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.

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.

Quantum transport and localization in 1d and 2d tight-binding lattices

  1. Amir H. Karamlou,
  2. Jochen Braumüller,
  3. Yariv Yanay,
  4. Agustin Di Paolo,
  5. Patrick Harrington,
  6. Bharath Kannan,
  7. David Kim,
  8. Morten Kjaergaard,
  9. Alexander Melville,
  10. Sarah Muschinske,
  11. Bethany Niedzielski,
  12. Antti Vepsäläinen,
  13. Roni Winik,
  14. Jonilyn L. Yoder,
  15. Mollie Schwartz,
  16. Charles Tahan,
  17. Terry P. Orlando,
  18. Simon Gustavsson,
  19. and William D. Oliver
Particle transport and localization phenomena in condensed-matter systems can be modeled using a tight-binding lattice Hamiltonian. The ideal experimental emulation of such a model
utilizes simultaneous, high-fidelity control and readout of each lattice site in a highly coherent quantum system. Here, we experimentally study quantum transport in one-dimensional and two-dimensional tight-binding lattices, emulated by a fully controllable 3×3 array of superconducting qubits. We probe the propagation of entanglement throughout the lattice and extract the degree of localization in the Anderson and Wannier-Stark regimes in the presence of site-tunable disorder strengths and gradients. Our results are in quantitative agreement with numerical simulations and match theoretical predictions based on the tight-binding model. The demonstrated level of experimental control and accuracy in extracting the system observables of interest will enable the exploration of larger, interacting lattices where numerical simulations become intractable.

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.

Deep Neural Network Discrimination of Multiplexed Superconducting Qubit States

  1. Benjamin Lienhard,
  2. Antti Vepsäläinen,
  3. Luke C.G. Govia,
  4. Cole R. Hoffer,
  5. Jack Y. Qiu,
  6. Diego Ristè,
  7. Matthew Ware,
  8. David Kim,
  9. Roni Winik,
  10. Alexander Melville,
  11. Bethany Niedzielski,
  12. Jonilyn Yoder,
  13. Guilhem J. Ribeill,
  14. Thomas A. Ohki,
  15. Hari K. Krovi,
  16. Terry P. Orlando,
  17. Simon Gustavsson,
  18. and William D. Oliver
Demonstrating the quantum computational advantage will require high-fidelity control and readout of multi-qubit systems. As system size increases, multiplexed qubit readout becomes
a practical necessity to limit the growth of resource overhead. Many contemporary qubit-state discriminators presume single-qubit operating conditions or require considerable computational effort, limiting their potential extensibility. Here, we present multi-qubit readout using neural networks as state discriminators. We compare our approach to contemporary methods employed on a quantum device with five superconducting qubits and frequency-multiplexed readout. We find that fully-connected feedforward neural networks increase the qubit-state-assignment fidelity for our system. Relative to contemporary discriminators, the assignment error rate is reduced by up to 25 % due to the compensation of system-dependent nonidealities such as readout crosstalk which is reduced by up to one order of magnitude. Our work demonstrates a potentially extensible building block for high-fidelity readout relevant to both near-term devices and future fault-tolerant systems.

Probing quantum information propagation with out-of-time-ordered correlators

  1. Jochen Braumüller,
  2. Amir H. Karamlou,
  3. Yariv Yanay,
  4. Bharath Kannan,
  5. David Kim,
  6. Morten Kjaergaard,
  7. Alexander Melville,
  8. Bethany M. Niedzielski,
  9. Youngkyu Sung,
  10. Antti Vepsäläinen,
  11. Roni Winik,
  12. Jonilyn L. Yoder,
  13. Terry P. Orlando,
  14. Simon Gustavsson,
  15. Charles Tahan,
  16. and William D. Oliver
Interacting many-body quantum systems show a rich array of physical phenomena and dynamical properties, but are notoriously difficult to study: they are challenging analytically and
exponentially difficult to simulate on classical computers. Small-scale quantum information processors hold the promise to efficiently emulate these systems, but characterizing their dynamics is experimentally challenging, requiring probes beyond simple correlation functions and multi-body tomographic methods. Here, we demonstrate the measurement of out-of-time-ordered correlators (OTOCs), one of the most effective tools for studying quantum system evolution and processes like quantum thermalization. We implement a 3×3 two-dimensional hard-core Bose-Hubbard lattice with a superconducting circuit, study its time-reversibility by performing a Loschmidt echo, and measure OTOCs that enable us to observe the propagation of quantum information. A central requirement for our experiments is the ability to coherently reverse time evolution, which we achieve with a digital-analog simulation scheme. In the presence of frequency disorder, we observe that localization can partially be overcome with more particles present, a possible signature of many-body localization in two dimensions.

Realization of high-fidelity CZ and ZZ-free iSWAP gates with a tunable coupler

  1. Youngkyu Sung,
  2. Leon Ding,
  3. Jochen Braumüller,
  4. Antti Vepsäläinen,
  5. Bharath Kannan,
  6. Morten Kjaergaard,
  7. Ami Greene,
  8. Gabriel O. Samach,
  9. Chris McNally,
  10. David Kim,
  11. Alexander Melville,
  12. Bethany M. Niedzielski,
  13. Mollie E. Schwartz,
  14. Jonilyn L. Yoder,
  15. Terry P. Orlando,
  16. Simon Gustavsson,
  17. and William D. Oliver
High-fidelity two-qubit gates at scale are a key requirement to realize the full promise of quantum computation and simulation. The advent and use of coupler elements to tunably control
two-qubit interactions has improved operational fidelity in many-qubit systems by reducing parasitic coupling and frequency crowding issues. However, two-qubit gate errors still limit the capability of near-term quantum applications. In particular, the existing framework for tunable couplers based on the dispersive approximation does not fully incorporate three-body multi-level dynamics, which are essential for addressing coherent leakage to the coupler and parasitic longitudinal (ZZ) interactions during two-qubit gates. Here, we present a new systematic approach that goes beyond the dispersive approximation and outlines how to optimize the coupler-control and exploit the engineered level structure of the coupler. Using this approach, we experimentally demonstrate a CZ gate with 99.76 ± 0.10 % fidelity and a ZZ-free iSWAP gate with 99.86 ± 0.32 % fidelity, which are close to their T1 limits.

Comparison of Dielectric Loss in Titanium Nitride and Aluminum Superconducting Resonators

  1. Alexander Melville,
  2. Greg Calusine,
  3. Wayne Woods,
  4. Kyle Serniak,
  5. Evan Golden,
  6. Bethany M. Niedzielski,
  7. David K. Kim,
  8. Arjan Sevi,
  9. Jonilyn L. Yoder,
  10. Eric A. Dauler,
  11. and William D. Oliver
Lossy dielectrics are a significant source of decoherence in superconducting quantum circuits. In this report, we model and compare the dielectric loss in bulk and interfacial dielectrics
in titanium nitride (TiN) and aluminum (Al) superconducting coplanar waveguide (CPW) resonators. We fabricate isotropically trenched resonators to produce a series of device geometries that accentuate a specific dielectric region’s contribution to resonator quality factor. While each dielectric region contributes significantly to loss in TiN devices, the metal-air interface dominates the loss in the Al devices. Furthermore, we evaluate the quality factor of each TiN resonator geometry with and without a post-process hydrofluoric (HF) etch, and find that it reduced losses from the substrate-air interface, thereby improving the quality factor.