Circle fit optimization for resonator quality factor measurements: point redistribution for maximal accuracy

  1. Paul G. Baity,
  2. Connor Maclean,
  3. Valentino Seferai,
  4. Joe Bronstein,
  5. Yi Shu,
  6. Tania Hemakumara,
  7. and Martin Weides
The control of material loss mechansims is playing an increasingly important role for improving coherence times in superconducting quantum devices. Such material losses can be characterized
through the measurement of planar superconducting resonators, which reflect losses through the resonance’s quality factor Ql. The resonance quality factor consists of both internal (material) losses as well as coupling losses when resonance photons escape back into the measurement circuit. The combined losses are then described as Q−1l=Q−1c+Q−1i, where Qc and Qi reflect the coupling and internal quality factors of the resonator, respectively. To separate the relative contributions of Qi and Qc to Ql, diameter-correcting circle fits use algebraic or geometric means to fit the resonance signal on the complex plane. However, such circle fits can produce varied results, so to address this issue, we use a combination of simulation and experiment to determine the reliability of a fitting algorithm across a wide range of quality factor values from Qi≪Qc to Qc≪Qi. In addition, we develop a novel measurement protocol that can not only reduce fitting errors by factors ≳2 but also mitigates the influence of the measurement background on the fit results. This technique can be generalized for other resonance systems beyond superconducting resonators.

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.

Quantum sensing with tuneable superconducting qubits: optimization and speed-up

  1. Sergey Danilin,
  2. Nicholas Nugent,
  3. and Martin Weides
Sensing and metrology play an important role in fundamental science and applications by fulfilling the ever-present need for more precise data sets and by allowing researchers to make
more reliable conclusions on the validity of theoretical models. Sensors are ubiquitous. They are used in applications across a diverse range of fields including gravity imaging, geology, navigation, security, timekeeping, spectroscopy, chemistry, magnetometry, healthcare, and medicine. Current progress in quantum technologies has inevitably triggered the exploration of the use of quantum systems as sensors with new and improved capabilities. This article describes the optimization of the quantum-enhanced sensing of external magnetic fluxes with a Kitaev phase estimation algorithm based on a sensor with tuneable transmon qubits. It provides the optimal flux biasing point for sensors with different maximal qubit transition frequencies. An estimation of decoherence rates is made for a given design. The use of 2− and 3−qubit entangled states for sensing are compared in simulation with the single qubit case. The flux sensing accuracy reaches 10−8⋅Φ0 and scales with time as ∼ 1/t which proves the speed-up of sensing with high ultimate accuracy.

Engineering the microwave to infrared noise photon flux for superconducting quantum systems

  1. Sergey Danilin,
  2. João Barbosa,
  3. Michael Farage,
  4. Zimo Zhao,
  5. Xiaobang Shang,
  6. Jonathan Burnett,
  7. Nick Ridler,
  8. Chong Li,
  9. and Martin Weides
Electromagnetic filtering is essential for the coherent control, operation and readout of superconducting quantum circuits at milliKelvin temperatures. The suppression of spurious modes
around the transition frequencies of a few GHz is well understood and mainly achieved by on-chip and package considerations. Noise photons of higher frequencies — beyond the pair-breaking energies — cause decoherence, and require spectral engineering before reaching the packaged quantum chip. The external wires through the refrigerator down to the quantum circuit provides a direct path, and this article contains quantitative analysis and experimental data for noise photon flux through the coaxial filtered wiring. The coaxial cable attenuation and noise photon flux for typical wiring configurations are provided, and compact cryogenic microwave low-pass filters with CR-110 and Esorb-230 absorptive dielectric fillings along with experimental data at room and cryogenic temperatures and up to 70 GHz presented. The filter cut-off frequencies between 1 to 10 GHz are set by the filter length, and the roll-off is material dependent. The relative dielectric permittivity and magnetic permeability for the Esorb-230 material in the pair-breaking frequency range from 75 to 110 GHz are measured, and the filter properties in this frequency range are calculated. The filter contribution to the noise photon flux implies a dramatic reduction, proving their usefulness for experiments with superconducting quantum systems.

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.

State preparation of a fluxonium qubit with feedback from a custom FPGA-based platform

  1. Richard Gebauer,
  2. Nick Karcher,
  3. Daria Gusenkova,
  4. Martin Spiecker,
  5. Lukas Grünhaupt,
  6. Ivan Takmakov,
  7. Patrick Winkel,
  8. Luca Planat,
  9. Nicolas Roch,
  10. Wolfgang Wernsdorfer,
  11. Alexey V. Ustinov,
  12. Marc Weber,
  13. Martin Weides,
  14. Ioan M. Pop,
  15. and Oliver Sander
We developed a versatile integrated control and readout instrument for experiments with superconducting quantum bits (qubits), based on a field-programmable gate array (FPGA) platform.
Using this platform, we perform measurement-based, closed-loop feedback operations with 428ns platform latency. The feedback capability is instrumental in realizing active reset initialization of the qubit into the ground state in a time much shorter than its energy relaxation time T1. We show experimental results demonstrating reset of a fluxonium qubit with 99.4% fidelity, using a readout-and-drive pulse sequence approximately 1.5μs long. Compared to passive ground state initialization through thermalization, with the time constant given by T1= 80μs, the use of the FPGA-based platform allows us to improve both the fidelity and the time of the qubit initialization by an order of magnitude.

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.

Transmon Qubit in a Magnetic Field: Evolution of Coherence and Transition Frequency

  1. Andre Schneider,
  2. Tim Wolz,
  3. Marco Pfirrmann,
  4. Martin Spiecker,
  5. Hannes Rotzinger,
  6. Alexey V. Ustinov,
  7. and Martin Weides
We report on spectroscopic and time-domain measurements on a fixed-frequency concentric transmon qubit in an applied in-plane magnetic field to explore its limits of magnetic field
compatibility. We demonstrate quantum coherence of the qubit up to field values of B=40mT, even without an optimized chip design or material combination of the qubit. The dephasing rate Γφ is shown to be not affected by the magnetic field in a broad range of the qubit transition frequency. For the evolution of the qubit transition frequency, we find the unintended second junction created in the shadow angle evaporation process to be non-negligible and deduce an analytic formula for the field-dependent qubit energies. We discuss the relevant field-dependent loss channels, which can not be distinguished by our measurements, inviting further theoretical and experimental investigation. Using well-known and well-studied standard components of the superconducting quantum architecture, we are able to reach a field regime relevant for quantum sensing and hybrid applications of magnetic spins and spin systems.

Correlating decoherence in transmon qubits: Low frequency noise by single fluctuators

  1. Steffen Schlör,
  2. Jürgen Lisenfeld,
  3. Clemens Müller,
  4. Andre Schneider,
  5. David P. Pappas,
  6. Alexey V. Ustinov,
  7. and Martin Weides
We report on long-term measurements of a highly coherent, non-tunable transmon qubit, revealing low-frequency burst noise in coherence times and transition frequency. We achieve this
through a simultaneous measurement of the qubits relaxation and dephasing rate as well as its resonance frequency and an analysis of their correlations. These yield information about the microscopic origin of the intrinsic decoherence mechanisms in Josephson qubits. Our data is consistent with a small number of microscopic two-level systems located at the edges of the superconducting film, which is further confirmed by a spectral noise analysis.

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.