Quantum decoherence dynamics of divacancy spins in silicon carbide

Abstract: Long coherence times are key to the performance of quantum bits (qubits). Here, we experimentally and theoretically show that the Hahn-echo coherence time (T2) of electron spins associated with divacancy defects in 4H-SiC reaches 1.3 ms, one of the longest T2 times of an electron spin in a naturally isotopic crystal. Using a first-principles microscopic quantum-bath model, we find that two factors determine the unusually robust coherence. First, in the presence of moderate magnetic fields (300 G and above), the 29Si and 13C paramagnetic nuclear spin baths are decoupled. In addition, because SiC is a binary crystal, homo-nuclear spin pairs are both diluted and forbidden from forming strongly coupled, nearest-neighbor spin pairs. Longer neighbor distances result in fewer nuclear spin flip-flops, a less fluctuating intra-crystalline magnetic environment, and thus a longer T2 time. Our results point to polyatomic crystals as promising hosts for coherent qubits in the solid state.

Quantum Decoherence Dynamics of Divacancy Spins in Silicon Carbide

The study under consideration presents a rigorous investigation into the spin coherence properties of divacancy defects within a silicon carbide (SiC) crystal lattice. Focusing on the dynamic behavior of electron spins, this research demonstrates one of the longest Hahn-echo coherence times recorded for electron spins in naturally isotopic crystals, reaching an impressive 1.3 milliseconds. The significance of this metric lies in its practical implications for quantum bit (qubit) performance, where prolonged coherence times are critical for executing complex quantum algorithms and advancing the development of quantum computing.

Experimental and Theoretical Framework

The authors employed both experimental methodologies (specifically, optically detected magnetic resonance or ODMR) and theoretical modeling, using a microscopic quantum-bath approach combined with the cluster-correlation expansion (CCE) method to accurately characterize the coherence dynamics. Their findings revolved around the influence of Si and C paramagnetic nuclear spin baths and how moderate magnetic fields can decouple these baths, suppressing decoherence.

Key experimental observations indicated that the Hahn-echo coherence time benefits from the polyatomic nature of the SiC lattice. Because SiC is a binary crystal, the predominant homo-nuclear spin pairs are isolated, resulting in fewer spin flip-flop interactions. This isolation contributes significantly to the stability of the spin qubit, as lattice sites are effectively distanced, reducing the intensity of nuclear spin-induced magnetic noise.

Numerical and Analytical Insights

The theoretical underpinning of the research used the CCE method to achieve numerical convergence in modeling the coherence function. The decoupling of nuclear spin baths at approximately 30 mT magnetic fields is a central contributor to the prolonged coherence time. Specifically, the study found that heterogeneous nuclear spin pairs, such as Si-C, do not significantly contribute to decoherence under these conditions. Instead, homogeneous spin interactions are the primary sources of coherence decay, though the binary nature of SiC leads to a notable dilution effect.

Implications and Future Directions

Practically, the paper suggests that SiC crystals hold substantial promise as hosts for coherent qubits in solid-state systems, potentially improving the performance of quantum sensors and memories. Additionally, the work implies the feasibility of isotopic purification as a means to further lengthen coherence times, although the scaling behavior does not follow a simple power-law due to the differing contributions from C and Si concentrations.

Theoretically, the insights into spin-bath decoupling can inform the design of qubit systems in other binary and polyatomic crystals, hinting at exciting possibilities in ternary and quaternary compounds such as nitrides and oxides with potentially even longer coherence times. By understanding the decoherence dynamics in SiC, researchers could explore hybrid quantum systems, incorporating phenomena such as piezoelectricity and ferromagnetism with coherent control.

Overall, this study provides a comprehensive framework for examining the quantum spin coherence in SiC, underscoring the material's potential for future quantum technology applications, including both solid-state spins and complex oxide systems. As researchers grapple with the challenges of quantum coherence, especially at varied temperatures and configurations, this paper offers critical insights that pave the way for advancements in quantum computational materials.