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 77 K 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 10 mV in the temperature range (75–83) K, leading to a magnetic flux noise of 0.2 μΦ₀/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 = 32 pA/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.