Based upon a superconducting transition edge sensor (TES), the Meissner-TES is a relatively new type of high resolution cryogenic thermometer which employs the magnetic transition of a superconductor to measure temperature. We have improved the signal-to-noise for DC sensing by a factor of 30 compared to our prior effort and developed a new AC mode which uses an oscillating magnetic field and a lock-in technique with much lower magnetic noise than the DC mode. The thermometer was tuned in situ over a range of operating temperatures 10–50 times larger than the transition width of the superconductor, using an applied persistent magnetic field. The DC mode can have sensitivity better than 1 nK for 100 s averaging, and the AC mode has sensitivity better than 120 nK for very small applied magnetic fields near 14 nT and 100 s averaging. The Meissner-TES can be applied to high resolution temperature control, high sensitivity infrared sensing, optical power scale realization, and the study of temperature-dependent phase transitions.
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The SQUID controller allows the external reset of the feedback integrator with a TTL signal within less than 5 µs. The digital acquisition boards trigger resets whenever the SQUID output signal changes by the voltage associated with a flux change of one fluxoid [i.e., when Vout > V(Φ0) or Vout < -V(Φ0)]. In this way the gain of the SQUID can be kept high, the total change in SQUID signal can range up to the equivalent of 105 V, and the SQUID output can always be kept from railing at its 10 V limit.
