Superconducting quantum interference device based resistance bridge for shot noise measurement on low impedance samples

We present a resistance bridge which uses a superconducting quantum interference device (SQUID) to measure the shot noise in low impedance samples (<1 Omega). The experimental requirements are high direct current (dc) bias currents (less than or similar to 10 mA), in order to obtain sufficiently high bias voltages, together with high alternating current (ac) sensitivity (approximate to pA/root Hz) to detect small changes of the noise away from the thermal noise ground level. These conditions are fulfilled by changing continuously the overall gain of the SQUID electronics and separating the dc and ac feedbacks. This system is used to investigate the shot noise in mesoscopic samples made with superconducting (S) and normal (N) metals (R(4.2K) approximate to 0.25 Omega). This value of source resistance is out of the range of classical noise measurement schemes. Because of its low intrinsic noise (less than or similar to 5 pA/root Hz), our system is dominated by the thermal noise of the resistors composing the bridge (similar to 24 pA/root Hz at 4.2 K); therefore, this method greatly simplifies the analysis of shot noise results. (C) 1999 American Institute of Physics. [S0034-6748(99)04006-X].