Abstract Non-Hermitian quantum systems, governed by nonunitary evolution, offer powerful tools for manipulating quantum states through engineered loss. A prime example is coherent absorption, where quantum states undergo phase-dependent partial or complete absorption in a lossy medium. Here, we demonstrate a fully programmable implementation of nonunitary transformations that emulate coherent absorption of quantum light using a programmable integrated linear photonic circuit, with loss introduced via coupling to an ancilla mode. Probing the circuit with a single-photon dual-rail state reveals phase-controlled coherent tunability between perfect transmission and perfect absorption. A two-photon NOON-state input, by contrast, exhibits switching between deterministic single-photon absorption and probabilistic two-photon absorption. Across a broad range of input phases and circuit configurations, we observe nonclassical effects including anti-coalescence and bunching, together with continuous and coherent tuning of output Fock-state probability amplitudes. Classical Fisher information analysis reveals phase sensitivity peaks of 1 for single-photon states and 3.4 for NOON states, exceeding the shot-noise limit of 2 and approaching the Heisenberg limit of 4 for two-photon states. The experiment integrates quantum state generation, programmable photonic circuitry, and photon-number-resolving detection, establishing ancilla-assisted circuits as powerful platforms for programmable quantum state engineering, filtering, multiplexed sensing, and nonunitary quantum simulation.
Krishna et al. (Sat,) studied this question.
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