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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)

Atomic Frequency Comb Memory in Warm Rubidium Vapour

Zakary Schofield, Alice Christan-Edwards, Ori Ezrah Mor, V. Laurindo, Patrick M. Ledingham

Published Jun 26, 2023• 0 citations
Publisher

Abstract

Quantum memories are used as a means of scaling quantum networks. Protocols based on warm atomic ensembles allow for a large number of atoms with technical simplicity allowing for efficient operation at ambient temperatures. Although protocols based on ladder transition configurations are prone to dephasing due to Doppler induced effects, Velocity Selective Pumping (VSP) provides a means to overcome this.

DOI
10.1109/cleo/europe-eqec57999.2023.10232409

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Quantum memories are used as a means of scaling quantum networks. Protocols based on warm atomic ensembles allow for a large number of atoms with technical simplicity allowing for efficient operation at ambient temperatures. Although protocols based on ladder transition configurations are prone to dephasing due to Doppler induced effects, Velocity Selective Pumping (VSP) provides a means to overcome this.
Quantum memories are a crucial technology for enabling large-scale quantum networks through synchronization of probabilistic operations. Such networks impose strict requirements on quantum memory, such as storage time, retrieval efficiency, bandwidth, and scalability. On- and off-resonant ladder protocols on warm atomic vapor platforms are promising candidates, combining efficient high-bandwidth operation with low-noise on-demand retrieval. However, their storage time is severely limited by motion-induced dephasing caused by the broad velocity distribution of atoms composing the vapor. In this paper, we demonstrate velocity selective optical pumping to overcome this decoherence mechanism. This will increase the achievable memory storage time of vapor memories. This technique can also be used for preparing arbitrarily shaped absorption profiles, for instance, preparing an atomic frequency comb absorption feature.
Abstract Quantum communication is an emerging field of the utmost importance. A critical component for scalable quantum communication, and other emerging quantum technologies is a robust practical quantum memory. We look at a theoretical model of a three level Λ quantum memory in a warm atomic gas complete with the Doppler broadening. A linearization procedure to compute solutions for the full set of Heisenberg-Langevin equations with the propagation equations is described. The atomic frequency comb (AFC) protocol together with the piecewise adiabatic passage (PAP) method in warm atomic gas is briefly discussed.
We have demonstrated the coherent storage and retrieval of single-photon-level light using the atomic frequency comb protocol in a room temperature rubidium vapor. Velocity-selective optical pumping is used to prepare the comb within the F = 2 hyperfine ground state of rubidium, with the spacing between peaks coinciding with half the F = 2 − F = 3 hyperfine splitting of the 5 P 3 / 2 2 excited state. Weak coherent states of average photon number μ in = 0.083 ( 5 ) are stored with preprogrammed recall time of 7.5 ns with an efficiency of η AFC = 6.59 ( 5 ) % , while two temporally distinct modes have been stored and recalled with η AFC = 2.6 ( 1 ) % , allowing for time-bin qubit storage. Finally, the storage efficiency is observed to be independent of the input pulse polarization, satisfying a necessary condition for polarization qubit storage.
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. Efficient memory systems are vital for the development of quantum communications technologies. Hosseini and colleagues describe an optical memory based on warm rubidium vapour that achieves 87% pulse recall efficiency, illustrating the potential of warm atomic vapour systems for quantum memory.
The Off-Resonant Cascaded Absorption (ORCA) protocol in warm atomic vapors offers a scalable platform for high-bandwidth, low noise quantum memories, but its coherence time is fundamentally limited by Doppler-induced dephasing. We introduce and experimentally demonstrate a dynamic rephasing protocol that counteracts Doppler dephasing in a telecom-band ORCA quantum memory. By transferring the stored excitation to an auxiliary shelving state, we effectively reverse the accumulated Doppler phase and extend the storage time by a factor of 50 while preserving the memory's GHz bandwidth and low noise. Using this protocol, we then demonstrate on-demand storage and retrieval of four independent time-bin modes within a single warm vapor memory -- showing that Doppler dephasing can alternatively be harnessed for high-dimensional temporal mode processing. Our results establish rephasing in warm atomic vapors as a viable route toward high-bandwidth, temporally multiplexed quantum memories operating at room temperature.