Fig. 1: Device overview and electron occupancy measurement.
From: Bell-state tomography in a silicon many-electron artificial molecule
a A 3D visualization of the Si-MOS device structure. A quantum dot is formed under gate G1 (blue) and G2 (red), with inter-dot tunnel rates controlled by J (green). Gate RG enables connection to an n-doped reservoir to load/unload electrons to/from the quantum dot, with tunnel rates controlled by BG. Gate CB serves as a confinement barrier in lateral direction. The cobalt structure at the top of the image acts as both a micromagnet and electrode for EDSR control (dark green), where a DC voltage bias Vbias and a microwave signal with frequency fESR is applied. b Top: cross-section diagram of a along the \([1\bar{1}0]\) crystallographic direction, indicated by the orange dashed line. Bottom: schematic showing the number of electrons in each of the two quantum dots, aligning with the metal gates in the panel above. The height of each electron represents its relative energy and the shell to which it belongs, with inter-dot detuning energy αε. Each orbital is labelled correspondingly. Yellow electrons form full shells and are inert, while the extra electron in each dot (blue and red) act as an effective single spin qubit. c Charge stability map of the double quantum dot at B0 = 0 T, showing the charge occupancies (N1,N2), produced by plotting the lock-in signal from SET sensor ISET as a function of detuning ε and VJ. The detuning ε = VG1 − VG2 is referenced by ε = 0 V at the charge readout transition (12,6) ⇔ (13,5). A square wave with peak-to-peak amplitude of 2 mV and frequency 487 Hz is applied to G1 for lock-in excitation. Dynamic compensation is applied to the SET sensor to maintain a high readout sensitivity. d Resonance frequency ΔfESR of Q1 and Q2 as a function of ε and ΔVJ = VJ − 1.58 V when a microwave control pulse of frequency fMW = 30.486 GHz + ΔfMW is applied. Each data point at any given ε and ΔVJ registers the ΔfMW when the adiabatic inversion probability is maximized for each qubit, i.e., when ΔfMW = ΔfESR. The qubits are initialized as \(\left|\downarrow \downarrow \right\rangle\). Colour scale represents the adiabatic inversion probability.
