Mirrors Improve the Quality of Photons

In NV centers in diamond the states of their electron spins can be determined from the photons they emit. Placing a system of this kind between two mirrors can considerably improve the rate and yield of emitted photons. As a result, key conditions are met for using NV centers in quantum technology applications. (Source: U. Basel, Dept. of Physics)

Scientists from the Uni­versity of Basel’s Department of Physics and the Swiss Nano­science Institute have succeeded in dramati­cally improving the quality of indi­vidual photons generated by a quantum system. The scientists have success­fully put a 10-year-old theo­retical prediction into practice. They have taken an important step towards future appli­cations in quantum infor­mation tech­nology.

For a number of years, scientists have been working on using electron spins to store and process infor­mation. A possible approach is to use a quantum system in which the quantum state of the electron spin is linked to that of the emitted photons. Nitrogen vacancy centers (NV centers) are consi­dered a proven structure for this approach, allowing electron spins to be read and mani­pulated easily. These NV centers are natural defects in the crystal lattice of diamond, over which researchers have gained control over the last decades.

NV centers are parti­cularly interesting in the field of quantum infor­mation proces­sing, as they emit individual photons that carry infor­mation about the state of their electron spin. These photons in turn can create quantum ent­anglement between various NV centers; this ent­anglement can be established even over large distances and can thus be used for data trans­mission. However, for applications in quantum infor­mation tech­nology, signi­ficant improve­ments will be needed in the quantity and above all the quality of the emitted photons, since so far only a fraction of the photons can be used to produce ent­anglement.

Daniel Riedel has now succeeded in boosting the yield of useful photons from these NV centers from 3% to a current value of 50%. In addition, he has been able to almost double the rate at which the photons are emitted. Riedel achieved these signi­ficant improve­ments by placing a nano­fabricated piece of diamond, measuring just a few hundred nano­meters across, between two tiny mirrors. It had already been described theo­retically 10 years ago that the placement of NV centers inside a cavity ought to increase the yield of photons. Until now, however, no research group had managed to put this theory into practice.

“We have overcome an impor­tant hurdle on the path towards the quantum internet,” says Richard War­burton of the Department of Physics at the Uni­versity of Basel. Patrick Male­tinsky, who also supervised the work, adds: “The unique combi­nation of exper­tise in the field of photonics, special diamond structures and nano­fabri­cation here in Basel meant it was possible to overcome this 10-year-old challenge for the first time.” (U. Basel)

Reference: D. Riedel et al.: Deterministic Enhancement of Coherent Photon Generation from a Nitrogen-Vacancy Center in Ultrapure Diamond, Phys. Rev. X 7, 031040 (2017); DOI: 10.1103/PhysRevX.7.031040

Link: Quantum Sensing Lab, Dept. of Physics, University of Basel, Basel, Switzerland

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