A Single-photon On-demand Source with Decoupled Single and Multiphoton Probabilities: Initial Results

The advent of photon-based quantum cryptography, communication and computation schemes is increasing the need for light sources that produce individual photons; as unwanted additional photons can render quantum cryptographic links insecure and degrade quantum-computation efficiencies. More specifically, it would be useful to have these photons created in response to an external trigger, or even to have them arrive repetitively at a fixed, but selectable rate. As currently implemented, single-photon sources cannot be made to produce single photons with high probability, while simultaneously suppressing the probability of yielding two or more photons. Because of this fact, single photon sources cannot really produce single photons on demand. We have previously described a multiplexed parametric downconversion (PDC) system that allows the probabilities of producing one and more photons to be adjusted independently, enabling a better approximation of a source of single photons on demand. Also described was a simplified version of that scheme that retains significant advantages, while greatly easing its implementation (Fig. 1). That implementation allows photons to be produced along with a quantitative “certification” that they are single photons. Some of the single-photon certifications can be significantly better than what is possible with conventional downconversion sources using a unified trigger detector region or with faint laser sources. We present a comparison of the relative merits of these methods along with the first experimental results obtained with a 4-delay-line setup. Single-photon production schemes, whether based on faint lasers or PDC, suffer from the problem that the rate of producing unwanted multiple photons is tied to the single-photon rate. The two-photon rate grows as the singlephoton rate squared. To reduce the multiphoton rate, the single photon rate must be kept small. Moreover it requires operation in a regime where it is mostly likely that no photons are produced at all! Of course, PDC schemes