  The implementation of optical quantum gates comes at the cost of
incorporating a source of nonclassical light, which suffers from a low flux of
photons, and thus, long acquisition times. Quantum-mimetic optical gates
combine the benefits of quantum systems with the convenience of using intense
light beams. Here, we are concerned with the classical implementation of a
controlled-SWAP (c-SWAP) gate using the tools of structured light. A c-SWAP
gate is a three-qubit gate, where one of the input qubits controls the exchange
of information between the other two qubits. We use Laguerre-Gauss beams to
realize the c-SWAP gate and demonstrate one of its primary applications: the
comparison of two signals without revealing their content, i.e., a simultaneous
message-passing protocol. We achieve signal-comparison measurements with
modulation speeds of kHz using a digital micromirror device as a spatial light
modulator. Our system is capable of performing dynamic error analysis and a
normalization procedure, which overcomes the necessity of data postprocessing
and largely reduces comparison times.
