Richard Schmidt

Quantum Matter Theory

Max Planck Institute of Quantum Optics

Hans-Kopfermann-Str. 1

85748 Garching

Tel: + 49 89 329 05242

richard.schmidt[at]mpq.mpg.de

Group Webpage

I am fascinated by the universality of many of the physical phenomena. My group investigates how this underlying simplicity of physics allows new discoveries to be made in the controlled environment of ultracold quantum gases, and then to implement and utilize these phenomena in everyday materials such as semiconductors and other solids.

Description

Research focus: quantum matter theory, 2D materials, ultracold quantum gases, functional methods

The research focus of the group “Theory of Quantum Matter” lies at the intersection of theoretical solid state and atomic physics. We are particularly interested in systems that feature a strong interplay of few- and many-body physics with the aim to understand its significance for the dynamics, spectroscopic and transport properties of quantum matter realized in ultracold atomic gases and semiconducting materials.


From cold atoms to semiconductors

In order to make progress in understanding complex quantum matter, it is important to identify systems which allow not only to study physics from different perspectives, but which also highlight universal aspects of the underlying dynamics. Discovering such universal connections can provide a basis to establish new phenomena that universally appear in artificial cold atomic quantum system and actual solid state materials with the potential of quantum technological applications

In this spirit cold atomic quantum systems can serve as a platform for an applied quantum simulation of solid state materials and the goal to discover universal connections between both fields drives our research agenda. Remarkably, two-dimensional van-der Waals materials — with graphene being a famous example from this rapidly growing field of research — feature such a unique similarity to ultracold atomic quantum gases. Specifically, excitons interacting with electrons in two-dimensional semiconductor heterostructures realize Bose-Fermi mixtures that are closely related to those studied in ultracold atoms. Indeed, one example for the synergy of the two fields of van-der-Waals materials and ultracold atoms is the measurement of repulsive polarons in two-dimensional semiconductors [1] following our theoretical prediction [2,3] and their first observation in ultracold atoms [4,5], for a review on recent progress see [6] and [7]. Building on our expertise at the intersection of solid state and atomic physics, our research group focusses on studying and exploiting such universal connections between solid state and cold atomic physics to theoretically discover novel states of quantum matter and finding ways to actually realize those in experiments.


Functional methods

In our theoretical pursuit of this goal we rely on a wide set of theoretical few- and many-body methods (including quantum field theory, diagrammatics, functional renormalization group, time-dependent variational wave functions, functional determinants, and exact approaches to few-body problems). We aim to both improve these methods as well as to develop new theoretical tools that allow to deepen our understanding of universal aspects of few- to many-body dynamics in quantum matter.

Our group has many active collaborations with theorists and experimentalists in the United States, Austria, Denmark, Switzerland and throughout Germany, and we also work in close connection with other groups within the research community in the Greater Munich area.


References:

[1] M. Sidler et al., Nat. Phys. 13, 255 (2017).
[2] R. Schmidt, and T. Enss, Phys. Rev. A 83, 063620 (2011).
[3] R. Schmidt, T. Enss, V. Pietila, and E. Demler, Phys. Rev. A 85, 021602(R) (2012).
[4] C. Kohstall et al., Nature 485, 615 (2012).
[5] M. Koschorreck et al., Nature 485, 619 (2012).
[6] R. Schmidt, M. Knap, D. A. Ivanov, J.-S. You, M. Cetina and E. Demler, Rep. Prog. Phys. 81, 024401 (2018).
[7] M. Cetina, M. Jag, R. S. Lous, I. Fritsche, J. T. M. Walraven, R. Grimm, J. Levinsen, M. M. Parish, R. Schmidt, M. Knap, E. Demler, Science 354, 96 (2016).


Publications

Site-selectively generated photon emitters in monolayer MoS2 via local helium ion irradiation

J. Klein, M. Lorke, M. Florian, F. Sigger, J. Wierzbowski, J. Cerne, K. Müller, T. Taniguchi, K. Watanabe, U. Wurstbauer, M. Kaniber, M. Knap, R. Schmidt, J. Finley, A. Holleitner.

Nature Communications 10, Article number: 2755 (2019).

Show Abstract

Quantum light sources in solid-state systems are of major interest as a basic ingredient for integrated quantum photonic technologies. The ability to tailor quantum emitters via site-selective defect engineering is essential for realizing scalable architectures. However, a major difficulty is that defects need to be controllably positioned within the material. Here, we overcome this challenge by controllably irradiating monolayer MoS2 using a sub-nm focused helium ion beam to deterministically create defects. Subsequent encapsulation of the ion exposed MoS2 flake with high-quality hBN reveals spectrally narrow emission lines that produce photons in the visible spectral range. Based on ab-initio calculations we interpret these emission lines as stemming from the recombination of highly localized electron–hole complexes at defect states generated by the local helium ion exposure. Our approach to deterministically write optically active defect states in a single transition metal dichalcogenide layer provides a platform for realizing exotic many-body systems, including coupled single-photon sources and interacting exciton lattices that may allow the exploration of Hubbard physics.

DOI: 0.1038/s41467-019-10632-z

Atomtronics with a spin: Statistics of spin transport and nonequilibrium orthogonality catastrophe in cold quantum gases

J. S. You, R. Schmidt, D. A. Ivanov, M. Knap, and E. Demler.

Physical Review B 99, 214505 (2019).

Show Abstract

We propose to investigate the full counting statistics of nonequilibrium spin transport with an ultracold atomic quantum gas. The setup makes use of the spin control available in atomic systems to generate spin transport induced by an impurity atom immersed in a spin-imbalanced two-component Fermi gas. In contrast to solid-state realizations, in ultracold atoms spin relaxation and the decoherence from external sources is largely suppressed. As a consequence, once the spin current is turned off by manipulating the internal spin degrees of freedom of the Fermi system, the nonequilibrium spin population remains constant. Thus one can directly count the number of spins in each reservoir to investigate the full counting statistics of spin flips, which is notoriously challenging in solid-state devices. Moreover, using Ramsey interferometry, the dynamical impurity response can be measured. Since the impurity interacts with a many-body environment that is out of equilibrium, our setup provides a way to realize the nonequilibrium orthogonality catastrophe. Here, even for spin reservoirs initially prepared in a zero-temperature state, the Ramsey response exhibits an exponential decay, which is in contrast to the conventional power-law decay of Anderson's orthogonality catastrophe. By mapping our system to a multistep Fermi sea, we are able to derive analytical expressions for the impurity response at late times. This allows us to reveal an intimate connection of the decay rate of the Ramsey contrast and the full counting statistics of spin flips.

DOI: 10.1103/PhysRevB.99.214505

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