⚛️ quantum physics

RF-free driving of nuclear spins with color centers in silicon carbide

This study demonstrates that coherent control of nuclear spins in silicon carbide divacancy centers can be achieved without radio-frequency fields by using microwave pulses and a tilted magnetic field, thereby enabling high-fidelity, scalable quantum devices with simplified experimental requirements.

Original authors: Raphael Wörnle, Jonathan Körber, Timo Steidl, Georgy V. Astakhov, Durga B. R. Dasari, Florian Kaiser, Vadim Vorobyov, Jörg Wrachtrup

Published 2026-01-30

📖 4 min read🧠 Deep dive

CC BY 4.0

Original authors: Raphael Wörnle, Jonathan Körber, Timo Steidl, Georgy V. Astakhov, Durga B. R. Dasari, Florian Kaiser, Vadim Vorobyov, Jörg Wrachtrup

Original paper licensed under CC BY 4.0 (http://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 Big Idea: Controlling Tiny Magnets Without a Radio

Imagine you have a tiny, invisible magnet inside a piece of silicon carbide (a hard, sand-like material used in electronics). This magnet is actually a "nuclear spin," which is a fundamental part of an atom that acts like a tiny compass needle.

Usually, to make this tiny compass spin or point in a specific direction (which is necessary for quantum computers and sensors), scientists need to blast it with strong radio waves (like a radio station signal). This is messy: it requires extra equipment, uses a lot of power, and can heat up the experiment, making it hard to control.

The Breakthrough:
This paper shows a new way to control that tiny nuclear magnet without using any radio waves at all. Instead, the researchers used a clever trick involving a "helper" magnet (an electron spin) and a very precise tilt of the main magnetic field.

The Characters in the Story

The Main Actor (The PL6 Center): Think of this as a tiny, glowing lightbulb inside the silicon carbide. It has an electron spin (a "helper" magnet) that is easy to talk to using microwaves (like the kind in a microwave oven, but much weaker and faster).
The Silent Partner (The Nuclear Spin): This is the tiny nuclear magnet sitting right next to the lightbulb. It's very stubborn and hard to talk to directly. In the past, you needed a "radio megaphone" to get its attention.
The Connection (Hyperfine Interaction): The lightbulb and the silent partner are holding hands. If you shake the lightbulb, the partner feels it.

The Magic Trick: The "Tilted Field"

The researchers discovered a way to shake the silent partner by only shaking the lightbulb, but only if they set the stage just right.

The Setup: They placed the silicon carbide in a magnetic field. Usually, this field points straight up.
The Tilt: They tilted the magnetic field slightly (by just 2 degrees).
The Result: Because of this slight tilt, when they used microwaves to spin the "helper" electron, the "helper" didn't just spin itself; it also dragged the "silent partner" (the nuclear spin) along with it.

The Analogy:
Imagine the electron spin is a large, heavy wheel, and the nuclear spin is a small, light ball attached to the edge of that wheel.

Old Way: To make the ball spin, you had to push the ball directly with a separate machine (the radio waves).
New Way: You tilt the whole axle of the wheel slightly. Now, when you spin the big wheel with a simple motor (microwaves), the tilt causes the wheel to wobble in a way that naturally spins the little ball attached to it. You don't need a second machine for the ball; the motion of the wheel does the work for you.

What They Achieved

High Fidelity (Accuracy): They were able to control the nuclear spin with 89% accuracy. In the world of quantum mechanics, this is like hitting a bullseye almost every time.
Long Memory: The nuclear spin is a great memory storage. While the "helper" electron forgets its state very quickly (in about 25 microseconds), the nuclear spin remembers for much longer (about 151 microseconds). It's like the difference between a sticky note that falls off in a second versus a memory that lasts for minutes.
Simplicity: By removing the need for radio waves, the experiment became simpler, used less power, and avoided heating issues.

Why This Matters (According to the Paper)

The paper claims this method is a "simplified and scalable route."

Simplified: You don't need complex radio equipment.
Scalable: Because it's simpler, it's easier to build many of these devices together to make bigger quantum computers or sensors.

The researchers also showed they could use this system to create a "Bell state," which is a special quantum link where the two magnets (electron and nucleus) become entangled. They proved they could read the state of this linked pair with high accuracy, all without ever turning on a radio transmitter.

Summary

The paper demonstrates that by using a specific type of defect in silicon carbide and tilting the magnetic field just a tiny bit, scientists can control a stubborn nuclear spin using only microwaves. This removes the need for complex radio equipment, making future quantum devices simpler, more efficient, and easier to build.

Technical Summary: RF-free driving of nuclear spins with color centers in silicon carbide

Problem Statement
Quantum devices based on color centers, such as those in silicon carbide (SiC), often utilize nearby nuclear spins as quantum memories or auxiliary qubits. However, conventional manipulation of these nuclear spins requires strong radio-frequency (RF) fields due to their weak magnetic moments. The introduction of RF fields increases experimental complexity, power consumption, and thermal drift, particularly in low-temperature environments. A significant challenge remains in finding systems where both electron and nuclear spins can be coherently controlled using a single microwave (MW) source, thereby eliminating the need for additional RF hardware.

Methodology
The authors investigate a modified divacancy center in 4H-silicon carbide (SiC), specifically the PL6 center. The study utilizes a single PL6 center strongly coupled to a nearby nuclear spin (identified as $^{13}$ C with a coupling strength of $\approx 6.7$ MHz). The experimental setup involves a home-built confocal microscopy system operating at room temperature, employing a 940 nm laser for excitation and MW pulses for spin manipulation.

The core methodology relies on precisely tilting an externally applied magnetic field relative to the defect's quantization axis. Theoretical modeling using Van Vleck perturbation theory derives an effective Hamiltonian for the nuclear spin within the $m_s=0$ ground-state subspace. This model demonstrates that a tilted magnetic field ( $B_\perp \neq 0$ ) activates hyperfine-mediated effects. Specifically, the transverse component of the field, combined with hyperfine coupling, induces a mixing between the $m_s=0$ and $m_s=\pm1$ manifolds. This mixing generates an effective transverse field that drives nuclear spin precession without direct RF excitation.

The experimental protocol involves:

Initialization: Polarizing the nuclear spin via a laser pulse and a spin-selective MW $\pi$ -pulse on the electron spin.
Driving: Applying MW pulses to drive the electron spin, which simultaneously manipulates the nuclear spin through the hyperfine-enhanced precession mechanism.
Readout: Mapping the nuclear spin state back to the electron spin for optical readout.
Characterization: Performing Ramsey and Hahn echo sequences to measure coherence times ( $T_2^*$ and $T_2$ ) and quantum state tomography (QST) to verify gate fidelities.

Key Contributions and Results

RF-free Nuclear Control: The authors demonstrate coherent control of a coupled nuclear spin using only MW pulses and a precisely tilted magnetic field, eliminating the need for RF sources. The nuclear spin precession is driven by the electron spin via hyperfine-enhanced effects.
High-Fidelity Control: The system achieves high-fidelity operations. Single-qubit gate fidelities were measured at 97% for the electron spin and 93% for the nuclear spin. Two-qubit quantum state tomography of the entangled electron-nuclear system yielded a fidelity of 89%.
Extended Coherence Times: The nuclear spin serves as a robust quantum memory. The measured pure dephasing time ( $T_2^{*, \text{Nucl}}$ ) is $102.2 \pm 7.2$ $\mu$ s, and the spin-echo coherence time ( $T_2^{\text{Nucl}}$ ) is $151.0 \pm 6.9$ $\mu$ s. These values are approximately 30 and 6 times longer, respectively, than the corresponding electron spin coherence times, indicating that the nuclear coherence is limited primarily by the electron spin's $T_1$ .
Magnetic Field Alignment: The nuclear spin precession frequency and contrast are shown to be highly sensitive to the tilt angle of the external magnetic field. This sensitivity allows the nuclear spin itself to be used as a probe for precise magnetic field alignment, identifying a "sweet spot" where the tilt maximizes the observable oscillation contrast while maintaining moderate precession frequencies.
Polarization: The study demonstrates that the nuclear spin can be polarized to a high degree (up to 95.5%) through repeated application of the initialization sequence, effectively suppressing transitions between nuclear spin states.

Significance
The paper claims that this approach offers a simplified and scalable route for future quantum applications. By enabling the control of nuclear spins without RF fields, the method reduces experimental complexity and power consumption. The ability to achieve high-fidelity control and long coherence times using a single MW source makes the PL6 center in SiC a promising candidate for quantum sensing, computing, and networking platforms, particularly those requiring room-temperature operation or integration into scalable photonic structures. The work establishes that the PL6 center can match the performance of long-studied diamond NV centers while offering the advantages of SiC's CMOS compatibility and biological window emission.