  We demonstrate a robust implementation of a deterministic linear-optical
Controlled-NOT (CNOT) gate for single-photon two-qubit quantum logic. A
polarization Sagnac interferometer with an embedded 45$^{\circ}$-oriented dove
prism is used to enable the polarization control qubit to act on the momentum
(spatial) target qubit of the same photon. The CNOT gate requires no active
stabilization because the two spatial modes share a common path, and it is used
to entangle the polarization and momentum qubits.
