Quantum emulation of the transient dynamics in the multistate Landau-Zener model

  1. Alexander Stehli,
  2. Jan David Brehm,
  3. Tim Wolz,
  4. Andre Schneider,
  5. Hannes Rotzinger,
  6. Martin Weides,
  7. and Alexey V. Ustinov
Quantum simulation is one of the most promising near term applications of quantum computing. Especially, systems with a large Hilbert space are hard to solve for classical computers
and thus ideal targets for a simulation with quantum hardware. In this work, we study experimentally the transient dynamics in the multistate Landau-Zener model as a function of the Landau-Zener velocity. The underlying Hamiltonian is emulated by superconducting quantum circuit, where a tunable transmon qubit is coupled to a bosonic mode ensemble comprising four lumped element microwave resonators. We investigate the model for different initial states: Due to our circuit design, we are not limited to merely exciting the qubit, but can also pump the harmonic modes via a dedicated drive line. Here, the nature of the transient dynamics depends on the average photon number in the excited resonator. The greater effective coupling strength between qubit and higher Fock states results in a quasi-adiabatic transition, where coherent quantum oscillations are suppressed without the introduction of additional loss channels. Our experiments pave the way for more complex simulations with qubits coupled to an engineered bosonic mode spectrum.

Slowing down light in a qubit metamaterial

  1. Jan David Brehm,
  2. Richard Gebauer,
  3. Alexander Stehli,
  4. Alexander N. Poddubny,
  5. Oliver Sander,
  6. Hannes Rotzinger,
  7. and Alexey V. Ustinov
The rapid progress in quantum information processing leads to a rising demand for devices to control the propagation of electromagnetic wave pulses and to ultimately realize a universal
and efficient quantum memory. While in recent years significant progress has been made to realize slow light and quantum memories with atoms at optical frequencies, superconducting circuits in the microwave domain still lack such devices. Here, we demonstrate slowing down electromagnetic waves in a superconducting metamaterial composed of eight qubits coupled to a common waveguide, forming a waveguide quantum electrodynamics system. We analyze two complementary approaches, one relying on dressed states of the Autler-Townes splitting, and the other based on a tailored dispersion profile using the qubits tunability. Our time-resolved experiments show reduced group velocities of down to a factor of about 1500 smaller than in vacuum. Depending on the method used, the speed of light can be controlled with an additional microwave tone or an effective qubit detuning. Our findings demonstrate high flexibility of superconducting circuits to realize custom band structures and open the door to microwave dispersion engineering in the quantum regime.

Highly coherent superconducting qubits from a subtractive junction fabrication process

  1. Alexander Stehli,
  2. Jan David Brehm,
  3. Tim Wolz,
  4. Paul Baity,
  5. Sergey Danilin,
  6. Valentino Seferai,
  7. Hannes Rotzinger,
  8. Alexey V. Ustinov,
  9. and Martin Weides
Josephson tunnel junctions are the centerpiece of almost any superconducting electronic circuit, including qubits. Typically, the junctions for qubits are fabricated using shadow evaporation
techniques to reduce dielectric loss contributions from the superconducting film interfaces. In recent years, however, sub-micron scale overlap junctions have started to attract attention. Compared to shadow mask techniques, neither an angle dependent deposition nor free-standing bridges or overlaps are needed, which are significant limitations for wafer-scale processing. This comes at the cost of breaking the vacuum during fabrication, but simplifies integration in multi-layered circuits, implementation of vastly different junction sizes, and enables fabrication on a larger scale in an industrially-standardized process. In this work, we demonstrate the feasibility of a subtractive process for fabrication of overlap junctions. We evaluate the coherence properties of the junctions by employing them in superconducting transmon qubits. In time domain experiments, we find that both, the qubit life- and coherence time of our best device, are on average greater than 20 μs. Finally, we discuss potential improvements to our technique. This work paves the way towards a more standardized process flow with advanced materials and growth processes, and constitutes an important step for large scale fabrication of superconducting quantum circuits.

Waveguide Bandgap Engineering with an Array of Superconducting Qubits

  1. Jan David Brehm,
  2. Alexander N. Poddubny,
  3. Alexander Stehli,
  4. Tim Wolz,
  5. Hannes Rotzinger,
  6. and Alexey V. Ustinov
In this work, we experimentally study a metamaterial made of eight superconducting transmon qubits with local frequency control coupled to the mode continuum of a superconducting waveguide.
By consecutively tuning the qubits to a common resonance frequency we observe the formation of super- and subradiant states as well as the emergence of a polaritonic bandgap. Making use of the qubits strong intrinsic quantum nonlinearity we study the saturation of the collective modes with increasing photon number and electromagnetically induce a transparency window in the bandgap region of the ensemble, allowing to directly control the band structure of the array. The moderately scaled circuit of this work extends experiments with one and two qubits towards a full-blown quantum metamaterial, thus paving the way for large-scale applications in superconducting waveguide quantum electrodynamics.

Amplitude and frequency sensing of microwave fields with a superconducting transmon qudit

  1. Maximilian Kristen,
  2. Andre Schneider,
  3. Alexander Stehli,
  4. Tim Wolz,
  5. Sergey Danilin,
  6. Hsiang S. Ku,
  7. David P. Pappas,
  8. Alexey V. Ustinov,
  9. and Martin Weides
Experiments with superconducting circuits require careful calibration of the applied pulses and fields over a large frequency range. This remains an ongoing challenge as commercial
semiconductor electronics are not able to probe signals arriving at the chip due to its cryogenic environment. Here, we demonstrate how the on-chip amplitude and frequency of a microwave field can be inferred from the ac Stark shifts of higher transmon levels. In our time-resolved measurements, we employ a simple quantum sensing protocol, i.e. Ramsey fringes, allowing us to detect the amplitude of the systems transfer function over a range of several hundreds of MHz with an energy sensitivity on the order of 10−4. Combined with similar measurements for the phase of the transfer function, our sensing method can facilitate the microwave calibration of high fidelity quantum gates necessary for working with superconducting quantum circuits. Additionally, the potential to characterize arbitrary microwave fields promotes applications in related areas of research, such as quantum optics or hybrid microwave systems including photonic, mechanical or magnonic subsystems.

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.

Analog quantum simulation of the Rabi model in the ultra-strong coupling regime

  1. Jochen Braumüller,
  2. Michael Marthaler,
  3. Andre Schneider,
  4. Alexander Stehli,
  5. Hannes Rotzinger,
  6. Martin Weides,
  7. and Alexey V. Ustinov
The quantum Rabi model describes the fundamental mechanism of light-matter interaction. It consists of a two-level atom or qubit coupled to a quantized harmonic mode via a transversal
interaction. In the weak coupling regime, a rotating wave approximation can be applied and the quantum Rabi Hamiltonian reduces to the well-known Jaynes-Cummings Hamiltonian. In the ultra-strong coupling regime, where the effective coupling strength g is comparable to the energy ω of the bosonic mode, the counter rotating terms can no longer be neglected, revealing remarkable features in the system dynamics. Here, we demonstrate an analog quantum simulation of the quantum Rabi model in the ultra-strong coupling regime of variable strength. The quantum hardware of the simulator is a superconducting circuit embedded in a cQED setup. The simulation scheme is based on the application of two transversal microwave drive tones used to engineer the desired effective Hamiltonian. We observe a fast quantum state collapse followed by periodically recurring quantum revivals of the initial qubit state, which is the most distinct signature of the synthesized model. We achieve a relative coupling ratio of g/ω∼0.7, approaching the deep strong coupling regime.