Temperature sensing using a hybrid quantum system consisting of nano-diamonds and a superconducting qubit*

A small temperature sensor has a small heat capacity and a small amount of heat flowing from the measured system into the thermometer. Therefore, the response of the thermometer is fast, and measurement can be performed with less influence on the measured system.

Generally, temperature sensors measure the temperature change of some physical quantity. Since the accuracy of the measured physical quantity is proportional to the square root of the quantity of the material of the thermometer, it was difficult to achieve both miniaturization and high accuracy in temperature sensors.

To solve this problem, we prepared a hybrid system that combines a superconducting flux qubit and paramagnetic spins in nano-diamonds. The magnetization induced by the electron spins in the nano-diamond can be detected by the flux qubit. Since the magnetization is sensitive to changes in temperature, we can measure the temperature changes using the flux qubit. Experiments with a magnetic field of 2.5 mT applied to the nano-diamonds demonstrated a temperature measurement with high sensitivity of 1.3 μK/√Hz at a minimum temperature of 9.1 mK in a dilution refrigerator.