Long coherence times are critical in evaluating the performance of quantum bits (qubits). Quantum algorithms need qubits with long coherence times to complete the algorithm before the qubits’ information is irreversibly lost to the surrounding environment. Electron spins in crystal defects, such as the nitrogen-vacancy (NV) center in diamond, donor spins in silicon, transition-metal ions, and rare-earth ions, have attracted interest as versatile solid-state qubits, in large part due to their long coherence times. The ensemble Hahn-echo coherence time, T2, of NV centers in naturally isotopic diamond has been measured to be 0.63 ms, while donor spins in natural silicon have T2 values reaching 0.8 ms.

Our work in the divacancy defect in silicon carbide (SiC), another crystal defect-based electron spin qubit, has shown coherence times approaching 1.3 ms. This value is one of the longest T2 times of an electron spin in a naturally isotopic crystal, despite SiC having a higher nuclear spin density than naturally isotopic diamond. An implication of this result is a more nuanced decoherence mechanism than simply the density of nuclear spin isotopes.

In conjunction with the Galli group here at the Institute for Molecular Engineering, we have combined experiment and theory to study the decoherence dynamics of spin-1 electronic spin ensembles of the neutral divacancy in 4H-SiC over a wide range of magnetic fields. We use Optically Detected Magnetic Resonance (ODMR) to probe the electronic spin state and measure the Hahn-echo coherence time. Quantum bath theory is used to understand the underlying physical reasons for the differences in coherence time between divacancy spin qubits in SiC and NV center spin qubits in diamond. As seen in the figures above, the agreement between experiment and theory is not only strong for the divacancy, but the theory also supports the T2 values seen in NV center spin qubits.

We propose an explanation for the higher coherence times in divacancy spins in SiC based on permitted nuclear spin interactions in the crystal lattice. As seen in the diagrams below, the diamond lattice (left) supports nearest-neighbor nuclear spin interactions, whereas these interactions are not possible in SiC (right). The increased distance between nuclear spins of the same species suppresses overall nuclear spin flips, reducing the amount of magnetic field fluctuations experienced by the electron.

These results introduce alternatives to isotopic purification as a method to extend the coherence times of spin qubits. Additionally, these results lay the foundation for the exploration of more coherent solid-state spin qubits in ternary and quaternary crystals.

To learn more about our studies, please see “Quantum decoherence dynamics of divacancy spins in silicon carbide”, H. Seo, A. L. Falk, P. V. Klimov, K. C. Miao, G. Galli, and D. D. Awschalom, Nature Communications 9, (2016) in press