HTS SQUID application as a Quantum Roulette Noise Thermometer

We describe a cryogenic primary thermometer called the Quantum Roulette Noi se Thermometer (QRNT), which is based on flux quantisation in a I-ITS super conducting ring. The basis of the QRNT is that the flux state of a double j unction SQUID ring (in thermal contact with a heat bath whose temperature i s to be measured) is allowed to evolve freely. The flux state is repeatedly interrogated to establish a probability distribution from which the absolu te 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 s ubstrate has been designed and fabricated. The superconducting ring of over all dimensions 6mm rr 8mm is broken by two grain boundary Josephson junctio ns of size 5 mu m x 3 mu m. The state of the junctions is switched between superconducting and resistive by means of small currents applied to a super conducting strip line located near the junctions, A de SQUID, which is used to read out the flux state of the QRNT ring, is operated at 77K while the QRNT device can be set at ally desired temperature above 10K. Ongoing exper imental measurements are described with reference to the expected theoretic al predictions.