Demonstration of Atomic Frequency Comb Memory for Light with Spin-Wave Storage

Abstract

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...

Control of Photon Storage Time in Photon Echoes using a Deshelving Process

“…In a rare-earth Pr 3+ (0.05 at. %) doped Y 2 SiO 5 , which has been used for most modified photon echo based quantum memories (Afzelius et al, 2010, Ham, 2010d, total atom number per unit volume (cm 3 ) is 4.7x10 18 (Maksimov et al, 1969). Either in the two-pulse photon echoes or in the stimulated photon echoes, at least one half the ground atoms are excited and spontaneously resulting in quantum noise.…”

“…To avoid coherence loss due to optical population decay during the storage process, the excited atoms must be intentionally transferred into a third state for an on-demand halt, such as an auxiliary spin state. The coherence transfer technique suggested by the Lund group has been modified to transfer the spectrally modulated atoms from the excited state to the auxiliary spin state (Afzelius et al, 2010;Ham, 2009b). Because spectral grating is free from the optical phase decay process, storage time can be lengthened if population decay-caused coherence loss is halted.…”

“…Because spectral grating is free from the optical phase decay process, storage time can be lengthened if population decay-caused coherence loss is halted. Unlike rephased atoms in the two-pulse photon echoes, the phase decay-dependent coherence loss can be frozen in the stimulated photon echoes using spectral grating, explaining why the coherence in AFC echoes (Afzelius et al, 2010) and phase locked echoes (Ham, 2009b) are degraded by spin dephasing. This will be discussed in more detail in Section 4.…”

“…Thus, the usage of an identical pulse set (Afzelius et al, 2010;Moiseevgeneration. The observation of delayed AFC echoes under this contradiction, however, can be explained as a result of coherence leakage due to imperfect population transfer in an optically dilute sample (Ham, 2010c;Ham, 2010d).…”

“…Unlike classical memories, quantum memory must satisfy a coherent process. Since the first observation of coherent retrieval of a stored optical pulse in a Bose Einstein condensate using slow light (Liu et al, 2001), interest in quantum memories has increased in the last decade (Alexander et al, 2006;Afzelius et al, 2010;Chaneliere et al, 2005. Choi et al, 2008110 Ham, 1998;Ham 2009a;Ham, 2010a;Hedges et al, 2010;Hetet et al, 2008;Hosseini et al, 2009;Julsgaard et al, 2004;Kocharovskaya et al, 2001;Kraus et al, 2006;Liu et al, 2001;Moiseev & Kroll, 2001;Moiseev et al, 2003;Neumann et al, 2009;Nilsson & Kroll, 2005;Sangouard et al, 2007;Turukhin et al, 2002;Van der Wal, et al, 2003).…”