Experimental implementation of a quantum computing algorithm strongly relies on the ability to construct required unitary transformations applied to the input quantum states. In particular, near-term linear optical computing requires universal programmable interferometers, capable of implementing an arbitrary transformation of input optical modes. So far these devices were composed as a circuit with well defined building blocks, such as balanced beamsplitters. This approach is vulnerable to manufacturing imperfections inevitable in any realistic experimental implementation, and the larger the circuit size grows, the more strict the tolerances become. In this work we demonstrate a new methodology for the design of the high-dimensional mode transformations, which overcomes this problem, and carefully investigate its features. The circuit in our architecture is composed of interchanging mode mixing layers, which may be almost arbitrary, and layers of variable phaseshifters, allowing to program the device to approximate any desired unitary transformation. arXiv:1906.06748v1 [quant-ph]
Photonic information processing often demands programmable multiport interferometers capable of implementing arbitrary transfer matrices, for which planar meshes of tunable Mach-Zehnder interferometers (MZIs) are usually exploited. However, these MZI-based interferometers require balanced static beam-splitter (BSs) that make them sensitive to manufacturing errors. Here, we abandon the error-sensitive MZI and propose an alternative design that uses a single static BS and a variable phase shift as the building block of the interferometer mesh. Our BS-based design has been shown to possess superior resilience to manufacturing errors, which is achieved without addition of extra elements into the schemes. Namely, the power transmissivities of the static BSs constituent the interferometers can take arbitrary values in the range from ≈ 1/2 to ≈ 4/5.
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