Variational Quantum Gate Optimization

  1. Kentaro Heya,
  2. Yasunari Suzuki,
  3. Yasunobu Nakamura,
  4. and Keisuke Fujii
We propose a gate optimization method, which we call variational quantum gate optimization (VQGO). VQGO is a method to construct a target multi-qubit gate by optimizing a parametrized
quantum circuit which consists of tunable single-qubit gates with high fidelities and fixed multi-qubit gates with limited controlabilities. As an example, we apply the proposed scheme to the models relevant to superconducting qubit systems. We show in numerical simulations that the high-fidelity CNOT gate can be constructed with VQGO using cross-resonance gates with finite crosstalk. We also demonstrate that fast and a high-fidelity four-qubit syndrome extraction can be implemented with simultaneous cross-resonance drives even in the presence of non-commutative crosstalk. VQGO gives a pathway for designing efficient gate operations for quantum computers.

Single-photon quantum regime of artificial radiation pressure on a surface acoustic wave resonator

  1. Atsushi Noguchi,
  2. Rekishu Yamazaki,
  3. Yutaka Tabuchi,
  4. and Yasunobu Nakamura
Electromagnetic fields carry momentum, which upon reflection on matter, gives rise to the radiation pressure of photons. The radiation pressure has recently been utilized in cavity
optomechanics for controlling mechanical motions of macroscopic objects at the quantum limit. However, because of the weakness of the interaction, attempts so far had to use a strong coherent drive to reach the quantum limit. Therefore, the single photon quantum regime, where even the presence of a totally off-resonant single photon alters the quantum state of the mechanical mode significantly, is one of the next milestones in cavity optomechanics. Here we demonstrate an artificial realization of the radiation pressure of microwave photons acting on phonons in a surface acoustic wave resonator. The order-of-magnitude enhancement of the interaction strength originates in the well-tailored strong second-order nonlinearity of a superconducting Josephson-junction circuit. The synthetic radiation pressure interaction adds a key element to the quantum optomechanical toolbox and can be applied to quantum information interfaces between electromagnetic and mechanical degrees of freedom.

Circuit QED-based measurement of vortex lattice order in a Josephson junction array

  1. R. Cosmic,
  2. Hiroki Ikegami,
  3. Zhirong Lin,
  4. Kunihiro Inomata,
  5. Jacob M. Taylor,
  6. and Yasunobu Nakamura
Superconductivity provides a canonical example of a quantum phase of matter. When superconducting islands are connected by Josephson junctions in a lattice, the low temperature state
of the system can map to the celebrated XY model and its associated universality classes. This has been used to experimentally implement realizations of Mott insulator and Berezinskii–Kosterlitz–Thouless (BKT) transitions to vortex dynamics analogous to those in type-II superconductors. When an external magnetic field is added, the effective spins of the XY model become frustrated, leading to the formation of topological defects (vortices). Here we observe the many-body dynamics of such an array, including frustration, via its coupling to a superconducting microwave cavity. We take the design of the transmon qubit, but replace the single junction between two antenna pads with the complete array. This allows us to probe the system at 10 mK with minimal self-heating by using weak coherent states at the single (microwave) photon level to probe the resonance frequency of the cavity. We observe signatures of ordered vortex lattice at rational flux fillings of the array.

Qubit-assisted transduction for a detection of surface acoustic waves near the quantum limit

  1. Atsushi Noguchi,
  2. Rekishu Yamazaki,
  3. Yutaka Tabuchi,
  4. and Yasunobu Nakamura
We demonstrate ultra-sensitive measurement of fluctuations in a surface-acoustic-wave~(SAW) resonator using a hybrid quantum system consisting of the SAW resonator, a microwave (MW)
resonator and a superconducting qubit. The nonlinearity of the driven qubit induces parametric coupling, which up-converts the excitation in the SAW resonator to that in the MW resonator. Thermal fluctuations of the SAW resonator near the quantum limit are observed in the noise spectroscopy in the MW domain.

Suppressing relaxation in superconducting qubits by quasiparticle pumping

  1. Simon Gustavsson,
  2. Fei Yan,
  3. Gianluigi Catelani,
  4. Jonas Bylander,
  5. Archana Kamal,
  6. Jeffrey Birenbaum,
  7. David Hover,
  8. Danna Rosenberg,
  9. Gabriel Samach,
  10. Adam P. Sears,
  11. Steven J. Weber,
  12. Jonilyn L. Yoder,
  13. John Clarke,
  14. Andrew J. Kerman,
  15. Fumiki Yoshihara,
  16. Yasunobu Nakamura,
  17. Terry P. Orlando,
  18. and William D. Oliver
Dynamical error suppression techniques are commonly used to improve coherence in quantum systems. They reduce dephasing errors by applying control pulses designed to reverse erroneous
coherent evolution driven by environmental noise. However, such methods cannot correct for irreversible processes such as energy relaxation. In this work, we investigate a complementary, stochastic approach to reducing errors: instead of deterministically reversing the unwanted qubit evolution, we use control pulses to shape the noise environment dynamically. In the context of superconducting qubits, we implement a pumping sequence to reduce the number of unpaired electrons (quasiparticles) in close proximity to the device. We report a 70% reduction in the quasiparticle density, resulting in a threefold enhancement in qubit relaxation times, and a comparable reduction in coherence variability.

Resolving magnon number states in quantum magnonics

  1. Dany Lachance-Quirion,
  2. Yutaka Tabuchi,
  3. Seiichiro Ishino,
  4. Atsushi Noguchi,
  5. Toyofumi Ishikawa,
  6. Rekishu Yamazaki,
  7. and Yasunobu Nakamura
Collective excitation modes in solid state systems play a central role in circuit quantum electrodynamics, cavity optomechanics, and quantum magnonics. In the latter, quanta of collective
excitation modes in a ferromagnet, called magnons, interact with qubits to provide the nonlinearity necessary to access quantum phenomena in magnonics. A key ingredient for future quantum magnonics systems is the ability to probe magnon states. Here we observe individual magnons in a millimeter-sized ferromagnet coherently coupled to a superconducting qubit. Specifically, we resolve magnon number states in spectroscopic measurements of a transmon qubit with the hybrid system in the strong dispersive regime. This enables us to detect a change in the magnetic dipole of the ferromagnet equivalent to a single spin flipped among more than 1019 spins. The strong dispersive regime of quantum magnonics opens up the possibility of encoding superconducting qubits into non-classical magnon states, potentially providing a coherent interface between a superconducting quantum processor and optical photons.

Flux-driven Josephson parametric amplifiers: Hysteretic flux response and nondegenerate gain measurements

  1. Stefan Pogorzalek,
  2. Kirill G. Fedorov,
  3. Ling Zhong,
  4. Jan Goetz,
  5. Friedrich Wulschner,
  6. Michael Fischer,
  7. Peter Eder,
  8. Edwar Xie,
  9. Kunihiro Inomata,
  10. Tsuyoshi Yamamoto,
  11. Yasunobu Nakamura,
  12. Achim Marx,
  13. Frank Deppe,
  14. and Rudolf Gross
Josephson parametric amplifiers (JPA) have become key devices in quantum science and technology with superconducting circuits. In particular, they can be utilized as quantum-limited
amplifiers or as a source of squeezed microwave fields. Here, we report on the detailed measurements of five flux-driven JPAs, three of them exhibiting a hysteretic dependence of the resonant frequency versus the applied magnetic flux. We model the measured characteristics by numerical simulations based on the two-dimensional potential landscape of the dc superconducting quantum interference devices (dc-SQUID), which provide the JPA nonlinearity, for a finite screening parameter βL>0 and demonstrate excellent agreement between the numerical results and the experimental data. Furthermore, we study the nondegenerate response of different JPAs and accurately describe the experimental results with our theory.

Single microwave-photon detector using an artificial Λ-type three-level system

  1. Kunihiro Inomata,
  2. Zhirong Lin,
  3. Kazuki Koshino,
  4. William D. Oliver,
  5. Jaw-Shen Tsai,
  6. Tsuyoshi Yamamoto,
  7. and Yasunobu Nakamura
Single photon detection is a requisite technique in quantum-optics experiments in both the optical and the microwave domains. However, the energy of microwave quanta are four to five
orders of magnitude less than their optical counterpart, making the efficient detection of single microwave photons extremely challenging. Here, we demonstrate the detection of a single microwave photon propagating through a waveguide. The detector is implemented with an „impedance-matched“ artificial Λ system comprising the dressed states of a driven superconducting qubit coupled to a microwave resonator. We attain a single-photon detection efficiency of 0.66±0.06 with a reset time of ∼400~ns. This detector can be exploited for various applications in quantum sensing, quantum communication and quantum information processing.

Dressed-state engineering for continuous detection of itinerant microwave photons

  1. Kazuki Koshino,
  2. Zhirong Lin,
  3. Kunihiro Inomata,
  4. Tsuyoshi Yamamoto,
  5. and Yasunobu Nakamura
We propose a scheme for continuous detection of itinerant microwave photons in circuit quantum electrodynamics. In the proposed device, a superconducting qubit is coupled dispersively
to two resonators: one is used to form an impedance-matched Λ system that deterministically captures incoming photons, and the other is used for continuous monitoring of the event. The present scheme enables efficient photon detection: for realistic system parameters, the detection efficiency reaches 0.9 with a bandwidth of about ten megahertz.

Quantum magnonics: magnon meets superconducting qubit

  1. Yutaka Tabuchi,
  2. Seiichiro Ishino,
  3. Atsushi Noguchi,
  4. Toyofumi Ishikawa,
  5. Rekishu Yamazaki,
  6. Koji Usami,
  7. and Yasunobu Nakamura
The techniques of microwave quantum optics are applied to collective spin excitations in a macroscopic sphere of ferromagnetic insulator. We demonstrate, in the single-magnon limit,
strong coupling between a magnetostatic mode in the sphere and a microwave cavity mode. Moreover, we introduce a superconducting qubit in the cavity and couple the qubit with the magnon excitation via the virtual photon excitation. We observe the magnon-vacuum-induced Rabi splitting. The hybrid quantum system enables generation and characterization of non-classical quantum states of magnons.