  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.
