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.

Extensible circuit-QED architecture via amplitude- and frequency-variable microwaves

  1. Agustin Di Paolo,
  2. Catherine Leroux,
  3. Thomas M. Hazard,
  4. Kyle Serniak,
  5. Simon Gustavsson,
  6. Alexandre Blais,
  7. and William D. Oliver
We introduce a circuit-QED architecture combining fixed-frequency qubits and microwave-driven couplers. In the appropriate frame, the drive parameters appear as tunable knobs enabling
selective two-qubit coupling and coherent-error suppression. We moreover introduce a set of controlled-phase gates based on drive-amplitude and drive-frequency modulation. We develop a theoretical framework based on Floquet theory to model microwave-activated interactions with time-dependent drive parameters, which we also use for pulse shaping. We perform numerical simulations of the gate fidelity for realistic circuit parameters, and discuss the impact of drive-induced decoherence. We estimate average gate fidelities beyond 99.9% for all-microwave controlled-phase operations with gate times in the range 50−120ns. These two-qubit gates can operate over a large drive-frequency bandwidth and in a broad range of circuit parameters, thereby improving extensibility. We address the frequency allocation problem for this architecture using perturbation theory, demonstrating that qubit, coupler and drive frequencies can be chosen such that undesired static and driven interactions remain bounded in a multi-qubit device. Our numerical methods are useful for describing the time-evolution of driven systems in the adiabatic limit, and are applicable to a wide variety of circuit-QED setups.

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.

Distinguishing multi-spin interactions from lower-order effects

  1. Thomas R. Bergamaschi,
  2. Tim Menke,
  3. William P. Banner,
  4. Agustin Di Paolo,
  5. Steven J. Weber,
  6. Cyrus F. Hirjibehedin,
  7. Andrew J. Kerman,
  8. and William D. Oliver
Multi-spin interactions can be engineered with artificial quantum spins. However, it is challenging to verify such interactions experimentally. Here we describe two methods to characterize
the n-local coupling of n spins. First, we analyze the variation of the transition energy of the static system as a function of local spin fields. Standard measurement techniques are employed to distinguish n-local interactions between up to five spins from lower-order contributions in the presence of noise and spurious fields and couplings. Second, we show a detection technique that relies on time dependent driving of the coupling term. Generalizations to larger system sizes are analyzed for both static and dynamic detection methods, and we find that the dynamic method is asymptotically optimal when increasing the system size. The proposed methods enable robust exploration of multi-spin interactions across a broad range of both coupling strengths and qubit modalities.

Mediated interactions beyond the nearest neighbor in an array of superconducting qubits

  1. Yariv Yanay,
  2. Jochen Braumüller,
  3. Terry P. Orlando,
  4. Simon Gustavsson,
  5. Charles Tahan,
  6. and William D. Oliver
We consider mediated interactions in an array of floating transmons, where each qubit capacitor consists of two superconducting pads galvanically isolated from ground. Each such pair
contributes two quantum degrees of freedom, one of which is used as a qubit, while the other remains fixed. However, these extraneous modes can generate coupling between the qubit modes that extends beyond the nearest neighbor. We present a general formalism describing the formation of this coupling and calculate it for a one-dimensional chain of transmons. We show that the strength of coupling and its range (that is, the exponential falloff) can be tuned independently via circuit design to realize a continuum from nearest-neighbor-only interactions to interactions that extend across the length of the chain. We present designs with capacitance and microwave simulations showing that various interaction configurations can be achieved in realistic circuits. Such coupling could be used in analog simulation of different quantum regimes or to increase connectivity in digital quantum systems. Thus mechanism must also be taken into account in other types of qubits with extraneous modes.

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.

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.

CircuitQ: An open-source toolbox for superconducting circuits

  1. Philipp Aumann,
  2. Tim Menke,
  3. William D. Oliver,
  4. and Wolfgang Lechner
We introduce CircuitQ, an open-source toolbox for the analysis of superconducting circuits implemented in Python. It features the automated construction of a symbolic Hamiltonian of
the input circuit, as well as a dynamic numerical representation of this Hamiltonian with a variable basis choice. Additional features include the estimation of the T1 lifetimes of the circuit states under various noise mechanisms. We review previously established circuit quantization methods and formulate them in a way that facilitates the software implementation. The toolbox is then showcased by applying it to practically relevant qubit circuits and comparing it to specialized circuit solvers. Our circuit quantization is both applicable to circuit inputs from a large design space and the software is open-sourced. We thereby add an important toolbox for the design of new quantum circuits for quantum information processing applications.

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.