We employ diamond-based vectorial magnetometry for imaging the penetration of magnetic field into a type II superconductor [1]. The technique of diamond magnetometry is based on optical detection of magnetic resonance (ODMR) of negatively charged nitrogen-vacancy defects inside a single crystal diamond.

Our prototype diamond magnetometer is designed to allow magnetic imaging of an electrically wired sample at cryogenic temperatures through a coherent fiber bundle with 30,000 cores, using a complementary metal-oxide semiconductor (CMOS) camera. We use the magnetometer for imaging the penetration of an externally applied magnetic field into a thin niobium (Nb) film. Magnetometry measurements of the film, whose critical temperature is 9.0K, are performed at a temperature of 4.3K.

The current distribution in such thin film type-II superconductor under perpendicular magnetic field is theoretically evaluated by employing the critical state model. This model predicts the superconducting shielding currents distribution, taking into account the penetration of magnetic flux into the superconductor and hysteresis. Comparison between the experimental findings and theoretical predictions yields an acceptable agreement.

The current experiment has demonstrated the ability of employing diamond magnetometry for low-temperature study of superconductors. Further improvements in the design of the magnetometer should enable operation at ultra-low temperatures. Such ability may open the way for a variety of new applications, for example the performance of a single-shot quantum state readout of a large array of superconducting Josephson qubits.

[1] Alfasi, Nir, et al. "Diamond Magnetometry of Meissner Currents in a Superconducting Film." AIP Advances 6, 075311 (2016).