1,911 research outputs found
Evanescent straight tapered-fiber coupling of ultra-high Q optomechanical micro-resonators in a low-vibration helium-4 exchange-gas cryostat
We developed an apparatus to couple a 50-micrometer diameter
whispering-gallery silica microtoroidal resonator in a helium-4 cryostat using
a straight optical tapered-fiber at 1550nm wavelength. On a top-loading probe
specifically adapted for increased mechanical stability, we use a
specifically-developed "cryotaper" to optically probe the cavity, allowing thus
to record the calibrated mechanical spectrum of the optomechanical system at
low temperatures. We then demonstrate excellent thermalization of a 63-MHz
mechanical mode of a toroidal resonator down to the cryostat's base temperature
of 1.65K, thereby proving the viability of the cryogenic refrigeration via heat
conduction through static low-pressure exchange gas. In the context of
optomechanics, we therefore provide a versatile and powerful tool with
state-of-the-art performances in optical coupling efficiency, mechanical
stability and cryogenic cooling.Comment: 8 pages, 6 figure
Resolved-sideband cooling and measurement of a micromechanical oscillator close to the quantum limit
The observation of quantum phenomena in macroscopic mechanical oscillators
has been a subject of interest since the inception of quantum mechanics.
Prerequisite to this regime are both preparation of the mechanical oscillator
at low phonon occupancy and a measurement sensitivity at the scale of the
spread of the oscillator's ground state wavefunction. It has been widely
perceived that the most promising approach to address these two challenges are
electro nanomechanical systems. Here we approach for the first time the quantum
regime with a mechanical oscillator of mesoscopic dimensions--discernible to
the bare eye--and 1000-times more massive than the heaviest nano-mechanical
oscillators used to date. Imperative to these advances are two key principles
of cavity optomechanics: Optical interferometric measurement of mechanical
displacement at the attometer level, and the ability to use measurement induced
dynamic back-action to achieve resolved sideband laser cooling of the
mechanical degree of freedom. Using only modest cryogenic pre-cooling to 1.65
K, preparation of a mechanical oscillator close to its quantum ground state
(63+-20 phonons) is demonstrated. Simultaneously, a readout sensitivity that is
within a factor of 5.5+-1.5 of the standard quantum limit is achieved. The
reported experiments mark a paradigm shift in the approach to the quantum limit
of mechanical oscillators using optical techniques and represent a first step
into a new era of experimental investigation which probes the quantum nature of
the most tangible harmonic oscillator: a mechanical vibration.Comment: 14 pages, 4 figure
Comparison of Quadratic- and Median-Based Roughness Penalties for Penalized-Likelihood Sinogram Restoration in Computed Tomography
We have compared the performance of two different penalty choices
for a penalized-likelihood sinogram-restoration strategy we have
been developing. One is a quadratic penalty we have employed
previously and the other is a new median-based penalty. We
compared the approaches to a noniterative adaptive filter that
loosely but not explicitly models data statistics. We found that
the two approaches produced similar resolution-variance tradeoffs
to each other and that they outperformed the adaptive filter in
the low-dose regime, which suggests that the particular choice of
penalty in our approach may be less important than the fact that
we are explicitly modeling data statistics at all. Since the
quadratic penalty allows for derivation of an algorithm that is
guaranteed to monotonically increase the penalized-likelihood
objective function, we find it to be preferable to the median-based penalty
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