HTS SQUID application as a quantum roulette noise thermometer

We describe a cryogenic primary thermometer called the Quantum Roulette Noise Thermometer (QRNT), which is based on flux quantisation in a HTS superconducting ring. The basis of the QRNT is that the flux state of a double junction SQUID ring (in thermal contact with a heat bath whose temperature is to be measured) is allowed to evolve freely. The flux state is repeatedly interrogated to establish a probability distribution from which the absolute temperature may be derived. The QRNT is in principle dissipationless and has the potential to provide high accuracy with short measuring time. The QRNT prototype device which employs a YBCO thin film on an MgO bi-crystal substrate has been designed and fabricated. The superconducting ring of overall dimensions 6 mm/spl times/8 mm is broken by two grain boundary Josephson junctions of size 5 /spl mu/m/spl times/3 /spl mu/m. The state of the junctions is switched between superconducting and resistive by means of small currents applied to a superconducting strip line located near the junctions. A dc SQUID, which is used to read out the flux state of the QRNT ring, is operated at 77 K while the QRNT device can be set at any desired temperature above 10 K. Ongoing experimental measurements are described with reference to the expected theoretical predictions.