19 Oct 2007
Researchers from Cambridge, UK, have stored the circular polarization of an optical field by transferring it to the spin-state of an individual electron confined in a single semiconductor quantum dot.
Efficient transfer of qubits is necessary in quantum information applications, and requires a powerful link between stored qubits and transmitted quantum data. Storing excitons excited by single photons in semiconductor quantum dots could provide a good interface between transmitted and stored qubits.
The optical memory developed by the Cambridge researchers uses a single quantum dot embedded in the intrinsic region of a diode as the active element. (2007 New J. Phys. 9 365)
The researchers applied a bias to make the tunnelling rate of heavy-holes from the quantum dot dominate over the radiative recombination rate of exciton states confined by the dot, while a tunnel barrier on the negative side of the diode inhibits electrons from leaving the dot. A weak laser pulse can therefore be used to populate a single electron with a pure spin state into the quantum dot.
The state is subsequently read out through the electronically-triggered emission of a single photon. Experiments by the team show that the emitted photon shares the same polarization as the initial pulse but has a different energy.
This could prove to be a very useful feature, as it allows the pump and emitted photons to be spectrally separated and the device to operate with a significant input bandwidth. This could allow the transfer of quantum information between systems operating at different wavelengths.