Jiri Minar

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

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 submicrosecond (450 ns) pulses for up to 20 mus. 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.

Telecommunication-wavelength solid-state memory at the single photon level.

We demonstrate experimentally the storage and retrieval of weak coherent light fields at telecommunication wavelengths in a solid. Light pulses at the single photon level are stored for a time up to 600 ns in an erbium-doped Y2SiO5 crystal at 2.6 K and retrieved on demand. The memory is based on photon echoes with controlled reversible inhomogeneous broadening, which is realized here for the first time at the single photon level. This is implemented with an external field gradient using the linear Stark effect. This experiment demonstrates the feasibility of a solid-state quantum memory for single photons at telecommunication wavelengths, which would represent an important resource in quantum information science.

Long-Distance Entanglement Distribution with Single-Photon Sources

The entangled-state distribution over long distances is a challenging task due to the limited transmission efficiencies of optical fibers. To overcome this problem, quantum repeaters are likely to be required 1. The basic principle of quantum repeaters consists in decomposing the full distance into shorter elementary links. Quantum memories allow the creation of entanglement independently for each link. This entanglement can then be extended to the full distance using entanglement swapping. The protocol proposed here is similar to the well-known Duan-Lukin-Cirac-Zoller DLCZ scheme 2 and to its recent modification based on photon pairs and multimode memories P 2 M 3 3, in that entanglement for an elementary link is created by the detection of a single photon. However, both protocols rely on sources that create correlated pairs of excitations, namely, one atomic excitation and one photon in the case of the DLCZ scheme and two photons in the case of P 2 M 3 . These correlations allow one to establish entanglement between distant memories based on the detection of a photon which could have come from either of two remote sources. Our protocol uses single-photon sources, making it possible to eliminate errors due to two-pair emission events, which are unavoidable for Refs. 2,3. This leads to a significant improvement in the achievable entanglement distribution rate. Moreover, our scheme is compatible with the use of multimode memories 3 and spatial and frequency multiplexing 4 which promise additional speedups. We begin by recalling the basic principles of the P 2 M 3 protocol. The DLCZ protocol is equivalent for the purposes of the present discussion. The architecture of an elementary link is represented in Fig. 1a. The procedure to entangle two remote locations A and B requires one photon-pair source and one memory at each location. The pair sources are coherently excited such that each of them can emit a pair with a small probability p / 2, corresponding to the state 1+ p/2a † a † + b † b † + Op0;

Quantum repeaters based on heralded qubit amplifiers

We present a quantum repeater scheme based on the recently proposed qubit amplifier [N. Gisin, S. Pironio, and N. Sangouard, Phys. Rev. Lett. 105, 070501 (2010)]. It relies on an on-demand entangled-photon-pair source which uses on-demand single-photon sources, linear optical elements, and atomic ensembles. Interestingly, the imperfections affecting the states created from this source, caused, for example, by detectors with nonunit efficiencies, are systematically purified from an entanglement swapping operation based on a two-photon detection. This allows the distribution of entanglement over very long distances with a high fidelity, that is, without vacuum components and multiphoton errors. Therefore, the resulting quantum repeater architecture does not necessitate final postselections and thus achieves high entanglement distribution rates. This also provides unique opportunities for device-independent quantum key distribution over long distances with linear optics and atomic ensembles.

Bounding quantum-gravity-inspired decoherence using atom interferometry

Hypothetical models have been proposed in which explicit collapse mechanisms prevent the superposition principle from holding at large scales. In particular, the model introduced by Ellis et al. [J. Ellis et al., Phys. Lett. B 221, 113 (1989)] suggests that quantum gravity might be responsible for the collapse of the wave function of massive objects in spatial superpositions. We consider here a recent experiment reporting on interferometry with atoms delocalized over half a meter for a time scale of 1 s [T. Kovachy et al., Nature (London) 528, 530 (2015)] and show that the corresponding data strongly bound quantum-gravity-induced decoherence and rule it out in the parameter regime considered originally.

Solid state quantum memories for quantum repeaters

Quantum memories are necessary for the implementation of quantum networks and repeaters. Recent progress towards photonic quantum storage in solid state atomic ensembles using photon echo techniques will be presented.