Imam Usmani

Quantum storage of photonic entanglement in a crystal

Entanglement is the fundamental characteristic of quantum physics—much experimental effort is devoted to harnessing it between various physical systems. In particular, entanglement between light and material systems is interesting owing to their anticipated respective roles as ‘flying’ and stationary qubits in quantum information technologies (such as quantum repeaters and quantum networks). Here we report the demonstration of entanglement between a photon at a telecommunication wavelength (1,338 nm) and a single collective atomic excitation stored in a crystal. One photon from an energy–time entangled pair is mapped onto the crystal and then released into a well-defined spatial mode after a predetermined storage time. The other (telecommunication wavelength) photon is sent directly through a 50-metre fibre link to an analyser. Successful storage of entanglement in the crystal is proved by a violation of the Clauser–Horne–Shimony–Holt inequality 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 multiplexed quantum repeaters for long-distance quantum networks.

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.

Mapping multiple photonic qubits into and out of one solid-state atomic ensemble

The future challenge of quantum communication is scalable quantum networks, which require coherent and reversible mapping of photonic qubits onto atomic systems (quantum memories). A crucial requirement for realistic networks is the ability to efficiently store multiple qubits in one quantum memory. In this study, we show a coherent and reversible mapping of 64 optical modes at the single-photon level in the time domain onto one solid-state ensemble of rare-earth ions. Our light-matter interface is based on a high-bandwidth (100 MHz) atomic frequency comb, with a predetermined storage time of ≳ 1 μs. We can then encode many qubits in short (<10 ns) temporal modes (time-bin qubits). We show the good coherence of mapping by simultaneously storing and analysing multiple time-bin qubits.

Towards an efficient atomic frequency comb quantum memory

We present an efficient photon-echo experiment based on atomic frequency combs [Phys. Rev. A 79 (2009) 052329]. Echoes containing an energy of up to 35% of that of the input pulse are observed in a Pr3+ -doped Y2SiO5 crystal. This material allows for the precise spectral holeburning needed to make a sharp and highly absorbing comb structure. We compare our results with a simple theoretical model with satisfactory agreement. Our results show that atomic frequency combs has the potential for high-efficiency storage of single photons as required in future long-distance communication based on quantum repeaters. (c) 2010 Elsevier B.V. All rights reserved.

Cavity-enhanced storage in an optical spin-wave memory

We report on the experimental demonstration of an optical spin-wave memory, based on the atomic frequency comb (AFC) scheme, where the storage efficiency is strongly enhanced by an optical cavity. The cavity is of low finesse, but operated in an impedance matching regime to achieve high absorption in our intrinsically low-absorbing Eu3+:Y2SiO5 crystal. For storage of optical pulses as an optical excitation (AFC echoes), we reach efficiencies of 53% and 28% for 2 μs and 10 μs delays, respectively. For a complete AFC spin-wave memory we reach an efficiency of 12%, including spin-wave dephasing, which is a 12-fold increase with respect to previous results in this material. This result is an important step towards the goal of making efficient and long-lived quantum memories based on spin waves, in the context of quantum repeaters and quantum networks.

Single-photon-level optical storage in a solid-state spin-wave memory

A long-lived quantum memory is a firm requirement for implementing a quantum repeater scheme. Recent progress in solid-state rare-earth-ion-doped systems justifies their status as very strong candidates for such systems. Nonetheless an optical memory based on spin-wave storage at the single-photon level has not been shown in such a system to date, which is crucial for achieving the long storage times required for quantum repeaters. In this paper we show that it is possible to execute a complete atomic frequency comb (AFC) scheme, including spin-wave storage, with weak coherent pulses of n¯¯=2.5±0.6 photons per pulse. We discuss in detail the experimental steps required to obtain this result and demonstrate the coherence of a stored time-bin pulse. We show a noise level of (7.1±2.3)×10−3 photons per mode during storage, and this relatively low noise level paves the way for future quantum optics experiments using spin waves in rare-earth-doped crystals.

Atomic frequency comb memory with spin-wave storage in 153Eu3 +:Y2SiO5

153Eu3 +:Y2SiO5 is a very attractive candidate for a long-lived, multimode quantum memory due to the long spin coherence time (?15 ms), the relatively large hyperfine splitting (100 MHz) and the narrow optical homogeneous linewidth (?100 Hz). Here we show an atomic frequency comb memory with spin-wave storage in a promising material 153Eu3 +:Y2SiO5, reaching storage times slightly beyond 10 ?s. We analyse the efficiency of the storage process and discuss ways of improving it. We also measure the inhomogeneous spin linewidth of 153Eu3 +:Y2SiO5, which we find to be 69 ? 3 kHz. These results represent a further step towards realizing a long-lived, multimode solid-state quantum memory.

Photonic controlled-phase gates through Rydberg blockade in optical cavities

We propose a novel scheme for high fidelity photonic controlled phase gates using Rydberg blockade in an ensemble of atoms in an optical cavity. The gate operation is obtained by first storing a photonic pulse in the ensemble and then scattering a second pulse from the cavity, resulting in a phase change depending on whether the first pulse contained a single photon. We show that the combination of Rydberg blockade and optical cavities effectively enhances the optical non-linearity created by the strong Rydberg interaction and thereby reduces the requirements for photonic quantum gates. The resulting gate can be implemented with cavities of moderate finesse which allows for highly efficient processing of quantum information encoded in photons. As a particular example of this, we show how the gate can be employed to increase the communication rate of quantum repeaters based on atomic ensembles.

Quantum statistics of light transmitted through an intracavity Rydberg medium

We theoretically investigate the quantum statistical properties of light transmitted through an atomic medium with strong optical nonlinearity induced by Rydberg-Rydberg van der Waals interactions. In our setup, atoms are located in a cavity and nonresonantly driven on a two-photon transition from their ground state to a Rydberg level via an intermediate state by the combination of the weak signal field and a strong control beam. To characterize the transmitted light, we compute the second-order correlation function

Spectral hole lifetimes and spin population relaxation dynamics in neodymium-doped yttrium orthosilicate

g^{(2)}(\tau)