Implementing a synthetic magnetic vector potential in a 2D superconducting qubit array

  1. Ilan T. Rosen,
  2. Sarah Muschinske,
  3. Cora N. Barrett,
  4. Arkya Chatterjee,
  5. Max Hays,
  6. Michael DeMarco,
  7. Amir Karamlou,
  8. David Rower,
  9. Rabindra Das,
  10. David K. Kim,
  11. Bethany M. Niedzielski,
  12. Meghan Schuldt,
  13. Kyle Serniak,
  14. Mollie E. Schwartz,
  15. Jonilyn L. Yoder,
  16. Jeffrey A. Grover,
  17. and William D. Oliver
Superconducting quantum processors are a compelling platform for analog quantum simulation due to the precision control, fast operation, and site-resolved readout inherent to the hardware.
Arrays of coupled superconducting qubits natively emulate the dynamics of interacting particles according to the Bose-Hubbard model. However, many interesting condensed-matter phenomena emerge only in the presence of electromagnetic fields. Here, we emulate the dynamics of charged particles in an electromagnetic field using a superconducting quantum simulator. We realize a broadly adjustable synthetic magnetic vector potential by applying continuous modulation tones to all qubits. We verify that the synthetic vector potential obeys requisite properties of electromagnetism: a spatially-varying vector potential breaks time-reversal symmetry and generates a gauge-invariant synthetic magnetic field, and a temporally-varying vector potential produces a synthetic electric field. We demonstrate that the Hall effect–the transverse deflection of a charged particle propagating in an electromagnetic field–exists in the presence of the synthetic electromagnetic field.

High-Fidelity, Frequency-Flexible Two-Qubit Fluxonium Gates with a Transmon Coupler

  1. Leon Ding,
  2. Max Hays,
  3. Youngkyu Sung,
  4. Bharath Kannan,
  5. Junyoung An,
  6. Agustin Di Paolo,
  7. Amir H. Karamlou,
  8. Thomas M. Hazard,
  9. Kate Azar,
  10. David K. Kim,
  11. Bethany M. Niedzielski,
  12. Alexander Melville,
  13. Mollie E. Schwartz,
  14. Jonilyn L. Yoder,
  15. Terry P. Orlando,
  16. Simon Gustavsson,
  17. Jeffrey A. Grover,
  18. Kyle Serniak,
  19. and William D. Oliver
We propose and demonstrate an architecture for fluxonium-fluxonium two-qubit gates mediated by transmon couplers (FTF, for fluxonium-transmon-fluxonium). Relative to architectures that
exclusively rely on a direct coupling between fluxonium qubits, FTF enables stronger couplings for gates using non-computational states while simultaneously suppressing the static controlled-phase entangling rate (ZZ) down to kHz levels, all without requiring strict parameter matching. Here we implement FTF with a flux-tunable transmon coupler and demonstrate a microwave-activated controlled-Z (CZ) gate whose operation frequency can be tuned over a 2 GHz range, adding frequency allocation freedom for FTF’s in larger systems. Across this range, state-of-the-art CZ gate fidelities were observed over many bias points and reproduced across the two devices characterized in this work. After optimizing both the operation frequency and the gate duration, we achieved peak CZ fidelities in the 99.85-99.9\% range. Finally, we implemented model-free reinforcement learning of the pulse parameters to boost the mean gate fidelity up to 99.922±0.009%, averaged over roughly an hour between scheduled training runs. Beyond the microwave-activated CZ gate we present here, FTF can be applied to a variety of other fluxonium gate schemes to improve gate fidelities and passively reduce unwanted ZZ interactions.

Dynamically Reconfigurable Photon Exchange in a Superconducting Quantum Processor

  1. Brian Marinelli,
  2. Jie Luo,
  3. Hengjiang Ren,
  4. Bethany M. Niedzielski,
  5. David K. Kim,
  6. Rabindra Das,
  7. Mollie Schwartz,
  8. David I. Santiago,
  9. and Irfan Siddiqi
Realizing the advantages of quantum computation requires access to the full Hilbert space of states of many quantum bits (qubits). Thus, large-scale quantum computation faces the challenge
of efficiently generating entanglement between many qubits. In systems with a limited number of direct connections between qubits, entanglement between non-nearest neighbor qubits is generated by a series of nearest neighbor gates, which exponentially suppresses the resulting fidelity. Here we propose and demonstrate a novel, on-chip photon exchange network. This photonic network is embedded in a superconducting quantum processor (QPU) to implement an arbitrarily reconfigurable qubit connectivity graph. We show long-range qubit-qubit interactions between qubits with a maximum spatial separation of 9.2 cm along a meandered bus resonator and achieve photon exchange rates up to gqq=2π×0.9 MHz. These experimental demonstrations provide a foundation to realize highly connected, reconfigurable quantum photonic networks and opens a new path towards modular quantum computing.

Learning-based Calibration of Flux Crosstalk in Transmon Qubit Arrays

  1. Cora N. Barrett,
  2. Amir H. Karamlou,
  3. Sarah E. Muschinske,
  4. Ilan T. Rosen,
  5. Jochen Braumüller,
  6. Rabindra Das,
  7. David K. Kim,
  8. Bethany M. Niedzielski,
  9. Meghan Schuldt,
  10. Kyle Serniak,
  11. Mollie E. Schwartz,
  12. Jonilyn L. Yoder,
  13. Terry P. Orlando,
  14. Simon Gustavsson,
  15. Jeffrey A. Grover,
  16. and William D. Oliver
Superconducting quantum processors comprising flux-tunable data and coupler qubits are a promising platform for quantum computation. However, magnetic flux crosstalk between the flux-control
lines and the constituent qubits impedes precision control of qubit frequencies, presenting a challenge to scaling this platform. In order to implement high-fidelity digital and analog quantum operations, one must characterize the flux crosstalk and compensate for it. In this work, we introduce a learning-based calibration protocol and demonstrate its experimental performance by calibrating an array of 16 flux-tunable transmon qubits. To demonstrate the extensibility of our protocol, we simulate the crosstalk matrix learning procedure for larger arrays of transmon qubits. We observe an empirically linear scaling with system size, while maintaining a median qubit frequency error below 300 kHz.

Evolution of 1/f Flux Noise in Superconducting Qubits with Weak Magnetic Fields

  1. David A. Rower,
  2. Lamia Ateshian,
  3. Lauren H. Li,
  4. Max Hays,
  5. Dolev Bluvstein,
  6. Leon Ding,
  7. Bharath Kannan,
  8. Aziza Almanakly,
  9. Jochen Braumüller,
  10. David K. Kim,
  11. Alexander Melville,
  12. Bethany M. Niedzielski,
  13. Mollie E. Schwartz,
  14. Jonilyn L. Yoder,
  15. Terry P. Orlando,
  16. Joel I-Jan Wang,
  17. Simon Gustavsson,
  18. Jeffrey A. Grover,
  19. Kyle Serniak,
  20. Riccardo Comin,
  21. and William D. Oliver
The microscopic origin of 1/f magnetic flux noise in superconducting circuits has remained an open question for several decades despite extensive experimental and theoretical investigation.
Recent progress in superconducting devices for quantum information has highlighted the need to mitigate sources of qubit decoherence, driving a renewed interest in understanding the underlying noise mechanism(s). Though a consensus has emerged attributing flux noise to surface spins, their identity and interaction mechanisms remain unclear, prompting further study. Here we apply weak in-plane magnetic fields to a capacitively-shunted flux qubit (where the Zeeman splitting of surface spins lies below the device temperature) and study the flux-noise-limited qubit dephasing, revealing previously unexplored trends that may shed light on the dynamics behind the emergent 1/f noise. Notably, we observe an enhancement (suppression) of the spin-echo (Ramsey) pure dephasing time in fields up to B=100 G. With direct noise spectroscopy, we further observe a transition from a 1/f to approximately Lorentzian frequency dependence below 10 Hz and a reduction of the noise above 1 MHz with increasing magnetic field. We suggest that these trends are qualitatively consistent with an increase of spin cluster sizes with magnetic field. These results should help to inform a complete microscopic theory of 1/f flux noise in superconducting circuits.

Demonstration of long-range correlations via susceptibility measurements in a one-dimensional superconducting Josephson spin chain

  1. Daniel M. Tennant,
  2. Xi Dai,
  3. Antonio J. Martinez,
  4. Robbyn Trappen,
  5. Denis Melanson,
  6. M. A. Yurtalan,
  7. Yongchao Tang,
  8. Salil Bedkihal,
  9. Rui Yang,
  10. Sergei Novikov,
  11. Jeffery A. Grover,
  12. Steven M. Disseler,
  13. James I. Basham,
  14. Rabindra Das,
  15. David K. Kim,
  16. Alexander J. Melville,
  17. Bethany M. Niedzielski,
  18. Steven J. Weber,
  19. Jonilyn L. Yoder,
  20. Andrew J. Kerman,
  21. Evgeny Mozgunov,
  22. Daniel A. Lidar,
  23. and Adrian Lupascu
Spin chains have long been considered an effective medium for long-range interactions, entanglement generation, and quantum state transfer. In this work, we explore the properties of
a spin chain implemented with superconducting flux circuits, designed to act as a connectivity medium between two superconducting qubits. The susceptibility of the chain is probed and shown to support long-range, cross chain correlations. In addition, interactions between the two end qubits, mediated by the coupler chain, are demonstrated. This work has direct applicability in near term quantum annealing processors as a means of generating long-range, coherent coupling between qubits.

Lindblad Tomography of a Superconducting Quantum Processor

  1. Gabriel O. Samach,
  2. Ami Greene,
  3. Johannes Borregaard,
  4. Matthias Christandl,
  5. David K. Kim,
  6. Christopher M. McNally,
  7. Alexander Melville,
  8. Bethany M. Niedzielski,
  9. Youngkyu Sung,
  10. Danna Rosenberg,
  11. Mollie E. Schwartz,
  12. Jonilyn L. Yoder,
  13. Terry P. Orlando,
  14. Joel I-Jan Wang,
  15. Simon Gustavsson,
  16. Morten Kjaergaard,
  17. and William D. Oliver
As progress is made towards the first generation of error-corrected quantum computers, careful characterization of a processor’s noise environment will be crucial to designing
tailored, low-overhead error correction protocols. While standard coherence metrics and characterization protocols such as T1 and T2, process tomography, and randomized benchmarking are now ubiquitous, these techniques provide only partial information about the dynamic multi-qubit loss channels responsible for processor errors, which can be described more fully by a Lindblad operator in the master equation formalism. Here, we introduce and experimentally demonstrate Lindblad Tomography, a hardware-agnostic characterization protocol for tomographically reconstructing the Hamiltonian and Lindblad operators of a quantum channel from an ensemble of time-domain measurements. Performing Lindblad Tomography on a small superconducting quantum processor, we show that this technique characterizes and accounts for state-preparation and measurement (SPAM) errors and allows one to place strong bounds on the degree of non-Markovianity in the channels of interest. Comparing the results of single- and two-qubit measurements on a superconducting quantum processor, we demonstrate that Lindblad Tomography can also be used to identify and quantify sources of crosstalk on quantum processors, such as the presence of always-on qubit-qubit interactions.

Improving qubit coherence using closed-loop feedback

  1. Antti Vepsäläinen,
  2. Roni Winik,
  3. Amir H. Karamlou,
  4. Jochen Braumüller,
  5. Agustin Di Paolo,
  6. Youngkyu Sung,
  7. Bharath Kannan,
  8. Morten Kjaergaard,
  9. David K. Kim,
  10. Alexander J. Melville,
  11. Bethany M. Niedzielski,
  12. Jonilyn L. Yoder,
  13. Simon Gustavsson,
  14. and William D. Oliver
Superconducting qubits are a promising platform for building a larger-scale quantum processor capable of solving otherwise intractable problems. In order for the processor to reach
practical viability, the gate errors need to be further suppressed and remain stable for extended periods of time. With recent advances in qubit control, both single- and two-qubit gate fidelities are now in many cases limited by the coherence times of the qubits. Here we experimentally employ closed-loop feedback to stabilize the frequency fluctuations of a superconducting transmon qubit, thereby increasing its coherence time by 26\% and reducing the single-qubit error rate from (8.5±2.1)×10−4 to (5.9±0.7)×10−4. Importantly, the resulting high-fidelity operation remains effective even away from the qubit flux-noise insensitive point, significantly increasing the frequency bandwidth over which the qubit can be operated with high fidelity. This approach is helpful in large qubit grids, where frequency crowding and parasitic interactions between the qubits limit their performance.

Fabrication of superconducting through-silicon vias

  1. Justin L. Mallek,
  2. Donna-Ruth W. Yost,
  3. Danna Rosenberg,
  4. Jonilyn L. Yoder,
  5. Gregory Calusine,
  6. Matt Cook,
  7. Rabindra Das,
  8. Alexandra Day,
  9. Evan Golden,
  10. David K. Kim,
  11. Jeffery Knecht,
  12. Bethany M. Niedzielski,
  13. Mollie Schwartz,
  14. Arjan Sevi,
  15. Corey Stull,
  16. Wayne Woods,
  17. Andrew J. Kerman,
  18. and William D. Oliver
Increasing circuit complexity within quantum systems based on superconducting qubits necessitates high connectivity while retaining qubit coherence. Classical micro-electronic systems
have addressed interconnect density challenges by using 3D integration with interposers containing through-silicon vias (TSVs), but extending these integration techniques to superconducting quantum systems is challenging. Here, we discuss our approach for realizing high-aspect-ratio superconducting TSVs\textemdash 10 μm wide by 20 μm long by 200 μm deep\textemdash with densities of 100 electrically isolated TSVs per square millimeter. We characterize the DC and microwave performance of superconducting TSVs at cryogenic temperatures and demonstrate superconducting critical currents greater than 20 mA. These high-aspect-ratio, high critical current superconducting TSVs will enable high-density vertical signal routing within superconducting quantum processors.

Microwave Package Design for Superconducting Quantum Processors

  1. Sihao Huang,
  2. Benjamin Lienhard,
  3. Greg Calusine,
  4. Antti Vepsäläinen,
  5. Jochen Braumüller,
  6. David K. Kim,
  7. Alexander J. Melville,
  8. Bethany M. Niedzielski,
  9. Jonilyn L. Yoder,
  10. Bharath Kannan,
  11. Terry P. Orlando,
  12. Simon Gustavsson,
  13. and William D. Oliver
Solid-state qubits with transition frequencies in the microwave regime, such as superconducting qubits, are at the forefront of quantum information processing. However, high-fidelity,
simultaneous control of superconducting qubits at even a moderate scale remains a challenge, partly due to the complexities of packaging these devices. Here, we present an approach to microwave package design focusing on material choices, signal line engineering, and spurious mode suppression. We describe design guidelines validated using simulations and measurements used to develop a 24-port microwave package. Analyzing the qubit environment reveals no spurious modes up to 11GHz. The material and geometric design choices enable the package to support qubits with lifetimes exceeding 350 {\mu}s. The microwave package design guidelines presented here address many issues relevant for near-term quantum processors.