Quantum Physics: Making Hidden Dynamics Visible with Complex Time

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Modern experiments increasingly exploit the interplay between quantum systems and their environment to create and control exotic many-body states. Yet the microscopic mechanisms underlying these phenomena remain largely hidden in strongly interacting systems. The reason is surprisingly fundamental: for open quantum systems, it is not sufficient to consider their energy levels alone. What matters is also how the system evolves together with its environment. This dynamics is described by a so-called Lindbladian. Its eigenmodes determine, among other things, how quickly different collective excitations of the system decay, making them particularly relevant for understanding its dynamical properties.

Existing methods for studying this interplay between a system and its environment beyond just a few quantum degrees of freedom have so far been largely limited to the stationary state and, in some cases, the dissipative gap. The latter describes how quickly an open quantum system returns to its stationary state after a perturbation. The physically interesting dynamical (excited) states, by contrast, have remained inaccessible because they are exponentially suppressed in existing methods.

The team led by LMU physicist Dr. Sebastian Paeckel has developed an approach that makes these hidden modes accessible and was recently published in Physical Review X. The crucial step is to extend the time evolution in a computer simulation into the realm of complex time, building on a long-established connection in physics between imaginary time and thermal properties. This makes it possible to selectively control which dynamical modes of the system contribute most strongly to the simulation. Continue