3,670 research outputs found
Discrete-time quadrature feedback cooling of a radio-frequency mechanical resonator
We have employed a feedback cooling scheme, which combines high-frequency
mixing with digital signal processing. The frequency and damping rate of a 2
MHz micromechanical resonator embedded in a dc SQUID are adjusted with the
feedback, and active cooling to a temperature of 14.3 mK is demonstrated. This
technique can be applied to GHz resonators and allows for flexible control
strategies.Comment: To appear in Appl. Phys. Let
Supernarrow spectral peaks near a kinetic phase transition in a driven, nonlinear micromechanical oscillator
We measure the spectral densities of fluctuations of an underdamped nonlinear
micromechanical oscillator. By applying a sufficiently large periodic
excitation, two stable dynamical states are obtained within a particular range
of driving frequency. White noise is injected into the excitation, allowing the
system to overcome the activation barrier and switch between the two states.
While the oscillator predominately resides in one of the two states for most
excitation frequencies, a narrow range of frequencies exist where the
occupations of the two states are approximately equal. At these frequencies,
the oscillator undergoes a kinetic phase transition that resembles the phase
transition of thermal equilibrium systems. We observe a supernarrow peak in the
power spectral densities of fluctuations of the oscillator. This peak is
centered at the excitation frequency and arises as a result of noise-induced
transitions between the two dynamical states.Comment: 4 pages, 4 figure
Quantum irreversible decoherence behaviour in open quantum systems with few degrees of freedom. Application to 1H NMR reversion experiments in nematic liquid crystals
An experimental study of NMR spin decoherence in nematic liquid crystals (LC)
is presented. Decoherence dynamics can be put in evidence by means of
refocusing experiments of the dipolar interactions. The experimental technique
used in this work is based on the MREV8 pulse sequence. The aim of the work is
to detect the main features of the Irreversible Quantum Decoherence (IQD) in
LC, on the basis of the theory presented by the authors recently. The focus is
laid on experimentally probing the eigen-selection process in the intermediate
time scale, between quantum interference of a closed system and thermalization,
as a signature of the IQD of the open quantum system, as well as on quantifying
the effects of non-idealities as possible sources of signal decays which could
mask the intrinsic IQD. In order to contrast experiment and theory, the theory
was adapted to obtain the IQD function corresponding to the MREV8 reversion
experiments. Non-idealities of the experimental setting are analysed in detail
within this framework and their effects on the observed signal decay are
numerically estimated. It is found that, though these non-idealities could in
principle affect the evolution of the spin dynamics, their influence can be
mitigated and they do not present the characteristic behavior of the IQD. As
unique characteristic of the IQD, the experimental results clearly show the
occurrence of eigen-selectivity in the intermediate timescale, in complete
agreement with the theoretical predictions. We conclude that the
eigen-selection effect is the fingerprint of IQD associated with a quantum open
spin system in LC. Besides, these features of the results account for the
quasi-equilibrium states of the spin system, which were observed previously in
these mesophases, and lead to conclude that the quasi-equilibrium is a definite
stage of the spin dynamics during its evolution towards equilibriu
A practical scheme for error control using feedback
We describe a scheme for quantum error correction that employs feedback and
weak measurement rather than the standard tools of projective measurement and
fast controlled unitary gates. The advantage of this scheme over previous
protocols (for example Ahn et. al, PRA, 65, 042301 (2001)), is that it requires
little side processing while remaining robust to measurement inefficiency, and
is therefore considerably more practical. We evaluate the performance of our
scheme by simulating the correction of bit-flips. We also consider
implementation in a solid-state quantum computation architecture and estimate
the maximal error rate which could be corrected with current technology.Comment: 12 pages, 3 figures. Minor typographic change
Are the laws of entanglement theory thermodynamical?
We argue that on its face, entanglement theory satisfies laws equivalent to
thermodynamics if the theory can be made reversible by adding certain bound
entangled states as a free resource during entanglement manipulation. Subject
to plausible assumptions, we prove that this is not the case in general, and
discuss the implications of this for the thermodynamics of entanglement.Comment: 4 pages, 1 figure, Revtex4; to appear in Phys. Rev. Let
Are there phase transitions in information space?
The interplay between two basic quantities -- quantum communication and
information -- is investigated. Quantum communication is an important resource
for quantum states shared by two parties and is directly related to
entanglement. Recently, the amount of local information that can be drawn from
a state has been shown to be closely related to the non-local properties of the
state. Here we consider both formation and extraction processes, and analyze
informational resources as a function of quantum communication. The resulting
diagrams in information space allow us to observe phase-like transitions when
correlations become classical.Comment: 4 pages, 3 epsi figures, to appear in Phys. Rev. Let
Unconditional privacy over channels which cannot convey quantum information
By sending systems in specially prepared quantum states, two parties can
communicate without an eavesdropper being able to listen. The technique, called
quantum cryptography, enables one to verify that the state of the quantum
system has not been tampered with, and thus one can obtain privacy regardless
of the power of the eavesdropper. All previous protocols relied on the ability
to faithfully send quantum states. In fact, until recently, they could all be
reduced to a single protocol where security is ensured though sharing maximally
entangled states. Here we show this need not be the case -- one can obtain
verifiable privacy even through some channels which cannot be used to reliably
send quantum states.Comment: Related to quant-ph/0608195 and for a more general audienc
Breakdown of a liquid filament into drops under the action of acoustic disturbances
Liquid filament breakdown into drops by acoustic vibratio
The quantum one-time pad in the presence of an eavesdropper
A classical one-time pad allows two parties to send private messages over a
public classical channel -- an eavesdropper who intercepts the communication
learns nothing about the message. A quantum one-time pad is a shared quantum
state which allows two parties to send private messages or private quantum
states over a public quantum channel. If the eavesdropper intercepts the
quantum communication she learns nothing about the message. In the classical
case, a one-time pad can be created using shared and partially private
correlations. Here we consider the quantum case in the presence of an
eavesdropper, and find the single letter formula for the rate at which the two
parties can send messages using a quantum one-time pad
Universal Features of Collective Interactions in Hard-Sphere Systems at Higher Volume Fractions
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