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Physica Scripta

Classical-nonclassical transition in cavity-field states induced by field-matter coupling

Anneswa Paul, Upendra Harbola

Published Aug 21, 2025• 1 citation
Publisher

Abstract

Abstract Electromagnetic field states in optical cavities can show classical-nonclassical transition due to interaction with matter in the cavity. We analyze non-classicality in cavity-field state using negativity in its phase-space (quasi) distribution. We show that induced coherences in the cavity-state always enhance the non-classical nature of the field. The presence of vacuum state, on the other hand, diminishes the non-classicality as it represents a classical state with zero negativity. For weak cavity-field, the effect of vacuum state increases as the photon-number fluctuations are increased. These results have direct applications in areas, such as quantum metrology and cavity-induced chemistry, where non-classicality of cavity-field states is known to be an important resource.

DOI
10.1088/1402-4896/adfe22

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Abstract Electromagnetic field states in optical cavities can show classical-nonclassical transition due to interaction with matter in the cavity. We analyze non-classicality in cavity-field state using negativity in its phase-space (quasi) distribution. We show that induced coherences in the cavity-state always enhance the non-classical nature of the field. The presence of vacuum state, on the other hand, diminishes the non-classicality as it represents a classical state with zero negativity. For weak cavity-field, the effect of vacuum state increases as the photon-number fluctuations are increased. These results have direct applications in areas, such as quantum metrology and cavity-induced chemistry, where non-classicality of cavity-field states is known to be an important resource.
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Abstract Observing non-classical properties of light is a long-standing interest to advance a wide range of quantum application from computing to metrology. Optical cavities are essential to generate and manipulate non-classical light. However, detecting changes in cavity properties induced by the quantum state remains a critical challenge in the optical domain due to the weak material nonlinearity, limiting our ability to observe quantum states generated in optical cavities. Here, we propose a framework for observing the dynamics of quantum states generated inside nonlinear optical cavities. We utilize symmetry-breaking to obtain high sensitivity to small perturbations introduced to the quantum state, resulting in an asymmetric equilibrium of a macroscopic observable. With a nonlinear response at the single photon level, our approach directly imprints the field distribution of the cavity quantum state onto the statistics of bistable cavity steady-states. We experimentally demonstrate our approach in a degenerate optical parametric oscillator, generating and reconstructing the quasi-probability distribution of different quantum states. As a validation, we reconstruct the Husimi Q function of the cavity squeezed vacuum state. In addition, we observe the evolution of the quantum vacuum state inside the cavity as it undergoes phase-sensitive amplification. By enabling generation and measurement of quantum states in a single nonlinear optical cavity, our method paves a way for studying exotic dynamics of quantum optical states in nonlinear driven-dissipative systems such as soliton generation and Kerr frequency combs.
In this paper, we investigate the dynamics of two coupled two-level systems (or qubits) that are resonantly interacting with a microwave cavity. We examine the effects of the intrinsic decoherence rate and the coupling between the two qubits on the non-classicality of different system partitions via quasi-probability functions. New definitions for the partial Q-function and its Wehrl entropy are used to investigate the information and the quantum coherence of the phase space. The amount of the quantum coherence and non-classicality can be appropriately tuned by suitably adopting the rates of the intrinsic-decoherence and the coupling between the two qubits. The intrinsic decoherence has a pronounced effect on the negativity and the positivity of the Wigner function. The coupling between the two qubits can control the negativity and positivity of the quasi-probability functions.
We propose a scheme for the generation of various nonclassical states for a cavity field with a driven atom. In the scheme the atom is sent through the cavity and driven by a classical field. The atom interacts dispersively with the cavity field, which induces photon-number dependent Stark shift. Only for a desired Fock state, the atomic transition is tuned in resonance with the classical field. Thus, the measurement of the atomic transition may directly collapse the cavity field onto the desired Fock state. For a two-mode cavity field, pair coherent states and SU(2) coherent states can be generated.
The creation of non-classical states of light is an interesting problem, that we solve sending atoms through an optical cavity. We show that it is possible to add or subtract many photons from a cavity field by interacting it resonantly with a two-level atom. The atom, after entangling with the field inside the cavity and exiting it, may be measured in one of the Schmidt states, producing a multiphoton process (in the sense that can add or annihilate more photons than a single transition allows), i.e. adding or subtracting several photons from the cavity field. By plotting the quadratures and the Husimi Q-function, we also show that the non-classical state produced by such measurements is a squeezed state.