Figure 2: Device characterization in the Fabry–Pérot and in the quantum Hall regime.

(a) Two-terminal conductance as a function of left and right gate voltage shows regular Fabry–Pérot oscillations at zero magnetic field. The inset reveals the narrow Dirac dip along the pp–nn diagonal (blue dashed line) from which a mobility of μ≈470 × 10³ cm² V⁻¹ s⁻¹ is deduced. (b) Cuts along the same diagonal at different magnetic field strengths exhibit the expected quantum Hall plateaus at 2, 6, 10 e²/h. The G=2e²/h plateau is already visible at 60 mT. (c) The colour plot as a function of V_left and V_right at 200 mT shows quantum Hall plateaus in the unipolar region. (d) The numerical derivative (in arbitrary units) is recorded as a function of gate voltage and B displaying the dispersion of the Fabry–Pérot oscillations. is the magnitude of gate voltage in the situation of antisymmetric charge densities n_left=−n_right (red dashed line in a, that is, the np–pn diagonal). The white dashed curve indicates the line along which the cyclotron radius R_c is equal to the cavity length L=0.8 μm, the region R_c<L is darkened and will be discussed in the main text. (e) The measured pattern is reproduced by a tight-binding quantum transport calculation based on the designed geometry of the measured device. The small inset shows a resonant electron trajectory at low magnetic field. Constructive interference occurs if the phase along this trajectory is an integer of 2π, leading to the numerical solution of the yellow lines (see Methods).