Patrick Ledingham

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Patrick Ledingham

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My research focuses on the development and implementation of a quantum optical memory. This is a device that can take a quantum state of light (e.g. a single photon), store it for an arbitrary length of time, and recall the state efficiently in a noise-free fashion, preserving the quantum nature. A quantum memory would bring clear and immediate impact to the quantum information processing community for large-scale quantum networking, particularly, being useful in the context of quantum communication and quantum computation.

Platforms for Quantum Memories

Atomic based systems provide a platform for the implementation of quantum optical memories. An atom or an atomic ensemble absorbs the quantum state of light, storing it as an atomic coherence which can be read out back to a light field at a later time defined by the user. An on-going challenge is to implement this efficiently, free of noise, with broad acceptance bandwidth and long storage time.

Rare-Earth-Ion-Doped Solids

An attractive platform for realising a quantum optical memory is rare earth ion doped crystals. The triply ionised rare earths are doped within a solid state matrix, where the optical transitions occurring within a partially filled 4f shell spatially reside within an effective Faraday cage provided by a full 5p shell. The result is that an individual ion has narrow homogeneous line (~1 kHz) which leads to long coherence times, provided that the system is reduced to a cryogenic temperature to negate dephasing via phonons.  Coherence times of several hours have been measured in these systems, providing great prospects for quantum communication applications.

The optical lines of ensembles of ions are susceptible to inhomogeneous broadening due to the crystal field. This motivates the use of memory protocol based on rephasing mechanisms, for example, the atomic frequency comb (AFC) protocol. AFC uses complex spectral holeburning techniques to shape the inhomogeneously broadened line into a comb-like structure. The comb can then absorb a photon, and at a time given by the inverse of the comb tooth separation, the photon will be reemitted. Given the narrow homogenous lines rare earths have, long AFC delay times (several microseconds) are possible. The full AFC protocol combines this delay with an on-demand aspect: before the photon is reemitted by the AFC, a control pulse maps the optical coherence to a spin-wave coherence, then at a user-defined time later, a second control pulse does the reverse mapping, allowing the photon to finally be emitted. This full AFC protocol has been used to demonstrate storage and retrieval of time-bin qubits, polarisation based qubits, and heralded single photons.

Atomic Vapours

In the context of local quantum networks, a high repetition rate is desirable. Therefore broadband realizations of quantum memories are required. The Raman memory protocol is an example of a truly broadband memory. The memory is characterised by two optical fields, signal and control, that are in a lambda configuration. The fields are detuned off single-photon resonance but are on two-photon resonance; such a configuration results in a broadband coupling of light to atoms. Atomic vapours provide a platform to implement such a memory in. Alkali gases, with large dipole moments and high atomic densities possible, provide a large light-matter coupling. The high efficiencies, high bandwidths, and operation at ambient temperatures provide a scalable platform for fast quantum memories in a quantum network.