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All-electrical Coherent Control of a Single Rare-earth Spin Qubit

This study demonstrates all-electrical coherent control of a single Er electron spin exchange-coupled to a Ti atom via scanning tunneling microscopy, achieving near-gigahertz Rabi frequencies by leveraging anisotropic exchange interactions to overcome the shielding challenges of rare-earth 4f electrons.

Original authors: Fabio Donati, Yaowu Liu, Dasom Choi, Stefano Reale, Jeongmin Oh, Seorhin Choi, Lei Fang, We-hyo Soe, Arzhang Ardavan, Andreas Heinrich, Soo-hyon Phark

Published 2026-08-12

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Fabio Donati, Yaowu Liu, Dasom Choi, Stefano Reale, Jeongmin Oh, Seorhin Choi, Lei Fang, We-hyo Soe, Arzhang Ardavan, Andreas Heinrich, Soo-hyon Phark

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Tiny, Invisible Dance of Quantum Bits

Imagine a world where computers don't just calculate numbers but dance to the rhythm of the universe's smallest particles. This is the realm of quantum computing, a field that promises to solve problems too complex for today's supercomputers. At the heart of this technology are "qubits," the quantum version of the bits in your phone. While regular bits are like light switches (either on or off), qubits can be both at the same time, allowing them to process vast amounts of information simultaneously. However, controlling these tiny qubits is like trying to conduct an orchestra where every musician is invisible and only responds to a specific, whisper-quiet signal.

For decades, scientists have been trying to control qubits using electricity because it's fast and easy to wire up. But there's a stubborn problem: some of the best candidates for qubits are made of "rare-earth" atoms. These atoms are like shy, armored knights; their most important parts (the electrons that hold the quantum information) are hidden deep inside layers of other electrons, shielding them from the outside world. Because of this armor, electric fields usually bounce right off them, making it nearly impossible to control them with the simple electrical switches we use in modern electronics. Until now, the only way to talk to these atoms was with giant, slow magnetic fields, which limits how fast and small these quantum computers can become.

The Paper's Big Breakthrough

In this study, a team of researchers has found a clever way to bypass that armor and control a single rare-earth atom using only electricity. They didn't try to force the electric field through the armor; instead, they built a tiny "relay team" to do the work for them.

The Setup: A Quantum Relay Race
The researchers created a microscopic stage on a surface of magnesium oxide. On this stage, they placed two atoms: a titanium (Ti) atom and an erbium (Er) atom. The erbium is the "star" of the show—it's the rare-earth atom with the armored electrons that holds the quantum information. The titanium is the "helper."

Here is the magic trick: The titanium atom is like a surfer who loves the electric waves. When the researchers zap the titanium with a radio-frequency electric field, the titanium starts to wiggle and dance. Because the titanium is holding hands with the erbium atom (a connection called "exchange coupling"), the titanium's dance shakes the erbium, too. Even though the erbium is armored, it feels the tug from its partner. By carefully tuning the direction of the magnetic field and the distance between the atoms, the researchers made the titanium's dance create a specific kind of "push" that gets the erbium spinning.

The Results: Speeding Up the Spin
The team measured how fast they could make the erbium spin using this method. They achieved a "Rabi frequency" of 190 MHz. To put that in perspective, this is ten times faster than the previous record for controlling rare-earth atoms. In the world of quantum computing, this is like upgrading from a dial-up modem to a fiber-optic connection. It means they can perform operations on the qubit incredibly quickly, which is essential for building powerful quantum computers before the information fades away.

What They Ruled Out
The researchers were very careful to figure out exactly how this worked. They tested a few ideas and ruled them out:

It's not the tip: They proved that the electric field coming from their microscope tip wasn't directly hitting the erbium. If it were, the speed would have changed when they moved the tip closer or further away. It didn't.
It's not just about distance: They tried moving the atoms closer together, thinking a tighter grip would make the dance faster. Surprisingly, making them closer didn't always make the control faster. This told them that the direction the atoms were facing relative to each other was just as important as how close they were.

The Secret Mechanism: The "Wobble"
The paper explains that the secret lies in the "anisotropy" of the atoms. Imagine the erbium atom has a magnetic shape that is stretched out like a peanut, while the titanium is more like a sphere. When the electric field makes the titanium wobble, it doesn't just push the erbium straight; it makes the erbium's magnetic "peanut" tilt and wobble in a specific way. This wobbling creates an effective magnetic field that the erbium responds to, even though the original electric field couldn't touch it directly.

How Sure Are They?
The authors are very confident in their findings. They didn't just guess; they measured the spin transitions, mapped out how the atoms responded to different magnetic angles, and ran computer simulations that matched their real-world data perfectly. They showed that this method works specifically because of the unique, stretched-out shape of the erbium atom's magnetic field and its tight bond with the titanium.

Why This Matters
This discovery is a big deal because it suggests a new way to build quantum computers. Instead of needing giant, slow magnetic fields to control these special atoms, we might be able to use tiny, fast electrical signals. This opens the door to creating dense, fast, and scalable quantum devices using rare-earth atoms, which are already known for being very stable and good at storing information. It's a step toward making quantum computers that are not just powerful, but also practical enough to fit on a chip.

Technical Summary: All-electrical Coherent Control of a Single Rare-earth Spin Qubit

Problem Statement
Rare-earth (RE) 4f-electron spins are highly promising for quantum technologies due to their long coherence times and optical addressability, particularly for spin-photon interfaces at telecom wavelengths. However, achieving all-electrical coherent control of individual RE spins remains a fundamental experimental challenge. The primary obstacle is the strong shielding of the 4f orbitals by outer electron shells, which severely limits the transduction of external electric fields into effective driving fields for the 4f spins. While mediator spins have been proposed to bridge this gap, engineering such mediators with the requisite precision in conventional solid-state or molecular platforms has proven difficult.

Methodology
The authors address this challenge using a Scanning Tunneling Microscope (STM) equipped with Electron Spin Resonance (ESR) capabilities (ESR-STM) on an MgO/Ag(100) surface.

System Engineering: A tailored spin pair was constructed atom-by-atom, consisting of a single Erbium (Er) atom adsorbed at a low-symmetry oxygen-oxygen bridge site [Er(B)] and a Titanium (Ti) atom placed at a specific distance (three lattice sites along the O-O direction).
Driving Mechanism: The Er atom, lacking unpaired 5d/6s electrons, is not directly electrically addressable. Instead, an RF electric field is applied to the nearby Ti atom. The Ti spin acts as a mediator, generating an effective exchange field ( $B_J$ ) on the Er spin. The RF excitation modulates this exchange interaction, creating a transverse driving field ( $B_1$ ) for the Er spin.
Characterization: The team utilized three-dimensional magnetic field control to map resonance frequencies and Rabi frequencies across various field orientations ( $\theta$ and $\phi$ ). They compared different pair geometries—specifically (3, 0), (3.5, 0.5), and (2.5, −0.5)—to isolate the effects of atomic separation and orientation.
Modeling: A spin Hamiltonian was constructed incorporating anisotropic g-tensors for both Er and Ti, and an anisotropic exchange tensor ( $J$ ). The model accounts for the time-dependence of the exchange interaction induced by RF modulation of atomic orbitals, described as a "wobbling" of magnetic moments.

Key Contributions and Results

All-Electrical Coherent Control: The study demonstrates all-electrical coherent control of a single Er 4f spin coupled to a Ti atom. By driving the coupled Ti spin, the authors successfully manipulate the shielded Er spin without direct optical or magnetic resonance driving of the Er atom itself, overcoming the challenge that such coherent electrical manipulation of individual rare-earth spins has previously remained elusive.
Record Rabi Frequencies: The system achieved Rabi frequencies of up to 190 MHz for the Er spin transition. This represents a ten-fold improvement over previous records for rare-earth spin qubits in solid-state platforms. In contrast, the isolated Ti spin or the Ti transition in the pair exhibited Rabi frequencies an order of magnitude lower (~19 MHz), confirming the enhancement is specific to the Er spin via the coupling mechanism.
Anisotropic Exchange as a Resource: The research identifies anisotropic exchange interaction as the critical resource enabling this control.
- The Er atom at the bridge site exhibits a strongly anisotropic g-tensor ( $g_{xx} \approx 1.05, g_{yy} \approx 2.22, g_{zz} \approx 9.59$ ) and an anisotropic exchange tensor ( $J_{xx} \approx 80, J_{yy} \approx 544, J_{zz} \approx 1273$ MHz).
- The driving efficiency depends not only on the distance between spins but critically on the pair's orientation relative to the magnetic field. Maximum driving occurs when the magnetic field is oriented such that the anisotropic g-tensors cause the magnetic moments to deviate from the field direction, allowing the RF-induced "wobbling" of the Ti moment to generate a strong transverse field for the Er spin.
Double Quantum Transitions (DQT): The authors successfully drove and observed Double Quantum Transitions (where both Er and Ti spins rotate simultaneously), a feature of interest for quantum entanglement and sensing, which had not been previously demonstrated for surface atoms.
Geometry Dependence: While the spin-spin interaction strength (splitting) increases monotonically with decreasing separation, the Rabi frequency does not. The (3, 0) pair showed the highest Rabi frequency, while the (2.5, −0.5) pair, despite having stronger coupling, did not show a proportional increase in Rabi frequency, highlighting the dominance of geometric orientation and anisotropy over simple distance.

Significance
The paper establishes that rare-earth atoms on surfaces, when coupled to a mediator spin with anisotropic exchange interactions, provide a viable platform for atom-scale, electrically controlled spin qubits. The demonstrated mechanism—transducing electrical excitations into coherent rotation via anisotropic exchange—offers a general design strategy for accessing shielded 4f spins. This approach enables ultrafast gate operations (near-gigahertz regime) and local control, potentially opening new routes for high-fidelity qubit architectures in solid-state quantum devices, molecular spin qubits, and semiconductor systems where similar mediator strategies could be engineered. The work validates the "bottom-up" design of quantum devices using scanning probe manipulation to overcome the intrinsic shielding limitations of rare-earth elements.

The Tiny, Invisible Dance of Quantum Bits

The Paper's Big Breakthrough

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