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New Source of Photons
May 31, 2012   
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Numerous experiments in quantum optics require the use of sources of single photons with precisely controlled properties. Scientists at the University of Warsaw’s Faculty of Physics have constructed a source that is far more efficient—and cheaper—than the standard ones.

“The device we have constructed is not only more efficient, but also makes it possible to better control the parameters of the photons generated,” says Prof. Czesław Radzewicz, head of the Ultrafast Phenomena Laboratory at the University of Warsaw Institute of Experimental Physics. “Quantum experiments can now be carried out faster, more easily and, most importantly, at a lower cost.”

Photon sources used in quantum optics experiments have so far relied on spontaneous frequency conversion in bulk nonlinear crystals, and have not been efficient. Even when a crystal is illuminated with a laser beam with a power of several dozen milliwatts—a relatively high power for quantum experiments—it generates just a few tens of thousands of appropriate photons per second.

For a decade, physicists have been trying to generate single photons not only in bulk crystals but also in nonlinear waveguides, using the phenomenon of spontaneous parametric frequency conversion, also called parametric fluorescence. Despite the name, parametric fluorescence has little to do with ordinary fluorescence, which is based on the gradual emission of energy by atoms that have been previously excited.

“During parametric fluorescence, emission is immediate and does not result from the excitation of atoms but from the specific nonlinear properties of the electric potential, which binds electrons in atoms,” says Michał Karpiński, a Ph.D. student at the Faculty of Physics and the main author of a publication in the journal Optics Letters, where the new source of photons has been described.

As a result of parametric fluorescence, the initial photon from a pump beam is split in the waveguide into two photons the energy of which is half the original. This means that if the pump photon is a blue light photon with a wavelength of 400 nm, then two red light photons with a wavelength close to 800 nm are generated as a result of parametric fluorescence.

The properties of such photons are correlated in such a way that if, for example, one of them is polarized in one plane, the other one has a perpendicular polarization. Having obtained such a pair of photons, physicists can direct them at a crystal to select photons with different polarizations. If a photon with a particular polarization is registered in one arm of a measurement system, it is immediately clear that its twin photon, perpendicularly polarized, was in the other arm.

The source constructed by the scientists at the University of Warsaw Faculty of Physics uses a commercially available nonlinear waveguide. In contrast to traditional optical fibers, it is not a flexible fiber but a crystal several millimeters in size. On its surface there is a waveguiding optical line, through which a pump beam is transmitted.

“The key to success was gaining an insight into the physics of the dispersion phenomena which occur in our nonlinear waveguide,” says Konrad Banaszek, Ph.D., a researcher at the University of Warsaw. “We have been studying similar systems for several years and in 2009 we published one of the first experimental studies focusing on these phenomena. Therefore we knew exactly what parameters a beam passed through the waveguide should have, what spectral filters we should place in front of and behind the waveguide, and what the end result would be.”

The number of photons emitted by the new source is around a hundred times higher than with standard sources so far. This means that a pump beam passed through a waveguide can be less powerful and can even come from an ordinary diode laser. There is an additional advantage; a weaker pump beam will scatter to a lesser extent in the experimental system and will therefore disturb measurement results to a lesser extent.

The research was financed from European Union funds under the Foundation for Polish Science’s TEAM program and from a research grant provided by the Polish Ministry of Science and Higher Education. The results may be used in experimental works related to quantum optics and its applications: quantum encryption, quantum memories or quantum metrology, in which the use of quantum states of light makes it possible to carry out highly precise measurements.
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