Introduction of spin centers in single crystals of Ba$_2$CaWO$_{6-delta}$

Developing the field of quantum information science (QIS) hinges upon designing viable qubits, the smallest unit in quantum computing. One approach to creating qubits is introducing paramagnetic defects into semiconductors or insulators. This class of qubits has seen success in the form of nitrogen-vacancy centers in diamond, divacancy defects in SiC, and P doped into Si. These materials feature paramagnetic defects in a low nuclear spin environment to reduce the impact of nuclear spin on electronic spin coherence. In this work, we report single crystal growth of Ba$_2$CaWO$_{6-delta}$, and the coherence properties of controllably introduced W$^{5+}$ spin centers generated by oxygen vacancies. Ba$_2$CaWO$_{6-delta}$ ($delta$ = 0) is a B-site ordered double perovskite with a temperature-dependent octahedral tilting wherein oxygen vacancies generate W$^{5+}$ (d$^1$), $S = frac{1}{2}, I$ = 0, centers. We characterized these defects by measuring the spin-lattice ($T_1$) and spin-spin relaxation ($T_2$) times from T = 5 to 150 K. At T = 5 K, $T_1$ = 310 ms and $T_2$ = 4 $mu$s, establishing the viability of these qubit candidates. With increasing temperature, $T_2$ remains constant up to T = 60 K and then decreases to $T_2$ $approx$ 1 $mu$s at T = 90 K, and remains roughly constant until T = 150 K, demonstrating the remarkable stability of $T_2$ with increasing temperature. Together, these results demonstrate that controlled defect generation in double perovskite structures can generate viable paramagnetic point centers for quantum applications and expand the field of potential materials for QIS.