Measurements of a quantum bulk acoustic resonator using a superconducting qubit

  1. M.-H. Chou,
  2. É. Dumur,
  3. Y. P. Zhong,
  4. G. A. Peairs,
  5. A. Bienfait,
  6. H.-S. Chang,
  7. C. R. Conner,
  8. J. Grebel,
  9. R. G. Povey,
  10. K. J. Satzinger,
  11. and A. N. Cleland
Phonon modes at microwave frequencies can be cooled to their quantum ground state using conventional cryogenic refrigeration, providing a convenient way to study and manipulate quantum
states at the single phonon level. Phonons are of particular interest because mechanical deformations can mediate interactions with a wide range of different quantum systems, including solid-state defects, superconducting qubits, as well as optical photons when using optomechanically-active constructs. Phonons thus hold promise for quantum-focused applications as diverse as sensing, information processing, and communication. Here, we describe a piezoelectric quantum bulk acoustic resonator (QBAR) with a 4.88 GHz resonant frequency that at cryogenic temperatures displays large electromechanical coupling strength combined with a high intrinsic mechanical quality factor Qi≈4.3×104. Using a recently-developed flip-chip technique, we couple this QBAR resonator to a superconducting qubit on a separate die and demonstrate quantum control of the mechanics in the coupled system. This approach promises a facile and flexible experimental approach to quantum acoustics and hybrid quantum systems.

Fast high fidelity quantum non-demolition qubit readout via a non-perturbative cross-Kerr coupling

  1. R. Dassonneville,
  2. T. Ramos,
  3. V. Milchakov,
  4. L. Planat,
  5. É. Dumur,
  6. F. Foroughi,
  7. J. Puertas,
  8. S. Leger,
  9. K. Bharadwaj,
  10. J. Delaforce,
  11. K. Rafsanjani,
  12. C. Naud,
  13. W. Hasch-Guichard,
  14. J.J. García-Ripoll,
  15. N. Roch,
  16. and O. Buisson
Qubit readout is an indispensable element of any quantum information processor. In this work we propose an original coupling scheme between qubit and cavity mode based on a non-perturbative
cross-Kerr interaction. It leads to an alternative readout mechanism for superconducting qubits. This scheme, using the same experimental techniques as the perturbative cross-Kerr coupling (dispersive interaction), leads to an alternative readout mechanism for superconducting qubits. This new process, being non-perturbative, maximizes speed of qubit readout, single-shot fidelity and its quantum non-demolition (QND) behavior at the same time, while minimizing the effect of unwanted decay channels such as, for example, the Purcell effect. We observed 97.4 % single-shot readout fidelity for short 50 ns pulses. Using long measurement, we quantified the QND-ness to 99 %.

Phonon-mediated quantum state transfer and remote qubit entanglement

  1. A. Bienfait,
  2. K. J. Satzinger,
  3. Y. P. Zhong,
  4. H.-S. Chang,
  5. M.-H. Chou,
  6. C. R. Conner,
  7. E. Dumur,
  8. J. Grebel,
  9. G. A. Peairs,
  10. R. G. Povey,
  11. and A. N. Cleland
Phonons, and in particular surface acoustic wave phonons, have been proposed as a means to coherently couple distant solid-state quantum systems. Recent experiments have shown that
superconducting qubits can control and detect individual phonons in a resonant structure, enabling the coherent generation and measurement of complex stationary phonon states. Here, we report the deterministic emission and capture of itinerant surface acoustic wave phonons, enabling the quantum entanglement of two superconducting qubits. Using a 2 mm-long acoustic quantum communication channel, equivalent to a 500 ns delay line, we demonstrate the emission and re-capture of a phonon by one qubit; quantum state transfer between two qubits with a 67\% efficiency; and, by partial transfer of a phonon between two qubits, generation of an entangled Bell pair with a fidelity of FB=84±1 %

Violating Bell’s inequality with remotely-connected superconducting qubits

  1. Y. P. Zhong,
  2. H.-S. Chang,
  3. K. J. Satzinger,
  4. M.-H. Chou,
  5. A. Bienfait,
  6. C. R. Conner,
  7. É. Dumur,
  8. J. Grebel,
  9. G. A. Peairs,
  10. R. G. Povey,
  11. D.I. Schuster,
  12. and A. N. Cleland
Quantum communication relies on the efficient generation of entanglement between remote quantum nodes, due to entanglement’s key role in achieving and verifying secure communications.
Remote entanglement has been realized using a number of different probabilistic schemes, but deterministic remote entanglement has only recently been demonstrated, using a variety of superconducting circuit approaches. However, the deterministic violation of a Bell inequality, a strong measure of quantum correlation, has not to date been demonstrated in a superconducting quantum communication architecture, in part because achieving sufficiently strong correlation requires fast and accurate control of the emission and capture of the entangling photons. Here we present a simple and scalable architecture for achieving this benchmark result in a superconducting system.

A V-shape superconducting artificial atom based on two inductively coupled transmons

  1. É. Dumur,
  2. B. Küng,
  3. A. K. Feofanov,
  4. T. Weissl,
  5. N. Roch,
  6. C. Naud,
  7. W. Guichard,
  8. and O. Buisson
Circuit quantum electrodynamics systems are typically built from resonators and two-level artificial atoms, but the use of multi-level artificial atoms instead can enable promising
applications in quantum technology. Here we present an implementation of a Josephson junction circuit dedicated to operate as a V-shape artificial atom. Based on a concept of two internal degrees of freedom, the device consists of two transmon qubits coupled by an inductance. The Josephson nonlinearity introduces a strong diagonal coupling between the two degrees of freedom that finds applications in quantum non-demolition readout schemes, and in the realization of microwave cross-Kerr media based on superconducting circuits.

Ultrafast QND measurements based on diamond-shape artificial atom

  1. I. Diniz,
  2. E. Dumur,
  3. O. Buisson,
  4. and A. Auffèves
We propose a Quantum Non Demolition (QND) read-out scheme for a superconducting artificial atom coupled to a resonator in a circuit QED architecture, for which we estimate a very high
measurement fidelity without Purcell effect limitations. The device consists of two transmons coupled by a large inductance, giving rise to a diamond-shape artificial atom with a logical qubit and an ancilla qubit interacting through a cross-Kerr like term. The ancilla is strongly coupled to a transmission line resonator. Depending on the qubit state, the ancilla is resonantly or dispersively coupled to the resonator, leading to a large contrast in the transmitted microwave signal amplitude. This original method can be implemented with state of the art Josephson parametric amplifier, leading to QND measurements in a few tens of nanoseconds with fidelity as large as 99.9 %.