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.
Figure 1: Schematic of the full multiplexed single photon source
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switching circuit shown in the diagram has been replaced by a single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.
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