Building codes with transversal CCZ using projective geometry and SAT solvers

We construct CSS codes with three logical qubits whose logical CCZ gate is implemented by transversal physical $T\mathrm{/}{T}^{†}$ gates and whose distance is at least $3$. These codes start with $8$-divisible $X$-stabilizers defined by projective geometry, which guarantees $d_Z\ge 3$. A SAT solver then finds three $X$-logical operators compatible with the transversal CCZ. We build codes at thirteen block lengths from $n=48$ to $n=496$, with $d_Z=3$ or $4$, and illustrate the method with the $[[48,3,3]]$ code $Q_{48}$. Additionally, we prove that no CSS code with three logical qubits, $Z$-distance at least $3$, and a quasi-transversal CCZ gate (transversal $T$ with a diagonal Clifford correction) exists below $n=39$. The $[[47,3,3]]$ code of Jacinto et al. serves as an upper bound, leaving $39\le n\le 46$ open.