Fast universal control of a flux qubit via exponentially tunable wave-function overlap

  1. Svend Krøjer,
  2. Anders Enevold Dahl,
  3. Kasper Sangild Christensen,
  4. Morten Kjaergaard,
  5. and Karsten Flensberg
Fast, high fidelity control and readout of protected superconducting qubits are fundamentally challenging due to their inherent insensitivity. We propose a flux qubit variation which
enjoys a tunable level of protection against relaxation to resolve this outstanding issue. Our qubit design, the double-shunted flux qubit (DSFQ), realizes a generic double-well potential through its three junction ring geometry. One of the junctions is tunable, making it possible to control the barrier height and thus the level of protection. We analyze single- and two-qubit gate operations that rely on lowering the barrier. We show that this is a viable method that results in high fidelity gates as the non-computational states are not occupied during operations. Further, we show how the effective coupling to a readout resonator can be controlled by adjusting the externally applied flux while the DSFQ is protected from decaying into the readout resonator. Finally, we also study a double-loop gradiometric version of the DSFQ which is exponentially insensitive to variations in the global magnetic field, even when the loop areas are non-identical.

Scheme for parity-controlled multi-qubit gates with superconducting qubits

  1. Kasper Sangild Christensen,
  2. Nikolaj Thomas Zinner,
  3. and Morten Kjaergaard
Multi-qubit parity measurements are at the core of many quantum error correction schemes. Extracting multi-qubit parity information typically involves using a sequence of multiple two-qubit
gates. In this paper, we propose a superconducting circuit device with native support for multi-qubit parity-controlled gates (PCG). These are gates that perform rotations on a parity ancilla based on the multi-qubit parity operator of adjacent qubits, and can be directly used to perform multi-qubit parity measurements. The circuit consists of a set of concatenated Josephson ring modulators and effectively realizes a set of transmon-like qubits with strong longitudinal nearest-neighbor couplings. PCGs are implemented by applying microwave drives to the parity ancilla at specific frequencies. We investigate the scheme’s performance with numerical simulation using realistic parameter choices and decoherence rates, and find that the device can perform four-qubit PCGs in 30 ns with process fidelity surpassing 99%. Furthermore, we study the effects of parameter disorder and spurious coupling between next-nearest neighboring qubits. Our results indicate that this approach to realizing PCGs constitute an interesting candidate for near-term quantum error correction experiments.

Nonreciprocal devices based on voltage-tunable junctions

  1. Catherine Leroux,
  2. Adrian Parra-Rodriguez,
  3. Ross Shillito,
  4. Agustin Di Paolo,
  5. William D. Oliver,
  6. Charles M. Marcus,
  7. Morten Kjaergaard,
  8. András Gyenis,
  9. and Alexandre Blais
We propose to couple the flux degree of freedom of one mode with the charge degree of freedom of a second mode in a hybrid superconducting-semiconducting architecture. Nonreciprocity
can arise in this architecture in the presence of external static magnetic fields alone. We leverage this property to engineer a passive on-chip gyrator, the fundamental two-port nonreciprocal device which can be used to build other nonreciprocal devices such as circulators. We analytically and numerically investigate how the nonlinearity of the interaction, circuit disorder and parasitic couplings affect the scattering response of the gyrator.

Entangling transmons with low-frequency protected superconducting qubits

  1. Andrea Maiani,
  2. Morten Kjaergaard,
  3. and Constantin Schrade
Novel qubits with intrinsic noise protection constitute a promising route for improving the coherence of quantum information in superconducting circuits. However, many protected superconducting
qubits exhibit relatively low transition frequencies, which could make their integration with conventional transmon circuits challenging. In this work, we propose and study a scheme for entangling a tunable transmon with a Cooper-pair parity-protected qubit, a paradigmatic example of a low-frequency protected qubit that stores quantum information in opposite Cooper-pair parity states on a superconducting island. By tuning the external flux on the transmon, we show that non-computational states can mediate a two-qubit entangling gate that preserves the Cooper-pair parity independent of the detailed pulse sequence. Interestingly, the entangling gate bears similarities to a controlled-phase gate in conventional transmon devices. Hence, our results suggest that standard high-precision gate calibration protocols could be repurposed for operating hybrid qubit devices.

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.

A Charge-Noise Insensitive Chiral Photonic Interface for Waveguide Circuit QED

  1. Yu-Xiang Zhang,
  2. Carles R. i Carceller,
  3. Morten Kjaergaard,
  4. and Anders S. Sørensen
A chiral photonic interface is a quantum system that has different probabilities for emitting photons to the left and right. An on-chip compatible chiral interface is attractive for
both fundamental studies of light-matter interactions and applications to quantum information processing. We propose such a chiral interface based on superconducting circuits, which has wide bandwidth, rich tunability, and high tolerance to fabrication variations. The proposed interface consists of a core that uses Cooper-pair-boxes (CPBs) to break time-reversal symmetry, and two superconducting transmons which connect the core to a waveguide in the manner reminiscent of a „giant atom“. The transmons form a state decoupled from the core, akin to dark state of atomic physics, rendering the whole interface insensitive to the CPB charge noise. The proposed interface can be extended to realize a broadband fully passive on-chip circulator for microwave photons.

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.

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.