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

A titanium-titanium-iron trimer positioned on a magnesium oxide layer is central to a newly proposed method for controlling quantum spin. Researchers from the Instituto de Modelado e Innovación Tecnológica and Centro Atómico Bariloche, both in Argentina, demonstrate electrical control of quantum states using Landau-Zener-Stückelberg-Majorana (LZSM) interferometry. The work details how this approach enables non-resonant manipulation of atomic-scale spins, reducing unwanted effects from tunneling electrons and opening the way to faster quantum operations while mitigating decoherence in scalable qubit architectures.

Ti-Ti Dimer Platform for Electrical Control

Engineered avoided crossings within a titanium dimer enable electrical tuning of quantum states, a step toward manipulating atomic-scale spins without resonance. Detailed spectroscopic study reveals that these crossings, gaps between energy levels, can be controlled by combining an in-plane magnetic field, the magnetic field from a scanning tunneling microscopy (STM) tip, and applied bias voltage. This electrical tuning allows researchers to precisely navigate these crossings, a key element in achieving coherent state control. Simulations show that this configuration facilitates a high degree of remote control, mitigating unwanted effects typically associated with tunneling current.

LZSM Interferometry for Coherent State Control

Spectroscopic study of a titanium dimer revealed the ability to engineer avoided level crossings by combining external magnetic fields with applied bias voltage, providing electrical tuning of the system. This precise control forms the basis for Landau-Zener-Stückelberg-Majorana (LZSM) protocols developed for coherent state control within the dimer, using non-resonant LZSM-STM techniques. This asymmetry allows for the transfer of population from one configuration to a state where only the remote titanium spin is flipped, proving LZSM interferometry’s capacity for remote control of an atomic spin qubit.

Even with larger energy gaps and increased driving frequencies, optimal operational regions remain viable, offering flexibility in voltage selection. The central result of this work is coherent remote spin control in a titanium-titanium-iron trimer using LZSM interferometry, a protocol for the non-resonant electric control of atomic spin qubits.

Engineering Avoided Crossings in Ti Dimers

Engineered level crossings within titanium dimers offer a pathway to electrical quantum state control, revealed by spectroscopic analysis of the systems on a magnesium oxide substrate. Researchers discovered that manipulating the external magnetic field alongside the field generated by a scanning tunneling microscope tip allows for precise tuning of energy gaps within the dimer, opening and closing avoided crossings directly. This control doesn’t rely on resonant driving, a technique often used to manipulate quantum states, but instead uses the inherent interaction between the titanium atoms and external fields.

This minimal system, a Ti dimer, also highlighted a limitation in achieving full control; while manipulating the spin directly beneath the STM tip proved effective, electrical driving of transitions impacting the remote spin was weak. To overcome this, the team expanded the system to a titanium-titanium-iron trimer, introducing an iron atom to generate a local magnetic field at the remote titanium atom.

This configuration enables non-local spin control, a step toward scalable quantum architectures. The research involved developing an effective model describing the low-energy levels and mechanisms responsible for opening avoided crossings and controlling the system. Analysis of the Ti dimer, with the iron atom set to zero interaction, provided a baseline for understanding the fundamental principles at play. The team’s organization involved first analyzing the atomic array deposited on MgO and its interaction with the STM tip, then exploring the potential for controlling quantum states using LZSM quantum interference.

Spectroscopic Study of Dimer Level Crossings

Detailed spectroscopic study of titanium dimers reveals a pathway to electrically tune quantum states by engineering avoided level crossings, an element for scalable quantum technologies. The ability to manipulate these crossings stems from a combination of the external magnetic field, the localized field from the scanning tunneling microscope tip, and the applied bias voltage, allowing for electrical control of the system.

By setting specific parameters to zero, researchers isolated the behavior of the titanium dimer itself, demonstrating the fundamental principles of electrical control before introducing additional elements. This approach allowed for the identification of how to open and control the relevant energy gaps, a necessary step toward building more detailed quantum systems.

Remote Spin Control via Ti-Fe Interaction

A titanium-titanium-iron trimer configuration enables electrical manipulation of an atomic spin at a distance, overcoming limitations of direct control methods. Introducing an iron atom near a remote titanium atom establishes an electrically tunable exchange interaction, facilitating spin flips on the titanium without directly addressing it. This approach offers a pathway to coherent control of atomic qubits and potential implementation of quantum gates, extending possibilities for topological chains.

The team demonstrated that the architecture supports avoided crossings, critical for remote spin control, with gaps tunable via the external magnetic field. Voltage-induced modulation of the titanium-iron exchange drives repeated passages through these crossings, enabling Landau-Zener-Stückelberg-Majorana (LZSM) interference to selectively reverse the remote titanium spin while leaving the spin directly beneath the scanning tunneling microscope tip unchanged. The results show that quantum interference can be used as a resource for electrically driven, non-resonant, and non-local spin control in atomic-scale systems.

LZSM Protocols Enable Non-Local Spin Manipulation

Electrically driven Landau-Zener-Stückelberg-Majorana (LZSM) interferometry offers a pathway to manipulate remote atomic spins on nanosecond timescales, according to new simulations. This approach relies on periodically modulating control parameters to sweep a system through avoided crossings, points where quantum states nearly intersect, and uses coherent interference to achieve control. The work demonstrates that this method can be implemented using the same experimental setup for controlling on-surface spin states, streamlining quantum operations.

However, initial analysis revealed a limitation: while LZSM interference efficiently manipulates the spin directly under the tip, driving the remote spin proved challenging due to weak electrical coupling. To overcome this, researchers focused on the exchange interaction between the iron atom and the remote titanium spin. By modulating this interaction, they induced spin flips on the remote titanium atom, achieving high-probability remote spin flips on timescales as short as 15 nanoseconds. The study reports.

This tunability allows for optimization of the control process and opens possibilities for faster quantum operations. The simulations show good population inversion, a key metric for successful spin manipulation, at these speeds, suggesting the viability of this approach for scalable atom-based qubit architectures. The ability to combine remote control with LZSM protocols represents an advancement in quantum control techniques.

This method mitigates decoherence, the loss of quantum information, while enabling fast, non-local quantum control in engineered atomic spin structures. Overall, their results demonstrate that electrically driven LZSM interferometry provides a viable route for manipulating quantum states in atomic-scale systems, potentially opening the way to more efficient and scalable quantum technologies.

Electrical Tuning of Ti-Fe Exchange Interaction

The ability to electrically tune the exchange interaction between titanium and iron atoms within a trimer configuration is key to achieving coherent spin control, according to new work detailing a method for manipulating quantum states. This approach uses a titanium-titanium-iron arrangement assembled on a magnesium oxide substrate, enabling a pathway toward faster quantum operations by modulating the coupling between spins.

The team modeled the exchange interaction, representing titanium-titanium and titanium-iron couplings as J_t and J_f respectively, and neglected dipolar interactions due to their relative insignificance in the observed regime. The effect of the scanning tunneling microscope tip is treated as an effective local magnetic field acting on the initial titanium atom, incorporating both dipolar and exchange contributions.

A low-energy effective Hamiltonian, derived from a full Hamiltonian detailed in the supporting information, consists of two decoupled four-dimensional blocks corresponding to the relevant quantum states. The modulation of longitudinal anisotropy is irrelevant in the LZSM regime, allowing researchers to focus on the dominant effects arising from the tip magnetic field and the exchange interaction.

Mitigating Decoherence for Scalable Qubit Architectures

The arrangement of a titanium-titanium-iron trimer facilitates non-local spin control by generating a localized magnetic field at a remote titanium atom, a step toward minimizing decoherence in quantum systems. This configuration separates the controlled qubit from the tunneling current inherent in scanning tunneling microscopy, offering a direct pathway to reduce a primary source of qubit instability. This remote control is further refined by the system’s sensitivity to the in-plane component of an external magnetic field, allowing for precise tuning of avoided crossing gaps.

The size of these gaps, critical for controlling qubit interactions, becomes almost exclusively dependent on the external field strength, approximately gμBB_x, removing reliance on microscopic details difficult to control experimentally. This design feature simplifies calibration and enhances the reliability of quantum operations. The Ti-Ti system is a platform for engineering these avoided crossings, while the addition of iron enables their exploitation for remote qubit manipulation, establishing LZSM-STM as a viable strategy.

The ability to manipulate qubits remotely, coupled with tunable operation times, addresses a significant challenge in scaling quantum architectures. By using quantum interference, the system enables electrically driven, non-resonant control of atomic spins at the nanoscale. This approach offers a route to scalable atom-based qubit architectures, mitigating decoherence and enhancing the potential for complex quantum computations.

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