A team of German researchers has developed a versatile and efficient source of photon pairs that shows promise as a means of generating definitively single photons for quantum information and cryptography applications.
The properties of single photons are themselves of great interest in fundamental studies, and the promise of absolute security in quantum cryptography using photons can only be fulfilled with a source of photons that are emitted singly every time.
Much work has therefore gone into developing a reliable single-photon source, rather than those that can emit in bunches or pairs. Researchers are getting closer to an ideal single-photon gun: one that emits single photons with impeccable reliability and good efficiency, narrow bandwidth, and wide tunability.
Spontaneous parametric down-conversion
One promising way to do that has come to the fore in recent years is spontaneous parametric down-conversion (SPDC). Part of the family of nonlinear wave-mixing processes, SPDC occurs in some crystals, turning a small fraction of input photons into pairs whose energies sum to the input photon energy.
In general, the photons are of different wavelengths, so it is straightforward to filter the output into the three different types—the pump, and the resulting signal and idler. The process is suited for applications in which single photons are required because one of the pair acts as a “herald”: Catch one wavelength with a detector and that detection is a guarantee that one and only one photon of the other wavelength has been generated.
Much recent work has made use of this trick, and recently researchers at Humboldt University (Berlin, Germany) established the statistics of the process when they carried it out in a periodically poled potassium titanium oxide phosphate (KTP) crystal.1 But for maximum utility, the ideal single-photon gun should have narrow but tunable bandwidth, so that atomic transitions can be “addressed” in quantum information and storage applications, for example. The photons’ wavelength should also be tunable, so that a number of different atoms might be used.
Now, researchers from the Max Planck Institute for the Science of Light (Erlangen, Germany), the University of Erlangen-Nuremburg, and the University of Paderborn (Paderborn, Germany) have come up with a design for a photon gun that fulfills these requirements.2
Their starting point is a whispering-gallery-mode resonator: a disk-shaped cavity made of lithium niobate, a material whose SPDC properties are well known. It is pumped with a few hundred nanowatts of light from a frequency-doubled Nd:YAG laser at 532 nm coupled in through a diamond prism. Residual 532 nm light, as well as the signal and idler wavelengths of around 1030 and 1100 nm, are coupled out using the same prism, which separates them spatially.
The results showed a good degree of “antibunching” of the output photons—proving the single-photon nature of the split signal and idler streams, with an output of 13 million photon pairs per milliwatt of pump power. But the real strength of the design is its flexibility. The monolithically produced resonator itself can be heated, resulting in a tunability of the output photons by a range of 100 nm (about 50 nm in each beam).
The lowest bandwidth the team measured was 7.2 MHz, which was tuned up to 13 MHz simply by changing the distance of the coupling prism for the resonator. Taken together, the results suggest that the design is perfectly suited to the many different technologies and approaches that the quantum information community has come up with in recent years.
“This compact source provides unprecedented possibilities to couple to different physical quantum systems and renders it ideal for the implementation of quantum repeaters and optical quantum information processing,” the researchers wrote in their preprint.
“Our results mark the starting point of a new class of resonator-enhanced SPDC sources, simultaneously easily tunable in bandwidth and wavelength while offering a compact, stable, and easy to implement design with remarkable efficiency.”
1. M. Scholz et al., Phys. Rev. Lett., 102, 063603 (2009).
2. M. Foertsch et al., arXiv preprint 1204.3056 (submitted to Nature).