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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.

Josephson junction based qubits

Superconducting qubits are quantum two-level systems, which consist of nanometer-sized 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 counter-clockwise 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 three-dimensional (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\ia-Ripoll, E. Solano, F. Deppe, A. Marx, R. Gross, arXiv:1302.0729 (2013)

Squeezing with a flux-driven 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 qubit-resonator 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

  • Multi-photon 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 qubit-resonator 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. Garcia-Ripoll, A. Marx, R. Gross, arXiv:1107.0810v1 (2011)

Circuit quantum electrodynamics in the ultrastrong-coupling regime
T. Niemczyk, F. Deppe, H. Huebl, E. P. Menzel, F. Hocke, M. J. Schwarz, J. J. Garcia-Ripoll, D. Zueco, T. Hümmer, E. Solano, A. Marx, R. Gross, Nature Physics 6, 772-776 (2010)

Two-photon Probe of the Jaynes-Cummings 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. García-Ripoll, Phys. Rev. B 87, 134504 (2013)

Two-resonator 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)

Two-resonator 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 all-optical 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:

Dual-Path 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 Continuous-Variable 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)

Dual-path 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 introduction into the field of superconducting quantum circuits can be found in the lecture notes "Applied Superconductivity".

The QIP research activites at the WMI have been/are embedded in the Collaborative Research Center (Sonderforschungsbereich) 631 entitled "Solid-State Quantum Information Processing: Physical Concepts and Material Aspects" (2003-2015), the Excellence Cluster "Nanosystems Initiative Munich", and the EU Horizon 2020 Project Magnetomechanical Platforms for Quantum Experiments and Quantum Enabled Sensing Technologies (MagQSens). WMI is also an cactive player in the Munich Quantum Center (MQC) as well as the International PhD Program of Excellence Exploring Quantum Matter (ExQM) and the International Max Planck Research School on Quantum Science and Technology (IMPRS-QST).