S. U. Cho

Single-photon cavity optomechanics mediated by a quantum two-level system

  1. J.-M. Pirkkalainen,
  2. S. U. Cho,
  3. F. Massel,
  4. J. Tuorila,
  5. T. T. Heikkila,
  6. P. J. Hakonen,
  7. and M. A. Sillanpaa
Coupling electromagnetic waves in a cavity and mechanical vibrations via the radiation pressure of the photons is a promising platform for investigations of quantum mechanical properties
of motion of macroscopic bodies and thereby the limits of quantum mechanics [3,4]. A drawback is that the effect of one photon tends to be tiny, and hence one of the pressing challenges is to substantially increase the interaction strength towards the scale of the cavity damping rate. A novel scenario is to introduce into the setup a quantum two-level system (qubit), which, besides strengthening the coupling, allows for rich physics via strongly enhanced nonlinearities [5-8]. Addressing these issues, here we present a design of cavity optomechanics in the microwave frequency regime involving a Josephson junction qubit. We demonstrate boosting of the radiation pressure interaction energy by six orders of magnitude, allowing to approach the strong coupling regime, where a single quantum of vibrations shifts the cavity frequency by more than its linewidth. We observe nonlinear phenomena at single-photon energies, such as an enhanced damping due to the two-level system. This work opens up nonlinear cavity optomechanics as a plausible tool for the study of quantum properties of motion.
arxiv pdf

Hybrid circuit cavity quantum electrodynamics with a micromechanical resonator

  1. J. M. Pirkkalainen,
  2. S. U. Cho,
  3. Jian Li,
  4. G. S. Paraoanu,
  5. P. J. Hakonen,
  6. and M. A. Sillanpää
Hybrid quantum systems with inherently distinct degrees of freedom play a key role in many physical phenomena. A strong coupling can make the constituents loose their individual character
and form entangled states. The properties of these collective excitations, such as polaritons of light and phonons in semiconductors, can combine the benefits of each subsystem. In the emerging field of quantum information control, a promising direction is provided by the combination between long-lived atomic states and the accessible electrical degrees of freedom in superconducting cavities and qubits. Here we demonstrate the possibility to integrate circuit cavity quantum electrodynamics with phonons. Besides coupling to a microwave cavity, our superconducting transmon qubit interacts with a resonant phonon mode in a micromechanical resonator, allowing the combination of long lifetime, strong tunable coupling, and ease of access. We measure the phonon Stark shift, as well as the splitting of the transmon qubit spectral line into motional sidebands representing transitions between electromechanical polaritons formed by phonons and the qubit. In the time domain, we observe coherent sideband Rabi oscillations between the qubit states and phonons. This advance may allow for storage of quantum information in long-lived phonon states, and for investigations of strongly coupled quantum systems near the classical limit.
arxiv pdf