Patrick M. Ledingham

Quantum storage of a photonic polarization qubit in a solid.

We report on the quantum storage and retrieval of photonic polarization quantum bits onto and out of a solid state storage device. The qubits are implemented with weak coherent states at the single photon level, and are stored for a predetermined time of 500 ns in a praseodymium doped crystal with a storage and retrieval efficiency of 10%, using the atomic frequency comb scheme. We characterize the storage by using quantum state tomography, and find that the average conditional fidelity of the retrieved qubits exceeds 95% for a mean photon number μ=0.4. This is significantly higher than a classical benchmark, taking into account the poissonian statistics and finite memory efficiency, which proves that our crystal functions as a quantum storage device for polarization qubits. These results extend the storage capabilities of solid state quantum light matter interfaces to polarization encoding, which is widely used in quantum information science.

Solid State Spin-Wave Quantum Memory for Time-Bin Qubits

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(3+):Y2SiO5 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(3+):Y2SiO5 crystal is used to filter out the excess noise created by control pulses to reach an unconditional noise level of (2.0±0.3)×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.

Photon-echo quantum memories in inhomogeneously broadened two-level atoms

Here, we propose a solid-state quantum memory that does not require spectral holeburning, instead using strong rephasing pulses like traditional photon-echo techniques. The memory uses external broadening fields to reduce the optical depth and so switch off the collective atom-light interaction when desired. The proposed memory should allow operation with reasonable efficiency in a much broader range of material systems, for instance Er{sup 3+} doped crystals which have a transition at 1.5 {mu}m. We present analytic theory supported by numerical calculations and initial experiments.

A cavity-enhanced room-temperature broadband Raman memory

Quantum memories enable the synchronisation of photonic operations. Raman memories are a promising platform, but are susceptible to four-wave mixing noise. We present a demonstration of a cavity-enhanced Raman memory, showing suppression of four-wave mixing.

Coherent storage of temporally multimode light using a spin-wave atomic frequency comb memory

We report on the coherent and multi-temporal mode storage of light using the full atomic frequency comb memory scheme. The scheme involves the transfer of optical atomic excitations in Pr3+:Y2SiO5 to spin waves in hyperfine levels using strong single-frequency transfer pulses. Using this scheme, a total of five temporal modes are stored and recalled on-demand from the memory. The coherence of the storage and retrieval is characterized using a time-bin interference measurement resulting in visibilities higher than 80%, independent of the storage time. This coherent and multimode spin-wave memory is promising as a quantum memory for light.

Quantum storage of heralded single photons in a praseodymium-doped crystal.

We report on experiments demonstrating the reversible mapping of heralded single photons to long-lived collective optical atomic excitations stored in a Pr3+:Y2SiO5 crystal. A cavity-enhanced spontaneous down-conversion source is employed to produce widely nondegenerate 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 Pr3+ 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 nonclassical 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.

Nonclassical photon streams using rephased amplified spontaneous emission

Laser Physics Centre, RSPhysSE, Australian National University, Canberra, ACT 0200, Australia(Dated: November 20, 2009)We present a fully quantum mechanical treatment of optically rephased photon echoes. Theseechoes exhibit noise due to amplified spontaneous emission, however this noise can be seen as aconsequence of the entanglement between the atoms and the output light. With a rephasing pulseone can get an “echo” of the amplified spontaneous emission, leading to light with nonclassicalcorrelations at points separated in time, which is of interest in the context of building wide bandwithquantum repeaters. We also suggest a wideband version of DLCZ protocol based on the same ideas.

Storage of up-converted telecom photons in a doped crystal

We report on an experiment that demonstrates the frequency up-conversion of telecommunication wavelength single-photon-level pulses to be resonant with a : crystal. We convert the telecom photons at to using a periodically-poled potassium titanyl phosphate nonlinear waveguide. The maximum device efficiency (which includes all optical loss) is inferred to be (internal efficiency ) 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 for predetermined storage times of up to . 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.

Experimental realization of light with time-separated correlations by rephasing amplified spontaneous emission.

Amplified spontaneous emission is a common noise source in active optical systems, it is generally seen as being an incoherent process. Here we excite an ensemble of rare earth ion dopants in a solid with a π pulse, resulting in amplified spontaneous emission. The application of a second π pulse leads to a coherent echo of the amplified spontaneous emission that is correlated in both amplitude and phase. For small optical thicknesses, we see evidence that the amplified spontaneous emission and its echo are entangled.

Quantum Correlations between Single Telecom Photons and a Multimode On-Demand Solid-State Quantum Memory

Quantum memories are very important in quantum communication and information as they provide a quantum interface between photons, used for long distance communication, and matter, in which stationary qubits can be stored. Moreover they are fundamental building blocks in quantum repeaters. Rare-earth doped crystals (REDC) are promising candidates for the implementation of quantum memories because of their excellent coherence properties at cryogenic temperatures and because solid state offers good prospect for scalability and integration.