Figure 2: Population inversion and asymmetric broadening of microwave spectra.

(a) Avoided crossing of the energy levels for the (0,1)–(1,0) transition under microwave excitation. Blue arrows indicate allowed microwave transitions. (b) Charge-stability diagram showing microwave sidebands on either side of the (0,1)–(1,0) transition. The slight broadening of the feature near the transition boundaries is owing to microwave excitation to the leads. (c) Readout probability P(0,1) for an electron in the (0,1) state as a function of detuning ε and microwave power, where 0 dB is arbitrarily set to a power that yields no effect on the data. Microwave frequency f=31.8 GHz, applied to gate C. One- and two-photon sidebands (marked 1γ and 2γ) are visible. (d–f), Slices through (c) at different microwave powers, as indicated by the vertical-dashed lines overlaying (c). (g) Energy levels of the two-electron system under microwave excitation. Blue and red lines indicate different rates for microwave driving when the (0,2) singlet S is the ground state, verse in (1,1) where the triplets T are present. (h) Stability diagram at the (0,2)–(1,1) transition with microwaves applied. Sidebands are visible in (0,2) but appear strongly suppressed in (1,1) owing to Pauli spin-blockade. (i) Readout probability P(0,2) for an electron in the (0,2) state as a function of detuning ε and microwave power. Microwave frequency is f=26.7 GHz, applied to gate P. One- and two-photon sidebands (marked 1γ and 2γ) are visible in (0,2) but are highly suppressed in the (1,1) regime. With increasing power these sidebands asymmetrically broaden on the blue-detuned side closest to ∈=0. (j–l), Slices through (i) at positions indicated by the dashed lines in (i).