
Superconducting Quantum Circuits
In superconducting circuits, all superelectrons 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, we call our circuits superconducting quantum circuits. The wide field of applications includes the astonishing demonstration of textbook quantum mechanics and quantum information processing (QIP) and quantum simulation. Our research does not only provide the foundations of quantum information systems and quantum technology, but also addresses key fundamental questions regarding quantum coherence, quantum dynamics and decoherence processes in solid state quantum systems. Furthermore, it requires extremely sensitive measurements at millikelvin temperatures.
The research activities at the WMI are focused on:
 Josephson junction based qubits.
Superconducting qubits are quantum twolevel systems, which consist of nanometersized Josephson junctions providing strong nonlinear elements and superconducting loops, with typical transition frequencies in the gigahertz regime. In superconducting flux qubits quantum mechanical superposition states of clockwise and counterclockwise circulating persistent currents are used for the realization of solid state qubits. We are steadily working on
 Optimizing the fabrication and measurement process of superconducting flux and transmon qubits
 Develop advanced qubit designs best suited for our experiments
 Realization of flux qubits with tunable tunnel splitting which offer more flexibility in circuit design
 The challenge is to engineer qubits with long enough coherence times to study quantum effects
Recent Publications:
Gradiometric flux qubits with tunable gap
M. J. Schwarz, J. Goetz, Z. Jiang, T. Niemczyk, F. Deppe, A. Marx, R. Gross, New Journal of Physics 15, 045001 (2013)
Fabrication technology of and symmetry breaking in superconducting quantum circuits
T. Niemczyk, F. Deppe, M. Mariantoni, E.P. Menzel, E. Hoffmann, G. Wild, L. Eggenstein, A. Marx, R. Gross, Supercond. Sci. Techn. 22, 034009 (2009)


 Superconducting resonators.
Superconducting resonators are boxes for microwave photons and behave as quantum harmonic oscillators. In contrast to the fermionic qubits they behave as bosons. Resonators are attractive systems to study quantum correlations. We work both on superconductung transmission line resonators which act as photon boxes on a chip and threedimensional (3D) superconducting cavites providing very high single photon lifetims. Here we focus on
 Microwave engineering of advanced single and multi resonator setups
 Control the decay rates
 Investigation of different coupling schemes
 Flux driven Josephson parametric amplifiers
Recent Publications:
Fast microwave beam splitters from superconducting resonators
M. Haeberlein, D. Zueco, P. Assum, T. Weißl, E. Hoffmann, B. Peropadre, J.J. Garc\iaRipoll, E. Solano, F. Deppe, A. Marx, R. Gross, arXiv:1302.0729 (2013)
Squeezing with a fluxdriven Josephson parametric amplifier
L. Zhong, E. P. Menzel, R. Di Candia, P. Eder, M. Ihmig, A. Baust, M. Haeberlein, E. Hoffmann, K. Inomata, T. Yamamoto, Y. Nakamura, E. Solano, F. Deppe, A. Marx, R. Gross, New Journal of Physics 15, 125013 (2013)
Networks of nonlinear superconducting transmission line resonators
M. Leib, F. Deppe, A. Marx, R. Gross, M. Hartmann, New Journal of Physics 14, 075024 (2012)


 Circuit quantum electrodynamics (cQED) in the strong and ultrastrong coupling regime: quantum optics on a chip.
Coupled qubitresonator systems are an ideal playground for investigating the interaction between light and matter and thus to perform quantum optics on a chip experiments. The coupling rates can be made larger than the decoherence rates of qubit and resonator (strong coupling regime). In particular for flux based circuits we have reached the new regime of ultrastrong coupling where the coupling rate becomes the dominating energy scale of the system. Main interests are
 Multiphoton physics
 Quantum state generation
 Josephson and kinetic inductance enhancement of the coupling
 Fascinating counter intuitive physics beyond the rotating wave approximation
Recent Publications:
Selection rules in a strongly coupled qubitresonator system
T. Niemczyk, F. Deppe, E. P. Menzel, M. J. Schwarz, H. Huebl, F. Hocke, M. Häberlein, M. Danner, E. Hoffmann, A. Baust, E. Solano, J. J. GarciaRipoll, A. Marx, R. Gross,
arXiv:1107.0810v1 (2011)
Circuit quantum electrodynamics in the ultrastrongcoupling regime
T. Niemczyk, F. Deppe, H. Huebl, E. P. Menzel, F. Hocke, M. J. Schwarz, J. J. GarciaRipoll, D. Zueco, T. Hümmer, E. Solano, A. Marx, R. Gross,
Nature Physics 6, 772776 (2010)
Twophoton Probe of the JaynesCummings Model and Controlled Symmetry Breaking in Circuit QED
F. Deppe, M. Mariantoni, E. Menzel, A. Marx, S. Saito, K. Kakuyanagi, H. Tanaka, T. Meno, K. Semba, H. Takayanagi, E. Solano, R. Gross,
Nature Physics 4, 686  691 (2008)


 Tunable coupling between microwave resonators.
To realize quantum gates and quantum information and simulation protocols, the coupling between the individual circuit elements needs to be tunable in situ. The coupling between two circuit QED building blocks can be mediated by additional coupling circuits. We work on tunable and switchable coupling between two superconducting transmission line resonators mediated by both superconducting flux qubits and RF SQUDS. We aim at
 Fabricating and characterizing tunable and switchable coupling
 Entanglement generation between two resonators
 Generation of interesting quantum states (Schrodinger cats, GHZ states)
Recent Publications:
Tunable and Switchable Coupling Between Two Superconducting Resonators
A. Baust, E. Hoffmann, M. Haeberlein, M. J. Schwarz, P. Eder, E. P. Menzel, K. Fedorov, J. Goetz, F. Wulschner, E. Xie, L. Zhong, F. Quijandria, B. Peropadre, D. Zueco, J.J. Garcia Ripoll, E. Solano, F. Deppe, A. Marx, R. Gross, arXiv:1405.1969 (2014)
Tunable coupling engineering between superconducting resonators: from sidebands to effective gauge fields
B. Peropadre, D. Zueco, F. Wulschner, F. Deppe, A. Marx, R. Gross, J.J. GarcíaRipoll, Phys. Rev. B 87, 134504 (2013)
Tworesonator circuit quantum electrodynamics: Dissipative theory
G. M. Reuther, D. Zueco, F. Deppe, E. Hoffmann, E. P. Menzel, T. Weißl, M. Mariantoni, S. Kohler, A. Marx, E. Solano, R. Gross, P. Hänggi, Phys. Rev. B 81, 144510 (2010)
Tworesonator Circuit Quantum Electrodynamics: A Superconducting Quantum Switch
M. Mariantoni, F. Deppe, E. Menzel, A. Marx, F.K. Wilhelm, R. Gross & E. Solano,
Phys. Rev. B 78, 104508 (2008)


 Propagating quantum microwaves.
In propagating quantum microwave photonics one aims at the the generation, control, detection, and interaction of quantum microwave beams using superconducting quantum circuits. Our goal is to merge the possibilities of alloptical quantum computing with superconducting circuits to perform useful tasks such as scalable quantum gates or quantum simulation. Our efforts employ superconducting quantum circuits to explore novel paths in engineering strong and ultrastrong controlled interactions between propagating microwave photons and their environment as well as among photons themselves. To this end, we aim at
 Tomography of arbitrary propagating quantum microwaves
 Manipulation of propagating quantum microwaves
 The development and understanding of microwave beam splitters with a similar degree of tunability
 Characterization of propagating squeezed microwave light
Recent Publications:
DualPath Methods for Propagating Quantum Microwaves
R. Di Candia, E. P. Menzel, L. Zhong, F. Deppe, A. Marx, R. Gross, E. Solano, New Journal of Physics 16, 015001 (2014)
Path Entanglement of ContinuousVariable Quantum Microwaves
E. P. Menzel, R. Di Candia, F. Deppe, P. Eder, L. Zhong, M. Ihmig, M. Haeberlein, A. Baust, E. Hoffmann, D. Ballester, K. Inomata, T. Yamamoto, Y. Nakamura, E. Solano, A. Marx, R. Gross, Phys. Rev. Lett. 109, 250502 (2012)
Dualpath state reconstruction scheme for propagating quantum microwaves and detector noise tomography
E. P. Menzel, F. Deppe, M. Mariantoni, M. Á. Araque Caballero, A. Baust, T. Niemczyk, E. Hoffmann, A. Marx, E. Solano, R. Gross, Phys. Rev. Lett. 105, 100401 (2010)
Planck Spectroscopy and the Quantum Noise of Microwave Beam Splitters
M. Mariantoni, E. P. Menzel, F. Deppe, M. A. Araque Caballero, A. Baust, T. Niemczyk, E. Hoffmann, E. Solano, A. Marx, R. Gross, Phys. Rev. Lett. 105, 133601 (2010


For all above described research areas we are constantly providing fascinating topics for diploma/master and bachelor students. If you are interested in doing exciting new physics in our QIP team please contact Frank Deppe, Achim Marx or Rudolf Gross.
An introducting into the field of superconducting quantum circuits can be found in the lecture notes "Applied Superconductivity".
The QIP research activites at the WMI are embedded in the Collaborative Research Center (Sonderforschungsbereich) 631 entitled "SolidState Quantum Information Processing: Physical Concepts and Material Aspects", the Excellence Cluster "Nanosystems Initiative Munich", the FP7 Project Circuit and Cavity Quantum ElectroDynamics (CCQED), and the FP7STReP Project PROMISCE: Quantum Propagating Microwaves in Strongly Coupled Environments.
Nanomechanics/Optomechanics
Nanomechanical quantum hybrid systems offer promising perspectives to test macroscopic objects for their quantum mechanical behavior. Key experiments are the cooling of mechanical harmonic oscillators to their groundstate, squeezing of quantum states and conversion of quantum information between energyscales (e.g. frequency conversion).
The research activities at the WMI are focussed on:
 Sideband control of electromechanical hybrids
We investigate electromechanical hybrids based on superconducting (Nb) microwave resonators combined with nanomechanical beams based on SiNNb sandwich layers. This combines important key parameters of the beam like the excellent conduction with high mechanical resonance frequencies and quality factors. The structures are precharacterized at liquid helium temperatures followed by microwave spectroscopy in dilutionrefrigerator environments.
Here we focus on:
 the investigation of the properties of the mechanical, electrical and coupled system at millikelvin temperatures.
 benchmarking the performance of sideband cooling and heating.


 Optomechanically induced transparency and absorption in electromechanical hybrids
Optomechanically induced transparency is an interference effect which occurs when the electromechanical system is investigated with multiple microwave tones. In particular, a socalled transparency window opens in the otherwise absorptive signature of the microwave cavity if the beam is driven by a microwave signal which is red detuned. In this experiment the microwave cavity is simultaneously probed by a low intensity microwave tone.
In the hybrid structures described above, we test the existing models of optomechanical induced transparency towards:
 the red and the blue detuned regime.
 high pump and probe intensities resulting in a nonlinear response of the system and duffing like oscillations.


For all above described research areas we are constantly providing fascinating topics for diploma/master and bachelor students. If you are interested in doing exciting new physics in our nanomechanics team please contact Hans Hübl, Achim Marx or Rudolf Gross.
The nanomechanical hybrid systems research activites at the WMI are embedded in the Excellence Cluster "Nanosystems Initiative Munich".
For further information contact Hans Hübl.
