Tunable coupling of transmission-line microwave resonators mediated by an rf SQUID

  1. F. Wulschner,
  2. J. Goetz,
  3. F. R. Koessel,
  4. E. Hoffmann,
  5. A. Baust,
  6. P. Eder,
  7. M. Fischer,
  8. M. Haeberlein,
  9. M. J. Schwarz,
  10. M. Pernpeintner,
  11. E. Xie,
  12. L. Zhong,
  13. C. W. Zollitsch,
  14. B. Peropadre,
  15. J.J. García-Ripoll,
  16. E. Solano,
  17. K. Fedorov,
  18. E. P. Menzel,
  19. F. Deppe,
  20. A. Marx,
  21. and R. Gross
We realize tunable coupling between two superconducting transmission line resonators. The coupling is mediated by a non-hysteretic rf SQUID acting as a flux-tunable mutual inductance
between the resonators. From the mode distance observed in spectroscopy experiments, we derive a coupling strength ranging between -320MHz and 37 MHz. In the case where the coupling strength is about zero, the microwave power cross transmission between the two resonators can be reduced by almost four orders of magnitude compared to the case where the coupling is switched on. In addition, we observe parametric amplification by applying a suitable additional drive tone.

Quantum teleportation of propagating quantum microwaves

  1. R. Di Candia,
  2. K. G. Fedorov,
  3. L. Zhong,
  4. S. Felicetti,
  5. E. P. Menzel,
  6. M. Sanz,
  7. F. Deppe,
  8. A. Marx,
  9. R. Gross,
  10. and E. Solano
Propagating quantum microwaves have been proposed and successfully implemented to generate entanglement, thereby establishing a promising platform for the realisation of a quantum communication
channel. However, the implementation of quantum teleportation with photons in the microwave regime is still absent. At the same time, recent developments in the field show that this key protocol could be feasible with current technology, which would pave the way to boost the field of microwave quantum communication. Here, we discuss the feasibility of a possible implementation of microwave quantum teleportation in a realistic scenario with losses. Furthermore, we propose how to implement quantum repeaters in the microwave regime without using photodetection, a key prerequisite to achieve long distance entanglement distribution.

Ultrastrong coupling in two-resonator circuit QED

  1. A. Baust,
  2. E. Hoffmann,
  3. M. Haeberlein,
  4. M. J. Schwarz,
  5. P. Eder,
  6. J. Goetz,
  7. F. Wulschner,
  8. E. Xie,
  9. L. Zhong,
  10. F. Quijandria,
  11. D. Zueco,
  12. J.J. García-Ripoll,
  13. L. Garcia-Alvarez,
  14. G. Romero,
  15. E. Solano,
  16. K. G. Fedorov,
  17. E. P. Menzel,
  18. F. Deppe,
  19. A. Marx,
  20. and R. Gross
We report on ultrastrong coupling between a superconducting flux qubit and a resonant mode of a system comprised of two superconducting coplanar stripline resonators coupled galvanically
to the qubit. With a coupling strength as high as 17% of the mode frequency, exceeding that of previous circuit quantum electrodynamics experiments, we observe a pronounced Bloch-Siegert shift. The spectroscopic response of our multimode system reveals a clear breakdown of the Jaynes-Cummings model. In contrast to earlier experiments, the high coupling strength is achieved without making use of an additional inductance provided by a Josephson junction.

Tunable and Switchable Coupling Between Two Superconducting Resonators

  1. A. Baust,
  2. E. Hoffmann,
  3. M. Haeberlein,
  4. M. J. Schwarz,
  5. P. Eder,
  6. E. P. Menzel,
  7. K. Fedorov,
  8. J. Goetz,
  9. F. Wulschner,
  10. E. Xie,
  11. L. Zhong,
  12. F. Quijandria,
  13. B. Peropadre,
  14. D. Zueco,
  15. J.J. García-Ripoll,
  16. E. Solano,
  17. F. Deppe,
  18. A. Marx,
  19. and R. Gross
We realize a device allowing for tunable and switchable coupling between two superconducting resonators mediated by an artificial atom. For the latter, we utilize a persistent current
flux qubit. We characterize the tunable and switchable coupling in frequency and time domain and find that the coupling between the relevant modes can be varied in a controlled way. Specifically, the coupling can be tuned by adjusting the flux through the qubit loop or by saturating the qubit. Our time domain measurements allow us to find parameter regimes for optimal switch performance with respect to qubit drive power and the dynamic range of the resonator input power

Dual-Path Methods for Propagating Quantum Microwaves

  1. R. Di Candia,
  2. E. P. Menzel,
  3. L. Zhong,
  4. F. Deppe,
  5. A. Marx,
  6. R. Gross,
  7. and E. Solano
We study quantum state tomography, entanglement detection, and channel noise reconstruction of propagating quantum microwaves via dual-path methods. The presented schemes make use of
the following key elements: propagation channels, beam splitters, linear amplifiers, and field quadrature detectors. Remarkably, our methods are tolerant to the ubiquitous noise added to the signals by phase-insensitive microwave amplifiers. Furthermore, we analyze our techniques with numerical examples and experimental data. Our methods provide key toolbox components that may pave the way towards quantum microwave teleportation and communication protocols.

Squeezing with a flux-driven Josephson parametric amplifier

  1. L. Zhong,
  2. E. P. Menzel,
  3. R. Di Candia,
  4. P. Eder,
  5. M. Ihmig,
  6. A. Baust,
  7. M. Haeberlein,
  8. E. Hoffmann,
  9. K. Inomata,
  10. T. Yamamoto,
  11. Y. Nakamura,
  12. E. Solano,
  13. F. Deppe,
  14. A. Marx,
  15. and R. Gross
Josephson parametric amplifiers (JPA) are promising devices for applications in circuit quantum electrodynamics (QED) and for studies on propagating quantum microwaves because of their
good noise performance. In this work, we present a systematic characterization of a flux-driven JPA at millikelvin temperatures. In particular, we study in detail its squeezing properties by two different detection techniques. With the homodyne setup, we observe squeezing of vacuum fluctuations by superposing signal and idler bands. For a quantitative analysis we apply dual-path cross-correlation techniques to reconstruct the Wigner functions of various squeezed vacuum and thermal states. At 10 dB signal gain, we find 4.9+-0.2 dB squeezing below vacuum. In addition, we discuss the physics behind squeezed coherent microwave fields. Finally, we analyze the JPA noise temperature in the degenerate mode and find a value smaller than the standard quantum limit for phase-insensitive amplifiers.

Fast microwave beam splitters from superconducting resonators

  1. M. Haeberlein,
  2. D. Zueco,
  3. P. Assum,
  4. T. Weißl,
  5. E. Hoffmann,
  6. B. Peropadre,
  7. J.J. Garcia-Ripoll,
  8. E. Solano,
  9. F. Deppe,
  10. A. Marx,
  11. and R. Gross
Coupled superconducting transmission line resonators have applications in quantum information processing and fundamental quantum mechanics. A particular example is the realization of
fast beam splitters, which however is hampered by two-mode squeezer terms. Here, we experimentally study superconducting microstrip resonators which are coupled over one third of their length. By varying the position of this coupling region we can tune the strength of the two-mode squeezer coupling from 2.4% to 12.9% of the resonance frequency of 5.44GHz. Nevertheless, the beam splitter coupling rate for maximally suppressed two-mode squeezing is 810MHz, enabling the construction of a fast and pure beam splitter.

Path Entanglement of Continuous-Variable Quantum Microwaves

  1. E. P. Menzel,
  2. R. Di Candia,
  3. F. Deppe,
  4. P. Eder,
  5. L. Zhong,
  6. M. Ihmig,
  7. M. Haeberlein,
  8. A. Baust,
  9. E. Hoffmann,
  10. D. Ballester,
  11. K. Inomata,
  12. T. Yamamoto,
  13. Y. Nakamura,
  14. E. Solano,
  15. A. Marx,
  16. and R. Gross
Path entanglement constitutes an essential resource in quantum information and communication protocols. Here, we demonstrate frequency-degenerate entanglement between continuous-variable
quantum microwaves propagating along two spatially separated paths. We combine a squeezed and a vacuum state using a microwave beam splitter. Via correlation measurements, we detect and quantify the path entanglement contained in the beam splitter output state. Our experiments open the avenue to quantum teleportation, quantum communication, or quantum radar with continuous variables at microwave frequencies.

Gradiometric flux qubits with tunable gap

  1. M. J. Schwarz,
  2. J. Goetz,
  3. Z. Jiang,
  4. T. Niemczyk,
  5. F. Deppe,
  6. A. Marx,
  7. and R. Gross
For gradiometric three-Josephson-junction flux qubits, we perform a systematic study on the tuning of the minimal transition frequency, the so-called qubit gap. By replacing one of
the qubit’s Josephson junctions by a dc SQUID, the critical current of this SQUID and, in turn, the qubit gap can be tuned in situ by a control flux threading the SQUID loop. We present spectroscopic measurements demonstrating a well-defined controllability of the qubit gap between zero and more than 10 GHz. In the future, this enables one to tune the qubit into and out of resonance with other superconducting quantum circuits, while operating the qubit at its symmetry point with optimal dephasing properties. The experimental data agree very well with model calculations based on the full qubit Hamiltonian. From a numerical fit, we determine the Josephson coupling and the charging energies of the qubit junctions. The derived values agree well with those measured for other junctions fabricated on the same chip. We also demonstrate the biasing of gradiometric flux qubits near the symmetry point by trapping an odd number of flux quanta in the gradiometer loop. In this way, we study the effect of the significant kinetic inductance, thereby obtaining valuable information for the qubit design.

Tunable coupling engineering between superconducting resonators: from sidebands to effective gauge fields

  1. B. Peropadre,
  2. D. Zueco,
  3. F. Wulschner,
  4. F. Deppe,
  5. A. Marx,
  6. R. Gross,
  7. and J.J. García-Ripoll
In this work we show that a tunable coupling between microwave resonators can be engineered by means of simple Josephson junctions circuits, such as dc- and rf-SQUIDs. We show that
by controlling the time dependence of the coupling it is possible to switch on and off and modulate the cross-talk, boost the interaction towards the ultrastrong regime, as well as to engineer red and blue sideband couplings, nonlinear photon hopping and classical gauge fields. We discuss how these dynamically tunable superconducting circuits enable key applications in the fields of all optical quantum computing, continuous variable quantum information and quantum simulation – all within the reach of state of the art in circuit-QED experiments.