"Quantum transducer in circuit optomechanics"
<br>
<br>
 
Mechanical resonators are becoming macroscopic quantum objects with great
potential. It is however difficult to measure and manipulate the phonon state
due to the tiny motion in the quantum regime. We show that a superconducting
microwave resonator linearly coupled to the mechanical mode constitutes a
powerful probe and an interesting quantum source. This coupling is rendered much
stronger than the usual radiation pressure interaction by adjusting a gate
voltage and gives rise to coherent oscillations between phonons and photons. The
phenomenon of phonon blockade is detected from the statistics of the light field
and a quantum tomography of the mechanical resonator is obtained after
transferring the state to the microwave cavity. Quantum phonon states can also
be synthesized from the cavity and hybrid entanglement can be engineered between
phonons and photons. Moreover, when two mechanical resonators are coupled to the
cavity, they can be entangled by the photons. Mechanical resonators can also be
coupled to a large variety of quantum systems such as spins, optical photons,
cold atoms, Bose Einstein condensates… They naturally act as a quantum
transducer between an auxiliary quantum system and a microwave resonator, which
is used as a quantum bus. The quantum communications are then controlled with
the individual gate voltages.
<br>
<br>
This work is supported by the QNEMS European project.