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.
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.
Quantum entanglement is essential for various quantum information technologies that are beyond the possibilities of classical physics. However, it is susceptible to decoherence that may arise from noisy channels and environmental noise. While this deleterious problem has been explored by using filtering techniques, such as photon ancillas and Rydberg atom blockade, a concise yet efficient approach to entanglement retrieval is still a formidable challenge. Here, we present that nonunitary material provides an alternative tool to flexibly control the anomalous Hong–Ou–Mandel (HOM) interference and enables the selective entanglement filter with coherent perfect absorption of anti-symmetric quantum states. An elaborate Cr–SiN–Cr film is prepared as a nonunitary beam splitter such that its tunability over the effective photon–photon interaction in frequency modes can be readily explored in the experiment, which may introduce sophisticated nonunitary realization into integrated optical platforms. In addition, we confirm its ability for selective filtering of high-dimensional frequency entanglement by observing the output quantum state of the conventional HOM interference with a unitary beam splitter and the anomalous HOM interference with a nonunitary beam splitter. These results may offer new insights into investigating the quantum light–matter interaction and facilitating the use of two-photon interference for addressing central challenges in quantum technologies.
The application of quantum states of light such as entangled photons, for example, created by parametric down conversion, has experienced tremendous progress in the almost 40 years since their first experimental realization. Initially, they were employed in the investigation of the foundations of quantum physics, such as the violation of Bell's inequalities and studies of quantum entanglement. They later emerged as basic platforms for quantum communication protocols and, in the recent experiments on single-photon interactions, in photonic quantum computation. These applications aim at the controlled manipulation of the photonic degrees of freedom, and therefore rely on simple models of matter, where the analysis is simpler. Furthermore, quantum imaging with entangled light can achieve enhanced resolution, and quantum metrology can overcome the shot noise limit for classical light. This Account focuses on an entirely different emerging class of applications using quantum light as a powerful spectroscopic tool to reveal novel information about complex molecules. These applications utilize two appealing properties of quantum light: its distinct intensity fluctuations and its nonclassical bandwidth properties. These give rise to new and surprising behavior of nonlinear optical signals. Nonclassical intensity fluctuations can enhance nonlinear optical signals relative to linear absorption. For instance, the two-photon absorption of entangled photon pairs scales linearly (rather than quadratically) in the photon flux, just like a single photon absorption. This enables nonlinear quantum spectroscopy of photosensitive, for example, biological, samples at low light intensities. We will discuss how the two-photon absorption cross section becomes a function of the photonic quantum state, which can be manipulated by properties of the entangled photon pairs. In addition, the quantum correlations in entangled photon states further influence the nonlinear signals in a variety of ways. Apart from affecting the signal's scaling with intensity, they also constitute an entirely new approach to shaping and controlling excitation pathways in molecular aggregates in a way that cannot be achieved with shaped classical pulses. This is because between the two absorption events in entangled two-photon absorption, the light and material system are entangled. Classical constraints for the simultaneous time and frequency resolution can thus be circumvented, since the two are not Fourier conjugates. Here we review the simplest manifestation of quantum light spectroscopy, two-photon absorption spectroscopy with entangled photons. This will allow us to discuss exemplarily the impact of quantum properties of light on a nonlinear optical signal and explore the opportunities for future applications.
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 [Phys. Rev. X 8, 021017; 2018]. 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 and probabilistic two-photon absorption. Across a range of input phases and circuit configurations, we observe nonclassical effects such as anti-coalescence and bunching, along 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, the latter 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 tools for programmable quantum state engineering, filtering, multiplexed sensing, and nonunitary quantum simulation.
Abstract Two-photon excitation spectroscopy is a nonlinear technique that has gained rapidly in interest and significance for studying the complex energy-level structure and transition probabilities of materials. While the conventional spectroscopy based on tunable classical light has been long established, quantum light provides an alternative way towards excitation spectroscopy with potential advantages in temporal and spectral resolution, as well as reduced photon fluxes. By using a quantum Fourier transform that connects the sum-frequency intensity and N00N-state temporal interference, we present an approach for quantum interferometric two-photon excitation spectroscopy. Our proposed protocol overcomes the difficulties of engineering two-photon joint spectral intensities and fine-tuned absorption-frequency selection. These results may significantly facilitate the use of quantum interferometric spectroscopy for extracting the information about the electronic structure of the two-photon excited-state manifold of atoms or molecules without any requirement for precise and complicated scanning in the spectral domain. This may be particularly relevant for photon-sensitive biological and chemical samples.
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.
We demonstrate the tunable quantum beat of single photons through the co-development of core nonlinear nanophotonic technologies for frequency-domain manipulation of quantum states in a common physical platform. Spontaneous four-wave mixing in a nonlinear resonator is used to produce non-degenerate, quantum-correlated photon pairs. One photon from each pair is then frequency shifted, without degradation of photon statistics, using four-wave mixing Bragg scattering in a second nonlinear resonator. Fine tuning of the applied frequency shift enables tunable quantum interference of the two photons as they are impinged on a beamsplitter, with an oscillating signature that depends on their frequency difference. Our work showcases the potential of nonlinear nanophotonic devices as a valuable resource for photonic quantum information science.