Simple DC-SQUID system based on a frequency modulated relaxation oscillator

Nanobridges with hysteretic I-V (current-voltage) characteristics can be operated as relaxation oscillators when connected to an R-L shunt. A DC SQUID (superconducting quantum interference device) configuration results if two such oscillators are incorporated in a superconducting ring. Operating such a self-oscillating SQUID (SOS) with a suitable bias current leads to an output signal of the order of 10 mV, whose frequency depends on the magnetic flux through the SQUID ring modulo Φ₀ . Due to this large output voltage, no impedance matching or special low-noise preamplifier is needed for the readout scheme. Standard integrated circuits for FM (frequency modulated) signal processing can be used, leading to matchbox-size electronics. Such DC SQUIDs have been made of thin Nb, NbN, and Nb₃Ge films. The highest sensitivity achieved so far is Nb devices with a white flux noise below 10^-5 Φ₀/√Hz at 4.2 K