| The development of a model to predict and understand the effects of quantum noise in the resistively shunted Josephson junction and experimental tests of this model are described. It is shown that the low-frequency spectral density of the voltage noise in a current-biased Josephson junction with critical current I(,o), shunt resistance R, and small capacitance is 2eI(,o)('2)R('3)/V in the limit eV >> k(,B)T(I/I(,o))('2) and I > I(,o), where V is the voltage and I is the current. The noise arises from zero-point current fluctuations in the shunt resistor that are mixed down from near the Josephson frequency to the much lower measurement frequency. Experimental data are in excellent agreement with these predictions, demonstrating clearly the measurability of zero-point fluctuations and the validity when I > I(,o) of the Langevin treatment combined with the Callen-Welton expression for the noise from a resistor. The rounding of the current-voltage characteristic when I(, )<(, )I(,o) caused by quantum noise and macroscopic quantum effects are briefly discussed.; The noise temperature of a dc superconducting quantum interference device (SQUID) coupled to a tuned input circuit is computed using the complete quantum expression for the equilibrium noise in the shunt resistance of each junction. At T = 0, where the noise reduces to zero-point fluctuations, the noise temperature for an optimized system is h(nu)/k(,B)ln2, where (nu) is the signal frequency and the noise energy, (epsilon)/1Hz, of the bare SQUID is approximately (DBLTURN)(H/2PI). The computation is extended to nonzero temperatures, and it is shown that a SQUID operated at 1K can approach the quantum limit. Tunnel junction dc SQUIDs designed to approach the quantum noise limit in the temperature range 1 to 4K were fabricated with an inductance of about 2 pH and a capacitance per junction of about 0.5 pF. The lowest measured noise energy was 3.2(H/2PI) at 1.4K at a frequency of 202 kHz. When the 1/f noise was subtracted, the white noise energy decreased from around 3(H/2PI) at 4.2K to below 2(H/2PI) at 1.4K. |
