Many quantum experiments are performed at cryogenic temperatures, wherein the same design choices that ensure low temperatures often work against measurement and control of the quantum system of interest.

Researchers from CQC²T and UNSW have overcome the challenge of efficient, low temperature microwave delivery to optically active spin defects – potentially enabling faster gate operations in quantum computers.

By using a compact and transparent potassium tantalate dielectric resonator that enhances the microwave field, while simultaneously allowing optical access to the sample, the researchers were able to demonstrate significant improvement in the microwave power to field conversion efficiency across millimeter length scales.

A diamond sample containing an ensemble of optically-active nitrogen vacancy (NV) colour centers was used to probe the low temperature performance of the dielectric resonator, i.e. intensity and spatial variation of the microwave field, by implementing optically-detected magnetic resonance (ODMR) techniques to coherently control the quantum states of the NV centers.

“The experimental results show a very high power conversion efficiency offered by the dielectric resonator, that allows coherent quantum experiments to be performed at a fraction of the input power at cryogenenic temperatures. This is especially exciting to for applications involving diamond based spin defects.” Says lead author, Hyma Vallabhapurapu, PhD student at CQC²T.

“For the future, we envision that this transparent dielectric resonator could perform a dual-role as a solid immersion lens and a microwave resonator, to further improve the optical signal efficiency whilst simultaneously combining the advantage of efficient microwave delivery.” Says co-author A/Prof Arne Laucht, Program Manager at CQC²T.

“This could potentially lead to more efficient quantum computers built on optically compatible solid state spin-based platforms such as diamond.”

The paper was published in Physical Review Applied.