F. Nori

Deterministic loading and phase shaping of microwaves onto a single artificial atom

  1. W.-J. Lin,
  2. Y. Lu,
  3. P. Y. Wen,
  4. Y.-T. Cheng,
  5. C.-P. Lee,
  6. K.-T. Lin,
  7. K.-H. Chiang,
  8. M. C. Hsieh,
  9. J. C. Chen,
  10. C.-S. Chuu,
  11. F. Nori,
  12. A. F. Kockum,
  13. G.-D. Lin,
  14. P. Delsing,
  15. and I.-C. Hoi
Loading quantum information deterministically onto a quantum node is an important step towards a quantum network. Here, we demonstrate that coherent-state microwave photons, with anoptimal temporal waveform, can be efficiently loaded onto a single superconducting artificial atom in a semi-infinite one-dimensional (1D) transmission-line waveguide. Using a weak coherent state (average photon number N<<1 with an exponentially rising waveform, whose time constant matches the decoherence time of the artificial atom, we demonstrate a loading efficiency of above 94% from 1D semi-free space to the artificial atom. We also show that Fock-state microwave photons can be deterministically loaded with an efficiency of 98.5%. We further manipulate the phase of the coherent state exciting the atom, enabling coherent control of the loading process. Our results open up promising applications in realizing quantum networks based on waveguide quantum electrodynamics (QED).[/expand]
arxiv pdf

Large collective Lamb shift of two distant superconducting artificial atoms

  1. P. Y. Wen,
  2. K.-T. Lin,
  3. A. F. Kockum,
  4. B. Suri,
  5. H. Ian,
  6. J. C. Chen,
  7. S. Y. Mao,
  8. C. C. Chiu,
  9. P. Delsing,
  10. F. Nori,
  11. G.-D. Lin,
  12. and I.-C. Hoi
Virtual photons can mediate interaction between atoms, resulting in an energy shift known as a collective Lamb shift. Observing the collective Lamb shift is challenging, since it can
be obscured by radiative decay and direct atom-atom interactions. Here, we place two superconducting qubits in a transmission line terminated by a mirror, which suppresses decay. We measure a collective Lamb shift reaching 0.8% of the qubit transition frequency and exceeding the transition linewidth. We also show that the qubits can interact via the transmission line even if one of them does not decay into it.
arxiv pdf

Reflective amplification without population inversion from a strongly driven superconducting qubit

  1. P. Y. Wen,
  2. A. F. Kockum,
  3. H. Ian,
  4. J. C. Chen,
  5. F. Nori,
  6. and I.-C. Hoi
Amplification of optical or microwave fields is often achieved by strongly driving a medium to induce population inversion such that a weak probe can be amplified through stimulated
emission. Here we strongly couple a superconducting qubit, an artificial atom, to the field in a semi-infinite waveguide. When driving the qubit strongly on resonance such that a Mollow triplet appears, we observe a 7\% amplitude gain for a weak probe at frequencies in-between the triplet. This amplification is not due to population inversion, neither in the bare qubit basis nor in the dressed-state basis, but instead results from a four-photon process that converts energy from the strong drive to the weak probe. We find excellent agreement between the experimental results and numerical simulations without any free fitting parameters. The device demonstrated here may have applications in quantum information processing and quantum-limited measurements.
arxiv pdf

Nonclassical microwave radiation from the dynamical Casimir effect

  1. J. R. Johansson,
  2. G. Johansson,
  3. C. M. Wilson,
  4. P. Delsing,
  5. and F. Nori
We investigate quantum correlations in microwave radiation produced by the dynamical Casimir effect in a superconducting waveguide terminated and modulated by a superconducting quantum
interference device. We apply nonclassicality tests and evaluate the entanglement for the predicted field states. For realistic circuit parameters, including thermal background noise, the results indicate that the produced radiation can be strictly nonclassical and can have a measurable amount of intermode entanglement. If measured experimentally, these nonclassicalilty indicators could give further evidence of the quantum nature of the dynamical Casimir radiation in these circuits.
arxiv pdf