Photons are an attractive alternative to electron spins as quantum bits in information processing because they can propagate over large distances with low losses and can operate at low powers. Quantum information processing using photons rests on the ability to manipulate each photon individually and employing a two-photon logic gate.

Until now, a suitable medium for a two-photon gate has proven difficult to realize. Researchers from Stanford University led by Jelena Vucckovic have made a breakthrough on the latter challenge by harnessing photonic crystal cavities [Fushman et al., Science (2008) 320, 769]. Instead of using quantum dots (QDs) as the qubits themselves, Vucckovic's team use an InAs QD inside the optical cavity of a photonic crystal to give a nonlinear phase shift to qubits carried by photons.

In this approach, a QD in the ground state is illuminated in the photonic crystal cavity using two laser beams. When a single photon couples to cavity at the QD resonance, it is absorbed. But because the QD is coupled strongly to the cavity, the photon is reemitted. If a signal photon arrives at the cavity when there is already a control photon present, because the QD it sees is no longer in the ground state there is a change in the response – or a phase shift. This can be mapped into a rotation of the photon polarization, say the researchers, which represents a controlled interaction between the two photons.

Although the researchers were only able to achieve relatively small phase shifts of the signal polarization in this way, repeating the interaction multiple times (in as ‘cascade’) could make much larger phase shifts possible.

“This cavity-QD system represents that highest nonlinearity optical medium operating at the lowest powers on a semiconductor chip,” says lead author Ilya Fushman. “This system is extremely promising for both low power switching in classical information processing and quantum gates in quantum information processing.”

Many engineering challenges remain to be overcome before this approach can be realized in practical terms, but the researchers are hopeful that they are not insurmountable and that their approach opens the way to chip-based logic using photons.