Solid State Optical Interface

We aim to utilize the complementary characteristics of optical and solid-state systems for quantum information processing applications by developing techniques to reversible transfer quantum information between the two. These techniques will allow us to create devices essential for the development of quantum communication networks. The devices we are developing include quantum memories for light, entangled photon sources, small-scale quantum processors and single photon detectors. The techniques we are developing utilize optically active centres in crystals, in particular rare-earth ions and excitonic transitions associated with dopant sites in semiconductors. These centres can exhibit transitions with extremely long quantum coherence times, enabling the precise manipulation and storage of quantum information. For rare-earth ions coherence times of milliseconds for optical transitions and seconds for nuclear magnetic transitions are possible (Fraval, Sellars and Longdell 2005). To reversibly transfer quantum information between the light and the optical centres we need to achieve strong coupling between the two quantum systems. We are pursuing two general strategies to obtain this strong coupling, the first is to use large ensembles of centres, the second is to place a single optical centre in the resonant mode of a high Q optical cavity.

Ensemble based quantum memory

A practical quantum memory for light is required for the development of quantum communication networks and optical based quantum computing. Like any memory, its role is to enable the synchronization of otherwise asynchronous processes. Sequences of light pulses entering the memory can be stored and then released on demand. The amplitude, phase and polarisation of the light can be recalled with a fidelity greater than is possible using a classical memory. We have recently demonstrated the first quantum memory to operate with fidelities above the quantum no-cloning limit. It was also the first demonstration of a solid-state quantum memory for light (Hedges, Longdell, Li and Sellars 2010). It is our aim to develop this prototype quantum memory into a practical device by moving to ultra low disorder crystals (Ahlefeldt, Smith and Sellars 2009) and by employing planar waveguide configurations. These modifications will enable the memory to operate with wider bandwidths, increased fidelity and storage times greater than a second.

Ensemble based entangled light sources

When an ensemble of atoms emits a photon the state of the ensemble is entangled with that of the photon. By manipulation the quantum coherence in the ensemble we aim to force the ensemble to emit a second photon in the same quantum state as the first but delayed in time (Ledingham, Naylor, Longdell, Beavan and Sellars 2010). This source combined with the quantum memory will be used to entangle the quantum state of two spatially separate crystals. The aim is to use this method of entanglement as the basis of a quantum repeater.

Single ion cavity work

By strongly coupling the transition of a single emitter to a mode of an optical cavity it is possible to reversibly transfer quantum information between the emitter and the optical field. This coupled emitter cavity device enables a range of critical quantum operations, including logic operations, single photon sources, quantum memories. We will be investigating coupling cavities to two types of solid-state emitters: rare-earth ions in insulating crystals and localized excitons in silicon.