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Physica Scripta

Deterministic conversion of a thermal state to a quasi-Fock state and the dynamics of number state-multi-atom interactions

Kiyoung Cho, CHULMIN CHOI, Hoonsoo Kang

Published Jun 2, 2026• 0 citations
Publisher

Abstract

Abstract The Fock state is widely regarded as the most nonclassical quantum state of light and has great potential in the areas of quantum computation and quantum sensing. Numerous methods have been proposed to generate Fock states and extremely number-squeezed states, or quasi-Fock states. To utilize natural light for quantum technologies, it is necessary to develop methods for transforming classical light into quasi-Fock states. Thus, we demonstrated that through the collective interaction of light and atoms in the Tavis–Cummings model, we can amplify classical light, such as thermal states and coherent states, and transform those states into quasi-Fock states. To analyze this process, we introduce theoretical tools for studying number state–multi-atom interactions and provide a comprehensive analysis of Tavis–Cummings dynamics, rather than relying on extensions of the Jaynes–Cummings model. Our results reveal that the emergence of nearly sinusoidal emission-absorption oscillations, periodic oscillation of photon number variance, and intrinsic nonlinear effects collectively establishes the necessary condition for deterministic conversion of classical light into quasi-Fock states.

DOI
10.1088/1402-4896/ae766d

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Abstract The Fock state is widely regarded as the most nonclassical quantum state of light and has great potential in the areas of quantum computation and quantum sensing. Numerous methods have been proposed to generate Fock states and extremely number-squeezed states, or quasi-Fock states. To utilize natural light for quantum technologies, it is necessary to develop methods for transforming classical light into quasi-Fock states. Thus, we demonstrated that through the collective interaction of light and atoms in the Tavis–Cummings model, we can amplify classical light, such as thermal states and coherent states, and transform those states into quasi-Fock states. To analyze this process, we introduce theoretical tools for studying number state–multi-atom interactions and provide a comprehensive analysis of Tavis–Cummings dynamics, rather than relying on extensions of the Jaynes–Cummings model. Our results reveal that the emergence of nearly sinusoidal emission-absorption oscillations, periodic oscillation of photon number variance, and intrinsic nonlinear effects collectively establishes the necessary condition for deterministic conversion of classical light into quasi-Fock states.
Coupling an oscillator to a single two-level system is one of the most fundamental interactions in quantum physics. We report on a dynamical effect during which a thermal state of an oscillator is unconditionally transformed to a highly nonclassical state with negative Wigner function values by mere absorbtion by a single uncontrolled two-level system. This complements the traditional test of Rabi oscillations and it serves as a simply measurable witness that the process in question is highly nonclassical. The process is experimentally feasible with possible experimental implementation in a number of experimental platforms with intrinsic Jaynes-Cummings interaction and it has the potential of enabling deterministic generation of nonclassical quantum states.
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Understanding quantum system dynamics driven by nonclassical light pulses is challenging, particularly for general light states with large photon numbers. Here we introduce an efficient framework that makes this task tractable. By introducing a pulse-shaped P-representation, the exact quantum evolution is decomposed into a mixture of many independent quasi-classical branches, each governed by a standard master equation with a classical pulse which can be solved efficiently. As an illustration, for a two-level system interacting with an exponential pulse, we first find out the exact analytical solutions to the Bloch equations in each quasi-classical branch, and then by taking proper P-function average over all branches, the full system dynamics driven by nonclassical light pulses is analytically obtained. For the one-photon and two-photon cases, our method well reproduces the previous exact results either analytically or numerically. Crucially, our approach scales efficiently to more complex light states (Fock, thermal, squeezed vacuum states) with large photon numbers (N∼100N\sim 100N∼100). We further provide a clear physical interpretation how the system dynamics is influenced through the high-order optical coherence of the nonclassical pulses. This work provides a unified and computationally efficient route and a useful starting point to explore more complex quantum dynamics driven by nonclassical light in quantum optics and quantum information processing.
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The quantization of the electromagnetic field leads directly to the existence of quantum mechanical states, called Fock states, with an exact integer number of photons. Despite these fundamental states being long-understood, and despite their many potential applications, generating them is largely an open problem. For example, at optical frequencies, it is challenging to deterministically generate Fock states of order two and beyond. Here, we predict the existence of an effect in nonlinear optics, which enables the deterministic generation of large Fock states at arbitrary frequencies. The effect, which we call an n -photon bound state in the continuum, is one in which a photonic resonance (such as a cavity mode) becomes lossless when a precise number of photons n is inside the resonance. Based on analytical theory and numerical simulations, we show that these bound states enable a remarkable phenomenon in which a coherent state of light, when injected into a system supporting this bound state, can spontaneously evolve into a Fock state of a controllable photon number. This effect is also directly applicable for creating (highly) squeezed states of light, whose photon number fluctuations are (far) below the value expected from classical physics (i.e., shot noise). We suggest several examples of systems to experimentally realize the effects predicted here in nonlinear nanophotonic systems, showing examples of generating both optical Fock states with large n ( n > 10), as well as more macroscopic photonic states with very large squeezing, with over 90% less noise (10 dB) than the classical value associated with shot noise.
The ability to tailor photon-number statistics is central to the generation and control of nonclassical states of light. Here, we investigate how Fock-state filtering of squeezed coherent states can be used to engineer their photon statistics. We consider the selective removal of the m=0m=0m=0 and m=1m=1m=1 Fock components and show that this process can enhance the sub-Poissonian character of the field, depending on the coherent amplitude. We then analyze the evolution of these engineered states in a thermal environment. Although all states eventually relax toward thermal statistics, the signatures of the filtering process may persist over a significant part of the evolution, while the purity of the filtered states is rapidly degraded by incoherent population redistribution. Our results illustrate both the potential and the limitations of photon-number engineering in realistic, dissipative settings.