Publications
In principle you can find all my publications on
arXiv. Moreover, my orcidID is
0000000314779855. Below all publications are listed in chronological order (beginning with the most recent one) together with their respective abstracts.
Preprints
 Quantum field simulator for dynamics in curved spacetime with Celia Viermann, Marius Sparn, Nikolas Liebster, Maurus Hans, Elinor Kath, Álvaro ParraLópez, Mireia TolosaSimeón, Natalia SánchezKuntz, Helmut Strobel, Stefan Flörchinger and Markus K. Oberthaler.
The observed largescale structure in our Universe is seen as a result of quantum fluctuations amplified by spacetime evolution. This, and related problems in cosmology, asks for an understanding of the quantum fields of the standard model and dark matter in curved spacetime. Even the reduced problem of a scalar quantum field in an explicitly timedependent spacetime metric is a theoretical challenge and thus a quantum field simulator can lead to new insights. Here, we demonstrate such a quantum field simulator in a twodimensional BoseEinstein condensate with a configurable trap and adjustable interaction strength to implement this model system. We explicitly show the realisation of spacetimes with positive and negative spatial curvature by wave packet propagation and confirm particle pair production in controlled powerlaw expansion of space. We find quantitative agreement with new analytical predictions for different curvatures in time and space. This benchmarks and thereby establishes a quantum field simulator of a new class. In the future, straightforward upgrades offer the possibility to enter new, so far unexplored, regimes that give further insight into relativistic quantum field dynamics.
Published Papers
 Curved and expanding spacetime geometries in BoseEinstein condensates with Mireia TolosaSimeón, Álvaro ParraLópez, Natalia SánchezKuntz, Celia Viermann, Marius Sparn, Nikolas Liebster, Maurus Hans, Elinor Kath, Helmut Strobel, Markus K. Oberthaler and Stefan Flörchinger.
Phonons have the characteristic linear dispersion relation of massless relativistic particles. They arise as low energy excitations of BoseEinstein condensates and, in nonhomogeneous situations, are governed by a space and timedependent acoustic metric. We discuss how this metric can be experimentally designed to realize curved spacetime geometries, in particular, expanding FriedmannLemaîtreRobertsonWalker cosmologies, with negative, vanishing, or positive spatial curvature. A nonvanishing Hubble rate can be obtained through a timedependent scattering length of the background condensate. For relativistic quantum fields this leads to the phenomenon of particle production, which we describe in detail. We explain how particle production and other interesting features of quantum field theory in curved spacetime can be tested in terms of experimentally accessible correlation functions.
 Scalar quantum fields in cosmologies with 2+1 spacetime dimensions with Natalia SánchezKuntz, Álvaro ParraLópez, Mireia TolosaSimeón and Stefan Flörchinger.
Motivated by the possibility to use BoseEinstein condensates as quantum simulators for spacetime curvature, we study a massless relativistic scalar quantum field in spatially curved FriedmannLemaîtreRobertsonWalker universes with d=2+1 spacetime dimensions. In particular, we investigate particle production caused by a timedependent background geometry, by means of the spectrum of fluctuations and several twopoint field correlation functions. We derive new analytical results for several expansion scenarios.
 Relative entropic uncertainty relation for scalar quantum fields with Markus Schröfl and Stefan Flörchinger.
Entropic uncertainty is a wellknown concept to formulate uncertainty relations for continuous variable quantum systems with finitely many degrees of freedom. Typically, the bounds of such relations scale with the number of oscillator modes, preventing a straightforward generalization to quantum field theories. In this work, we overcome this difficulty by introducing the notion of a functional relative entropy and show that it has a meaningful field theory limit. We present the first entropic uncertainty relation for a scalar quantum field theory and exemplify its behavior by considering few particle excitations and the thermal state. Also, we show that the relation implies the multidimensional Heisenberg uncertainty relation.
 Entropic entanglement criteria in phase space with Oliver Stockdale, Martin Gärttner and Stefan Flörchinger.
We derive entropic inseparability criteria for the phasespace representation of quantum states. In contrast to criteria involving differential entropies of marginal phasespace distributions, our criteria are based on a joint distribution known as the Husimi Q distribution. This distribution is experimentally accessible in cold atoms, circuit QED architectures, and photonic systems, and bears practical advantages compared to the detection of marginals. We exemplify the strengths of our entropic approach by considering several classes of nonGaussian states where secondorder criteria fail. We show that our criteria certify entanglement in previously undetectable regions, highlighting the strength of using the Husimi Q distribution for entanglement detection.

Wehrl entropy, entropic uncertainty relations and entanglement witnessing with Henrik MüllerGroeling and Stefan Flörchinger.
The Wehrl entropy is an entropy associated to the Husimi quasiprobability distribution. We discuss how it can be used to formulate entropic uncertainty relations and for a quantification of entanglement in continuous variables. We show that the WehrlLieb inequality is tighter than the usual BiałynickiBirula and Mycielski entropic uncertainty relation almost everywhere. Furthermore, we show how a Wehrl mutual information can be used to obtain a measurable perfect witness for pure state bipartite entanglement, which additionally provides a lower bound on the entanglement entropy.

Relative entropic uncertainty relation with Ben Höber and Stefan Flörchinger.
Quantum uncertainty relations are formulated in terms of relative entropy between distributions of measurement outcomes and suitable reference distributions with maximum entropy. This type of entropic uncertainty relation can be applied directly to observables with either discrete or continuous spectra. We find that a sum of relative entropies is bounded from above in a nontrivial way, which we illustrate with some examples.

Thermodynamics from relative entropy with Stefan Flörchinger.
Thermodynamics is usually developed starting from entropy and the maximum entropy principle. We investigate here to what extent one can replace entropy with relative entropy which has several advantages, for example in the context of local quantum field theory. We find that the principle of maximum entropy can be replaced by a principle of minimum expected relative entropy. Various ensembles and their thermodynamic potentials can be defined through relative entropy. We also show that thermal fluctuations are in fact governed by a relative entropy. Furthermore we reformulate the third law of thermodynamics using relative entropy only.
 Second law of thermodynamics for relativistic fluids formulated with relative entropy with Neil Dowling and Stefan Flörchinger.
The second law of thermodynamics is discussed and reformulated from a quantum information theoretic perspective for open quantum systems using relative entropy. Specifically, the relative entropy of a quantum state with respect to equilibrium states is considered and its monotonicity property with respect to an open quantum system evolution is used to obtain second lawlike inequalities. We discuss this first for generic quantum systems in contact with a thermal bath and subsequently turn to a formulation suitable for the description of local dynamics in a relativistic quantum field theory. A local version of the second law similar to the one used in relativistic fluid dynamics can be formulated with relative entropy or even relative entanglement entropy in a spacetime region bounded by two light cones. We also give an outlook toward isolated quantum field theories and discuss the role of entanglement for relativistic fluid dynamics.