# Real-time density-matrix coupled-cluster approach for closed and open systems at finite temperature

## TL;DR

This paper introduces a real-time density-matrix coupled-cluster method using thermo-field formalism for simulating quantum dynamics of closed and open systems at finite temperature, ensuring trace-preservation and computational simplicity.

## Contribution

It extends coupled-cluster theory to finite-temperature quantum dynamics with an exponential ansatz and Keldysh contour, maintaining trace-preservation and simplifying calculations.

## Key findings

- Successfully captures correlated dynamics of the Anderson model
- Maintains exact trace-preservation during evolution
- Demonstrates effectiveness for both closed and open systems

## Abstract

We extend the coupled-cluster method to correlated quantum dynamics of both closed and open systems at finite temperatures using the thermo-field formalism. The approach expresses the time-dependent density matrix in an exponential ansatz and describes time-evolution along the Keldysh path contour. A distinct advantage of the approach is exact trace-preservation as a function of time, ensuring conservation of probability and particle number. Furthermore, the method avoids the computation of correlated bra-states, simplifying the computational implementation. We develop the method in a thermal quasi-particle representation, which allows seamless connection to the projection method and diagrammatic techniques of the traditional coupled-cluster formalism. For comparison, we also apply the thermo-field framework to renormalization-group methods to obtain reference results for closed and open systems at finite temperatures. We test the singles and doubles approximation to the density-matrix coupled-cluster method on the correlated electronic dynamics of the single-impurity Anderson model, demonstrating that the new method successfully captures the correlated dynamics of both closed systems at finite temperature and driven-dissipative open systems. This encouraging performance motivates future applications to non-equilibrium quantum many-body dynamics in realistic systems.