Applications of a laser-driven superconducting switch to fundamental measurements and to low-frequency noise reduction in SQUID measurements

A SQUID is used to study laser-switches, 10-40 nm thick niobium microbridges which are driven into the normal state by laser pulses sent through a multimode optical fiber. Laser pulses induce a random change in the quantized flux state of the SQUID's input circuit. From the correlation between successive flux states, we have established that the laser-switch is able to follow 6 ns laser pulses. A technique based on flux quantization is used to measure precisely the inductance of superconducting circuit elements, as well as the magnetic penetration depth of niobium. Using a specially shielded toroidal solenoid, we have demonstrated that magnetic flux is quantized absolutely in superconducting circuits, refuting an assertion that fractional quanta of magnetic flux might exist. We observe that magnetic flux is trapped in the laser-switch occasionally if the laser's driving current is reduced to zero, but never if the laser beam is interrupted mechanically. This observation is consistent with the hypothesis that flux is trapped when the laser-switch cools through its transition in the presence of a stable modal interference pattern produced by the fiber.; The noise energy spectrum of a SQUID is white at high frequencies and is 1/f (inversely proportional to frequency) at low frequencies; the limiting behaviors intersect at the 1/f knee. We have demonstrated two techniques which improve the signal to noise ratio in low-frequency SQUID measurements by modulating the signal at a frequency above the 1/f knee. In the first technique, a double-pole double-throw network of laser-switches is interposed between a pickup coil and the SQUID's input coil. For a modulated signal, the 1/f knee is reduced by more than an order of magnitude from the unmodulated 1/f knee with no loss in signal amplitude. In the second technique, the SQUID's input coil and a pickup coil are connected in series with a single laser-switch modulated inductor. The 1/f knee is reduced by more than two orders of magnitude, but with a factor of {dollar}sim{dollar}40 loss in signal amplitude due to incomplete inductance modulation.