KEYWORDS: Field programmable gate arrays, Data transmission, Logic, Switches, Sensors, Clocks, Control systems, Light emitting diodes, Photons, Data acquisition
We present a multi-channel coincidence-counting module for use in quantum optics experiments. The circuit takes up to
four TTL pulse inputs and counts either 2-, 3-, or 4-fold coincidences, within a user-selected coincidence time window
as short as 12 ns. The module can accurately count eight sets of multi-channel coincidences, for input rates of up to 84
MHz. Due to their low cost and small size, multiple modules can easily be combined to count arbitrary M-order
coincidences among N inputs.
We present an experimental realization of a "sudden mirror replacement" thought experiment, in which a mirror that is inhibiting spontaneous emission is quickly replaced by a photodetector. The question is, can photons be counted immediately, or only after a retardation time that allows the emitter to couple to the changed modes of the cavity, and for light to propagate to the detector? Our results, obtained with a parametric downconverter, are consistent with the cavity QED prediction that photons can be counted immediately, and are in conflict with the retardation time prediction.
A number of optical technologies remain to be developed and
optimized for various applications in quantum information processing,
especially quantum communication. We will give an overview of our
approach to some of these, including periodic heralded single-photon sources based on spontaneous parametric down-conversion, ultrabright sources of tunable entangled photons, near unit efficiency single- and multi-photon detectors based on an atomic vapor interaction, quantum state transducers based on high efficiency frequency up-conversion, and low-loss optical quantum memories.
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 describe 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. The scheme uses a heralded photon source based on parametric downconversion, but by effectively breaking the trigger detector area into multiple regions, we are able 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, as well as being better than faint laser sources. With such a source of more tightly certified single photons, it should be possible to improve the maximum secure bit rate possible over a quantum cryptographic link. We present an analysis of the relative merits of this method over the conventional arrangement.
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