We quantify the quantum-to-classical transition of the single-mode Kerr nonlinear dynamics in the presence of loss. We establish three time scales that govern the dynamics, each with distinct characteristics. For times short compared to the Ehrenfest time, the evolution is classical, characterized by Gaussian dynamics. For sufficiently long times, as we increase the initial photon number, unitary Kerr evolution would generate macroscopic superpositions of coherent states (so-called kitten states), but this is severely restricted in the presence of small photon loss so that expectation values of observables coincide with their classical values. The intermediate time scale, however, shows resilient quantum behavior in the macroscopic limit. We show that in the mean-field non-Gaussian regime, the Kerr Hamiltonian (with small photon loss) generates a significant amount of Wigner-negativity, and classical flow is recovered only if the loss rate grows with system size. Our results broaden the usual understanding of quantum-to-classical transitions and demonstrate the potential for creating robust nonclassical resources for continuous-variable quantum information processing in the presence of loss.
We quantify the quantum-to-classical transition of the single-mode Kerr nonlinear dynamics in the presence of loss. We establish three time scales that govern the dynamics, each with distinct characteristics. For times short compared to the Ehrenfest time, the evolution is classical, characterized by Gaussian dynamics. For sufficiently long times, as we increase the initial photon number, unitary Kerr evolution would generate macroscopic superpositions of coherent states (so-called kitten states), but this is severely restricted in the presence of small photon loss so that expectation values of observables coincide with their classical values. The intermediate time scale, however, shows resilient quantum behavior in the macroscopic limit. We show that in the mean-field non-Gaussian regime, the Kerr Hamiltonian (with small photon loss) generates a significant amount of Wigner-negativity, and classical flow is recovered only if the loss rate grows with system size. Our results broaden the usual understanding of quantum-to-classical transitions and demonstrate the potential for creating robust nonclassical resources for continuous-variable quantum information processing in the presence of loss.
Driven-dissipative Kerr cavities serve as a versatile platform for generating nonclassical states, such as quantum cat states and high-photon-number Fock states, which are essential for quantum information processing. Building on the quantized Lugiato-Lefever equation (QLLE), which predicts steady-state coherent state superpositions in multi-mode Kerr resonators, and the framework for engineering Kerr nonlinearity ratios to mitigate spectral crowding in coupled oscillators, we propose an integrated approach for precise quantum control. By engineering the Kerr ratio K1/K2 to approximate an incommensurate value using complex rational approximations, we eliminate parasitic degeneracies and enable selective addressing of transitions in the QLLE steady state. Employing a Magnus expansion, we derive an effective Hamiltonian that incorporates Stark-shift corrections, facilitating deterministic synthesis of entangled cat states and Fock states with fidelities exceeding 99.9%. Numerical simulations using QuTiP validate robustness against dissipation and thermal noise, demonstrating quantum hysteresis and Wigner negativity in multi-mode configurations. This synthesis bridges dissipative phase transitions with architectural control, offering a blueprint for scalable bosonic quantum processors in circuit QED. The Wigner function plays a pivotal role in characterizing these nonclassical states, providing a phase-space representation that reveals quantum interference effects. In our framework, the emergence of negative values in the Wigner function for the central mode confirms the non-Gaussian nature of the cat states, as dictated by Hudson’s theorem, which states that only Gaussian pure states have non-negative Wigner functions everywhere [21]. This negativity arises from the interference term in the cat-state Wigner expression, quantifying macroscopic quantum coherence and serving as a witness for entanglement in multi-mode systems.For instance, our simulations at F = 2.0 yield a minimum Wigner value of approximately -0.006, highlighting the system’s departure from classical behavior and its utility for bosonic error correction, where cat codes leverage phase-space separation to suppress bit-flip errors exponentially with cat size. Furthermore, the relation to Wigner functions extends to dynamical properties: the steady-state Wigner negativity is robust against thermal noise due to the dissipative stabilization mechanism, aligning with results from driven Bose-Hubbard models where two-mode cat states maintain negativity under loss [46]. In the QLLE context, the multi-mode extension introduces spatial localization, where the Wigner function’s negative regions correspond to soliton-like structures, as evidenced by photon population distributions [0.33, 0.90, 0.33]. This negativity is not merely a feature but a consequence of fundamental limits on classical simulability, relating to the Gottesman-Knill theorem’s extension to continuous variables, where non-Gaussian operations are necessary for universal quantum computation. Quantum hysteresis, observed in our hysteresis plots, is intimately linked to the Wigner representation through the bifurcation dynamics. Near the codimension-2 point, the system’s bistability manifests as distinct paths in phase space, with Wigner functions switching between Gaussian-like (low-photon) and cat-like (high-photon) forms, supported by spectral theory of Liouvillians that guarantees metastable states in dissipative phase transitions [30]. The forward and backward sweeps in our simulations demonstrate this path dependence, with the Wigner negativity peaking in the high-drive regime, underscoring the role of Kerr nonlinearity in engineering quantum resources. Overall, the Wigner function’s negativity serves as a diagnostic tool for the efficacy of our engineered ratios, directly tying to experimental verifiability in circuit QED setups, where homodyne measurements can reconstruct Wigner distributions to confirm cat-state fidelity. This work not only advances control protocols but also highlights the profound interplay between phase-space quasi-probabilities and dissipative quantum engineering, paving the way for fault-tolerant bosonic qubits.
Non-Gaussian states of light are a critical resource for fault-tolerant quantum computing and enhanced metrology, but are typically faint and often obtained via post-selection. Here, we demonstrate the deterministic generation of a bright non-Gaussian state by introducing a Kerr nonlinearity to a macroscopic state of light called bright squeezed vacuum (BSV). To characterize the resulting state, we use a single-shot f-2f interferometer to sample its Husimi function. We observe a clear transformation from a 2D Gaussian distribution to an 'S'-shaped non-Gaussian profile, which is the direct statistical evidence of the intensity-dependent nonlinear phase. The negativity of the Wigner function, which is an intrinsic property of any pure non-Gaussian state, cannot be observed because BSV is a mixed state even under minute optical loss. However, we show that BSV can be considered as a mixture of pure squeezed coherent states, for some of which Kerr-induced Wigner-function negativity is quite tolerant to loss. This work bridges the gap between quantum optics and ultrafast nonlinear optics, opening a path to quantum applications that require high photon flux.
Abstract When radiation fields propagate through mediums and interact with environments, the quantum characteristics of the field states usually undergo deterioration, in particular, the nonclassicality (quantumness) often decays due to photon loss. To address this issue in a quantitative way, we invoke a nonclassicality quantifier based on the Wigner-Yanase skew information and explore the dynamics of nonclassicality in the photon-loss channel for several typical initial states. We illustrate pictorially the decoherence and quantum to classical transition in the photon-loss channel from an informational perspective. It is remarkable that for certain initial states with small average photon numbers, the nonclassicality, as quantified in terms of the Wigner-Yanase skew information, may exhibit nonmonotonic behavior (revival), as compared with the general case of monotonic decaying of nonclassicality.
Characterizing macroscopic quantum states of light is a frontier challenge at the interface of quantum optics and high-intensity photonics. Here, we demonstrate the first, to our knowledge, direct phase-space tomography of a bright, ultrafast quantum state. We adapt a single-shot f-2f interferometer to sample the Husimi Q-function of a 25 fs bright squeezed vacuum (BSV) pulse (mean photon number N ∼10 12 ) subject to Kerr nonlinearity. Our measurement reveals a clear transformation from a Gaussian distribution with pronounced quadrature antisqueezing to a characteristic "S"-shaped profile as the intensity-dependent nonlinear phase increases. Crucially, we show theoretically that despite optical loss, the state does not degrade into classically modulated light but is rigorously described as a statistical mixture of squeezed coherent states. While global Wigner negativity is masked by technical noise, our results show theoretically that the constituent states individually retain strong Wigner-negative features. This work bridges quantum optics and high-intensity photonics, establishing a platform for diagnosing bright quantum resources.
It is generally difficult to study the dynamical properties of a quantum system with both inherent quantum noises and nonperturbative nonlinearity. Due to the possibly drastic intensity increase of an input coherent light in gain-loss waveguide couplers with parity-time (PT) symmetry, the Kerr effect from a nonlinearity added into the system can be greatly enhanced and is expected to create macroscopic entangled states of the output light fields with huge photon numbers. Meanwhile, quantum noises also coexist with the amplification and dissipation of the light fields. Under the interplay between the quantum noises and nonlinearity, the quantum dynamical behaviors of the systems become rather complicated. However, the important quantum noise effects have been mostly neglected in previous studies about nonlinear PT-symmetric systems. Here we present a solution to this nonperturbative quantum nonlinear problem, showing the real-time evolution of the system observables. The enhanced Kerr nonlinearity is found to give rise to a previously unknown decoherence effect that is irrelevant to the quantum noises and imposes a limit on the emergence of macroscopic nonclassicality. In contrast to what happens in linear systems, the quantum noises exert significant impact on the system dynamics and can create nonclassical light field states in conjunction with the enhanced Kerr nonlinearity. This study on the noise involved in quantum nonlinear dynamics of coupled gain-loss waveguides can help to better understand the quantum noise effects in many nonlinear systems.
In this paper, a quantitative investigation of the non-classical and quantum non-Gaussian characters of the photon-subtracted displaced Fock state [Formula: see text], where k is the number of photons subtracted and n is Fock parameter, is performed by using a collection of measures like Wigner logarithmic negativity, linear entropy potential, skew information-based measure, and relative entropy of quantum non-Gaussianity. It is noticed that the number of photons subtracted ( k) changes the nonclassicality and quantum non-Gaussianity in a significant amount in the regime of small values of the displacement parameter, whereas the Fock parameter ( n) presents a notable change in the large regime of the displacement parameter. In this respect, the role of the Fock parameter is found to be stronger as compared to the photon subtraction number. Finally, the Wigner function dynamics considering the effects of photon loss channel is used to show that the Wigner negativity can only be detected by highly efficient detectors.