Nature 478, 360–363 (2011)

Entangled photons are key elements of quantum information processing and communications technology such as quantum key distribution. Unfortunately, current schemes for achieving entangled photon generation are inefficient because of the probabilistic processes involved. Now, Nathan Langford and co-workers from Austria, UK, Canada and Japan have proposed a deterministic process called coherent photon conversion that could be more suitable for practical applications. The key principle of coherent photon conversion is the use of classically pumped nonlinearities to induce coherent oscillations between different multi-excitation states. The resulting approach offers a new way to generate and process quantum states for quantum information processing tasks such as multiphoton entanglement and the realization of optically switched quantum circuits. The researchers demonstrated their approach by using a four-colour nonlinear process in a standard commercial, polarization-maintaining photonic crystal fibre with a 532 nm pulsed laser and a 710 nm diode laser.