A Superconducting Qubit Sensor for Shear-Coupled Dephasing

A superconducting qubit is an exquisitely sensitive detector of its own decoherence: anything in its environment that shifts its transition frequency shortens its coherence in a measurable way. This paper designs a qubit sensor that uses that sensitivity deliberately, reading shear-coupled environmental structure through echo dephasing. With an echo coherence time of 120 microseconds, a coupling of 2 pi times 8 MHz and 8000 measurement shots, the phase resolution is 0.0112 rad and the computed coupling sensitivity is 1.85e-06 per root hertz in fractional terms. The device is a laboratory sensor design for characterising shear-coupled noise in cryogenic environments. All figures are computed from the stated assumptions; no device has been fabricated.