Figure 1: Few-electron double quantum dot under microwave excitation.
(a) Cartoon of the double dot potential showing a single-electron wavefunction coherently tunnelling between the ground |g› and excited state |e› under microwave excitation. In a microwave analogue of the Raman effect, photon-stimulated emission of phonons (ripples) is modulated by the mode spectrum set by the intra-dot spacing, which for our device is ~280 nm. (b) Energy-level diagram for the single-electron charge qubit showing the stimulated-phonon emission process (light blue) that leads to asymmetric line shapes and population inversion. At a later time, spontaneous emission of a phonon (orange) leads to qubit relaxation. Grey shading depicts virtual states. (c) Micrograph of the double dot device showing surface gates and ohmic contacts to the electron gas (crossed squares). Scale bar, 300 nm. Microwaves are applied to the plunger (P) or centre (C) gate. The conductance GQPC of a proximal rf-QPC detects the average charge state of the dot and modulates the amount of reflected-rf power, Prf, from a resonant-tank circuit, enabling fast readout (see Methods for details). (d) Charge-stability diagram of the double dot, detected using the rf-QPC. Labels (n,m) denote the number of electrons in the left and right quantum dots, respectively. The demodulated signal Vrf is proportional to the QPC conductance and thus the double dot charge configuration. Gate voltages VL and VR are applied to gates L and R in (c). Red arrows indicate the direction of allowed transitions under resonant-microwave excitation.
