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).

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.

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.

Engineering symmetry-selective couplings of a superconducting artificial molecule to microwave waveguides

  1. Mohammed Ali Aamir,
  2. Claudia Castillo Moreno,
  3. Simon Sundelin,
  4. Janka Biznárová,
  5. Marco Scigliuzzo,
  6. Kowshik Erappaji Patel,
  7. Amr Osman,
  8. D. P. Lozano,
  9. and Simone Gasparinetti
Tailoring the decay rate of structured quantum emitters into their environment opens new avenues for nonlinear quantum optics, collective phenomena, and quantum communications. Here
we demonstrate a novel coupling scheme between an artificial molecule comprising two identical, strongly coupled transmon qubits, and two microwave waveguides. In our scheme, the coupling is engineered so that transitions between states of the same (opposite) symmetry, with respect to the permutation operator, are predominantly coupled to one (the other) waveguide. The symmetry-based coupling selectivity, as quantified by the ratio of the coupling strengths, exceeds a factor of 30 for both the waveguides in our device. In addition, we implement a two-photon Raman process activated by simultaneously driving both waveguides, and show that it can be used to coherently couple states of different symmetry in the single-excitation manifold of the molecule. Using that process, we implement frequency conversion across the waveguides, mediated by the molecule, with efficiency of about 95%. Finally, we show that this coupling arrangement makes it possible to straightforwardly generate spatially-separated Bell states propagating across the waveguides. We envisage further applications to quantum thermodynamics, microwave photodetection, and photon-photon gates.

Building Blocks of a Flip-Chip Integrated Superconducting Quantum Processor

  1. Sandoko Kosen,
  2. Hang-Xi Li,
  3. Marcus Rommel,
  4. Daryoush Shiri,
  5. Christopher Warren,
  6. Leif Grönberg,
  7. Jaakko Salonen,
  8. Tahereh Abad,
  9. Janka Biznárová,
  10. Marco Caputo,
  11. Liangyu Chen,
  12. Kestutis Grigoras,
  13. Göran Johansson,
  14. Anton Frisk Kockum,
  15. Christian Križan,
  16. Daniel Pérez Lozano,
  17. Graham Norris,
  18. Amr Osman,
  19. Jorge Fernández-Pendás,
  20. Anita Fadavi Roudsari,
  21. Giovanna Tancredi,
  22. Andreas Wallraff,
  23. Christopher Eichler,
  24. Joonas Govenius,
  25. and Jonas Bylander
We have integrated single and coupled superconducting transmon qubits into flip-chip modules. Each module consists of two chips – one quantum chip and one control chip –
that are bump-bonded together. We demonstrate time-averaged coherence times exceeding 90μs, single-qubit gate fidelities exceeding 99.9%, and two-qubit gate fidelities above 98.6%. We also present device design methods and discuss the sensitivity of device parameters to variation in interchip spacing. Notably, the additional flip-chip fabrication steps do not degrade the qubit performance compared to our baseline state-of-the-art in single-chip, planar circuits. This integration technique can be extended to the realisation of quantum processors accommodating hundreds of qubits in one module as it offers adequate input/output wiring access to all qubits and couplers.

Robust preparation of Wigner-negative states with optimized SNAP-displacement sequences

  1. Marina Kudra,
  2. Mikael Kervinen,
  3. Ingrid Strandberg,
  4. Shahnawaz Ahmed,
  5. Marco Scigliuzzo,
  6. Amr Osman,
  7. Daniel Pérez Lozano,
  8. Giulia Ferrini,
  9. Jonas Bylander,
  10. Anton Frisk Kockum,
  11. Fernando Quijandría,
  12. Per Delsing,
  13. and Simone Gasparinetti
Hosting non-classical states of light in three-dimensional microwave cavities has emerged as a promising paradigm for continuous-variable quantum information processing. Here we experimentally
demonstrate high-fidelity generation of a range of Wigner-negative states useful for quantum computation, such as Schrödinger-cat states, binomial states, Gottesman-Kitaev-Preskill (GKP) states, as well as cubic phase states. The latter states have been long sought after in quantum optics and were never achieved experimentally before. To do so, we use a sequence of interleaved selective number-dependent arbitrary phase (SNAP) gates and displacements. We optimize the state preparation in two steps. First we use a gradient-descent algorithm to optimize the parameters of the SNAP and displacement gates. Then we optimize the envelope of the pulses implementing the SNAP gates. Our results show that this way of creating highly non-classical states in a harmonic oscillator is robust to fluctuations of the system parameters such as the qubit frequency and the dispersive shift.