Unpolarized Single Photons Generated With True Randomness

The generation of single photons in random polarization states from diamond could have significance in the development of quantum cryptography and the testing of fundamental problems in quantum mechanics. Until now, much of the research has been focused on the generation of single photons in pure polarization states. To ensure true randomness, it was essential for the team to show that the polarizations between photons were not correlated with each other.

Unpolarized single-photon generation with true randomness from diamond. Courtesy of Tohoku University.

Researchers at Tohoku University explored dynamically unpolarized single-photon emission from a (111)-oriented nitrogen-vacancy (N-V) center in diamond, in which the single-photon stream exhibited intrinsic randomness with vanishing polarization correlation between time adjacent photons. These properties allowed true random number generation and enabled fundamental tests in quantum physics.

Researchers evaluated static polarization properties of emitted photons by measuring polarization angular dependencies and applying quantum state tomography and quantum process tomography. The excitation by a 532-nm laser and the collection of the phonon sideband emission from single (111)-oriented N-V centers were carried out using standard confocal microscopy. The team used a diamond synthesized by a high pressure high temperature method.

Schematic picture of unpolarized single-photon generation using a compound defect, a nitrogen-vacancy center (N-V center), in a diamond. Spheres, designated N and V respectively, indicate a nitrogen atom and a vacancy which comprises an N-V center in the diamond lattice. Dynamically and statically unpolarized single-photon emission is induced by laser excitation for a [111]-oriented NV center in (111) diamond. Courtesy of Naofumi Abe.

Researchers measured detection polarization angular dependencies of the fluorescence intensity in both linear and circular polarization bases. In the linear polarization basis measurements, they measured the polarizer angle dependence of the fluorescence intensity by rotating a polarizer for excitation laser polarizations set to horizontal, vertical, diagonal, anti-diagonal, right circular and left circular angles. They observed almost no angular dependence, with an average visibility of 6 percent for an excitation power of one milliwatt (mW). The team confirmed similar unpolarized properties for several other (111)-oriented N-V centers.

In circular polarization basis measurements, the team found only minimal angular dependence, with an average visibility of 1.3 percent for an excitation power of 1 mW.

The researchers believe their work to be the first demonstration of unpolarized single photons emitted from a (111)-oriented N-V center in diamond in both a static and a dynamic sense. The novel “dynamically unpolarized” property, i.e. the property of no polarization correlation between time adjacent single photons, constitutes a new criterion of randomness for unpolarized single photons which could be useful for true random number generation as well as for tests of fundamental quantum mechanics pertaining to mixed states.

Quantum information technology, such as quantum computing and quantum cryptography, has the potential to exceed classical information technology in security and capability. In quantum information technology, single photons play an especially important role.

The research was published in Scientific Reports (doi: 10.1038/srep46722).