SQUID-based current sensing noise thermometry for quantum resistors at dilution refrigerator temperatures

Abstract

We present a dc Superconducting QUantum Interference Device (SQUID)-based current amplifier with an estimated input referred noise of only 2.3 fA/Hz. Because of such a low amplifier noise, the circuit is useful for Johnson noise thermometry of quantum resistors in the kΩ range down to mK temperatures. In particular, we demonstrate that our circuit does not contribute appreciable noise to the Johnson noise of a 3.25 kΩ resistor down to 16 mK. Our circuit is a useful alternative to the commonly used High Electron Mobility Transistor-based amplifiers, but in contrast to the latter, it offers a much reduced 1/f noise. In comparison to SQUIDs interfaced with cryogenic current comparators, our circuit has similar low noise levels, but it is easier to build and to shield from magnetic pickup.

Stabilizing a SQUID current amplifier in high magnetic fields

“…We use the current-sensing circuit described in [26], which consists of a commercial dc-SQUID [30] connected to an impedance matching superconducting transformer of effective turn ratio 212. We found that when a R = 3.25 kΩ source resistor is connected to this current amplifier, the overall gain is R T = V out /I pr = 2.25 GΩ, the circuit bandwidth is 600 Hz, and the amplifier noise is 2.3 fA ( √ Hz) −1 .…”

“…Here V out is the output voltage of the SQUID electronics and I pr the primary current of the transformer. In this circuit, the impedance matching transformer was mounted on the mixing chamber of a dilution refrigerator, located in the cancellation region of a 10 T superconducting solenoid [26].…”

“…Furthermore, when impedance matching devices are used, such as transformers [3][4][5][6][7][8][9][10][11] or cryogenic current comparators [12][13][14][15][16][17][18][19], noise levels in the fA ( √ Hz) −1 range are accessible. Due to their exquisite sensitivity, such SQUID-based current sensors are often used for Johnson noise thermometry [20][21][22][23][24][25][26] or as null detectors in high precision bridge measurements in quantum metrology [12][13][14][15][16][17][18][19].…”

“…A dc-SQUID interfaced with a coreless transformer used for fA ( √ Hz) −1 level current noise measurements of kΩ level resistors was recently described in [26]. This current amplifier is stable when there are no externally applied magnetic fields.…”

Noise Thermometry for Ultralow Temperatures

“…The much higher sensitivity of dc-SQUIDs allowed one to open up the bandwidth and thus speed up the measurement process by several orders of magnitude. Following up this work about 15 years later, practical current noise thermometers were developed and characterized over a wide range of temperatures [35][36][37][38][39][40][41].…”