Quantum memories for light will be essential elements in future long-range quantum communication networks. These memories operate by reversibly mapping the quantum state of light onto the quantum transitions of a material system. For networks, the quantum coherence times of these transitions must be long compared to the network transmission times, approximately 100 ms for a global communication network. Due to a lack of a suitable storage material, a quantum memory that operates in the 1550 nm optical fiber communication band with a storage time greater than 1 us has not been demonstrated. Here we describe the spin dynamics of 167 Er 3+ : Y 2 SiO 5 in a high magnetic field and demonstrate that this material has the characteristics for a practical quantum memory in the 1550 nm communication band. We observe a hyperfine coherence time of 1.3 seconds. Further, we demonstrate efficient optical pumping of the entire ensemble into a single hyperfine state, the first such demonstration in a rare-earth system and a requirement for broadband spin-wave storage. With an absorption of 70 dB/cm at 1538 nm and Λ transitions enabling spin-wave storage, this material is the first candidate identified for an efficient, broadband quantum memory at telecommunication wavelengths.Any future globally deployed quantum communication network [1,2] will require nodes connected by optical fiber. To minimize transmission loss and maintain high data throughput, all elements of such a network, particularly quantum repeater nodes, should transmit in one of the low loss telecom bands for optical fiber at 1310 and 1550 nm. In its simplest implementation, a quantum repeater relies on an efficient, long-lived quantum memory [3].Developing such a memory has proven very challenging. None of the proposed systems that operate directly in the low loss telecom band have the potential for long-term storage [4,5]. For this reason, more complex ways of interfacing these candidate quantum memories with telecom are being investigated, including frequency conversion [6][7][8][9] or nondegenerate photon pairs [10][11][12][13][14].One of the most promising candidate memory systems * Corresponding author: (milos.rancic@anu.edu.au)is rare earth ions in solids. The potential for developing practical memories in these systems has been highlighted through a series of recent demonstrations using non-Kramers ions (ions with an even number of electrons Compatibility with the telecom bands is offered by Kramers ions, with an odd number of electrons. In particular, erbium has an optical transition in the telecom band at 1538 nm. However, it is much more difficult to make quantum memories with Kramers ions, and not a single Kramers system has demonstrated an on-demand quantum memory. The root of the difficulty is that, unlike for nonKramers ions, the electronic magnetic moment of Kramers ions cannot be quenched by a crystal field as they possess a half-integer spin. For these ions there is a rapid electronic spin relaxation which shortens the hyperfine state lifetimes. B...

The detection of electron spins associated with single defects in solids is a critical operation for a range of quantum information and measurement applications under development. So far, it has been accomplished for only two defect centres in crystalline solids: phosphorus dopants in silicon, for which electrical read-out based on a single-electron transistor is used, and nitrogen-vacancy centres in diamond, for which optical read-out is used. A spin read-out fidelity of about 90 per cent has been demonstrated with both electrical read-out and optical read-out; however, the thermal limitations of the former and the poor photon collection efficiency of the latter make it difficult to achieve the higher fidelities required for quantum information applications. Here we demonstrate a hybrid approach in which optical excitation is used to change the charge state (conditional on its spin state) of an erbium defect centre in a silicon-based single-electron transistor, and this change is then detected electrically. The high spectral resolution of the optical frequency-addressing step overcomes the thermal broadening limitation of the previous electrical read-out scheme, and the charge-sensing step avoids the difficulties of efficient photon collection. This approach could lead to new architectures for quantum information processing devices and could drastically increase the range of defect centres that can be exploited. Furthermore, the efficient electrical detection of the optical excitation of single sites in silicon represents a significant step towards developing interconnects between optical-based quantum computing and silicon technologies.

Three-level atomic gradient echo memory ( -GEM) is a proposed candidate for efficient quantum storage and for linear optical quantum computation with time-bin multiplexing [Hosseini et al., Nature (London) 461, 241 (2009)]. In this paper we investigate the spatial multimode properties of a -GEM system. Using a high-speed triggered CCD, we demonstrate the storage of complex spatial modes and images. We also present an in-principle demonstration of spatial multiplexing by showing selective recall of spatial elements of a stored spin wave. Using our measurements, we consider the effect of diffusion within the atomic vapor and investigate its role in spatial decoherence. Our measurements allow us to quantify the spatial distortion due to both diffusion and inhomogeneous control field scattering and compare these to theoretical models.