11,940 research outputs found
Entanglement-enhanced measurement of a completely unknown phase
The high-precision interferometric measurement of an unknown phase is the
basis for metrology in many areas of science and technology. Quantum
entanglement provides an increase in sensitivity, but present techniques have
only surpassed the limits of classical interferometry for the measurement of
small variations about a known phase. Here we introduce a technique that
combines entangled states with an adaptive algorithm to precisely estimate a
completely unspecified phase, obtaining more information per photon that is
possible classically. We use the technique to make the first ab initio
entanglement-enhanced optical phase measurement. This approach will enable
rapid, precise determination of unknown phase shifts using interferometry.Comment: 6 pages, 4 figure
Quantum-enhanced capture of photons using optical ratchet states
Natural and artificial light harvesting systems often operate in a regime
where the flux of photons is relatively low. Besides absorbing as many photons
as possible it is therefore paramount to prevent excitons from annihilation via
photon re-emission until they have undergone an irreversible energy conversion
process. Taking inspiration from photosynthetic antenna structures, we here
consider ring-like systems and introduce a class of states we call ratchets:
excited states capable of absorbing but not emitting light. This allows our
antennae to absorb further photons whilst retaining the excitations from those
that have already been captured. Simulations for a ring of four sites reveal a
peak power enhancement by up to a factor of 35 under ambient conditions owing
to a combination of ratcheting and the prevention of emission through
dark-state population. In the slow extraction limit the achievable power
enhancement due to ratcheting alone exceeds 20%.Comment: major revision with improved model (all data and figures updated
Adaptive Measurements in the Optical Quantum Information Laboratory
Adaptive techniques make practical many quantum measurements that would
otherwise be beyond current laboratory capabilities. For example: they allow
discrimination of nonorthogonal states with a probability of error equal to the
Helstrom bound; they allow measurement of the phase of a quantum oscillator
with accuracy approaching (or in some cases attaining) the Heisenberg limit;
and they allow estimation of phase in interferometry with a variance scaling at
the Heisenberg limit, using only single qubit measurement and control. Each of
these examples has close links with quantum information, in particular
experimental optical quantum information: the first is a basic quantum
communication protocol; the second has potential application in linear optical
quantum computing; the third uses an adaptive protocol inspired by the quantum
phase estimation algorithm. We discuss each of these examples, and their
implementation in the laboratory, but concentrate upon the last, which was
published most recently [Higgins {\em et al.}, Nature vol. 450, p. 393, 2007].Comment: 12 pages, invited paper to be published in IEEE Journal of Selected
Topics in Quantum Electronics: Quantum Communications and Information Scienc
Superabsorption of light via quantum engineering
Almost 60 years ago Dicke introduced the term superradiance to describe a
signature quantum effect: N atoms can collectively emit light at a rate
proportional to N^2. Even for moderate N this represents a significant increase
over the prediction of classical physics, and the effect has found applications
ranging from probing exciton delocalisation in biological systems, to
developing a new class of laser, and even in astrophysics. Structures that
super-radiate must also have enhanced absorption, but the former always
dominates in natural systems. Here we show that modern quantum control
techniques can overcome this restriction. Our theory establishes that
superabsorption can be achieved and sustained in certain simple nanostructures,
by trapping the system in a highly excited state while extracting energy into a
non-radiative channel. The effect offers the prospect of a new class of quantum
nanotechnology, capable of absorbing light many times faster than is currently
possible; potential applications of this effect include light harvesting and
photon detection. An array of quantum dots or a porphyrin ring could provide an
implementation to demonstrate this effect
Effects of rotation on coolant passage heat transfer. Volume 1: Coolant passages with smooth walls
An experimental program was conducted to investigate heat transfer and pressure loss characteristics of rotating multipass passages, for configurations and dimensions typical of modern turbine blades. The immediate objective was the generation of a data base of heat transfer and pressure loss data required to develop heat transfer correlations and to assess computational fluid dynamic techniques for rotating coolant passages. Experiments were conducted in a smooth wall large scale heat transfer model
Mixed state discrimination using optimal control
We present theory and experiment for the task of discriminating two
nonorthogonal states, given multiple copies. We implement several local
measurement schemes, on both pure states and states mixed by depolarizing
noise. We find that schemes which are optimal (or have optimal scaling) without
noise perform worse with noise than simply repeating the optimal single-copy
measurement. Applying optimal control theory, we derive the globally optimal
local measurement strategy, which outperforms all other local schemes, and
experimentally implement it for various levels of noise.Comment: Corrected ref 1 date; 4 pages & 4 figures + 2 pages & 3 figures
supplementary materia
Using weak values to experimentally determine "negative probabilities" in a two-photon state with Bell correlations
Bipartite quantum entangled systems can exhibit measurement correlations that
violate Bell inequalities, revealing the profoundly counter-intuitive nature of
the physical universe. These correlations reflect the impossibility of
constructing a joint probability distribution for all values of all the
different properties observed in Bell inequality tests. Physically, the
impossibility of measuring such a distribution experimentally, as a set of
relative frequencies, is due to the quantum back-action of projective
measurements. Weakly coupling to a quantum probe, however, produces minimal
back-action, and so enables a weak measurement of the projector of one
observable, followed by a projective measurement of a non-commuting observable.
By this technique it is possible to empirically measure weak-valued
probabilities for all of the values of the observables relevant to a Bell test.
The marginals of this joint distribution, which we experimentally determine,
reproduces all of the observable quantum statistics including a violation of
the Bell inequality, which we independently measure. This is possible because
our distribution, like the weak values for projectors on which it is built, is
not constrained to the interval [0, 1]. It was first pointed out by Feynman
that, for explaining singlet-state correlations within "a [local] hidden
variable view of nature ... everything works fine if we permit negative
probabilities". However, there are infinitely many such theories. Our method,
involving "weak-valued probabilities", singles out a unique set of
probabilities, and moreover does so empirically.Comment: 9 pages, 3 figure
Steep sharp-crested gravity waves on deep water
A new type of steady steep two-dimensional irrotational symmetric periodic
gravity waves on inviscid incompressible fluid of infinite depth is revealed.
We demonstrate that these waves have sharper crests in comparison with the
Stokes waves of the same wavelength and steepness. The speed of a fluid
particle at the crest of new waves is greater than their phase speed.Comment: 4 pages, 2 figures, submitted to Phys. Rev. Let
Complementary Organic Logic Gates on Plastic Formed by Self-Aligned Transistors with Gravure and Inkjet Printed Dielectric and Semiconductors
Complementary organic field-effect transistors, inverters, NAND and NOR logic on plastic are demonstrated using a combination of nanoimprint lithography, self-alignment, gravure, and inkjet printing. Sub-micrometer channel lengths, electrode overlaps and sub-100 nm dielectrics are compared to photolithographically patterned equivalents, as are inkjet and gravure printed semiconductors
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