Fig. 3: Detection of Bismuth donors.
From: Strong coupling of a superconducting flux qubit to single bismuth donors
a Protocol for filtering out two-level systems with short relaxation times (\({T}_{s}^{1}\, < \!\! < \,{t}_{{{{\rm{relax}}}}}\)). To achieve this, we repeat the sequence shown in Fig. 2a and compare the results of the first (R1) and second (R2) qubit readouts. b Qubit state readout versus Rabi frequency (Ω) and interaction time (t) after the first pulse sequence (R1), after the second pulse sequence (R2) and the difference between the readout results (R1 − R2). Only spins with a relaxation time longer than the relaxation time trelax are still visible. Three Si: Bi donors are detected at ωs1/2π = 7.3712 GHz, ωs2/2π = 7.3692 GHz, ωs3/2π = 7.3687 GHz. These three spectral lines disappear when the bias voltage is tuned to VDC = −0.5 V. c Coherent oscillations between the driven flux qubit and spin 3. The dashed lines are the results of a Linblad simulation assuming jump operators \({L}_{qb}^{1}=\sqrt{{\Gamma }_{qb}^{1}}{\sigma }_{qb}^{-}\) and \({L}_{qb}^{\varphi }=\sqrt{{\Gamma }_{qb}^{\varphi }}{\sigma }_{qb}^{z}\) for the flux qubit, with \({\Gamma }_{qb}^{1}=150\,{{{\rm{kHz}}}}\) and \({\Gamma }_{qb}^{\varphi }=40\,{{{\rm{kHz}}}}\) and assuming no decoherence or relaxation from the bismuth spin. From this measurement, one can extract the coupling constant between the qubit and the spin gs3/2π = 84 kHz.
