We present a resistance bridge which uses a superconducting quantum interference device (SQUID) to measure the shot noise in low impedance samples (<1 Ω). The experimental requirements are high direct current (dc) bias currents (≲10 mA), in order to obtain sufficiently high bias voltages, together with high alternating current (ac) sensitivity (≈pA/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 (R4.2 K≈0.25 Ω). This value of source resistance is out of the range of classical noise measurement schemes. Because of its low intrinsic noise (≲5 pA/Hz), our system is dominated by the thermal noise of the resistors composing the bridge (∼24 pA/Hz at 4.2 K); therefore, this method greatly simplifies the analysis of shot noise results.

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