, mostly with the aim of fulfilling the requirements of their first two applications, namely quantum repeaters and linear-optical quantum computation. A better understanding of those requirements then motivated several different experimental approaches. Along the way, other exciting applications emerged, such as as quantum metrology, single-photon detection, tests of the foundations of quantum physics, deviceindependent quantum information processing and nonlinear processing of quantum information. Here we review several prospective applications of optical quantum memories with a focus on recent experimental achievements pertaining to these applications. This review highlights that optical quantum memories have become essential for the development of optical quantum information processing.