Electron qubits surfing on acoustic waves: review of recent progress

TL;DR

The reviewed work analyzes SAW-driven transport of electron qubits as a path to scalable solid-state quantum interconnects. It integrates demonstrations of high-fidelity single-electron transfer, coherent spin transport, and two-electron interactions with advances in transducer design and programmable SAW waveforms, including single-cycle pulses and Fourier-synthesized pulses. Key findings include >$99\%$ transfer probability over long distances, entanglement-preserving spin shuttling of singlet-triplet qubits, and Coulomb-mediated two-qubit gates in flying-electron experiments, as well as progress toward electron–photon conversion and exotic substrates like superfluid helium and solid neon. These results establish SAW-based flying qubits as a versatile platform for on-chip quantum networking, with real-time control during flight and roadmap-dependent integration into large-scale quantum processors.

Abstract

The displacement of a single electron enables exciting avenues for nanotechnology with vast application potential in quantum metrology, quantum communication and quantum computation. Surface acoustic waves (SAW) have proven itself as a surprisingly useful solution to perform this task over large distance with outstanding precision and reliability. Over the last decade, important milestones have been achieved bringing SAW-driven single-electron transport from first proof-of-principle demonstrations to accurate, highly-controlled implementations, such as coherent spin transport, charge-to-photon conversion, or antibunching of charge states. Beyond the well-established piezoelectric gallium-arsenide platform, first realisations of acousto-electronic transport have also been carried out on the surface of liquid helium that promises unique stability and coherence. In this review article, we aim to keep track of this remarkable progress in SAW-driven transport of electron qubits by explaining these recent achievements from basic principles, with an outlook on follow-up experiments and near-term applications.

Figures (19)

• Figure 1: SAW-driven single-electron transport.(a) Depleted potential landscape (grey regions) along the transport channel. The surrounding red regions indicate the Fermi sea. The red dot at the source QD (left) indicates an electron. The receiver QD (right) is empty. Next to each QD a quantum point contact (QPC) is placed as electrometer. (b-e) Electron potential energy $U$ along the quasi-one-dimensional channel for the following situations: (b) Loading an electron from the Fermi sea. (c) Preparation of the isolated electron for SAW-driven transport. (d) Transport of the electron by a finite SAW train. (e) Catching the flying electron at the receiver QD. Figure reproduced from edlbauer2019electron with permission from the author.
• Figure 2: Sending and catching maps. QPC current change $\Delta I_{\rm QPC}$ at the source QD (left) and receiver QD (right) as function of the sending configuration on the source QD given by the voltage variations on the gate next to the reservoir ($\delta V_\text{R}$) and next to the transport channel ($\delta V_\text{C}$). The color-map is set such that a black (white) pixel indicates the presence (absence) of an electron. (a,b) No SAW is launched during the sequence. (c,d) A 30 ns SAW train is launched while the electron is in the sending configuration. (e,f) In addition, a short voltage pulse is applied at the plunger gate at the time of the SAW arrival. Figure reproduced from edlbauer2019electron with permission from the author.
• Figure 3: Pulse-triggered single-electron transfer.(a) SEM image of the source QD showing the pulsing gate highlighted in yellow. (b) Measurement scheme showing the potential modulation $\delta U$ in the QD. The delay $\tau$ of a fast voltage pulse is swept within the arrival window of the SAW at the source QD. (c) Measured probability $P$ to transfer a single electron with the SAW from the source QD to the receiver QD as a function of $\tau$. (d) Zoom in a time range of four SAW periods $T_\textrm{SAW}$. Figure reproduced from Takada2019 with permission from Springer Nature.
• Figure 4: In-flight distribution within the SAW train. Distribution $D(t)$ of the electron within the SAW minima for different values of the peak-to-peak SAW amplitude $A_\textrm{SAW}$. $t_1$ indicates the expected arrival time at the barrier gate. The data is obtained via the normalised derivative of transport probability data. Figure adapted from Edlbauer2021 with permission from the American Institute of Physics.
• Figure 5: Different spin-transfer strategies.(a) Schematic representation of an electron sequentially tunneling from a source dot to a reception dot. The intermediate dots may have small variations in size and shape due to the local electrostatic environment, which in turn may affect the tunneling rates $t_{i,j}$, the electron $\bm{g}$-tensor, or the spin-orbit field $\vec{B}_\textrm{SO}$. (b,c) Schematic representation of an electron in a gate-induced (b) and SAW-induced (c) moving potential. These transfer schemes are designed to avoid the successive tunneling events (except at the source and receiver dots).