Heralded pure single photons

If individual photons are to be harnessed for quantum information processing, they must not only be indistinguishable but also be in pure quantum states, not in incoherent mixed states. Creating such single photons is not easy, however. One of the most common approaches for generating individual photons involves nonlinear optics—four-wave mixing in optical fibers or three-wave mixing in crystals. But the resulting photons, produced in pairs, have spatial and temporal correlations that make them undesirable for scalable quantum information processing applications. Their suitability can be improved by using filters, but obtaining sufficient purity requires narrow filters that greatly slow the photon production rate and compromise the probability that detecting one photon of a pair reliably predicts the availability of its sibling. Now the University of Oxford’s Ian Walmsley and colleagues have demonstrated that by choosing a nonlinear crystal with appropriate dispersion characteristics, they can side-step those limitations and produce pairs of independent, pure photons through a process called parametric down-conversion. The figure shows the calculated spectral distribution. Detecting one of the paired photons announces, or heralds, the presence of the other, yet in this case reveals nothing about its quantum state. Testing their heralded single photons by interfering the outputs from two independent sources, the researchers find a photon purity of 95% or higher, which they say should make the technique useful for future quantum-enhanced technologies. (P. J. Mosley et al., Phys. Rev. Lett. 100 , 133601, 2008 http://dx.doi.org/10.1103/PhysRevLett.100.133601 .)