Quantum nonreciprocal switching effect in a triple-quantum-dot structure

We investigate the eigenenergy spectra and quantum transport of a triple-quantum-dot (TQD) system with nonreciprocal couplings (t_c ±γ) between the two side dots. For the magnetic-flux phase factor φ = nπ, parity-time (PT) symmetry breaking occurs as γ increases, and a smaller γ favors the formation of the exceptional point (EP) when the side dots are weakly coupled to the main-channel dot. Notably, this weak coupling induces a higher-order EP when φ ≠ nπ. Nonreciprocal couplings also modulate the Fano lineshape of the transmission spectrum in a nontrivial manner: besides shifting the lineshapes, they induce Fano deformation when γ satisfies the EP condition. More interestingly, when the couplings between the main-channel dot and the side dots are detuned, nonreciprocal couplings trigger a bound state in the continuum (BIC) and a reversal of the Fano lineshape, both of which become more pronounced as the detuning increases. Our results reveal the distinctive role of nonreciprocal couplings in regulating quantum transport in a TQD structure.