Figure 3

Construction and characterization of the 60-mode photonic network. (a) The 60-mode interferometer consists of one rectangular piece and two triangular pieces. The rectangular is fabricated by bonding (through intermolecular force) six trapezoidal pieces with a size of 28.28 × 28.28 × 4.00 mm 3 . The triangular is constructed in a similar way, with a size of 24.00 × 24.00 × 20.00 mm 3 . The bonding interfaces are coated with random polarization-dependent beam-splitting ratio. The trapezoidal pieces are cut and bounded with a dimension tolerance of < 5 μ m and parallel precision better than 5”. The design ensures that any possible spatial mismatch is much smaller than the coherence length of the quantum-dot single photons ( ∼ 30 mm ). (b) Illustration of light propagation inside the 3D photonic circuit. The rectangular piece has six horizontal ten-mode layers, while the two triangular ones have ten vertical six-mode layers. In the rectangular piece, only the photons in the same horizontal layer can interfere with each other but not with vertically different layers. After that, ten vertical layers are incorporated, which are to make the photons from different horizontal layers interfere with each other. Therefore, the interferometer is fully connected. (c) The measured 1200 elements of amplitude. These values are determined by the recorded counts of the 60 single-photon detectors when we inject photons in every input port one by one. (d) The measured 1200 elements of amplitude. These elements are measured using Mach-Zehnder-type interference with a narrow-band laser. (e) Unitarity test of the reconstructed matrix. All output probabilities are normalized to unity, corresponding to 20 diagonal elements. The average of all off-diagonal elements is as small as 0.01(1), confirming that the matrix is well reconstructed. (f) Statistical histogram of 1200 elements of amplitude. (g) Statistical histogram of 1200 elements of phase. The phase is uniformly distributed from − π to π .