Flying electron spin control gates

14.09.2022

Electron spins are attractive qubits for the implementation of quantum functionalities in semiconductor devices. In this context, the spin transistor proposed by Datta and Das [Datta and Das, Appl. Phys. Lett. 56, 665, (1990)] has been a guiding concept towards the implementation of spin-based functionalities in III-V semiconductor structures. The concept relies on the control of the spin vector of a moving (or flying) electron spin by an electrostatic field applied perpendicular to the motion. While eliminating the need for magnetic fields from spin devices is desirable, the most appealing feature of electrostatic spin control is compatibility with the field effect transistors widely used in semiconductor chips. Electrostatic spin control also requires, however, a precise control of the spin motion on the microscopic scale to avoid fluctuations in the spin orientation (spin decoherence). Consequently, Datta and Das spin transistors have, so far, only been demonstrated for ballistic spin transport along short (<2 μm) channels [Koo et al., Science 325, 1515 (2009), Chuang et al., Nat. Nanotechnol. 10, 35 (2015)]. A major challenge for efficient spin control is to devise suitable approaches to both drive spin motion and, simultaneously, control the spin orientation while avoiding decoherence.

In a recent publication, Helgers et al. have introduced an elegant solution for this challenge based on spin transport using moving potentials produced by a surface acoustic wave (SAW). SAWs are acoustic vibrations that travel along a surface in a manner analogous to earthquake waves. SAWs with micrometer-sized wavelengths can be electrically generated using interdigitated transducers (IDTs) fabricated on a semiconductor chip, as illustrated in Fig. 1. When applied along a narrow semiconductor channel, the SAW fields create moving potential dots, which capture the electrons and transport them with the acoustic velocity. The limited motion within the dots suppresses spin decoherence and enables electron spin transfer over distances up to 100 μm. [Stotz et al., Nat. Mater. 4, 585 (2005)]. Concomitantly with the transport, the SAW fields forming the dots and moving congruently with them can also manipulate the electron spin vector through spin precession. By changing the strength of the SAW, the precession rate will also be changed. As a result, the moving dots act as contactless, flying spin gates, which simultaneously drive the motion of the spins and dynamically control the rate of spin precession during transport.

Helgers et al. have now experimentally demonstrated the feasibility of this approach by controlling the spin precession rate using the SAW amplitude during transport. In the experiment, spin polarized electrons and holes were optically excited within the dots using a circularly polarized laser beam (cf. red beam in Fig. 1). These carriers were then acoustically transported over distances of up to 100 μm before they are forced to recombine emitting photons (green beam). From the analysis of the photon polarization, which reflects the spin state of the carriers prior to recombination, the authors determined the dependence of the spin vector on the transport distance, and the spins were observed to precess over many cycles during transport. More important, by electrically changing amplitude of the SAW field, the authors show that the precession rate can be change by over 200%, thus demonstrating an efficient flying gate for spin control.

The acoustically driven flying spin gates enable a high degree of dynamic spin control as well as on-chip spin transfer over several tens of micrometers by simply changing the amplitude of the carrier acoustic wave. The approach is compatible with planar technology and also offers a convenient interface for the interconversion between electron spins and polarized phonons for long-distance quantum information transfer. Furthermore, it can be extended for the control of single spin qubits by reducing the dot sizes to enclose single electron spins and, in this way, enable the generation on-demand of single photons with controlled polarization. The flying spin gates can thus act as a single spin qubit control gate, a key element for on-chip quantum information processing, with a photonic interface.

We acknowledge the financial support by the Natural Science and Engineering Research Council of Canada and the Alexander von Humboldt Foundation, Germany. This project has received funding from the European Union's Horizon 2020 research and innovation program under grant agreement No 642688.

Authors: Paul L. J. Helgers, James A. H. Stotz, Haruki Sanada, Yoji Kunihashi, Klaus Biermann, Paulo V. Santos
Title: Flying electron spin control gates
Source: Nat. Commun. 13, 5384 (2022)
DOI: 10.1038/s41467-022-32807-x