Benjamin Sparkes

High efficiency coherent optical memory with warm rubidium vapour

By harnessing aspects of quantum mechanics, communication and information processing could be radically transformed. Promising forms of quantum information technology include optical quantum cryptographic systems and computing using photons for quantum logic operations. As with current information processing systems, some form of memory will be required. Quantum repeaters, which are required for long distance quantum key distribution, require quantum optical memory as do deterministic logic gates for optical quantum computing. Here, we present results from a coherent optical memory based on warm rubidium vapour and show 87% efficient recall of light pulses, the highest efficiency measured to date for any coherent optical memory suitable for quantum information applications. We also show storage and recall of up to 20 pulses from our system. These results show that simple warm atomic vapour systems have clear potential as a platform for quantum memory.

Coherent optical pulse sequencer for quantum applications.

The bandwidth and versatility of optical devices have revolutionized information technology systems and communication networks. Precise and arbitrary control of an optical field that preserves optical coherence is an important requisite for many proposed photonic technologies. For quantum information applications, a device that allows storage and on-demand retrieval of arbitrary quantum states of light would form an ideal quantum optical memory. Recently, significant progress has been made in implementing atomic quantum memories using electromagnetically induced transparency, photon echo spectroscopy, off-resonance Raman spectroscopy and other atom–light interaction processes. Single-photon and bright-optical-field storage with quantum states have both been successfully demonstrated. Here we present a coherent optical memory based on photon echoes induced through controlled reversible inhomogeneous broadening. Our scheme allows storage of multiple pulses of light within a chosen frequency bandwidth, and stored pulses can be recalled in arbitrary order with any chosen delay between each recalled pulse. Furthermore, pulses can be time-compressed, time-stretched or split into multiple smaller pulses and recalled in several pieces at chosen times. Although our experimental results are so far limited to classical light pulses, our technique should enable the construction of an optical random-access memory for time-bin quantum information, and have potential applications in quantum information processing.

Unconditional room-temperature quantum memory

Optical quantum memories—storage devices for the data encoded in light pulses—will be vital for buffering the flow of quantum information. Researchers now demonstrate such a device that can operate at room temperature. The quantum state is stored in a vapour of rubidium atoms and then recalled with a fidelity in excess of 98%.

Photon echoes generated by reversing magnetic field gradients in a rubidium vapor

We propose a photon echo quantum memory scheme using detuned Raman coupling to long-lived ground states. In contrast to previous three-level schemes based on controlled reversible inhomogeneous broadening that use sequences of pi pulses, the scheme does not require accurate control of the coupling dynamics to the ground states. We present a proof-of-principle experimental realization of our proposal using rubidium atoms in a warm vapor cell. The Raman resonance line is broadened using a magnetic field that varies linearly along the direction of light propagation. Inverting the magnetic field gradient rephases the atomic dipoles and re-emits the light pulse in the forward direction.

Spatial-mode storage in a gradient-echo memory

Three-level atomic gradient echo memory (

Experimental investigation of the transition between Autler-Townes splitting and electromagnetically-induced transparency models

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Gradient echo memory in an ultra-high optical depth cold atomic ensemble

-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

Time- and frequency-domain polariton interference

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Storage and manipulation of light using a Raman gradient-echo process

-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.

Electromagnetically induced transparency and fourwave mixing in a cold atomic ensemble with large optical depth

Summary form only given. If in general the transparency of an initially absorbing medium for a probe field is increased by the presence of a control field on an adjacent transition, two very different processes can be invoked to explain it. One of them is a quantum Fano interference between two paths in the three-level system, which occurs even at low control intensity and gives rise to electromagnetically-induced transparency (EIT), the other one is the appearance of two dressed states in the excited level at higher control intensity, corresponding to the Autler-Townes splitting (ATS). This distinction is particularly critical for instance for the implementation of slow light or optical quantum memories. In a recent paper, P. M. Anisimov, J. P. Dowling and B. C. Sanders proposed a quantitative test to objectively discerning ATS from EIT. We experimentally investigated this test with cold atoms and demonstrated that it is very sensitive to the specific properties of the medium. In this study, we use an ensemble of cold Cesium atoms trapped in a MOT, interacting with light via a Λ-type scheme on the D2 line. Absorption profiles are obtained for various values of the control Rabi frequency Ω between 0.1Γ and 4Γ, where Γ is the natural linewidth.