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Nature Communications

Emulation of coherent absorption of Fock-state quantum light in a programmable linear photonic circuit

Govind Krishna, Jun Gao, Sam O’Brien, Rohan Yadgirkar, Venkatesh Deenadayalan, Stefan F. Preble, Val Zwiller, Ali W. Elshaari

Published May 9, 2026• 1 citation
Publisher

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.

DOI
10.1038/s41467-026-72850-6

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Quantum beats, the fact that periodic oscillations arise from coherent superposition of frequency-distinguishable states, have enabled various quantum information technologies beyond the possibilities of classical physics. Although the conventional quantum beats are based on unitary optical elements, we present that nonunitary material provides an alternative degree-of-freedom to control the quantum interference and even enables the observation of anomalous quantum beating signals. An elaborate lossy film is used to explore its tunability over the effective photon–photon interaction in frequency modes, which may introduce sophisticated nonunitary realization into integrated optical platforms. In addition, we confirm its ability of harnessing the suppression and enhancement of single- and two-photon absorption by manipulating the symmetric property of frequency entanglement, and this would provide significant benefits in entanglement-assisted absorption spectroscopy. These results show an alternative method to investigate the quantum light–matter interaction, facilitating the use of two-photon interference for quantum light-based spectroscopy and microscopy.