Design and Magnetic Field Noise of Magnetometer Using Digital SQUID With Subflux Quantum Feedback

Digital superconducting quantum interference devices (SQUIDs) based on single-flux quantum (SFQ) logic have attractive characteristics such as wide dynamic ranges and high slew rates. However, the resolution of a typical digital SQUID is limited by SFQ feedback. To realize lower magnetic field noise by magnetometers using digital SQUIDs, we have adopted two feedback combinations: one, utilizing the SFQ input/output for a main loop and the other, using a mutually coupled loop. The main loop receives direct Φ
₀
feedback and subflux quantum feedback Φ
₀
/n, where n is an integer from the mutually coupled loop. The flux resolution for the digital SQUID shows improvement with increasing n and reaches below 40 mΦ
₀
at n = 2
¹⁰
; however, the slew rate decreases with increasing n and reaches 1.22 × 10
⁶
Φ
₀
/s at f
_c
= 1.25 GHz and n = 2
¹⁰
. For a magnetometer using a digital SQUID with a pickup loop, we confirmed a magnetic field noise of less than 100 fT/√Hz with a signal bandwidth of 1 MHz for operation frequency f
_c
of 20 GHz.