Implementation of a simultaneous message passing protocol using optical vortices

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.