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Physica A Statistical Mechanics and its Applications

A theorem on bath-system correlation functions

Federico Casagrande, M. Milani

Published May 1, 1977• 1 citation
Publisher
DOI
10.1016/0378-4371(77)90027-9

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The system-bath entanglement theorem (SBET) was established in terms of linear response functions [Du et al., J. Chem. Phys. 152, 034102 (2020)] and generalized to correlation functions [Su et al., J. Chem. Phys. 160, 084104 (2024)] in our previous studies. This theorem connects the entangled system-bath properties to the local system and bare-bath ones. In this work, we extend the SBET to field-dressed conditions with multiple baths at different temperatures. As in reality, the external fields may interact with not only the system but also environments. The extended SBET facilitates, for example, photo-acoustic, photo-thermal, pump-probe related studies. The theorem under the field-free condition (multiple baths) and its counterpart in the classical limit is also presented.
The system-bath entanglement theorem (SBET) was established in terms of linear response functions [J. Chem. Phys. 152, 034102 (2020)] and generalized to correlation functions [arXiv: 2312.13618 (2023)] in our previous works. This theorem connects the entangled system-bath properties to the local system and bare bath ones. In this work, firstly we extend the SBET to field-dressed conditions with multiple bosonic Gaussian environments at different temperatures. Not only the system but also environments are considered to be of optical polarizability, as in reality. With the aid of the extended SBET developed here, for the evaluation of the nonlinear spectroscopy such as the pump-probe, the entangled system-bath contributions can be obtained upon reduced system evolutions via certain quantum dissipative methods. The extended SBET in the field-free condition and its counterpart in the classical limit is also presented. The SBET for fermionic environments is elaborated within the transport scenarios for completeness.
For open quantum systems, the Gaussian environmental dissipative effect can be represented by statistical quasi-particles, namely, dissipatons. We exploit this fact to establish the dissipaton thermofield theory. The resulting generalized Langevin dynamics of absorptive and emissive thermofield operators are effectively noise-resolved. The system-bath entanglement theorem is then readily followed between an important class of nonequilibrium steady-state correlation functions. All these relations are validated numerically. A simple corollary is the transport current expression, which exactly recovers the result obtained from the nonequilibrium Green's function formalism.
While the quantum regression theorem (QRT) is the standard tool for computing multi-time correlation functions in open quantum systems, it relies on system-bath separability and an environment that remains in equilibrium, assumptions that are violated once dynamical correlations develop. Using the projection operator formalism, we derive an extension to the QRT that explicitly incorporates these correlation-induced corrections, enabling us to capture multi-time correlation functions in non-Markovian regimes. We apply this framework to the variational polaron master equation for the spin-boson model in ohmic and super-ohmic regimes, where the polaron transformation mixes system-bath degrees of freedom to produce a non-thermal effective environment. Benchmarking against numerically exact tensor-network simulations, we demonstrate quantitative agreement for single- and two-time observables, including linear-response spectra, even at strong coupling. Our approach broadens the reach of analytic master equations to multi-time observables in strong-coupling and non-Markovian regimes, where environmental memory effects and system-bath correlations are crucial.