SQUID-arrays coupled to on-chip integrated thin-film superconducting input coils operating coherently

Recently, Superconducting Quantum Interference Device (SQUID) arrays operating in a coherent voltage-modulation state at 77K showed a flux-noise 10 times lower than that of single-SQUIDs at similar temperatures. To exploit the flux-noise superiority of SQUID-arrays in applications, however, it is essential to preserve the coherent state, while the magnetic field to be measured, B-z, is highly inhomogeneous along the array as being generated by thin film integrated superconducting input coils or flux-transformers located in close proximity. Indeed, the flux coupled to each individual SQUID may vary significantly along the array, leading to a rapid degradation in the coherency. Here, we present several solutions to avoid that based on a methodology we developed to assess the efficiency of signal coupling to SQUID-arrays while maintaining a highly coherent state. As a proof of concept, we applied it to highly integrated YBa2Cu3O7 800/770 SQUID-arrays inductively coupled to on-chip integrated thin film superconducting input coils. Each SQUID in the array is directly coupled to two individual flux focusers, leading to an increase in the effective area for which we derived an analytical formula. Consequently, we achieved SQUID-like voltage oscillation amplitudes above 10mV in the temperature range (75-83) K, leading to a magnetic flux noise of 0.2 mu Phi (0)/Hz(1/2), consistent with an ultra-enhanced coherent operation reached. For the strongest coupling scheme implemented experimentally, a current white noise of S-I(1/2)=32pA/Hz(1/2) was measured. This scheme can be used as the input coil of a flux-transformer, resulting in a SQUID-array-based magnetometer with an estimated field sensitivity of 13 fT/Hz(1/2).

Automated summary

The article discusses the performance of Superconducting Quantum Interference Device (SQUID) arrays in a coherent voltage-modulation state and proposes solutions to tackle the issue of highly inhomogeneous magnetic fields along the array. The study presents a methodology to assess signal coupling efficiency while maintaining a highly coherent state, showcasing successful experimental results with increased magnetic flux noise sensitivity and improved operational efficiency.