Optical quantum computing has received considerable interest over the past few years. In particular, linear optics with photon counting offers a promising approach towards practical quantum computers. The basic idea is to harness the quantum interference between two indistinguishable photons, but to do this it is crucial to have light sources that generate identical photons. And in order to be able to transmit these photons over large distributed networks, ideally one would use standard telecom-wavelength optical fibres. Now a team of scientists from Northwestern University in the US have demonstrated that both of these things are possible.

Reporting in Optics Letters1 Jun Chen and colleagues have for the first time generated entangled photon pairs of the same frequency in a standard optical fibre. They did this by exploiting a nonlinear effect known as reverse four-wave mixing. The phenomenon, which has been demonstrated before but only in the infrared, yields correlated photons of identical frequency. By measuring the temporal coincidences of the pulses, Chen and co-workers demonstrated the frequency overlap of photons with a wavelength of 1,550 nm — making their source suitable for use in telecom-band fibres.

Armed with this highly pure source of light, the team then went on to split the photons into two orthogonally polarized components with equal power. Polarization beam splitters were used to entangle the polarization states of the photons, and two-photon quantum interference was observed with a visibility greater than 97%. With further refinement of this telecom-based source of entangled light, practical applications of linear optical quantum computing could be on the horizon.