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

Color centers that enable nuclear-spin control without RF fields offer a powerful route towards simplified and scalable quantum devices. Such capabilities are especially valuable for quantum sensing and computing platforms that already find applications in biology, materials science, and geophysics. A key challenge is the coherent manipulation of nearby nuclear spins, which serve as quantum memories and auxiliary qubits but conventionally require additional high-power RF fields which increase the experimental complexity and overall power consumption. Finding systems where both electron and nuclear spins can be controlled using a single MW source is therefore highly desirable. Here, using a modified divacancy center in silicon carbide, we show that coherent control of a coupled nuclear spin is possible without any RF fields. Instead, MW pulses driving the electron spin also manipulate the nuclear spin through hyperfineenhanced effects, activated by a precisely tilted external magnetic field. We demonstrate high-fidelity nuclear-spin control, achieving 89% two-qubit tomography fidelity and nearly T1-limited nuclear coherence times. This approach offers a simplified and scalable route for future quantum applications.

💡 Research Summary

In this work the authors demonstrate a novel method for controlling nuclear spins coupled to a silicon‑carbide (SiC) color center without the need for any radio‑frequency (RF) fields. Using a modified divacancy defect known as the PL6 center in 4H‑SiC, they show that a single microwave (MW) source can simultaneously manipulate both the electron spin of the defect and a nearby nuclear spin (either ^13C or ^29Si) through hyperfine‑enhanced processes. The key to this RF‑free control is a precisely tilted external magnetic field (≈2° from the crystal axis). The tilt introduces a transverse magnetic component (B⊥) that, together with the transverse hyperfine coupling (A⊥), generates second‑order corrections (ν⊥, νz) in the effective Hamiltonian. These corrections mix the ms = 0 and ms = ±1 electron manifolds, effectively amplifying the hyperfine interaction and creating an “effective” magnetic field for the nucleus. As a result, the nuclear Larmor frequency is shifted and a sizable transverse component drives nuclear precession, observable as slow Rabi‑type oscillations in the fluorescence signal.

Experimentally, a single PL6 center is identified by its zero‑field splitting (D ≈ 1351.8 MHz, E ≈ 5.6 MHz) and high optical count rate (≈212 kcps). Pulsed ODMR measurements yield electron spin coherence times of T1 ≈ 243 µs, T2 ≈ 25 µs, and T2* ≈ 2.7 µs. A nearby nuclear spin with hyperfine couplings Az ≈ 6.7 MHz and A⊥ ≈ 5.5 MHz is found. By initializing the electron spin with a short laser pulse, applying a spin‑selective π‑pulse on the |−1, ↑⟩ transition, waiting for a variable time τ, and then applying a second π‑pulse before readout, the authors observe nuclear oscillations with a frequency of ~0.9 MHz at a magnetic field of 240 G. Adding a laser‑based nuclear polarization step after each π‑pulse further enhances the oscillation contrast. The measured nuclear coherence time approaches the electron T1 limit, indicating that the nuclear spin is well isolated from decoherence sources.

A theoretical model based on the full spin Hamiltonian H = DSz² + E(Sx²−Sy²) + γe B·S + S·A·I − γn B·I is reduced via Van Vleck perturbation theory to an effective Hamiltonian for the ms = 0 subspace. The resulting expressions for the effective nuclear field (B_eff) and the nuclear precession frequency f_nucl match the experimental data across a range of tilt angles and magnetic field strengths. The contrast of the nuclear oscillation is shown to be the product of the electron readout contrast (which degrades with increasing B⊥) and a factor proportional to the ratio of the transverse to total effective field components. The optimal tilt angle (~2°) yields the highest contrast, in agreement with the observed 89 % two‑qubit tomography fidelity.

The significance of this work lies in its demonstration that a single microwave source can replace the conventional dual‑source (MW + RF) architecture for electron‑nuclear spin control. This simplification reduces experimental complexity, power consumption, and RF‑induced heating—critical advantages for low‑temperature operation and for scaling up to large arrays of color centers. Moreover, the nuclear spin itself serves as a sensitive probe of magnetic‑field alignment, enabling self‑calibrated field orientation without auxiliary sensors. The RF‑free approach is compatible with CMOS fabrication, wafer‑scale integration, and the near‑infrared emission of PL6 centers (1000–1300 nm), making it attractive for bio‑compatible quantum sensing, distributed quantum networks, and hybrid quantum devices.

In summary, the authors achieve high‑fidelity, RF‑free nuclear spin control in SiC, reporting 89 % two‑qubit tomography fidelity and near‑T1 limited nuclear coherence. Their method opens a pathway toward simplified, scalable quantum technologies that leverage the favorable optical and spin properties of SiC color centers while eliminating the need for bulky RF hardware.