Figure 1. Two on-chip microwave power detection schemes for superconducting (hybrid) devices.

(a,c) Scanning electron micrograph of a device nominally identical to Device A (scale bar, 1 μm) and schematic drawings of the two measurement set-ups. (Data shown are from Device A unless otherwise stated.) In both cases, a static magnetic field, H is applied parallel to a superconducting bar (S, Al) and a sinusoidal signal of root mean squared amplitude V_RF and frequency f_RF in the microwave range applied across the length of S (with a lossy coaxial cable in series), resulting in a high-frequency field perpendicular to H. To detect the spin precession of the quasiparticles in S, two on-chip detection methods are used. (a) Detection scheme 1: a voltage V_d.c. is applied between S and a normal electrode (N1, thick Al) with which it is in contact via an insulating tunnel barrier (I, Al₂O₃). The differential conductance G=dI/dV_d.c. is measured, where I is the current between N1 and S. (b) G as a function of V_d.c. and nominal V_RF (at the output of the generator and not accounting for attenuation in the lines). The red dot indicates the operation point of the detector for the data in Fig. 2: , V_d.c.=−288 μV. For any given frequency, we define as the V_RF (at the output of the generator) at which the effective voltage at the device is the same as that for f_RF=7.14 GHz and V_RF=16.81 mV. (See Supplementary Note 2 and Supplementary Fig. 2). (c) A slice of b at V_d.c.=−288 μV (blue dashed line in b) with the operation point indicated. (d) Detection scheme 2: a current I_d.c. is injected along the length of S. We measure either the voltage V between the ends of the S bar or the differential resistance R=dV/dI_d.c.. We record in particular the switching current I_S at which S first becomes resistive. (e) R as a function of I_d.c. and nominal V_RF (not accounting for attenuation in the lines). The switching current I_S at which S become resistive appears here as a peak in R. I_S can be seen to decrease monotonically with V_RF. The red dashed line indicates the operation point of the detector for the data in Fig. 3: V_RF=0.8. (f) The blue trace is the first slice of e (blue dashed line in e) at V_RF=0.1 mV. The black trace is a two terminal measurement of the S bar, in the absence of microwaves, with a constant corresponding to the resistance of the lines subtracted. The difference in I_S between the two indicates that the S bar is strongly out of equilibrium in our second (switching current) detection scheme. (See Supplementary Note 4).