Quantum 2025 – wissenschaftliches Programm

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SYEC: Symposium Entanglement and Complexity – How “Complex” is Nature?

SYEC 1: Entanglement and Complexity – How “Complex” is Nature?

SYEC 1.3: Hauptvortrag

Donnerstag, 11. September 2025, 11:45–12:15, ZHG010

Complexity in thermodynamics from first principles — •Philippe Faist — Freie Universität Berlin, Berlin, Germany

Quantum complexity measures the difficulty of realizing a quantum process, such as preparing a state or implementing a unitary. I will present techniques based on first principles in quantum information theory to quantify operational physical properties of many-body systems that are undergoing complex dynamics, and review concepts of computational and complexity distinguishability of states. I will focus on the prototypical task of information erasure, or Landauer erasure, where an n-qubit memory is reset to the all-zero state. In this setting, the trade-off between the thermodynamic work and computational complexity required for erasure is determined by the complexity entropy. This measure more generally quantifies the entropy a system appears to have to an observer of limited complexity power and which is closely related to complexity distinguishability; it can also quantify long-time growth of complexity in random circuits. In a second part of my talk, I will introduce methods for identifying quantum channels that best model complex dynamics with partial information, going beyond the study of its final equilibrium state. To this end, we extend to channels several fundamental principles that define the thermal state, including a maximum entropy principle and a microcanonical argument; I will discuss the potential relevance of these methods to channel learning, going beyond thermodynamics and complex many-body systems.

Keywords: quantum complexity; many-body systems; circuit complexity; thermodynamics; quantum processes