$N$-dimensional discrete Fourier transform via bosonic Hamiltonian
The discrete Fourier transform (DFT) underpins many classical algorithms and is a fundamental unitary operator for quantum information processing. Implementing the $N$-dimensional DFT in photonic integrated circuits (PICs) is limited by the cascades of Mach-Zehnder interferometers that current architectures require. Here we propose bosonic Hamiltonians that realize the $N$-dimensional DFT through a single stage of multimode evolution, complemented only by phase shifters before and after the interaction region, in a geometry suited to 3D waveguides. Modeling the system as a graph, where edges correspond to couplings and the vertices are the waveguides, we obtain analytical solutions for complete graphs up to $N=6$ and numerical solutions up to $N=31$. For non-complete graphs, different propagation constants are required in the Hamiltonian. We report all solutions for $N\leq 8$, partial exploration for $N=9$, and selected cases for $N=10$, together with three conjectures that guide the numerical search for $N \geq 11$. These configurations circumvent the vanishing evanescent coupling strength imposed by the waveguide separation, and a closed-form sensitivity criterion selects those that are admissible as a waveguide layout and least sensitive to fabrication error. We also uncover the missing non-affine parameters of the $6$-dimensional DFT, and show that the scaling law for implementing the $N$-dimensional DFT with our building blocks is $\mathcal{O}(N\log\log{N})$. This allows assembling the $2520$-dimensional DFT with only $2625$ interferometers, in contrast to the $\approx 3\times 10^6$ of Reck and Clements architectures.