Universität zu Köln

Müller, Thomas Martin ORCID: 0000-0002-6924-6741 (2024). Phase transitions in monitored fermions. PhD thesis, Universität zu Köln.

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Abstract

The recently developed possibility to implement non-destructive mid circuit measurements in synthetic quantum matter provides a new angle to understand quantum manybody dynamics. Measurements can now be seen not only as a way to extract information about a system, but also as a dynamical resource. This sparks a lot of interest in studying the interplay of the three generators of dynamics of quantum states, unitaries, dissipation and measurements, in many-body systems. Historically first, random dynamics of this type in absence of conservation laws have been studied, giving rise to so called measurement-induced phase transitions. Based on this, we study the effects of symmetries in monitored dynamics, focusing on the paradigmatic model of spinless fermions in one dimension. We develop a systematic field theoretical description of the relevant degrees of freedom of this model. In particular, the entanglement entropy is studied both using this framework and numerical simulations. We find, that this quantity qualitatively changes its behavior at a finite critical measurement strength, from logarithmic growth with system size for small measurement strength, to area law for strong measurement. The transition is confirmed to be of Berezinskii-Kosterlitz-Thouless universality. To explore the scope of applicability of the developed replica field theory method, we also extend this model including long-range interactions, giving rise to a third phase with algebraic sub-volume entanglement scaling. A even more recent development is the experimental realization of adaptive quantum circuits. While this is required on the path towards efficient quantum error correction and therefore fault tolerant quantum computing, it also provides yet another new type of quantum dynamics. We therefore study measurement-based active feedback in the light of non-equilibrium universality. This gives rise to a type of absorbing state phase transitions with a strong notion of quantum physics. This is seen in the fact that an entanglement- and an absorbing state phase transition happen at the same critical measurement rate. Besides being interesting for a broader understanding of non-equilibrium criticality, we argue that the correspondence of measurement-induced phase transitions and quantum absorbing state phase transitions provides a way to overcome the post-selection problem.

Item Type:	Thesis (PhD thesis)
Creators:	Creators Email ORCID ORCID Put Code Müller, Thomas Martin thomueller@hotmail.de orcid.org/0000-0002-6924-6741 UNSPECIFIED
URN:	urn:nbn:de:hbz:38-734440
Date:	2024
Language:	English
Faculty:	Faculty of Mathematics and Natural Sciences
Divisions:	Faculty of Mathematics and Natural Sciences > Department of Physics > Institute for Theoretical Physics
Subjects:	Physics
Uncontrolled Keywords:	Keywords Language Measurement-induced phase transition UNSPECIFIED Non-equilibrium quantum physics UNSPECIFIED
Date of oral exam:	10 July 2024
Referee:	Name Academic Title Diehl, Sebastian Prof. Dr. Rizzi, Matteo Prof. Dr. Egger, Reinhold Prof. Dr.
Refereed:	Yes
URI:	http://kups.ub.uni-koeln.de/id/eprint/73444