Anja Ulrich

Materials that feature very high permittivity and very low losses at low temperatures offer interesting opportunities for improving the performance of a variety of microwave-optical devices such as phase shifters and quantum transducers. In this Letter, superconducting Al coplanar waveguide resonators between 4 and 6 GHz were used to determine the complex permittivity and loss of thin film SrTiO3 on Si at cryogenic temperatures. We report a permittivity of 1950 ± 140 and a high-power loss tangent (tan δSTO) of 3.4 ± 0.1×10−4 at millikelvin temperatures. To further investigate the potential of SrTiO3 for use in emerging quantum technology applications, the transition of tan δSTO into the single photon regime was analyzed. Our findings reveal a loss scaling that aligns closely with a two-level system loss mechanism. We observe a nearly two orders of magnitude difference in tan δSTO between high and low input power levels, consistent with that two-level system loss picture.

Pockels materials are notable for their strong electro-optic interaction and rapid response times and are therefore used extensively in optical communications. However, at cryogenic temperatures, Pockels coefficients are reduced in many materials optimized for room-temperature operation, which is a major hurdle for emerging quantum technologies. Here, we show that strontium titanate (SrTiO ₃ ) can be engineered to exhibit a Pockels coefficient of 345 picometers per volt at 20 hertz at cryogenic temperatures, a value twice as high as any other thin-film electro-optic material. By adjusting the stoichiometry, we were able to increase the Curie temperature and realize a ferroelectric phase yielding a high Pockels coefficient, so far with limited optical losses of decibels per centimeter. Our findings position SrTiO ₃ as a promising material for cryogenic quantum photonics applications.