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Proceedings of the IEEE

A new technique for RF measurements using superconductors

R.A. Kamper, M. B. Simmonds, Robert T. Adair, Cletus A. Hoer

Published Jan 1, 1973• 24 citations
Publisher

Abstract

The quantization of magnetic flux in a superconducting circuit can serve for the measurement of electrical quantities in the same way that we use the wavelength of light to measure length. We report a demonstration of this function in the measurement of attenuation ratio at a frequency of 30 MHz.
DOI
10.1109/proc.1973.8979

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We have demonstrated experimentally that magnetic-flux quantization in a superconducting circuit is obeyed in an absolute sense, and have ruled out the possibility of a fractional quantum of magnetic flux. We use an optically switched superconducting microbridge to destroy and reestablish the phase coherence of the order parameter around a superconducting circuit. Changes in the vector potential while the global phase coherence is destroyed are still observed when the coherence is reestablished, even though the magnetic field is inacessible to the superconductor.
With improved understanding of loss mechanisms in superconducting resonant circuits (SRC), it is possible to design a variety of devices which take advantage of the high Q. Superconducting surface resistance, trapped-flux loss, and residual losses which are annealable can all be related to the superconductor itself. Dielectric dissipation is well enough understood to permit this source of loss to be minimized. Coupling and radiation losses can be made negligible. SRC's are described with a variety of geometries to fill the range from 10 MHz to 10 GHz. Type-I superconductors provide the highest Q's and are readily fabricated by several techniques. The superconducting resonator can be tuned accurately in frequency by temperature control of the surface reactance as well as the dielectric constant of liquid helium. An external magnetic field can be used to tune the bandwidth as well as the resonant frequency. A quarter-wave reentrant cavity is ideally suited for use as an optically tuned frequency-control element. This is accomplished using the photodielectric effect in high-resistivity semiconductor wafers which terminate the stub. Wide-bandwidth optical communication detectors are described using semiconductors with short recombination lifetime. Oscillators have been built which are optically tuned, and others which have frequency stabilities as good as commercial frequency counters. Use of SRC's to study material properties at low temperature is emphasized.
We describe a new method for observing magnetic periodicities, both spatial and temporal, ultimately capable of resolving wavelengths between 100 and 105{10}^{5}105 \AA{} and frequencies between 0.5 and 500 MHz. We have used this method to observe the periodic magnetic field above the surface of a type-II superconductor containing a lattice of flux lines which moves under transport currents. We believe that the measurements are the first direct observations of the flux-flow velocity in a type-II superconductor.
An on-chip superconducting microwave pulse generator (MPG), incorporating rapid single-flux-quantum (RSFQ) logic gates and passive radio frequency (RF) components, is an energy-efficient solution for controlling superconducting quantum bits (qubits). Traditional RSFQ MPG designs, however, suffer from a large footprint as multiple RF components (including impedance matching networks, filters, etc.) are separately implemented one after another. In this Letter, we propose a compact on-chip MPG by introducing an integrated RF module. Using a generalized filter synthesis technique, the integrated RF module combines the functionalities of impedance matching and bandpass filtering as a single inductor-capacitor (LC) lumped network. Plenty of circuit elements can, thus, be saved, and the resultant LC network is applicable to both real and complex terminal impedance cases. For validation, a superconducting MPG adopting a fourth-order, 5 GHz centered, 100 MHz bandwidth integrated RF module is designed and fabricated, whose correct operations are confirmed through both simulation and measurement. These results raise the availability of controlling large-scale quantum systems by low-power superconductor technology in the future.