Scientists Achieve 89% Coherent Nuclear Spin Control Without RF Fields

Scientists are developing novel methods for controlling nuclear spins without relying on traditional radiofrequency fields, paving the way for simpler and more scalable quantum technologies. Raphael Wörnle, Jonathan Körber, and Timo Steidl, from the 3rd Institute of Physics at the University of Stuttgart, alongside colleagues including Georgy V. Astakhov and Durga B. R. Dasari et al., demonstrate coherent control of a nuclear spin coupled to a modified divacancy center in silicon carbide, achieved solely through microwave pulses. This breakthrough is significant because it eliminates the need for complex and power-hungry RF equipment typically required for nuclear spin manipulation, offering a potentially transformative advance for quantum computing and sensing applications in diverse fields such as biology and materials science.

This innovation utilises modified divacancy centers in silicon carbide, offering a simplified and scalable approach for quantum devices. The research team successfully manipulated a coupled nuclear spin using only microwave (MW) pulses driving the electron spin, activated by a precisely tilted external magnetic field, circumventing the conventional requirement for additional high-power RF fields. This method significantly reduces experimental complexity and overall power consumption, paving the way for more efficient quantum systems.

The study focused on a specific type of defect, the PL6 center, within the 4H-silicon carbide lattice, chosen for its promising properties including a high count rate of approximately 200 kcps and a spin readout contrast of 30% at room temperature. Researchers demonstrated high-fidelity nuclear-spin control, achieving an impressive 89% two-qubit tomography fidelity and coherence times approaching the limits dictated by the nuclear spin’s T1 relaxation. Detailed characterisation of a single PL6 center revealed a saturation intensity of 211.7 ±1.8 kcps, confirming its single-defect nature through second-order correlation measurements. Furthermore, a CW-ODMR spectrum was obtained, exhibiting a two-Lorentzian fit centered around 1351.8MHz, indicating the sensitivity of the center to external magnetic fields.

Experiments showcased coherent control through Rabi oscillations and precise measurements of spin-lattice relaxation time T1, yielding a value of 242.8±22.1μs in a 210 Gauss magnetic field. Hahn echo measurements provided insights into spin dephasing, with a T2 value of 25.0 ±1.3μs, while Ramsey measurements determined a pure spin-dephasing time T∗2 of 2.7 ±0.3μs. This level of control, achieved without RF fields, is enabled by hyperfine-enhanced effects, where the MW pulses driving the electron spin also influence the nuclear spin through their interaction. The ability to manipulate nuclear spins with a single MW source represents a substantial advancement, offering a pathway towards full CMOS compatibility and wafer-scale fabrication of quantum devices.

This innovative approach has broad implications for quantum sensing and computing platforms, with potential applications in biology, materials science, and geophysics. The PL6 centers’ emission spectrum within the second biological window (1000-1300nm) makes them particularly well-suited for biological studies, reducing scattering and absorption in tissue. Ongoing research focuses on integrating these color centers into nanophotonic structures to further enhance count rates and improve their performance, while cryogenic optical studies continue to refine understanding of their optical properties. The work opens exciting possibilities for simplified, scalable, and energy-efficient quantum technologies, leveraging the unique properties of modified divacancy centers in silicon carbide.

Nuclear spin control via tilted magnetic fields enables

Scientists harnessed a modified divacancy center in silicon carbide to achieve coherent control of a coupled nuclear spin without radio frequency fields. The study pioneered a method where microwave pulses driving the electron spin simultaneously manipulate the nuclear spin via hyperfine-enhanced effects, activated by a precisely tilted external magnetic field. Researchers meticulously aligned the external magnetic field using nuclear spin precession, a complementary technique to established methods like ODMR transitions and spin Hamiltonian analysis. This innovative approach exploits the interplay between nuclear precession frequency and oscillation contrast, both strongly dependent on the tilt angle of the external field relative to the defect axis.

Experiments employed an effective Hamiltonian to model the system, demonstrating excellent agreement between theoretical values and experimental results, determining a magnetic tilt of φ = 2° with hyperfine couplings of Az = 6.7MHz and A⊥= 5.5MHz. The team discovered a “sweet spot” for magnetic field alignment, where a small, finite tilt maximizes nuclear spin contrast while maintaining a moderate oscillation frequency. Fitting experimentally measured nuclear precession frequency and contrast to the theoretical model allowed extraction of both the tilt angle and hyperfine couplings. To quantify nuclear coherence, the research team modified a pulse sequence, simultaneously applying microwave transitions to drive the nuclear spin, resulting in a measured spin decoherence time T∗,Nucl 2 of 102.2±7.2μs.

This technique achieves a nuclear T∗ 2 that is over 30times longer than the corresponding electron-spin coherence, significantly improving magnetic sensitivity. Scientists further extended nuclear-spin coherence by inserting two fast π pulses on the selected ODMR transition, separated by a time interval corresponding to the hyperfine splitting of 2π · 6.7MHz. The resulting sequence yielded a nuclear-spin relaxation time of T Nucl 2 = 151.0 ±6.9μs, approximately six times longer than the electron-spin T2. The study demonstrated high-fidelity nuclear-spin control, achieving 89% two-qubit tomography fidelity and nearly T1-limited nuclear coherence times, offering a simplified and scalable route for future applications.

PL6 Centre Shows Coherent Spin Control capabilities

Scientists have demonstrated coherent control of a coupled nuclear spin without requiring radio-frequency (RF) fields, utilising a modified divacancy center in silicon carbide. This breakthrough simplifies experimental setups and reduces power consumption, offering a pathway towards scalable quantum technologies. The research team measured a saturation intensity of 211.7 ±1.8 kcps during a power-dependent study of a single PL6 center, confirming a single defect origin with a g(2)(0) value significantly less than 0.5. This saturation behaviour is consistent with previously reported values for bulk samples and validates the single-emitter nature of the studied defect.

Experiments revealed a characteristic zero-field splitting (ZFS) through continuous-wave optically detected magnetic resonance (CW-ODMR) measurements, identifying individual PL6 centers. The CW-ODMR spectrum exhibited two distinct dips at frequencies of 1346.1MHz and 1357.4MHz, corresponding to transitions between spin states, with an ODMR contrast of approximately 19 percent. These measurements establish the fundamental spin properties of the PL6 center and provide a basis for subsequent nuclear spin control experiments. The team then performed Rabi oscillations, fitting the data with a damped cosine function to characterise the electron spin coherence.

Further measurements focused on quantifying the spin dynamics, with the team recording a spin-lattice relaxation time (T1) of 242.8 ±22.1μs in a magnetic field of 210 Gauss. This value indicates the rate at which the electron spin returns to its equilibrium state after excitation. Hahn echo measurements yielded a spin coherence time (T2) of 25.0 ±1.3μs, representing the duration for which quantum information can be stored in the electron spin. A Ramsey measurement, conducted at a detuning of 3MHz, determined a pure spin-dephasing time (T*2) of 2.7 ±0.3μs, providing insight into the environmental factors affecting coherence.

The breakthrough delivers high-fidelity nuclear-spin control, achieving a two-qubit tomography fidelity of 89 percent and nearly T1-limited nuclear coherence times. By precisely tilting an external magnetic field, scientists activated hyperfine-enhanced effects, enabling manipulation of the nuclear spin solely with microwave pulses. This approach eliminates the need for RF fields, simplifying the experimental setup and reducing power consumption, and offers a scalable route for future applications in quantum computing, biology, materials science, and geophysics. Measurements confirm that the coupled nuclear spin serves as a sensitive probe of magnetic field orientation, enabling accurate field alignment.

Hyperfine Control Achieves High Fidelity Nuclear Spins manipulation

Scientists have demonstrated coherent control of a nuclear spin within silicon carbide, achieving this without the need for direct radio frequency (RF) driving of the nuclear spin. Utilizing a strongly coupled PL6 center, researchers manipulated the nuclear spin through hyperfine-enhanced effects activated by a precisely tilted external magnetic field. This approach enables high-fidelity control, with a two-qubit tomography fidelity of 89% and nearly T1-limited nuclear coherence times. The achievement of robust nuclear-spin control is significant as it simplifies experimental setups and reduces power consumption, particularly benefiting quantum computing platforms with applications in diverse fields like biology, materials science, and geophysics.

Notably, the team attained a nuclear spin coherence time of 151.0 ±6.9μs, exceeding the electron spin coherence time by a factor of six, and also demonstrated the potential for extended information storage using the nuclear spin. The authors acknowledge that their work is based on a specific sample fabricated through ion implantation and annealing, which may introduce limitations in defect density and homogeneity. Future research could focus on extending this method to couple multiple nuclear spins, thereby facilitating the scalable development of defects in silicon carbide for room temperature quantum technologies.