All-Pass Readout for Robust and Scalable Quantum Measurement

  1. Alec Yen,
  2. Yufeng Ye,
  3. Kaidong Peng,
  4. Jennifer Wang,
  5. Gregory Cunningham,
  6. Michael Gingras,
  7. Bethany M. Niedzielski,
  8. Hannah Stickler,
  9. Kyle Serniak,
  10. Mollie E. Schwartz,
  11. and Kevin P. O'Brien
Robust and scalable multiplexed qubit readout will be essential to the realization of a fault-tolerant quantum computer. To this end, we propose and demonstrate transmission-based dispersive
readout of a superconducting qubit using an all-pass resonator that preferentially emits readout photons in one direction. This is in contrast to typical readout schemes, which intentionally mismatch the feedline at one end so that the readout signal preferentially decays toward the output. We show that this intentional mismatch creates scaling challenges, including larger spread of effective resonator linewidths due to non-ideal impedance environments and added infrastructure for impedance matching. Our proposed „all-pass readout“ architecture avoids the need for intentional mismatch and aims to enable reliable, modular design of multiplexed qubit readout, thus improving the scaling prospects of quantum computers. We design and fabricate an all-pass readout resonator that demonstrates insertion loss below 1.17 dB at the readout frequency and a maximum insertion loss of 1.53 dB across its full bandwidth for the lowest three states of a transmon qubit. We demonstrate qubit readout with an average single-shot fidelity of 98.1% in 600 ns; to assess the effect of larger dispersive shift, we implement a shelving protocol and achieve a fidelity of 99.0% in 300 ns.

Synchronous Detection of Cosmic Rays and Correlated Errors in Superconducting Qubit Arrays

  1. Patrick M. Harrington,
  2. Mingyu Li,
  3. Max Hays,
  4. Wouter Van De Pontseele,
  5. Daniel Mayer,
  6. H. Douglas Pinckney,
  7. Felipe Contipelli,
  8. Michael Gingras,
  9. Bethany M. Niedzielski,
  10. Hannah Stickler,
  11. Jonilyn L. Yoder,
  12. Mollie E. Schwartz,
  13. Jeffrey A. Grover,
  14. Kyle Serniak,
  15. William D. Oliver,
  16. and Joseph A. Formaggio
Quantum information processing at scale will require sufficiently stable and long-lived qubits, likely enabled by error-correction codes. Several recent superconducting-qubit experiments,
however, reported observing intermittent spatiotemporally correlated errors that would be problematic for conventional codes, with ionizing radiation being a likely cause. Here, we directly measured the cosmic-ray contribution to spatiotemporally correlated qubit errors. We accomplished this by synchronously monitoring cosmic-ray detectors and qubit energy-relaxation dynamics of 10 transmon qubits distributed across a 5x5x0.35 mm3 silicon chip. Cosmic rays caused correlated errors at a rate of 1/(10 min), accounting for 17±1% of all such events. Our qubits responded to essentially all of the cosmic rays and their secondary particles incident on the chip, consistent with the independently measured arrival flux. Moreover, we observed that the landscape of the superconducting gap in proximity to the Josephson junctions dramatically impacts the qubit response to cosmic rays. Given the practical difficulties associated with shielding cosmic rays, our results indicate the importance of radiation hardening — for example, superconducting gap engineering — to the realization of robust quantum error correction.

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 tunable three-body interactions between superconducting qubits

  1. Tim Menke,
  2. William P. Banner,
  3. Thomas R. Bergamaschi,
  4. Agustin Di Paolo,
  5. Antti Vepsäläinen,
  6. Steven J. Weber,
  7. Roni Winik,
  8. Alexander Melville,
  9. Bethany M. Niedzielski,
  10. Danna Rosenberg,
  11. Kyle Serniak,
  12. Mollie E. Schwartz,
  13. Jonilyn L. Yoder,
  14. Alán Aspuru-Guzik,
  15. Simon Gustavsson,
  16. Jeffrey A. Grover,
  17. Cyrus F. Hirjibehedin,
  18. Andrew J. Kerman,
  19. and William D. Oliver
Nonpairwise multi-qubit interactions present a useful resource for quantum information processors. Their implementation would facilitate more efficient quantum simulations of molecules
and combinatorial optimization problems, and they could simplify error suppression and error correction schemes. Here we present a superconducting circuit architecture in which a coupling module mediates 2-local and 3-local interactions between three flux qubits by design. The system Hamiltonian is estimated via multi-qubit pulse sequences that implement Ramsey-type interferometry between all neighboring excitation manifolds in the system. The 3-local interaction is coherently tunable over several MHz via the coupler flux biases and can be turned off, which is important for applications in quantum annealing, analog quantum simulation, and gate-model quantum computation.

On-Demand Directional Photon Emission using Waveguide Quantum Electrodynamics

  1. Bharath Kannan,
  2. Aziza Almanakly,
  3. Youngkyu Sung,
  4. Agustin Di Paolo,
  5. David A. Rower,
  6. Jochen Braumüller,
  7. Alexander Melville,
  8. Bethany M. Niedzielski,
  9. Amir Karamlou,
  10. Kyle Serniak,
  11. Antti Vepsäläinen,
  12. Mollie E. Schwartz,
  13. Jonilyn L. Yoder,
  14. Roni Winik,
  15. Joel I-Jan Wang,
  16. Terry P. Orlando,
  17. Simon Gustavsson,
  18. Jeffrey A. Grover,
  19. and William D. Oliver
Routing quantum information between non-local computational nodes is a foundation for extensible networks of quantum processors. Quantum information can be transferred between arbitrary
nodes by photons that propagate between them, or by resonantly coupling nearby nodes. Notably, conventional approaches involving propagating photons have limited fidelity due to photon loss and are often unidirectional, whereas architectures that use direct resonant coupling are bidirectional in principle, but can generally accommodate only a few local nodes. Here, we demonstrate high-fidelity, on-demand, bidirectional photon emission using an artificial molecule comprising two superconducting qubits strongly coupled to a waveguide. Quantum interference between the photon emission pathways from the molecule generate single photons that selectively propagate in a chosen direction. This architecture is capable of both photon emission and capture, and can be tiled in series to form an extensible network of quantum processors with all-to-all connectivity.

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.

Hexagonal Boron Nitride (hBN) as a Low-loss Dielectric for Superconducting Quantum Circuits and Qubits

  1. Joel I.J. Wang,
  2. Megan A. Yamoah,
  3. Qing Li,
  4. Amir Karamlou,
  5. Thao Dinh,
  6. Bharath Kannan,
  7. Jochen Braumüller,
  8. David Kim,
  9. Alexander J. Melville,
  10. Sarah E. Muschinske,
  11. Bethany M. Niedzielski,
  12. Kyle Serniak,
  13. Youngkyu Sung,
  14. Roni Winik,
  15. Jonilyn L. Yoder,
  16. Mollie Schwartz,
  17. Kenji Watanabe,
  18. Takashi Taniguchi,
  19. Terry P. Orlando,
  20. Simon Gustavsson,
  21. Pablo Jarillo-Herrero,
  22. and William D. Oliver
Dielectrics with low loss at microwave frequencies are imperative for high-coherence solid-state quantum computing platforms. We study the dielectric loss of hexagonal boron nitride
(hBN) thin films in the microwave regime by measuring the quality factor of parallel-plate capacitors (PPCs) made of NbSe2-hBN-NbSe2 heterostructures integrated into superconducting circuits. The extracted microwave loss tangent of hBN is bounded to be at most in the mid-10-6 range in the low temperature, single-photon regime. We integrate hBN PPCs with aluminum Josephson junctions to realize transmon qubits with coherence times reaching 25 μs, consistent with the hBN loss tangent inferred from resonator measurements. The hBN PPC reduces the qubit feature size by approximately two-orders of magnitude compared to conventional all-aluminum coplanar transmons. Our results establish hBN as a promising dielectric for building high-coherence quantum circuits with substantially reduced footprint and, with a high energy participation that helps to reduce unwanted qubit cross-talk.