The responsivity (η1) of a photon counting detector can be determined using pairs of correlated photons by positioning two detectors to intercept each of photons in the pair. The counting rate of each detector (N1 and N2) is recorded along with the coincidence rate (NC) between the two detectors. The ratio of the coincidence rate to the single rate of one detector is the absolute quantum efficiency of the other detector and vice versa. The rate of photon production (N) cancels in this procedure. Put another way, the output pulses of one of the detectors can be thought of as a trigger which indicates the existence of a second photon headed for the other detector. The quantum efficiency of the detector is then just the fraction of photons detected at the second detector in conjunction with a trigger from the first.
To test this method, a parametric down-conversion source has been set up to allow the absolute spatial responsivity of a photomultiplier to be measured at a range of wavelengths in the visible. The method was verified using independent calibration methods available within the division. The goal is to determine the ultimate accuracy that may be achieved with this method. The first results showed agreement at the 0.5% level, which was the estimated 1-sigma uncertainty of that comparison. Improvements are underway to test this comparison at the 0.1% level.
We are using a correlated photon technique to measures the absolute radiance of an IR source by comparison to fundamental constants. This system starts with a down-conversion setup designed to produce visible-IR photon pairs. The IR source output beam to be measured is superimposed onto the IR correlated photon output. By overlapping the two beams, spatially and spectrally, the IR source effectively stimulates the production of down-converted photons along that direction as well as along the correlated visible photon direction. The enhancement is measured along the visible direction with a visible detector. The ratio of the visible down-converted signal with the IR beam on versus IR beam off provides an absolute determination of the IR radiance.
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, single photon sources cannot really produce single photons on demand. We are building a multiplexed system that allows the probabilities of producing one and more photons to be adjusted independently, enabling a much better approximation of a source of single photons on demand. This is accomplished using an array of downconverters and detectors (Fig. 1). All of the downconverters are pumped simultaneously by the same laser pulse. The pump laser power is chosen so each downconverter has some small probability of producing a photon pair, while the number of downconverters is chosen so there is a high likelihood of at least one pair being created somewhere in the array. The detector associated with each downconverter allows us to determine which of the downconverters has fired. This information is used to control an optical switching circuit directing the other photon of the pair onto the single output channel. This arrangement allows a much truer approximation of a single photon on-demand source than is possible with other methods.
Currently, we have implemented a simplified version of this scheme where the optical switching circuit shown in the diagram has been replaced by a single collection lens. This simplified scheme effectively breaks the trigger detector area into multiple regions, which allows us to extract more information about a heralded photon than is possible with a conventional arrangement. This scheme 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), as well as being better than faint laser sources.
The following paper details the current implementation of the scheme: Single photon source with individualized single photon certifications.