10,862 research outputs found
Interaction-free quantum computation
In this paper, we study the quantum computation realized by an
interaction-free measurement (IFM). Using Kwiat et al.'s interferometer, we
construct a two-qubit quantum gate that changes one particle's trajectory
according to whether or not the other particle exists in the interferometer. We
propose a method for distinguishing Bell-basis vectors, each of which consists
of a pair of an electron and a positron, by this gate. (This is called the
Bell-basis measurement.) This method succeeds with probability 1 in the limit
of , where N is the number of beam splitters in the
interferometer. Moreover, we can carry out a controlled-NOT gate operation by
the above Bell-basis measurement and the method proposed by Gottesman and
Chuang. Therefore, we can prepare a universal set of quantum gates by the IFM.
This means that we can execute any quantum algorithm by the IFM.Comment: 11 pages, 7 figures, LaTex2
Structure of strongly coupled, multi-component plasmas
We investigate the short-range structure in strongly coupled fluidlike plasmas using the hypernetted chain approach generalized to multicomponent systems. Good agreement with numerical simulations validates this method for the parameters considered. We found a strong mutual impact on the spatial arrangement for systems with multiple ion species which is most clearly pronounced in the static structure factor. Quantum pseudopotentials were used to mimic diffraction and exchange effects in dense electron-ion systems. We demonstrate that the different kinds of pseudopotentials proposed lead to large differences in both the pair distributions and structure factors. Large discrepancies were also found in the predicted ion feature of the x-ray scattering signal, illustrating the need for comparison with full quantum calculations or experimental verification
Tunable asymmetric reflectance in silver films near the percolation threshold
We report on the optical characterization of semicontinuous nanostructured
silver films exhibiting tunable optical reflectance asymmetries. The films are
obtained using a multi-step process, where a nanocrystalline silver film is
first chemically deposited on a glass substrate and then subsequently coated
with additional silver via thermal vacuum-deposition. The resulting films
exhibit reflectance asymmetries whose dispersions may be tuned both in sign and
in magnitude, as well as a universal, tunable spectral crossover point. We
obtain a correlation between the optical response and charge transport in these
films, with the spectral crossover point indicating the onset of charge
percolation. Such broadband, dispersion-tunable asymmetric reflectors may find
uses in future light-harvesting systems.Comment: 18 pages, 5 figures, accepted by Journal of Applied Physic
Effect of electrical bias on spin transport across a magnetic domain wall
We present a theory of the current-voltage characteristics of a magnetic
domain wall between two highly spin-polarized materials, which takes into
account the effect of the electrical bias on the spin-flip probability of an
electron crossing the wall. We show that increasing the voltage reduces the
spin-flip rate, and is therefore equivalent to reducing the width of the domain
wall. As an application, we show that this effect widens the temperature window
in which the operation of a unipolar spin diode is nearly ideal.Comment: 11 pages, 3 figure
Collision of Polymers in a Vacuum
In a number of experimental situations, single polymer molecules can be
suspended in a vacuum. Here collisions between such molecules are considered.
The limit of high collision velocity is investigated numerically for a variety
of conditions. The distribution of contact times, scattering angles, and final
velocities are analyzed. In this limit, self avoiding chains are found to
become highly stretched as they collide with each other, and have a
distribution of scattering times that depends on the scattering angle. The
velocity of the molecules after the collisions is similar to predictions of a
model assuming thermal equilibration of molecules during the collision. The
most important difference is a significant subset of molecules that
inelastically scatter but do not substantially change direction.Comment: 7 pages, 6 figure
Non-locality and gauge freedom in Deutsch and Hayden's formulation of quantum mechanics
Deutsch and Hayden have proposed an alternative formulation of quantum
mechanics which is completely local. We argue that their proposal must be
understood as having a form of `gauge freedom' according to which
mathematically distinct states are physically equivalent. Once this gauge
freedom is taken into account, their formulation is no longer local.Comment: 3 page
Quantum algorithms know in advance 50% of the solution they will find in the future
Quantum algorithms require less operations than classical algorithms. The
exact reason of this has not been pinpointed until now. Our explanation is that
quantum algorithms know in advance 50% of the solution of the problem they will
find in the future. In fact they can be represented as the sum of all the
possible histories of a respective "advanced information classical algorithm".
This algorithm, given the advanced information (50% of the bits encoding the
problem solution), performs the operations (oracle's queries) still required to
identify the solution. Each history corresponds to a possible way of getting
the advanced information and a possible result of computing the missing
information. This explanation of the quantum speed up has an immediate
practical consequence: the speed up comes from comparing two classical
algorithms, with and without advanced information, with no physics involved.
This simplification could open the way to a systematic exploration of the
possibilities of speed up.Comment: The example of new quantum speed up that was just outlined in the
previous version (finding the character of a permutation) is fully deployed
in the present version. There are minor distributed changes to the writin
Design of On-Target FAAH Inhibitors
In this issue of Chemistry & Biology, Alexander and Cravatt [1] propose a model for the binding of carbamate inhibitors to fatty acid amide hydrolase (FAAH), the enzyme that breaks down signaling lipids. Using competitive activity-based protein profiling and click chemistry, they designed potent and selective FAAH inhibitors and characterized their off-target reactions
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