Open quantum systems often interact with complex, intractable quantum environments. Establishing a minimal solvable model that captures the essential effects of these environments on central quantum systems has long been of interest in physics, dating back to the origins of quantum mechanics. The Caldeira-Leggett oscillator bath model has become a prominent minimal model that effectively represents many quantum environments. However, it has been argued that this model cannot capture the main effects of spin environments on central two-level systems when the couplings between the spins and the central systems are strong. In contrast, I adopt an appropriate spectral density function for the Caldeira-Leggett model and demonstrate that it can effectively replicate the essential effects of spin environments in the regime of interest. This suggests that the class of the Caldeira-Leggett model is more universal than previously recognized.
Open quantum systems often interact with complex, intractable quantum environments. Establishing a minimal solvable model that captures the essential effects of these environments on central quantum systems has long been of interest in physics, dating back to the origins of quantum mechanics. The Caldeira-Leggett oscillator bath model has become a prominent minimal model that effectively represents many quantum environments. However, it has been argued that this model cannot capture the main effects of spin environments on central two-level systems when the couplings between the spins and the central systems are strong. In contrast, I adopt an appropriate spectral density function for the Caldeira-Leggett model and demonstrate that it can effectively replicate the essential effects of spin environments in the regime of interest. This suggests that the class of the Caldeira-Leggett model is more universal than previously recognized.
Open quantum systems are subject to interaction with their surrounding environment. In many applications, at low temperatures, quantum environments fall into two universality classes of models: Caldeira-Leggett oscillator bath models and Prokof'ev-Stamp spin bath models. The two classes are commonly recognized to be distinct and to have strikingly different effects on principal systems, except at weak coupling limits. However, I show here, in contrast, that oscillator bath models can simulate the effect of spin bath models in strong coupling limit of the spin bath. I choose parameters of the oscillator bath models such that they produce incoherent relaxation, just as in the spin bath models, with relaxation rates for a two-state system (a qubit) equivalent to those of the spin bath models in the strong coupling limit.
Quantum mechanics now underpins much of physics and chemistry. In recent years, a new field has emerged: quantum biology, highlighting the fundamental problem of explaining how molecular processes within living cells exhibit quantum behaviour in such warm, wet and complex cellular environments. Making use of the so-called Caldeira-Leggett model for open quantum systems, where the quantum system of interest is linearly coupled to its external environment, modelled as a heat bath of harmonics oscillators, we study the effect of environmental noise on quantum behaviour. We present a new and novel adaptation to the Caldeira-Leggett model through the introduction of dissipation to create a more realistically modelled environment.
The quantum dynamics of mesoscopic or macroscopic systems is always complicated by their coupling to many `environmental' modes. At low T these environmental effects are dominated by localized modes, such as nuclear and paramagnetic spins, and defects (which also dominate the entropy and specific heat). This environment, at low energies, maps onto a `spin bath' model. This contrasts with `oscillator bath' models (originated by Feynman and Vernon) which describe delocalized environmental modes such as electrons, phonons, photons, magnons, etc. The couplings to N spin bath modes are independent of N (rather than the ~O(1/( N ) 1/2 ) dependence typical of oscillator baths), and often strong. One cannot in general map a spin bath to an oscillator bath (or vice versa); they constitute distinct `universality classes' of quantum environment. We show how the mapping to spin bath models is made, and then discuss several examples in detail, including moving particles, magnetic solitons, nanomagnets, and SQUIDs, coupled to nuclear and paramagnetic spin environments. We then focus on the `central spin' model, which couples a central two-level system to a background spin bath. It is the spin bath analogue of the famous `spin-boson' oscillator model, and describes, e.g., the tunnelling dynamics of nanoscopic and mesoscopic magnets and superconductors. We show how to average over (or `integrate out') spin bath modes, using an operator instanton technique, to find the central spin dynamics. The formal manouevres involve four separate averages - each average corresponds physically to a different `decoherence' mechanism acting on the central spin dynamics. Each environmental spin has its own topological `spin phase', which by interacting with the phase of the central system, decoheres it - this can happen even without dissipation. We give analytic results for the central spin correlation functions, under various conditions. We then describe the application of this theory to magnetic and superconducting systems. Particular attention is given to recent work on tunnelling magnetic macromolecules, where the role of the nuclear spin bath in controlling the tunnelling is very clear; we also discuss other magnetic systems in the quantum regime, and the influence of nuclear and paramagnetic spins on flux dynamics in SQUIDs. Finally, we discuss decoherence mechanisms and coherence experiments in superconductors and magnets. We show that a spin bath environment causes decoherence even in the T 0 limit. Control of this decoherence will be essential in the effort to construct `qubits' for quantum computers.
We develop a complete theoretical framework for open quantum systems coupled to scale-invariant environments. We show that such environments are universally described by unparticle baths characterized by a single scaling dimension dU. This work provides the proof of the uniqueness theorem, the formalism of the resulting non-Markovian dynamics, and applications to several physical systems.
From the uniqueness theorem, we derive the non-Markovian memory kernels, the exact noise kernel including vacuum and thermal contributions, and a fractional generalization of the Caldeira-Leggett master equation for arbitrary dU. The scaling dimension governs a rich phase structure, including a thermalization transition at dU=3/2, the Ohmic boundary at dU=2, and a decoherence transition at dU=5/2 in the thermal regime, beyond which long-time quantum coherence is protected.
Three realizations are studied. For the quantum Ising model at criticality, coupling to the energy operator in (1+1) dimensions gives dU=3/2, producing 1/f noise, while the (2+1)D case yields dU≈1.413 from the conformal bootstrap. In inflationary cosmology, massless scalar and graviton baths in de Sitter spacetime give dU=2, predicting linear decoherence growth consistent with the quantum-to-classical transition. For high-energy astrophysical neutrinos, the decoherence rate Γdecoh∝B(E,TU)L5−2dU provides an observable signature of the scaling dimension.
We also compare the framework with Caldeira-Leggett and Lindblad approaches, analyze the validity regimes, and discuss experimental implications for trapped-ion simulators, neutrino telescopes, and superconducting qubits.
In this work, we investigate the non-Markovian features of the Adapted Caldeira-Leggett model, a computationally efficient framework recently proposed to capture the essential physics of the standard Caldeira-Leggett model. While this effective model has been previously validated for decoherence and einselection, its ability to reproduce memory effects remains to be explored. By exploiting the model's capability to explicitly track both system and environment degrees of freedom, we provide a detailed characterization of non-Markovianity through the lens of information backflow. We evaluate the buildup of system-environment correlations and the corresponding modifications of the environmental state, assessing a quantitative upper bound for the revival of distinguishability in the reduced dynamics. Our results, obtained by comparing different distinguishability quantifiers such as trace distance and the square root of the Jensen-Shannon divergence, show that while correlations are primarily sensitive to coupling strength, environmental state changes are more heavily influenced by temperature. Our analysis substantiates the physical interpretation of the distinguishability-based approach to non-Markovianity, and confirms this variant of the Caldeira-Leggett model as a reliable tool for exploring the microscopic origins of different fundamental phenomena in quantum mechanics.
We calculate the reduced density matrix for a system of coupled harmonic oscillators in a bosonic heat bath using the Born-Markov approximation and show that the expectation values of position and momentum evolve like classical quantities. We consider the cases where every oscillator is coupled to its own reservoir and where all oscillators are coupled to the same heat bath, pointing out the differences between these two models. In particular, for systems coupled to a common reservoir, we present an uncommon dissipation mechanism, which disappears, when the dynamics of the system are gouverned by the low frequency modes of the environment (Caldeira-Leggett limit). We also address the interesting phenomenon of reservoir-induced interactions.