Cross-correlation noise measurements of a graphene-based SQUID magnetometer*

Lateral superconductor/graphene structures can be used to make Josephson junctions with low contact resistances and gate-tuneable critical currents [1]. These junctions have the potential to provide new functionality for superconducting devices. For most devices, e.g. transmon qubits and SQUID sensors [2], it is important to quantify the intrinsic noise of the junctions. The voltage noise of low resistance junctions is typically below the noise floor of room temperature amplifiers. By cross-correlating the signals from two parallel amplifiers, we can detect signals down to ~100 pV/√Hz, well below the noise floor of each amplifier. Using this technique, we characterise the voltage noise of a NbTi DC SQUID with graphene junctions in a frequency range from ~Hz to ~kHz. Combined with measurements of the SQUID’s gain, we map its sensitivity across a range of operating conditions and find that the best-case sensitivity of the device is similar to traditional low temperature SQUIDs with oxide tunnel junctions.

[1] M. Ben Shalom et al., Nature Physics 12, p318 (2016)
[2] M. D. Thompson et al., Applied Physics Letters 110, 162602 (2017)