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

Abstract

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.

Figures (5)

• Figure 1: Properties of a single PL6 center.(a) Schematic representation of a PL6 center with ground state level structure and the experimental apparatus with wire for MW, and external magnetic field and objective for excitation and collection of the fluorescence emission. (b) Saturation study of a single PL6 center with a saturation intensity of $211.7 \pm 1.8$ kcps. Inset shows the second order correlation function g$^{(2)}(\tau)$ confirming a clear single defect behavior. (c) CW-ODMR spectrum of a single PL6 center in zero magnetic field with a two-Lorentzian fit function centered around 1351.8 MHz. (d) Rabi oscillation fitted with a damped cosine function. (e) Measurement of the spin-lattice relaxation time $T_1$ in a magnetic field of $B = 210$ G. A single exponential fitting function was used to determine $T_1 = 242.8 \pm 22.1 \,\upmu$s. (f) Hahn echo measured in a magnetic field of $B = 210$ G. $T_2$ is estimated from the damped cosine fitting to $T_2 = 25.0 \pm 1.3 \,\upmu$s. Inset shows Ramsey measurement measured at a detuning of 3 MHz yielding a pure spin-dephasing time $T_2^* = 2.7 \pm 0.3 \,\upmu$s.
• Figure 2: PL6 coupled to nearby nuclear spin.(a) Schematic representation of a single color center coupled to a nearby nuclear spin and a coupling strength of 6.7 MHz with shown hyperfine splitting. (b) ODMR spectra of the coupled PL6 center spin without and with applied magnetic field up to a field strength of 20 G and corresponding Zeeman splitting. (c) Pulse sequence for nuclear oscillation. The $\ket{-1, \uparrow}$ transition from the ODMR measurement is used as a frequency for the MW pulse. (d) Modified version of the pulse sequence shown in (c) with laser polarization step to further polarize the nuclear spin. (e) Experimental results measured at an external magnetic field of 240 G for the shown pulse sequences without (green) and with (blue) polarized oscillation with corresponding fit functions.
• Figure 3: Theoretical derivation of nuclear oscillation.(a) Schematic representation of the tilt of the magnetic field causing the precision of the nuclear spin with enhanced oscillation amplitude due to gyromagnetic enhancement. (b) Nuclear oscillation experimental data (blue) & fit function (blue line) with derived theoretical solution of the Hamiltonian (red dashed line) at an external magnetic field of 240 G and a tilt angle $\phi = 2~^\circ$. (c) Field angle $\phi$ calculated from experimental data for different planar alignments of the magnetic field. (d) Larmor frequency in dependence of the tilt angle $\phi$ of the magnetic field with theoretical data of the Hamiltonian. Inset shows the Larmor frequency of the nuclear oscillation in dependence of the external magnetic field strength with linear fit function $f_\mathrm{L} \propto B$. (e) Contrast of the nuclear oscillation in dependence of the magnetic tilt angle $\phi$ in comparison to the theoretical values.
• Figure 4: Nuclear Ramsey and spin echo.(a) & (b) Schematic pulse sequence for the Ramsey ($T_2^{*, \mathrm{Nucl}}$) and nuclear spin echo ($T_2^{\text{Nucl}})$ measurement. For the Ramsey measurement, both microwave transitions $\ket{1,\uparrow}$ and $\ket{-1,\uparrow}$ are applied at the same time for driving the nuclear spin. For the nuclear spin echo measurement, two "fast" $\pi$ pulses are applied for Hahn echo, as first performed by Dutt et al. Dutt2007QuantumDiamond. (c) & (d) Experimental result for the Ramsey and nuclear spin echo measurement yielding a spin dephasing time $T_2^{*, \mathrm{Nucl}} = 102.2 \pm 7.2 \,\upmu$s and a spin echo time for the nuclear spin of $T_2^{\mathrm{Nucl}} = 151.0 \pm 6.9 \,\upmu$s. Each inset shows a zoomed in version of the first 50 $\upmu$s.
• Figure 5: Quantum state tomography. Single qubit tomography for the electron spin (a) and the nuclear spin (b) with reconstructed density matrix shown and a corresponding gate fidelity of 97 % and 93 %, respectively. (c) Schematic representation of entanglement generation. A global $\pi/2$ MW pulse on the electron spin and a subsequent $\pi$ pulse applied as a waiting time are used to create the Bell state from the polarized initial state. (d) Two-qubit tomography for the nuclear spin coupled PL6 center yielding a fidelity of 89 %.