Signal crosstalk in a flip-chip quantum processor

  1. Sandoko Kosen,
  2. Hang-Xi Li,
  3. Marcus Rommel,
  4. Robert Rehammar,
  5. Marco Caputo,
  6. Leif Grönberg,
  7. Jorge Fernández-Pendás,
  8. Anton Frisk Kockum,
  9. Janka Biznárová,
  10. Liangyu Chen,
  11. Christian Križan,
  12. Andreas Nylander,
  13. Amr Osman,
  14. Anita Fadavi Roudsari,
  15. Daryoush Shiri,
  16. Giovanna Tancredi,
  17. Joonas Govenius,
  18. and Jonas Bylander
Quantum processors require a signal-delivery architecture with high addressability (low crosstalk) to ensure high performance already at the scale of dozens of qubits. Signal crosstalk
causes inadvertent driving of quantum gates, which will adversely affect quantum-gate fidelities in scaled-up devices. Here, we demonstrate packaged flip-chip superconducting quantum processors with signal-crosstalk performance competitive with those reported in other platforms. For capacitively coupled qubit-drive lines, we find on-resonant crosstalk better than -27 dB (average -37 dB). For inductively coupled magnetic-flux-drive lines, we find less than 0.13 % direct-current flux crosstalk (average 0.05 %). These observed crosstalk levels are adequately small and indicate a decreasing trend with increasing distance, which is promising for further scaling up to larger numbers of qubits. We discuss the implication of our results for the design of a low-crosstalk, on-chip signal delivery architecture, including the influence of a shielding tunnel structure, potential sources of crosstalk, and estimation of crosstalk-induced qubit-gate error in scaled-up quantum processors.

Characterization of process-related interfacial dielectric loss in aluminum-on-silicon by resonator microwave measurements, materials analysis, and imaging

  1. Lert Chayanun,
  2. Janka Biznárová,
  3. Lunjie Zeng,
  4. Per Malmberg,
  5. Andreas Nylander,
  6. Amr Osman,
  7. Marcus Rommel,
  8. Pui Lam Tam,
  9. Eva Olsson,
  10. August Yurgens,
  11. Jonas Bylander,
  12. and Anita Fadavi Roudsari
We systematically investigate the influence of the fabrication process on dielectric loss in aluminum-on-silicon superconducting coplanar waveguide resonators with internal quality
factors (Qi) of about one million at the single-photon level. These devices are essential components in superconducting quantum processors; they also serve as proxies for understanding the energy loss of superconducting qubits. By systematically varying several fabrication steps, we identify the relative importance of reducing loss at the substrate-metal and the substrate-air interfaces. We find that it is essential to clean the silicon substrate in hydrogen fluoride (HF) prior to aluminum deposition. A post-fabrication removal of the oxides on the surface of the silicon substrate and the aluminum film by immersion in HF further improves the Qi. We observe a small, but noticeable, adverse effect on the loss by omitting either standard cleaning (SC1), pre-deposition heating of the substrate to 300°C, or in-situ post-deposition oxidation of the film’s top surface. We find no improvement due to excessive pumping meant to reach a background pressure below 6×10−8 mbar. We correlate the measured loss with microscopic properties of the substrate-metal interface through characterization with X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectroscopy (ToF-SIMS), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS), and atomic force microscopy (AFM).

Intermodulation spectroscopy and the nonlinear response of two-level systems in superconducting coplanar waveguide resonators

  1. Janka Biznárová,
  2. J. C. Rivera Hernández,
  3. Daniel Forchheimer,
  4. Jonas Bylander,
  5. David B. Haviland,
  6. and Gustav Andersson
Two-level system (TLS) loss is typically limiting the coherence of superconducting quantum circuits. The loss induced by TLS defects is nonlinear, resulting in quality factors with
a strong dependence on the circulating microwave power. We observe frequency mixing due to this nonlinearity by applying a two-tone drive to a coplanar waveguide resonator and measuring the intermodulation products using a multifrequency lock-in technique. This intermodulation spectroscopy method provides an efficient approach to characterizing TLS loss in superconducting circuits. Using harmonic balance reconstruction, we recover the nonlinear parameters of the device-TLS interaction, which are in good agreement with the standard tunnelling model for TLSs.

Loss and decoherence in superconducting circuits on silicon: Insights from electron spin resonance

  1. Aditya Jayaraman,
  2. Andrey V. Danilov,
  3. Jonas Bylander,
  4. and Sergey E. Kubatkin
Solid-state devices used for quantum computation and quantum sensing applications are adversely affected by loss and noise caused by spurious, charged two-level systems (TLS) and stray
paramagnetic spins. These two sources of noise are interconnected, exacerbating the impact on circuit performance. We use an on-chip electron spin resonance (ESR) technique, with niobium nitride (NbN) superconducting resonators, to study surface spins on silicon and the effect of post-fabrication surface treatments. We identify two distinct spin species that are characterized by different spin-relaxation times and respond selectively to various surface treatments (annealing and hydrofluoric acid). Only one of the two spin species has a significant impact on the TLS-limited resonator quality factor at low-power (near single-photon) excitation. We observe a 3-to-5-fold reduction in the total density of spins after surface treatments, and demonstrate the efficacy of ESR spectroscopy in developing strategies to mitigate loss and decoherence in quantum systems.

Mitigation of interfacial dielectric loss in aluminum-on-silicon superconducting qubits

  1. Janka Biznárová,
  2. Amr Osman,
  3. Emil Rehnman,
  4. Lert Chayanun,
  5. Christian Križan,
  6. Per Malmberg,
  7. Marcus Rommel,
  8. Christopher Warren,
  9. Per Delsing,
  10. August Yurgens,
  11. Jonas Bylander,
  12. and Anita Fadavi Roudsari
We demonstrate aluminum-on-silicon planar transmon qubits with time-averaged T1 energy relaxation times of up to 270μs, corresponding to Q = 5 million, and a highest observed value
of 501μs. We use materials analysis techniques and numerical simulations to investigate the dominant sources of energy loss, and devise and demonstrate a strategy towards mitigating them. The mitigation of loss is achieved by reducing the presence of oxide, a known host of defects, near the substrate-metal interface, by growing aluminum films thicker than 300 nm. A loss analysis of coplanar-waveguide resonators shows that the improvement is owing to a reduction of dielectric loss due to two-level system defects. We perform time-of-flight secondary ion mass spectrometry and observe a reduced presence of oxygen at the substrate-metal interface for the thicker films. The correlation between the enhanced performance and the film thickness is due to the tendency of aluminum to grow in columnar structures of parallel grain boundaries, where the size of the grain depends on the film thickness: transmission electron microscopy imaging shows that the thicker film has larger grains and consequently fewer grain boundaries containing oxide near this interface. These conclusions are supported by numerical simulations of the different loss contributions in the device.

Fast analytic and numerical design of superconducting resonators in flip-chip architectures

  1. Hang-Xi Li,
  2. Daryoush Shiri,
  3. Sandoko Kosen,
  4. Marcus Rommel,
  5. Lert Chayanun,
  6. Andreas Nylander,
  7. Robert Rehammer,
  8. Giovanna Tancredi,
  9. Marco Caputo,
  10. Kestutis Grigoras,
  11. Leif Grönberg,
  12. Joonas Govenius,
  13. and Jonas Bylander
In superconducting quantum processors, the predictability of device parameters is of increasing importance as many labs scale up their systems to larger sizes in a 3D-integrated architecture.
In particular, the properties of superconducting resonators must be controlled well to ensure high-fidelity multiplexed readout of qubits. Here we present a method, based on conformal mapping techniques, to predict a resonator’s parameters directly from its 2D cross-section, without computationally heavy simulation. We demonstrate the method’s validity by comparing the calculated resonator frequency and coupling quality factor with those obtained through 3D finite-element-method simulation and by measurement of 15 resonators in a flip-chip-integrated architecture. We achieve a discrepancy of less than 2% between designed and measured frequencies, for 6-GHz resonators. We also propose a design method that reduces the sensitivity of the resonant frequency to variations in the inter-chip spacing.

Mitigation of frequency collisions in superconducting quantum processors

  1. Amr Osman,
  2. Jorge Fernàndez-Pendàs,
  3. Chris Warren,
  4. Sandoko Kosen,
  5. Marco Scigliuzzo,
  6. Anton Frisk Kockum,
  7. Giovanna Tancredi,
  8. Anita Fadavi Roudsari,
  9. and Jonas Bylander
The reproducibility of qubit parameters is a challenge for scaling up superconducting quantum processors. Signal crosstalk imposes constraints on the frequency separation between neighboring
qubits. The frequency uncertainty of transmon qubits arising from the fabrication process is attributed to deviations in the Josephson junction area, tunnel barrier thickness, and the qubit capacitor. We decrease the sensitivity to these variations by fabricating larger Josephson junctions and reduce the wafer-level standard deviation in resistance down to 2%. We characterize 32 identical transmon qubits and demonstrate the reproducibility of the qubit frequencies with a 40 MHz standard deviation (i.e. 1%) with qubit quality factors exceeding 2 million. We perform two-level-system (TLS) spectroscopy and observe no significant increase in the number of TLSs causing qubit relaxation. We further show by simulation that for our parametric-gate architecture, and accounting only for errors caused by the uncertainty of the qubit frequency, we can scale up to 100 qubits with an average of only 3 collisions between quantum-gate transition frequencies, assuming 2% crosstalk and 99.9% target gate fidelity.

Three-wave mixing traveling-wave parametric amplifier with periodic variation of the circuit parameters

  1. Anita Fadavi Roudsari,
  2. Daryoush Shiri,
  3. Hampus Renberg Nilsson,
  4. Giovanna Tancredi,
  5. Amr Osman,
  6. Ida-Maria Svensson,
  7. Marina Kudra,
  8. Marcus Rommel,
  9. Jonas Bylander,
  10. Vitaly Shumeiko,
  11. and Per Delsing
We report the implementation of a near-quantum-limited, traveling-wave parametric amplifier that uses three-wave mixing (3WM). To favor amplification by 3WM, we use the superconducting
nonlinear asymmetric inductive element (SNAIL) loops, biased with a dc magnetic flux. In addition, we equip the device with dispersion engineering features to create a stop-band at the second harmonic of the pump and suppress the propagation of the higher harmonics that otherwise degrade the amplification. With a chain of 440 SNAILs, the amplifier provides up to 20 dB gain and a 3-dB bandwidth of 1 GHz. The added noise by the amplifier is found to be less than one photon.

Transmon qubit readout fidelity at the threshold for quantum error correction without a quantum-limited amplifier

  1. Liangyu Chen,
  2. Hang-Xi Li,
  3. Yong Lu,
  4. Christopher W. Warren,
  5. Christian J. Križan,
  6. Sandoko Kosen,
  7. Marcus Rommel,
  8. Shahnawaz Ahmed,
  9. Amr Osman,
  10. Janka Biznárová,
  11. Anita Fadavi Roudsari,
  12. Benjamin Lienhard,
  13. Marco Caputo,
  14. Kestutis Grigoras,
  15. Leif Grönberg,
  16. Joonas Govenius,
  17. Anton Frisk Kockum,
  18. Per Delsing,
  19. Jonas Bylander,
  20. and Giovanna Tancredi
High-fidelity and rapid readout of a qubit state is key to quantum computing and communication, and it is a prerequisite for quantum error correction. We present a readout scheme for
superconducting qubits that combines two microwave techniques: applying a shelving technique to the qubit that effectively increases the energy-relaxation time, and a two-tone excitation of the readout resonator to distinguish among qubit populations in higher energy levels. Using a machine-learning algorithm to post-process the two-tone measurement results further improves the qubit-state assignment fidelity. We perform single-shot frequency-multiplexed qubit readout, with a 140ns readout time, and demonstrate 99.5% assignment fidelity for two-state readout and 96.9% for three-state readout – without using a quantum-limited amplifier.

Extensive characterization of a family of efficient three-qubit gates at the coherence limit

  1. Christopher W. Warren,
  2. Jorge Fernández-Pendás,
  3. Shahnawaz Ahmed,
  4. Tahereh Abad,
  5. Andreas Bengtsson,
  6. Janka Biznárová,
  7. Kamanasish Debnath,
  8. Xiu Gu,
  9. Christian Križan,
  10. Amr Osman,
  11. Anita Fadavi Roudsari,
  12. Per Delsing,
  13. Göran Johansson,
  14. Anton Frisk Kockum,
  15. Giovanna Tancredi,
  16. and Jonas Bylander
While all quantum algorithms can be expressed in terms of single-qubit and two-qubit gates, more expressive gate sets can help reduce the algorithmic depth. This is important in the
presence of gate errors, especially those due to decoherence. Using superconducting qubits, we have implemented a three-qubit gate by simultaneously applying two-qubit operations, thereby realizing a three-body interaction. This method straightforwardly extends to other quantum hardware architectures, requires only a „firmware“ upgrade to implement, and is faster than its constituent two-qubit gates. The three-qubit gate represents an entire family of operations, creating flexibility in quantum-circuit compilation. We demonstrate a gate fidelity of 97.90%, which is near the coherence limit of our device. We then generate two classes of entangled states, the GHZ and W states, by applying the new gate only once; in comparison, decompositions into the standard gate set would have a two-qubit gate depth of two and three, respectively. Finally, we combine characterization methods and analyze the experimental and statistical errors on the fidelity of the gates and of the target states.