131 research outputs found
Comment on: Nonlocal Realistic Leggett Models Can be Considered Refuted by the Before-Before Experiment
It is shown here that Suarez [Found. Phys. 38, 583 (2008)] wrongly presents
the assumptions behind the Leggett's inequalities, and their modified form used
by Groeblacher et al. [Nature 446, 871 (2007)] for an experimental
falsification of a certain class of non-local hidden variable models.Comment: comment submitted to Found. Phy
Experimental test of nonlocal realistic theories without the rotational symmetry assumption
We analyze the class of nonlocal realistic theories that was originally
considered by Leggett [Found. Phys. 33, 1469 (2003)] and tested by us in a
recent experiment [Nature (London) 446, 871 (2007)]. We derive an
incompatibility theorem that works for finite numbers of polarizer settings and
that does not require the previously assumed rotational symmetry of the
two-particle correlation functions. The experimentally measured case involves
seven different measurement settings. Using polarization-entangled photon
pairs, we exclude this broader class of nonlocal realistic models by
experimentally violating a new Leggett-type inequality by 80 standard
deviations.Comment: Published versio
Creating and Verifying a Quantum Superposition in a Micro-optomechanical System
Micro-optomechanical systems are central to a number of recent proposals for
realizing quantum mechanical effects in relatively massive systems. Here we
focus on a particular class of experiments which aim to demonstrate massive
quantum superpositions, although the obtained results should be generalizable
to similar experiments. We analyze in detail the effects of finite temperature
on the interpretation of the experiment, and obtain a lower bound on the degree
of non-classicality of the cantilever. Although it is possible to measure the
quantum decoherence time when starting from finite temperature, an unambiguous
demonstration of a quantum superposition requires the mechanical resonator to
be in or near the ground state. This can be achieved by optical cooling of the
fundamental mode, which also provides a method to measure the mean phonon
number in that mode. We also calculate the rate of environmentally induced
decoherence and estimate the timescale for gravitational collapse mechanisms as
proposed by Penrose and Diosi. In view of recent experimental advances,
practical considerations for the realization of the described experiment are
discussed.Comment: 19 pages, 8 figures, published in New J. Phys. 10 095020 (2008);
minor revisions to improve clarity; fixed possibly corrupted figure
Testing Leggett's Inequality Using Aharonov-Casher Effect
Bell's inequality is established based on local realism. The violation of
Bell's inequality by quantum mechanics implies either locality or realism or
both are untenable. Leggett's inequality is derived based on nonlocal realism.
The violation of Leggett's inequality implies that quantum mechanics is neither
local realistic nor nonlocal realistic. The incompatibility of nonlocal realism
and quantum mechanics has been urrently confirmed by photon experiments. In our
work, we propose to test Leggett's inequality using the Aharonov-Casher effect.
In our scheme, four entangled particles emitted from two sources manifest a
two-qubit-typed correlation that may result in the violation of the Leggett
inequality, while satisfying the no-signaling condition for spacelike
separation. Our scheme is tolerant to some local inaccuracies due to the
topological nature of the Aharonov-Casher phase. The experimental
implementation of our scheme can be possibly realized by a calcium atomic
polarization interferometer experiment.Comment: 7 pages, 2 figures. Accepted by Scientific Report
Modeling Green's function measurements with two-tip scanning tunneling microscopy
A double-tip scanning tunneling microscope with nanometer-scale tip separation has the ability to access the single-electron Green's function in real and momentum spaces based on second-order tunneling processes. Experimental realization of such measurements has been limited to quasi-one-dimensional systems due to the extremely small signal size. Here we propose an alternative approach to obtain such information by exploiting the current-current correlations from the individual tips and present a theoretical formalism to describe it. To assess the feasibility of our approach we make a numerical estimate for an ∼25-nm Pb nanoisland and show that the wave function in fact extends from tip to tip and the signal depends less strongly on increased tip separation in the diffusive regime than the one in alternative approaches relying on tip-to-tip conductance.Quantum Matter and Optic
Radiation-pressure self-cooling of a micromirror in a cryogenic environment
We demonstrate radiation-pressure cavity-cooling of a mechanical mode of a
micromirror starting from cryogenic temperatures. To achieve that, a
high-finesse Fabry-Perot cavity (F\approx 2200) was actively stabilized inside
a continuous-flow 4He cryostat. We observed optical cooling of the fundamental
mode of a 50mu x 50 mu x 5.4 mu singly-clamped micromirror at \omega_m=3.5 MHz
from 35 K to approx. 290 mK. This corresponds to a thermal occupation factor of
\approx 1x10^4. The cooling performance is only limited by the mechanical
quality and by the optical finesse of the system. Heating effects, e.g. due to
absorption of photons in the micromirror, could not be observed. These results
represent a next step towards cavity-cooling a mechanical oscillator into its
quantum ground state
Dissipative Optomechanics in High-Frequency Nanomechanical Resonators
The coherent transduction of information between microwave and optical
domains is a fundamental building block for future quantum networks. A
promising way to bridge these widely different frequencies is using
high-frequency nanomechanical resonators interacting with low-loss optical
modes. State-of-the-art optomechanical devices rely on purely dispersive
interactions that are enhanced by a large photon population in the cavity.
Additionally, one could use dissipative optomechanics, where photons can be
scattered directly from a waveguide into a resonator hence increasing the
degree of control of the acousto-optic interplay. Hitherto, such dissipative
optomechanical interaction was only demonstrated at low mechanical frequencies,
precluding prominent applications such as the quantum state transfer between
photonic and phononic domains. Here, we show the first dissipative
optomechanical system operating in the sideband-resolved regime, where the
mechanical frequency is larger than the optical linewidth. Exploring this
unprecedented regime, we demonstrate the impact of dissipative optomechanical
coupling in reshaping both mechanical and optical spectra. Our figures
represent a two-order-of-magnitude leap in the mechanical frequency and a
tenfold increase in the dissipative optomechanical coupling rate compared to
previous works. Further advances could enable the individual addressing of
mechanical modes and help mitigate optical nonlinearities and absorption in
optomechanical devices.Comment: 10 pages, 4 figures, supplemental materia
Achieving ground state and enhancing entanglement by recovering information
For cavity-assisted optomechanical cooling experiments, it has been shown in
the literature that the cavity bandwidth needs to be smaller than the
mechanical frequency in order to achieve the quantum ground state of the
mechanical oscillator, which is the so-called resolved-sideband or good-cavity
limit. We provide a new but physically equivalent insight into the origin of
such a limit: that is information loss due to a finite cavity bandwidth. With
an optimal feedback control to recover those information, we can surpass the
resolved-sideband limit and achieve the quantum ground state. Interestingly,
recovering those information can also significantly enhance the optomechanical
entanglement. Especially when the environmental temperature is high, the
entanglement will either exist or vanish critically depending on whether
information is recovered or not, which is a vivid example of a quantum eraser.Comment: 9 figures, 18 page
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