Three-dimensional strain fields in realistic qubit devices

Electron spin qubits in Si1-xGex/Si/Si1-xGex heterostructures are based on quantum dots (QDs) defined by the strain of the Si quantum well (QW) plus electrostatic gates. Spin coherent shuttling over several microns is possible by applying time-dependent waveforms to the gates. Strain fluctuations on the 100 nm level deteriorate the uniformity of electron potential and may lead to decoherence. As shown by non-destructive nano-beam XRD measurements, a significant source of strain is the stress exerted by the gate electrodes themselves. Here, we propose to use coherent Bragg diffraction to further enhance the resolution and thus be able to investigate real shuttling devices (“QuBus”) with structure sizes smaller than the nano-beam. This will involve focused ion beam (FIB) preparation to provide samples of optimal size for coherent diffraction.