Quantum photonic full adder gate based on an eight-mode integrated optical circuit

We propose a method to implement a compact quantum photonic full adder (FA) gate on a photonic integrated circuit (PIC). The proposed FA gate consists of two 8×8 photonic interferometers interconnected via attenuators. Multiport interferometers are designed using the Clements decomposition algorithm to generate the sum ( s ) and carry ( c out ) outputs. The gate operation is modeled through a four-photon scattering process on an eight-mode integrated optical circuit, where permanents of submatrices determine the output probabilities. Simulation results demonstrate a fidelity of 1 and a success probability of 2.22×10 −4 for the FA gate. We further analyze the impact of phase-shifter and attenuator variations on fidelity and success probability. Finally, the performance of the proposed FA gate is compared with a benchmark Toffoli gate designed using the same approach.