Nonexponential decay of a giant artificial atom

  1. Gustav Andersson,
  2. Baladitya Suri,
  3. Lingzhen Guo,
  4. Thomas Aref,
  5. and Per Delsing
In quantum optics, light-matter interaction has conventionally been studied using small atoms interacting with electromagnetic fields with wavelength several orders of magnitude larger
than the atomic dimensions. In contrast, here we experimentally demonstrate the vastly different giant atom regime, where an artificial atom interacts with acoustic fields with wavelength several orders of magnitude smaller than the atomic dimensions. This is achieved by coupling a superconducting qubit to surface acoustic waves at two points with separation on the order of 100 wavelengths. This approach is comparable to controlling the radiation of an atom by attaching it to an antenna. The slow velocity of sound leads to a significant internal time-delay for the field to propagate across the giant atom, giving rise to non-Markovian dynamics. We demonstrate the non-Markovian character of the giant atom in the frequency spectrum as well as nonexponential relaxation in the time domain.

Probing the Tavis-Cummings level splitting with intermediate-scale superconducting circuits

  1. Ping Yang,
  2. Jan David Brehm,
  3. Juha Leppäkangas,
  4. Lingzhen Guo,
  5. Michael Marthaler,
  6. Isabella Boventer,
  7. Alexander Stehli,
  8. Tim Wolz,
  9. Alexey V. Ustinov,
  10. and Martin Weides
We demonstrate the local control of up to eight two-level systems interacting strongly with a microwave cavity. Following calibration, the frequency of each individual two-level system
(qubit) is tunable without influencing the others. Bringing the qubits one by one on resonance with the cavity, we observe the collective coupling strength of the qubit ensemble. The splitting scales up with the square root of the number of the qubits, being the hallmark of the Tavis-Cummings model. The local control circuitry causes a bypass shunting the resonator, and a Fano interference in the microwave readout, whose contribution can be calibrated away to recover the pure cavity spectrum. The simulator’s attainable size of dressed states is limited by reduced signal visibility, and -if uncalibrated- by off-resonance shifts of sub-components. Our work demonstrates control and readout of quantum coherent mesoscopic multi-qubit system of intermediate scale under conditions of noise.

Resonance inversion in a superconducting cavity coupled to artificial atoms and a microwave background

  1. Juha Leppäkangas,
  2. Jan David Brehm,
  3. Ping Yang,
  4. Lingzhen Guo,
  5. Michael Marthaler,
  6. Alexey V. Ustinov,
  7. and Martin Weides
We demonstrate how heating of an environment can invert the line shape of a driven cavity. We consider a superconducting coplanar cavity coupled to multiple artificial atoms. The measured
cavity transmission is characterized by Fano-type resonances with a shape that is continuously tunable by bias current through nearby (magnetic flux) control lines. In particular, the same dispersive shift of the microwave cavity can be observed as a peak or a dip. We find that this Fano-peak inversion is possible due to a tunable interference between a microwave transmission through a background, with reactive and dissipative properties, and through the cavity, affected by bias-current induced heating. The background transmission occurs due to crosstalk with the multiple control lines. We show how such background can be accounted for by a Jaynes- or Tavis-Cummings model with modified boundary conditions between the cavity and transmission-line microwave fields. A dip emerges when cavity transmission is comparable with background transmission and dissipation. We find generally that resonance positions determine system energy levels, whereas resonance shapes give information on system fluctuations and dissipation.

Local Sensing with an AC Stark Spectrum Analyzer

  1. Andre Schneider,
  2. Jochen Braumüller,
  3. Lingzhen Guo,
  4. Patrizia Stehle,
  5. Hannes Rotzinger,
  6. Michael Marthaler,
  7. Alexey V. Ustinov,
  8. and Martin Weides
Analyzing weak microwave signals in the GHz regime is a challenging task if the signal level is very low and the photon energy widely undefined. Due to its discrete level structure,
a superconducting qubit is only sensitive to photons of certain energies. With a multi-level quantum system (qudit) in contrast, the unknown photon frequency can be deduced from the higher level AC Stark shift. The measurement accuracy is given by the signal amplitude, its detuning from the discrete qudit energy level structure and the anharmonicity. We demonstrate an energy sensitivity in the order of 10−4 with a measurement range of 1 GHz. Here, using a transmon qubit, we experimentally observe shifts in the transition frequencies involving up to three excited levels. These shifts are in good agreement with an analytic circuit model and master equation simulations. For large detunings, we find the shifts to scale linearly with the power of the applied microwave drive.

The giant acoustic atom — a single quantum system with a deterministic time delay

  1. Lingzhen Guo,
  2. Arne Grimsmo,
  3. Anton Frisk Kockum,
  4. Mikhail Pletyukhov,
  5. and Göran Johansson
We investigate the quantum dynamics of a single transmon qubit coupled to surface acoustic waves (SAWs) via two distant connection points. Since the acoustic speed is five orders of
magnitude slower than the speed of light, the travelling time between the two connection points needs to be taken into account. Therefore, we treat the transmon qubit as a giant atom with a deterministic time delay. We find that the spontaneous emission of the system, formed by the giant atom and the SAWs between its connection points, initially follows a polynomial decay law instead of an exponential one, as would be the case for a small atom. We obtain exact analytical results for the scattering properties of the giant atom up to two-phonon processes by using a diagrammatic approach. The time delay gives rise to novel features in the reflection, transmission, power spectra, and second-order correlation functions of the system. Furthermore, we find the short-time dynamics of the giant atom for arbitrary drive strength by a numerically exact method for open quantum systems with a finite-time-delay feedback loop.

Multi-photon dressing of an anharmonic superconducting many-level quantum circuit

  1. Jochen Braumüller,
  2. Joel Cramer,
  3. Steffen Schlör,
  4. Hannes Rotzinger,
  5. Lucas Radtke,
  6. Alexander Lukashenko,
  7. Ping Yang,
  8. Michael Marthaler,
  9. Lingzhen Guo,
  10. Alexey V. Ustinov,
  11. and Martin Weides
We report on the investigation of a superconducting anharmonic multi-level circuit that is coupled to a harmonic readout resonator. We observe multi-photon transitions via virtual energy
levels of our system up to the fifth excited state. The back-action of these higher-order excitations on our readout device is analyzed quantitatively and demonstrated to be in accordance with theoretical expectation. By applying a strong microwave drive we achieve multi-photon dressing of our system which is dynamically coupled by a weak probe tone. The emerging higher-order Rabi sidebands and associated Autler-Townes splittings involving up to five levels of the investigated anharmonic circuit are observed. Experimental results are in good agreement with master equation simulations.

Emission spectrum of the driven nonlinear oscillator

  1. Stephan André,
  2. Lingzhen Guo,
  3. Vittorio Peano,
  4. Michael Marthaler,
  5. and Gerd Schön
Motivated by recent „circuit QED“ experiments we investigate the noise properties of coherently driven nonlinear resonators. By using Josephson junctions in superconducting
circuits, strong nonlinearities can be engineered, which lead to the appearance of pronounced effects already for a low number of photons in the resonator. Based on a master equation approach we determine the emission spectrum and observe for typical circuit QED parameters, in addition to the primary Raman-type peaks, second-order peaks. These peaks describe higher harmonics in the slow noise-induced fluctuations of the oscillation amplitude of the resonator and provide a clear signature of the nonlinear nature of the system.