Directionally unbiased linear optics enable novel high-resolution sensors and quantum network routers

Grover multiports are higher-dimensional generalizations of beam splitters in which inputs to any of the four ports have an equal probability of exiting at any of the four ports, including the input port. This offers direct access to a greater number of degrees of freedom [1]: We demonstrate that traditional interferometers can obtain novel features when directionally unbiased multi-ports replace traditional beam splitters. It will be shown that the Grover-based counterpart of the conventional Michelson interferometer can substantially enhance the phase measurement resolution. The Grover-Michelson interferometer demonstrates a tunable intensity-phase slope instead of a traditional fixed cos2 profile, thus enabling super-resolution phase measurement while utilizing only linear optical elements and classical states of light [2]. The traditional two-photon Hong-Ou-Mandel (HOM) effect could be expanded onto a higherdimensional set of four spatial modes when combining the Grover coin with quantum-correlated twophoton input and coincidence measurements. We introduce a novel quantum photonic state router design that allows controllable redistribution (switching) of (|2,0>+|0,2>) quantum states over these four modes using directionally unbiased linear-optical four-ports without post-selection [3]. The original HOM effect only enables photon pairs to exit in two directions in space. However, when beam splitters and phase shifters accompany the Grover coin, the result is a directionally controllable two-photon HOM effect in four spatial modes, with the outgoing photon direction controlled by changing the phases in the system. By this means, spatial and temporal control of the propagation of the two-photon superposition state through a network or quantum nodes (quantum state routing) can be achieved.
[1] Shuto Osawa, David S. Simon, and Alexander V. Sergienko "Directionally-Unbiased Unitary Optical Devices in Discrete-Time Quantum Walks,” Entropy, v. 21, p. 853 (2019).
[2] Christopher R. Schwarze, David S. Simon, and Alexander V. Sergienko, "Enhanced-Sensitivity Interferometry With Phase-Sensitive Unbiased Multiports,” Phys. Rev. A 107, 052615 (2023).
[3] Shuto Osawa, David S. Simon, and Alexander V. Sergienko "Higher-Dimensional Hong-Ou-Mandel Effect and State Redistribution With Linear-Optical Multiports,” Phys. Rev. A 102, 063712 (2020).