Description

2009/09/17, Dr. Ignacio Wilson-Rae (Technische Universität München)

Place: Sala de Seminarios del Departamento de Física Teórica e Historia de la Ciencia
Time: 12h.
Title: Quantum dissipation in optomechanical and nanomechanical systems
Abstract
 State of the art optomechanical and nanomechanical setups are
close to allowing for the observation of quantum effects in a
"macroscopic" mechanical system. Major challenges that remain to
be addressed are understanding and controlling mechanical
dissipation, and finding practical ways to demonstrate quantum
signatures. Here we address such issues and study mechanical
dissipation in the quantum regime understood in a broad sense to
include both: (i) dissipation and decoherence of high-quality
mechanical resonators and (ii) vibrational decoherence induced
by mesoscopic phonons on two level transducers used to monitor
them. These physical scenarios afford, respectively,
realizations of quantum Brownian motion and the spin-boson
model, i.e. the two main paradigms of "quantum dissipation".

In the context of (i) we analyze the dissipation mechanism
induced by the unavoidable coupling of the resonator to the
substrate (known as clamping losses). We derive the
Caldeira-Leggett model that determines the quantum Brownian
motion of a given resonance. Our treatment provides the leading
contribution in the aspect ratio and is applicable to a wide
range of structures including single-walled carbon nanotubes
(CNT) and microtoroids. This yields fundamental limits for the
Q-values that are strongly dependent on the geometry and are
relevant for state of the art resonators. We also discuss
high-stress structures and how the geometries can be optimized.

In the context of (ii) we propose a scheme to tailor the exciton
confinement in a suspended CNT that generates optically active
quantum dots with a level spacing in the milli-electronvolt
range. This technique is based on the spatial modulation of the
DC Stark-shift induced by an inhomogeneous axial electric
field. In addition a transverse field can be used to induce a
tunable parametric coupling between the quantum dot and the
fundamental mechanical mode of the nanotube. We show that this
interaction enables efficient optical ground-state
cooling. Finally, we analyze the fluorescence spectrum of this
system in the limit of large CNT length.  At low temperature and
for suitable parameters, coupling to the bending phonons
dominates. As a result of their divergent DOS the absorption
lineshape exhibits anomalous non-Lorentzian features. These are
characterized by a frequency scale that can lie in the 100 MHz
range and they relate to the possibility of optically tuning
across a quantum-dissipative phase transition in this system.