Yuichiro Matsuzaki

Observation of collective coupling between an engineered ensemble of macroscopic artificial atoms and a superconducting resonator

  1. Kosuke Kakuyanagi,
  2. Yuichiro Matsuzaki,
  3. Corentin Deprez,
  4. Hiraku Toida,
  5. Kouichi Semba,
  6. Hiroshi Yamaguchi,
  7. William J. Munro,
  8. and Shiro Saito
The hybridization of distinct quantum systems is now seen as an effective way to engineer the properties of an entire system leading to applications in quantum metamaterials, quantum
simulation, and quantum metrology. One well known example is superconducting circuits coupled to ensembles of microscopic natural atoms. In such cases, the properties of the individual atom are intrinsic, and so are unchangeable. However, current technology allows us to fabricate large ensembles of macroscopic artificial atoms such as superconducting flux qubits, where we can really tailor and control the properties of individual qubits. Here, we demonstrate coherent coupling between a microwave resonator and several thousand superconducting flux qubits, where we observe a large dispersive frequency shift in the spectrum of 250 MHz induced by collective behavior. These results represent the largest number of coupled superconducting qubits realized so far. Our approach shows that it is now possible to engineer the properties of the ensemble, opening up the way for the controlled exploration of the quantum many-body system.
arxiv pdf

A strict experimental test of macroscopic realism in a superconducting flux qubit

  1. George C. Knee,
  2. Kosuke Kakuyanagi,
  3. Mao-Chuang Yeh,
  4. Yuichiro Matsuzaki,
  5. Hiraku Toida,
  6. Hiroshi Yamaguchi,
  7. Anthony J. Leggett,
  8. and William J. Munro
Macroscopic realism is the name for a class of modifications to quantum theory that allow macroscopic objects to be described in a measurement-independent fashion, while largely preserving
a fully quantum mechanical description of the microscopic world. Objective collapse theories are examples which attempt to provide a physical mechanism for wavefunction collapse, and thereby aim to solve the quantum measurement problem. Here we describe and implement an experimental protocol capable of constraining theories of this class, and show how it is related to the original Leggett-Garg test, yet more noise tolerant and conceptually transparent. By conducting a set of simple ‚prepare, shuffle, measure‘ tests in a superconducting flux qubit, we rule out (by over 77 standard deviations) those theories which would deny coherent superpositions of 170 nA currents over a 9 ns timescale. Further, we address the ‚clumsiness loophole‘ by determining classical disturbance in a set of control experiments.
arxiv pdf

Improving the lifetime of the NV center ensemble coupled with a superconducting flux qubit by applying magnetic fields

  1. Yuichiro Matsuzaki,
  2. Xiaobo Zhu,
  3. Kosuke Kakuyanagi,
  4. Hiraku Toida,
  5. Takaaki Shimooka,
  6. Norikazu Mizuochi,
  7. Kae Nemoto,
  8. Kouichi Semba,
  9. W. J. Munro,
  10. Hiroshi Yamaguchi,
  11. and Shiro Saito
One of the promising systems to realize quantum computation is a hybrid system where a superconducting flux qubit plays a role of a quantum processor and the NV center ensemble is used
as a quantum memory. We have theoretically and experimentally studied the effect of magnetic fields on this hybrid system, and found that the lifetime of the vacuum Rabi oscillation is improved by applying a few mT magnetic field to the NV center ensemble. Here, we construct a theoretical model to reproduce the vacuum Rabi oscillations with/without magnetic fields applied to the NV centers, and we determine the reason why magnetic fields can affect the coherent properties of the NV center ensemble. From our theoretical analysis, we quantitatively show that the magnetic fields actually suppress the inhomogeneous broadening from the strain in the NV centers.
arxiv pdf

Ising interaction between capacitively-coupled superconducting flux qubits

  1. Takahiko Satoh,
  2. Yuichiro Matsuzaki,
  3. Kosuke Kakuyanagi,
  4. Koichi Semba,
  5. Hiroshi Yamaguchi,
  6. and Shiro Saito
Here, we propose a scheme to generate a controllable Ising interaction between superconducting flux qubits. Existing schemes rely on inducting couplings to realize Ising interactions
between flux qubits, and the interaction strength is controlled by an applied magnetic field On the other hand, we have found a way to generate an interaction between the flux qubits via capacitive couplings. This has an advantage in individual addressability, because we can control the interaction strength by changing an applied voltage that can be easily localized. This is a crucial step toward the realizing superconducting flux qubit quantum computation.
arxiv pdf

Towards Realizing a Quantum Memory for a Superconducting Qubit: Storage and Retrieval of quantum states

  1. Shiro Saito,
  2. Xiaobo Zhu,
  3. Robert Amsüss,
  4. Yuichiro Matsuzaki,
  5. Kosuke Kakuyanagi,
  6. Takaaki Shimo-Oka,
  7. Norikazu Mizuochi,
  8. Kae Nemoto,
  9. William J. Munro,
  10. and Kouichi Semba
We have built a hybrid system composed of a superconducting flux qubit (the processor) and an ensemble of nitrogen-vacancy centers in diamond (the memory) that can be directly coupled
to one another and demonstrated how information can be transferred from the flux qubit to the memory, stored and subsequently retrieved. We have established the coherence properties of the memory, and succeeded in creating an entangled state between the processor and memory, demonstrating how the entangled state’s coherence is preserved. Our results are a significant step towards using an electron spin ensemble as a quantum memory for superconducting qubits.
arxiv pdf