Quantum Systems

We study the fundamental physics of solid-state based quantum systems and advance their fabrication technology to lay the basis for applications in quantum computing, quantum communication, and quantum sensing.
Recent projects
Frank Deppe, Rudolf Gross, Qi-Ming Chen, Kirill Fedorov, Florian Fesquet, Achim Marx, Yuki Nojiri, Michael Renger, Kedar Honasoge
Rudolf Gross, Stefan Filipp, Frank Deppe, Hans Huebl, Matthias Althammer, Qi-Ming Chen, Frank Deppe, Kirill Fedorov, Florian Fesquet, Kedar Honasoge, Achim Marx, Yuki Nojiri, Michael Renger, Nadezhda Kukharchyk, Stephan Geprägs, Thomas Luschmann, Ana Strinic
Rudolf Gross, Frank Deppe, Kirill Fedorov, Rudolf Gross, Hans Huebl, Achim Marx
Frank Deppe, Kirill Fedorov, Rudolf Gross, Nadezhda Kukharchyk, Achim Marx, Qi-Ming Chen, Florian Fesquet, Yuki Nojiri, Michael Renger
Rudolf Gross, Frank Deppe, Stefan Filipp, Rudolf Gross, Hans Huebl, Nadezhda Kukharchyk
Rudolf Gross, Frank Deppe, Stefan Filipp, Rudolf Gross, Hans Huebl, Nadezhda Kukharchyk
Rudolf Gross, Stefan Filipp
Recent publications
K. G. Fedorov, M. Renger, S. Pogorzalek, R. Di Candia, Q. Chen, Y. Nojiri, K. Inomata, Y. Nakamura, M. Partanen, A. Marx, R. Gross, F. Deppe
Research Article | arXiv:2103.04155
Qi-Ming Chen, Florian Fesquet, Kedar E. Honasoge, Fabian Kronowetter, Yuki Nojiri, Michael Renger, Kirill G. Fedorov, Achim Marx, Frank Deppe, Rudolf Gross
Research Article | arXiv:2107.01842
M. Renger, S. Pogorzalek, Q. Chen, Y. Nojiri, K. Inomata, Y. Nakamura, M. Partanen, A. Marx, R. Gross, F. Deppe, K. G. Fedorov
Research Article | npj Quantum Information 7, 160  (2021)
Preprint: arXiv:2011.00914
Jasmin Graf, Sanchar Sharma, Hans Huebl, and Silvia Viola Kusminskiy
Research Article | Physical Review Research 3, 013277  (2021)
Manuel Müller, Raphael Hoepfl, Lukas Liensberger, Stephan Geprägs, Hans Huebl, Mathias Weiler, Rudolf Gross, Matthias Althammer
Research Article | Materials for Quantum Technology 1, 045001  (2021)
Preprint: arXiv:2102.09018

Today we are witnessing a scientific and technological revolution, which has been enabled by the realization and manipulation of engineered quantum systems and their use in quantum information processing, communication and sensing. This development has led to the novel research field of Quantum Science and Technology (QST), which advances the understanding of fundamental principles of quantum systems and explores their practical applications.

Quantum systems based on engineered solid-state systems and circuits have attracted enormous interest because they provide one of the most promising hardware platforms for quantum computers, quantum communication systems, and applications in quantum sensing. We study the fundamental properties of various solid-state systems based on superconducting circuits, spin systems, nano-mechanical systems as well as hybrid systems thereof. We also use them for the realization of quantum bits and circuits. Key topics addressed by our research are decoherence mechanisms, the realization of strong and ultra-strong coupling between quantized solid-state excitations and microwave photons (e.g. magnon-photon, magnon-phonon, phonon-photon coupling), as well as the optimization of the manipulation, control and readout of solid-state quantum bits. Highlights of our research have been the first demonstration of ultra-strong coupling in superconducting circuit QED, the first demonstration of strong magnon-photon coupling, or the observation of echo trains in pulsed electron spin resonance of a strongly coupled spin ensemble.

For key quantum technology platforms (e.g. superconducting circuits, NV centers, quantum dots), microwaves intrinsically allow for zero frequency conversion loss, since they are the natural frequency scale. Therefore, our research aims at developing novel components, experimental techniques, and theory models building on the quantum properties of continuous-variable propagating microwaves. Our long-term visions include distributed quantum computing & communication via microwave quantum local area networks (QLANs) as well as sensing applications based on the illumination of an object with quantum microwaves (quantum radar). To this end, we already developed the dual path method for state tomography of propagating quantum microwaves and demonstrated path entanglement, one- and two-mode squeezing, the displacement operation or the implementation of the remote state preparation protocol.

Subtopics
Superconducting Quantum Circuits
In superconducting circuits, the superconducting condensate can be described by a quantum mechanical wave function with a single amplitude and phase. As a consequence, superconducting circuits can be engineered in a way to behave as macroscopic artificial atoms or quantum harmonic oscillators. For this reasons, they are called superconducting quantum circuits.

We study the foundations and applications of superconducting quantum circuits. The latter include the astonishing demonstration of textbook quantum mechanics as well as quantum information processing (QIP) and quantum simulation. Our research does not only address the foundations of quantum information systems and superconducting quantum technology, but also key fundamental questions regarding quantum coherence, quantum dynamics, and decoherence mechanisms in solid state quantum systems. Furthermore, it requires extremely sensitive measurements at millikelvin temperatures.

Recent projects
Frank Deppe, Rudolf Gross, Qi-Ming Chen, Kirill Fedorov, Florian Fesquet, Achim Marx, Yuki Nojiri, Michael Renger, Kedar Honasoge
Rudolf Gross, Stefan Filipp, Frank Deppe, Hans Huebl, Matthias Althammer, Qi-Ming Chen, Frank Deppe, Kirill Fedorov, Florian Fesquet, Kedar Honasoge, Achim Marx, Yuki Nojiri, Michael Renger, Nadezhda Kukharchyk, Stephan Geprägs, Thomas Luschmann, Ana Strinic
Frank Deppe, Kirill Fedorov, Rudolf Gross, Nadezhda Kukharchyk, Achim Marx, Qi-Ming Chen, Florian Fesquet, Yuki Nojiri, Michael Renger
Rudolf Gross, Stefan Filipp
Recent publications
Qi-Ming Chen, Florian Fesquet, Kedar E. Honasoge, Fabian Kronowetter, Yuki Nojiri, Michael Renger, Kirill G. Fedorov, Achim Marx, Frank Deppe, Rudolf Gross
Research Article | arXiv:2107.01842
M. Renger, S. Pogorzalek, Q. Chen, Y. Nojiri, K. Inomata, Y. Nakamura, M. Partanen, A. Marx, R. Gross, F. Deppe, K. G. Fedorov
Research Article | npj Quantum Information 7, 160  (2021)
Preprint: arXiv:2011.00914
Philip Schmidt, Mohammad T. Amawi, Stefan Pogorzalek, Frank Deppe, Achim Marx, Rudolf Gross, Hans Huebl
Research Article | Communications Physics 3, 233  (2020)
Preprint: arXiv:1912.08731
Hybrid Systems
Hybrid systems aim to couple two properties to gain novel functionalities. They are the basis of transducers and are presently considered for various quantum applications. Our research focusses on achieving strong coupling between various subsystems, such as magnons and photons, magnons and phonons, photons and phonons, as well as coupled excitations of the same nature. The dynamics of the hybrid system can vastly differ from from the behavior of their constituents, which is highly interesting from fundamental science perspective and is of key importance for applications based on hybrid concepts.

At the Walther-Meissner-Institute we pioneered research in the direction of strong magnon-photon interaction, which is key to investigate magnons on the quantum level, but also gives deep insight in the hybridization of magonic and phonic states, an aspect which we have intensely researched by combining this hybrid system with electical readout techniques. 

Spin ensembles based on paramagnetic centers are complementary to their exchange coupled counterpart. Although, the coupling is less intense, they still can be operated in the strong coupling regime. Due to their extreme coherence times, these spin systems are discussed for quantum memory or quantum transduction applications. 

In addition, we study magnon-phonon hybrids based on acoustic resonators and magnetic thin films, as well as coupled nano-string resonator networks. 

Recent publications
Jasmin Graf, Sanchar Sharma, Hans Huebl, and Silvia Viola Kusminskiy
Research Article | Physical Review Research 3, 013277  (2021)
Manuel Müller, Raphael Hoepfl, Lukas Liensberger, Stephan Geprägs, Hans Huebl, Mathias Weiler, Rudolf Gross, Matthias Althammer
Research Article | Materials for Quantum Technology 1, 045001  (2021)
Preprint: arXiv:2102.09018
Stefan Weichselbaumer, Christoph W. Zollitsch, Martin S. Brandt, Rudolf Gross, Hans Huebl
Research Article | Physical Review Letters 125, 137701  (2020)
Nano-Electromechanics
The field of nano-electromechanics explores the interaction of excitations in an electrical circuit with a displacement of a nano-mechanical object. The resulting sensing concept allows to investigate quantum mechanical phenomena in the literal sense, while interaction itself can be harnessed to synthesize quantum states in the electrical and mechanical entity. We realize nano-electromechanical systems in the form of nano-strings coupled to superconducting quantum circuits. The displacement of the nano-string controls the resonance frequency of the superconducting microwave circuit, realizing this interaction. While this coupling concept is vastly utilized in sensing applications ranging from simple force sensors to gravitational wave detectors, the nature of the coupling also allows for controlling and preparing mechanical oscillation states down to the quantum regime.

At the Walther-Meissner-Institute, we explore two coupling stategies for the realisation of the nano-electromechanical interaction:

  1. a capacitive coupling scheme, where the displacement of the nano-string resonator changes the overall capacitance of the superconducting electronic circuit, and
  2. an inductive coupling scheme, where we utilize the tunable inductance of a superconducting interference device to realize this purpose.

With these integrated nano-electromechanical devices, we have demonstrated force sensitivities down to aN/sqrt(Hz) and coupling rates in the tens of kHz range. We utilize these platforms to investigate and understand the interaction itself, study the mechanical properties of the materials involved and realize literal quantum mechanical states.

Recent projects
Rudolf Gross, Stefan Filipp, Frank Deppe, Hans Huebl, Matthias Althammer, Qi-Ming Chen, Frank Deppe, Kirill Fedorov, Florian Fesquet, Kedar Honasoge, Achim Marx, Yuki Nojiri, Michael Renger, Nadezhda Kukharchyk, Stephan Geprägs, Thomas Luschmann, Ana Strinic
Recent publications
Philip Schmidt, Mohammad T. Amawi, Stefan Pogorzalek, Frank Deppe, Achim Marx, Rudolf Gross, Hans Huebl
Research Article | Communications Physics 3, 233  (2020)
Preprint: arXiv:1912.08731
Mehdi Abdi, Matthias Pernpeintner, Rudolf Gross, Hans Huebl, Michael J. Hartmann
Research Article | Physical Review Letters 114, 173602  (2015)
Xiaoqing Zhou, Fredrik Hocke, Albert Schliesser, Achim Marx, Hans Huebl, Rudolf Gross, Tobias J. Kippenberg
Research Article | Nature Physics 9, 179-184  (2013)
Quantum Materials
The properties of a large class of materials are determined by quantum effects. In such quantum materials, interesting novel phases emerge due to a subtle interplay between different microscopic degrees of freedom.

Like magnetism and superconductivity, those quantum phases are not only of high interest from the fundamental science perspective, but also have potential applications in quantum science and technology. We fabricate thin film and multilayer quantum materials, including magnetically ordered insulators (e.g. Y3Fe5O12 or α-Fe2O3), spin-orbit driven materials (e.g. Sr2IrO4), or Dzyaloshinskii-Moriya-active interfaces...

Recent projects
Rudolf Gross, Stefan Filipp, Frank Deppe, Hans Huebl, Matthias Althammer, Qi-Ming Chen, Frank Deppe, Kirill Fedorov, Florian Fesquet, Kedar Honasoge, Achim Marx, Yuki Nojiri, Michael Renger, Nadezhda Kukharchyk, Stephan Geprägs, Thomas Luschmann, Ana Strinic
Rudolf Gross, Stefan Filipp
Recent publications
Tobias Wimmer, Janine Gückelhorn, Sebastian Wimmer, Sergiy Mankovsky, Hubert Ebert, Matthias Opel, Stephan Geprägs, Rudolf Gross, Hans Huebl, Matthias Althammer
Research Article | Physical Review B 104, L140404  (2021)
Preprint: arXiv:2103.12697
Luis Flacke, Valentin Ahrens, Simon Mendisch, Lukas Körber, Tobias Böttcher, Elisabeth Meidinger, Misbah Yaqoob, Manuel Müller, Lukas Liensberger, Attila Kákay, Markus Becherer, Philipp Pirro, Matthias Althammer, Stephan Geprägs, Hans Huebl, Rudolf Gross, Mathias Weiler
Research Article | Physical Review B 104, L100417  (2021)
Preprint: arXiv:2102.11117
Kira Riedl, Elena Gati, David Zielke, Steffi Hartmann, Oleg M. Vyaselev, Nataliya D. Kushch, Harald O. Jeschke, Michael Lang, Roser Valentí, Mark V. Kartsovnik, Stephen M. Winter
Research Article | Physical Review Letters 127, 147204  (2021)
Preprint: arXiv:2106.02130