Flying qubits Surfing on Plasmons

Abstract

The rapid emergence of flying qubits in graphene and other low-dimensional conductors is pushing quantum electronics into an ultrafast regime where conventional transport theories no longer apply. In these systems, single-electron wave packets propagate coherently over micrometer scales while interacting with collective charge excitations on comparable time scales. Yet existing theoretical frameworks describe either fermionic single-particle dynamics or bosonic plasmonic modes, without reconciling the two. Here we introduce a unified theory of dynamical quantum transport that bridges this long-standing divide. Starting from a gauge-invariant scattering approach, we show how a time-dependent single-electron excitation self-consistently generates a propagating internal potential that behaves as a collective plasmonic mode. Electrons propagate at the Fermi velocity while simultaneously 'surfing' on this self-induced plasmon wave, whose velocity is renormalized by Coulomb interactions and screening. This dynamical mean-field framework captures photon-assisted transport, charge relaxation, and edge magnetoplasmon dynamics within a single description and remains valid far beyond the low-frequency limit. By unifying single-electron and plasmonic pictures, our results provide a timely foundation for the interpretation and control of flying-qubit experiments in graphene at gigahertz/terahertz frequencies.

Figures (4)

• Figure 1: Open mesoscopic capacitor. The upper chiral edge channel (blue line), connecting ohmic contacts, is fully transmitted and propagates beneath a screening metallic gate. Together, the edge channel and the gate form a quantum capacitor. The gate is driven by a harmonic potential, and the resulting AC output current $I_{\mathrm{out}}$ is measured. The lower edge channel acts as a spectator and does not contribute to the AC current.
• Figure 2: Going beyond the low-frequency limit. The metallic gate is partitioned into a series of $N$ shorter gates, all driven at the same potential $V\cos(\Omega t)$. Each segment induces an internal potential $U_n$ that is uniform over its length $L/N$. Within the frozen Floquet scattering approach, this spatial discretization extends the range of validity to $\Omega \ll Nv/L$, thereby circumventing the single-gate constraint $\Omega \ll v/L$.
• Figure 3: Schematic of the chiral gated edge channel. Electrons injected from left contact propagate ballistically toward grounded contact. The gate-induced internal potential $U(x,t)$ mediates Coulomb interaction and supports collective edge magnetoplasmon modes.
• Figure 4: Hong--Ou--Mandel interference in a graphene $p$-$n$ junction. Two time-dependent voltage pulses $V_1(t)$ and $V_2(t)$ are applied to opposite contacts, generating single-electron wave packets that propagate chirally toward a central $p$--$n$ junction acting as an electronic beam splitter. The two excitations collide at the junction with a controllable time delay, leading to two-particle quantum interference. The resulting current correlations at the outputs provide a measure of fermionic Hong--Ou--Mandel interference.