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.

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.

Microwave amplification via interfering multi-photon processes in a half-waveguide quantum electrodynamics system

  1. Fahad Aziz,
  2. Kuan-Ting Lin,
  3. Ping-Yi Wen,
  4. Samina,
  5. Yu Chen Lin,
  6. Emely Wiegand,
  7. Ching-Ping Lee,
  8. Yu-Ting Cheng,
  9. Ching-Yeh Chen,
  10. Chin-Hsun Chien,
  11. Kai-Min Hsieh,
  12. Yu-Huan Huang,
  13. Ian Hou,
  14. Jeng-Chung Chen,
  15. Yen-Hsiang Lin,
  16. Anton Frisk Kockum,
  17. Guin-Dar Lin,
  18. and Io-Chun Hoi
We investigate the amplification of a microwave probe signal by a superconducting artificial atom, a transmon, strongly coupled to the end of a one-dimensional semi-infinite transmission
line. The end of the transmission line acts as a mirror for microwave fields. Due to the weak anharmonicity of the artificial atom, a strong pump field creates multi-photon excitations among the dressed states. Transitions between these dressed states, Rabi sidebands, give rise to either amplification or attenuation of the weak probe. We obtain a maximum amplitude amplification of about 18 %, higher than in any previous experiment with a single artificial atom, due to constructive interference between Rabi sidebands. We also characterize the noise properties of the system by measuring the spectrum of spontaneous emission.

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.

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.

Nonequilibrium heat transport and work with a single artificial atom coupled to a waveguide: emission without external driving

  1. Yong Lu,
  2. Neill Lambert,
  3. Anton Frisk Kockum,
  4. Ken Funo,
  5. Andreas Bengtsson,
  6. Simone Gasparinetti,
  7. Franco Nori,
  8. and Per Delsing
We observe the continuous emission of photons into a waveguide from a superconducting qubit without the application of an external drive. To explain this observation, we build a two-bath
model where the qubit couples simultaneously to a cold bath (the waveguide) and a hot bath (a secondary environment). Our results show that the thermal-photon occupation of the hot bath is up to 0.14 photons, 35 times larger than the cold waveguide, leading to nonequilibrium heat transport with a power of up to 132 zW, as estimated from the qubit emission spectrum. By adding more isolation between the sample output and the first cold amplifier in the output line, the heat transport is strongly suppressed. Our interpretation is that the hot bath may arise from active two-level systems being excited by noise from the output line. We also apply a coherent drive, and use the waveguide to measure thermodynamic work and heat, suggesting waveguide spectroscopy is a useful means to study quantum heat engines and refrigerators. Finally, based on the theoretical model, we propose how a similar setup can be used as a noise spectrometer which provides a new solution for calibrating the background noise of hybrid quantum systems.

Ultimate quantum limit for amplification: a single atom in front of a mirror

  1. Emely Wiegand,
  2. Ping-Yi Wen,
  3. Per Delsing,
  4. Io-Chun Hoi,
  5. and Anton Frisk Kockum
We investigate three types of amplification processes for light fields coupling to an atom near the end of a one-dimensional semi-infinite waveguide. We consider two setups where a
drive creates population inversion in the bare or dressed basis of a three-level atom and one setup where the amplification is due to higher-order processes in a driven two-level atom. In all cases, the end of the waveguide acts as a mirror for the light. We find that this enhances the amplification in two ways compared to the same setups in an open waveguide. Firstly, the mirror forces all output from the atom to travel in one direction instead of being split up into two output channels. Secondly, interference due to the mirror enables tuning of the ratio of relaxation rates for different transitions in the atom to increase population inversion. We quantify the enhancement in amplification due to these factors and show that it can be demonstrated for standard parameters in experiments with superconducting quantum circuits.

Engineering the Level Structure of a Giant Artificial Atom in Waveguide Quantum Electrodynamics

  1. A. M. Vadiraj,
  2. Andreas Ask,
  3. T.G. McConkey,
  4. I. Nsanzineza,
  5. C.W. Sandbo Chang,
  6. Anton Frisk Kockum,
  7. and C. M. Wilson
Engineering light-matter interactions at the quantum level has been central to the pursuit of quantum optics for decades. Traditionally, this has been done by coupling emitters, typically
natural atoms and ions, to quantized electromagnetic fields in optical and microwave cavities. In these systems, the emitter is approximated as an idealized dipole, as its physical size is orders of magnitude smaller than the wavelength of light. Recently, artificial atoms made from superconducting circuits have enabled new frontiers in light-matter coupling, including the study of „giant“ atoms which cannot be approximated as simple dipoles. Here, we explore a new implementation of a giant artificial atom, formed from a transmon qubit coupled to propagating microwaves at multiple points along an open transmission line. The nature of this coupling allows the qubit radiation field to interfere with itself leading to some striking giant-atom effects. For instance, we observe strong frequency-dependent couplings of the qubit energy levels to the electromagnetic modes of the transmission line. Combined with the ability to in situ tune the qubit energy levels, we show that we can modify the relative coupling rates of multiple qubit transitions by more than an order of magnitude. By doing so, we engineer a metastable excited state, allowing us to operate the giant transmon as an effective lambda system where we clearly demonstrate electromagnetically induced transparency.