OPA855: ±4.9 V is experimental cryogenic operation only; total supply exceeds the 5.5 V datasheet absolute maximum. NEVER apply ±4.9 V at room temperature.
SNSPD bias: use J1 SNSPD_BIAS through R5=100 kΩ; target about 1.4–1.5 V for approximately 14–15 µA (Isw=16 µA), not 2 V/100 kΩ.