All-electrostatic valley qubit gates in tilted Dirac-Weyl semimetals

Abstract

Valley degrees of freedom in tilted Dirac materials offer a route toward fully electrical quantum control, but previous electrostatic barrier schemes have used the valley index only as a classical filtering resource. Here, we show that a smooth electrostatic barrier operated in a quantum point contact geometry at normal incidence instead realizes coherent valley phase control. In the single-mode regime, both valleys retain near-unit transmission while the tilt-induced valley-dependent traversal phase generates a controllable relative phase shift $Δd = δ_K - δ_{K'}$ between the $|K\rangle$ and $|K'\rangle$ components of the wavefunction. The resulting electrostatic element implements a tunable valley $Z$ rotation whose accessible phase range covers 99.5\% of the full $2π$ interval while maintaining a transmission-balance metric $B$ above 0.99 over a broad parameter window. Combined with a fixed valley-mixing element that supplies an $X$ rotation, this enables universal single-qubit control through a $Z$--$X$--$Z$ Euler decomposition. For realistic parameters, the ballistic gate time is $\sim$50\,fs, with particularly favourable operating windows in 8-$Pmmn$ borophene and WTe$_2$. These results establish tilted Dirac semimetals as a platform for coherent, all-electrical valley manipulation.