A trimer on MgO layer enables fast, electrical quantum spin control

Fast Electrical Spin Control in a Ti‑Ti‑Fe Trimer on MgO

A team from the Instituto de Modelado e Innovación Tecnológica and Centro Atómico Bariloche has shown that a titanium‑titanium‑iron (Ti‑Ti‑Fe) trimer placed on a magnesium‑oxide (MgO) surface can be manipulated electrically at nanosecond speeds. By engineering avoided level crossings in the trimer and applying Landau‑Zener‑Stückelberg‑Majorana (LZSM) interferometry, the researchers achieve non‑resonant, remote spin flips without the need for direct tunneling‑current excitation.

Engineering Avoided Crossings for Electrical Tuning

The core of the approach is the creation of controllable avoided crossings within a Ti dimer. Combining an in‑plane magnetic field, the magnetic field generated by a scanning tunneling microscopy (STM) tip, and an applied bias voltage allows the energy gap between quantum states to be opened or closed on demand. Spectroscopic measurements reveal that these gaps can be tuned purely by the external magnetic field, simplifying calibration and reducing sensitivity to microscopic variations.

Extending Control with an Iron Atom

While the Ti dimer alone permits precise control of the spin directly beneath the STM tip, remote spin manipulation proved weak. Adding a single Fe atom next to the far‑end Ti atom introduces a localized exchange interaction (J_f) that can be modulated electrically. This interaction enables the remote Ti spin to be flipped while the tip‑proximal spin remains unchanged, demonstrating true non‑local control.

LZSM Interferometry as a Fast Gate Mechanism

The researchers drive the system periodically through the engineered avoided crossings. In the LZSM regime, coherent interference between successive passages yields population inversion on timescales as short as 15 ns. Simulations show high‑probability spin flips and robust operation across a range of bias voltages, indicating that the protocol can be adapted to different experimental constraints.

Mitigating Decoherence for Scalable Architectures

A key advantage of the trimer design is the separation of the controlled qubit from the tunneling current that normally induces decoherence in STM‑based experiments. By manipulating the remote Ti spin electrically, the method reduces the primary source of qubit instability, paving the way for larger atom‑based arrays where each qubit can be addressed without direct current injection.

Outlook

The work provides a concrete experimental platform for fast, electrically driven quantum operations at the atomic scale. While the study is currently limited to a single trimer, the underlying principles—engineered avoided crossings, exchange‑mediated remote control, and LZSM interferometry—are transferable to more complex spin chains and could inform the design of scalable atom‑based quantum processors.

--- The authors of the study are Sergio Santiago Gómez, Alejandro Ferrón, Gustavo Mauricio Meneses Ancalle, Daniel Domínguez, and María José Sánchez.