Robin P. Giffard

Observations with a Low-Temperature, Resonant Mass, Gravitational Radiation Detector

The noise level at the output of a highly sensitive antenna has been used to establish a new observational upper limit on the flux of 1 kHz gravity bursts.

Input noise in the hysteretic rf SQUID: Theory and experiment

The spectral density of the low‐frequency open‐circuit intrinsic input noise of the hysteretic rf‐biased SQUID is calculated. The noise level, which affects the optimized power sensitivity of the SQUID, is found to be strongly model dependent and thus also constitutes an interesting probe of device physics. The results of the calculation are significantly different from previous predictions. A direct measurement of the total input noise of a typical rf‐biased SQUID system is reported. The results, which are consistent our calculation, correspond to a system noise temperature of 3.1±0.4 mK at a frequency of 1 kHz. This value is consistent with the Manley‐Rowe equations and with previous experimentally determined upper limits.

Method for calibrating resonant‐mass gravitational wave detectors

A simple and accurate method of calibrating the response of a resonant‐mass gravitational wave detector to an impulsive signal is discussed. The procedure automatically includes the effects of the detection electronics chain and any filtering algorithm that is applied. The method provides a direct measure of the frequency dependence of the signal‐to‐noise ratio (SNR) referred to a signal that has a white frequency spectrum. Measurement of this quantity allows us to determine the pulse detection noise temperature that can be obtained by optimum linear filtering of the detector output. Experimental results are presented.

Nine gigahertz impedance measurements on Ta and Nb point constants

The 9 GHz complex impedance of a number of niobium and tantalum superconducting point contacts has been measured as a function of dc bias current. In order to obtain more information, the I-V curve of each junction was also recorded. The impedance measurements are in qualitative agreement with predictions based on the resistively shunted junction model modified to include noise.

Microwave impedance measurements of point-contact Josephson weak links

We describe measurements of the 10 GHz dynamic impedance of niobium point-contact weak links at 2 K, biased below their critical currents. The measured quantity is Γ(ω), the complex reflection coefficient Of the weak link in parallel with a superconducting cavity resonator terminating a waveguide. Measurements were made at several frequencies close to resonance. In order to interpret the results it is only necessary to assume that the weak link is a two-terminal device consisting of a phase-dependent susceptance in parallel with a phase-dependent conductivity, which is connected to the waveguide through a lossless, phase-independent two-port. The data are consistent with the resistively shunted junction model including a phase-dependent conductance with a negative sign. An important feature of this study was the ability to measure the d.c. IV characteristic simultaneously. The point-contacts investigated had nearly ideal I c R products, and exhibited reproducibile structure in the differential conductance at the superconducting energy gap voltage.