A SQUID-Based Picovoltmeter for Quantum Resistors

We present a sensitive voltage amplifier suited for measurements of source impedances in the k$$\Omega$$ range at dilution refrigerator temperatures. The circuit is based on a commercial dc SQUID, an impedance matching transformer, and it works on the principle of negative feedback. At 10 mK, the amplifier contribution to the noise is only 17 $$\text{pV/}\sqrt{\text{ Hz }}$$, which is negligible in comparison with the fluctuations of the thermal voltage of a 3.25 k$$\Omega$$ metallic source resistor. Various circuit parameters of the amplifier are discussed.