We demonstrate the first solid-state spin-wave optical quantum memory with on-demand read-out. Using the full atomic frequency comb scheme in a Pr 3þ ∶Y 2 SiO 5 crystal, we store weak coherent pulses at the single-photon level with a signal-to-noise ratio > 10. Narrow-band spectral filtering based on spectral hole burning in a second Pr 3þ ∶Y 2 SiO 5 crystal is used to filter out the excess noise created by control pulses to reach an unconditional noise level of ð2.0 AE 0.3Þ × 10 −3 photons per pulse. We also report spin-wave storage of photonic time-bin qubits with conditional fidelities higher than achievable by a measure and prepare strategy, demonstrating that the spin-wave memory operates in the quantum regime. This makes our device the first demonstration of a quantum memory for time-bin qubits, with on-demand read-out of the stored quantum information. These results represent an important step for the use of solid-state quantum memories in scalable quantum networks. DOI: 10.1103/PhysRevLett.114.230501 PACS numbers: 03.67.Hk, 42.50.Ex, 42.50.Gy, 78.55.Qr Photonic quantum memories are essential in quantum information science (QIS) where they are used as quantum interfaces between flying and stationary qubits. They enable the synchronization of probabilistic quantum processes, e.g., in quantum communication [1,2] and computing [3]. The implementation of quantum memories (QMs) for light requires strong interactions between individual photons and matter. This can be achieved by placing individual quantum systems (e.g., single atoms) in high finesse cavities [4] or by using ensembles of atoms, where the photons are mapped onto collective atomic excitations. Atomic systems are natural candidates as QMs [5][6][7][8][9][10][11][12][13][14], but solid state systems offer interesting perspectives for scalability and integration into existing technology [15][16][17][18][19][20][21].Rare-earth ion doped solids are promising candidates for high performance solid state QMs since they have excellent coherence properties at cryogenic temperatures [22]. They also exhibit large static inhomogeneous broadening of the optical transitions, which can be tailored and used as a resource for various storage protocols, e.g., enabling temporally [23] and spectrally [24] multiplexed quantum memories. Recent experimental progress includes qubit storage [15,[24][25][26][27], highly efficient quantum storage of weak coherent states [16], storage of entangled and single photons [17,18,28], entanglement between two crystals [29], and quantum teleportation [30].Yet, nonclassical states have so far only been stored as collective optical atomic excitations with fixed storage times [17,18,28]. While this may provide a useful resource if combined with massive multiplexing and deterministic quantum light sources [24], the ability to read-out the stored state on demand is essential for applications where the quantum memory is used as a synchronizing device. On-demand read-out can be achieved by actively controlling the optical collective exc...

We report on experiments demonstrating the reversible mapping of heralded single photons to long lived collective optical atomic excitations stored in a Pr 3+ :Y2SiO5 crystal. A cavity-enhanced spontaneous down-conversion source is employed to produce widely non-degenerate narrow-band (≈ 2 MHz) photon-pairs. The idler photons, whose frequency is compatible with telecommunication optical fibers, are used to herald the creation of the signal photons, compatible with the Pr 3+ transition. The signal photons are stored and retrieved using the atomic frequency comb protocol. We demonstrate storage times up to 4.5 µs while preserving non-classical correlations between the heralding and the retrieved photon. This is more than 20 times longer than in previous realizations in solid state devices, and implemented in a system ideally suited for the extension to spin-wave storage.PACS numbers: 03.67. Hk,42.50.Gy,42.50.Md Many protocols in quantum information science rely on the efficient and reversible interaction between photons and matter [1]. The interaction lays the basis for the realization of quantum memories for light and of their application, e.g. in quantum repeaters [2,3]. Possible choices for the system used to store light are single atoms in cavities [4], cold or hot atomic gases [5-13], or rare earth (RE) doped solid state systems [14]. Thanks to the weak interaction between the optical active ions and the environment, RE doped crystals offer, when cryogenically cooled, the long optical and spin coherence times typical of atomic systems, free of the drawbacks deriving from atomic motion [15]. Moreover they possess the benefits of the solid state systems, such as strong interaction with light, allowing for efficient storage of photons [16,17] and prospect for integrated devices. Furthermore, their inhomogeneously broadened absorption lines can be tailored in appropriate structures, like atomic frequency combs (AFCs), to enable storage protocols with remarkable properties (e.g. temporal or frequency multiplexing) [18][19][20][21][22][23][24].Single photon level weak coherent pulses [25,26] and qubits [18,27,28] have been stored in the excited state of rare-earth doped crystals using the AFC scheme. This has recently been extended to the ground state, in the regime of a few photons per pulse [29]. The storage of non-classical light generated by spontaneous parametric down-conversion (SPDC) has also been demonstrated and enabled entanglement between one photon and one collective optical atomic excitation in a crystal [30,31], entanglement between two crystals [32], and single photon qubit storage [33,34]. However, the mapping of nonclassical light using AFC in rare earth doped crystals was obtained so far only in systems with two ground state levels, thus inherently limited to the optical coherence and not directly extendable to spin-wave storage.On the contrary, Pr 3+ or Eu 3+ doped crystals have the required level structure for spin-wave storage [22,23,29].In particular, Pr 3+ :Y 2 SiO 5 is one of the optical ...

Interfacing fundamentally different quantum systems is key to building future hybrid quantum networks. Such heterogeneous networks offer capabilities superior to those of their homogeneous counterparts, as they merge the individual advantages of disparate quantum nodes in a single network architecture. However, few investigations of optical hybrid interconnections have been carried out, owing to fundamental and technological challenges such as wavelength and bandwidth matching of the interfacing photons. Here we report optical quantum interconnection of two disparate matter quantum systems with photon storage capabilities. We show that a quantum state can be transferred faithfully between a cold atomic ensemble and a rare-earth-doped crystal by means of a single photon at 1,552  nanometre telecommunication wavelength, using cascaded quantum frequency conversion. We demonstrate that quantum correlations between a photon and a single collective spin excitation in the cold atomic ensemble can be transferred to the solid-state system. We also show that single-photon time-bin qubits generated in the cold atomic ensemble can be converted, stored and retrieved from the crystal with a conditional qubit fidelity of more than 85 per cent. Our results open up the prospect of optically connecting quantum nodes with different capabilities and represent an important step towards the realization of large-scale hybrid quantum networks.

The generation and distribution of quantum correlations between photonic qubits is a key resource in quantum information science. For applications in quantum networks and quantum repeaters, it is required that these quantum correlations be stored in a quantum memory. In 2001, Duan, Lukin, Cirac, and Zoller (DLCZ) proposed a scheme combining a correlated photon-pair source and a quantum memory in atomic gases, which has enabled fast progress towards elementary quantum networks. In this Letter, we demonstrate a solid-state source of correlated photon pairs with embedded spin-wave quantum memory, using a rare-earth-ion-doped crystal. We show strong quantum correlations between the photons, high enough for performing quantum communication. Unlike the original DLCZ proposal, our scheme is inherently multimode thanks to a built-in rephasing mechanism, allowing us to demonstrate storage of 11 temporal modes. These results represent an important step towards the realization of complex quantum networks architectures using solid-state resources.

We report on an experiment that demonstrates the frequency up-conversion of telecommunication wavelength single-photon-level pulses to be resonant with a + Pr 3 :Y SiO 2 5 crystal. We convert the telecom photons at 1570 nm to 606 nm using a periodically-poled potassium titanyl phosphate nonlinear waveguide. The maximum device efficiency (which includes all optical loss) is inferred to be η = ± 22 1% dev max (internal efficiency η = ± 75 8% int ) with a signal to noise ratio exceeding 1 for single-photon-level pulses with durations of up to 560 ns. The converted light is then stored in the crystal using the atomic frequency comb scheme with storage and retrieval efficiencies exceeding η = 20% AFC for predetermined storage times of up to μ 5 s. The retrieved light is time delayed from the noisy conversion process allowing us to measure a signal to noise ratio exceeding 100 with telecom single-photon-level inputs. These results represent the first demonstration of single-photon-level optical storage interfaced with frequency up-conversion.