In situ measurement of the permittivity of helium using microwave NbN resonators

By measuring the electrical transport properties of superconducting NbN quarter-wave resonators in direct contact with a helium bath, we have demonstrated a high-speed and spatially sensitive sensor for the permittivity of helium. In our implementation an similar to 10-3 mm(3) sensing volume is measured with a bandwidth of 300 kHz in the temperature range of 1.8-8.8 K. The minimum detectable change in the permittivity of helium is calculated to be similar to 6 x 10(-11) epsilon(0)/Hz(1/2) with a sensitivity of the order of 10(-13) epsilon(0)/Hz(1/2) easily achievable. Potential applications include operation as a fast, localized helium thermometer and as a transducer in superfluid hydrodynamic experiments. c 2008 American Institute of Physics.