5,922 research outputs found
Quantum Disentangled Liquids
We propose and explore a new finite temperature phase of translationally
invariant multi-component liquids which we call a "Quantum Disentangled Liquid"
(QDL) phase. We contemplate the possibility that in fluids consisting of two
(or more) species of indistinguishable quantum particles with a large mass
ratio, the light particles might "localize" on the heavy particles. We give a
precise, formal definition of this Quantum Disentangled Liquid phase in terms
of the finite energy density many-particle wavefunctions. While the heavy
particles are fully thermalized, for a typical fixed configuration of the heavy
particles, the entanglement entropy of the light particles satisfies an area
law; this implies that the light particles have not thermalized. Thus, in a QDL
phase, thermal equilibration is incomplete, and the canonical assumptions of
statistical mechanics are not fully operative. We explore the possibility of
QDL in water, with the light proton degrees of freedom becoming "localized" on
the oxygen ions. We do not presently know whether a local, generic Hamiltonian
can have eigenstates of the QDL form, and if it can not, then the non-thermal
behavior discussed here will exist as an interesting crossover phenomena at
time scales that diverge as the ratio of the mass of the heavy to the light
species also diverges.Comment: 14 page
Quantum computers can search rapidly by using almost any transformation
A quantum computer has a clear advantage over a classical computer for
exhaustive search. The quantum mechanical algorithm for exhaustive search was
originally derived by using subtle properties of a particular quantum
mechanical operation called the Walsh-Hadamard (W-H) transform. This paper
shows that this algorithm can be implemented by replacing the W-H transform by
almost any quantum mechanical operation. This leads to several new applications
where it improves the number of steps by a square-root. It also broadens the
scope for implementation since it demonstrates quantum mechanical algorithms
that can readily adapt to available technology.Comment: This paper is an adapted version of quant-ph/9711043. It has been
modified to make it more readable for physicists. 9 pages, postscrip
Quantum Mechanics helps in searching for a needle in a haystack
Quantum mechanics can speed up a range of search applications over unsorted
data. For example imagine a phone directory containing N names arranged in
completely random order. To find someone's phone number with a probability of
50%, any classical algorithm (whether deterministic or probabilistic) will need
to access the database a minimum of O(N) times. Quantum mechanical systems can
be in a superposition of states and simultaneously examine multiple names. By
properly adjusting the phases of various operations, successful computations
reinforce each other while others interfere randomly. As a result, the desired
phone number can be obtained in only O(sqrt(N)) accesses to the database.Comment: Postscript, 4 pages. This is a modified version of the STOC paper
(quant-ph/9605043) and is modified to make it more comprehensible to
physicists. It appeared in Phys. Rev. Letters on July 14, 1997. (This paper
was originally put out on quant-ph on June 13, 1997, the present version has
some minor typographical changes
Nested quantum search and NP-complete problems
A quantum algorithm is known that solves an unstructured search problem in a
number of iterations of order , where is the dimension of the
search space, whereas any classical algorithm necessarily scales as . It
is shown here that an improved quantum search algorithm can be devised that
exploits the structure of a tree search problem by nesting this standard search
algorithm. The number of iterations required to find the solution of an average
instance of a constraint satisfaction problem scales as , with
a constant depending on the nesting depth and the problem
considered. When applying a single nesting level to a problem with constraints
of size 2 such as the graph coloring problem, this constant is
estimated to be around 0.62 for average instances of maximum difficulty. This
corresponds to a square-root speedup over a classical nested search algorithm,
of which our presented algorithm is the quantum counterpart.Comment: 18 pages RevTeX, 3 Postscript figure
Observation of tunable exchange bias in SrYbRuO
The double perovskite compound, SrYbRuO, displays reversal in the
orientation of magnetic moments along with negative magnetization due to an
underlying magnetic compensation phenomenon. The exchange bias (EB) field below
the compensation temperature could be the usual negative or the positive
depending on the initial cooling field. This EB attribute has the potential of
getting tuned in a preselected manner, as the positive EB field is seen to
crossover from positive to negative value above .Comment: 4 Pages, 4 Figure
Heisenberg chains cannot mirror a state
Faithful exchange of quantum information can in future become a key part of
many computational algorithms. Some Authors suggest to use chains of mutually
coupled spins as channels for quantum communication. One can divide these
proposals into the groups of assisted protocols, which require some additional
action from the users, and natural ones, based on the concept of state
mirroring. We show that mirror is fundamentally not the feature chains of
spins-1/2 coupled by the Heisenberg interaction, but without local magnetic
fields. This fact has certain consequences in terms of the natural state
transfer
Grover Algorithm with zero theoretical failure rate
In standard Grover's algorithm for quantum searching, the probability of
finding the marked item is not exactly 1. In this Letter we present a modified
version of Grover's algorithm that searches a marked state with full successful
rate. The modification is done by replacing the phase inversion by two phase
rotation through angle . The rotation angle is given analytically to be
, where
, the number of items in the database, and
an integer equal to or greater than the integer part of . Upon measurement at -th iteration, the marked state
is obtained with certainty.Comment: 5 pages. Accepted for publication in Physical Review
The quantum correlation between the selection of the problem and that of the solution sheds light on the mechanism of the quantum speed up
In classical problem solving, there is of course correlation between the
selection of the problem on the part of Bob (the problem setter) and that of
the solution on the part of Alice (the problem solver). In quantum problem
solving, this correlation becomes quantum. This means that Alice contributes to
selecting 50% of the information that specifies the problem. As the solution is
a function of the problem, this gives to Alice advanced knowledge of 50% of the
information that specifies the solution. Both the quadratic and exponential
speed ups are explained by the fact that quantum algorithms start from this
advanced knowledge.Comment: Earlier version submitted to QIP 2011. Further clarified section 1,
"Outline of the argument", submitted to Phys Rev A, 16 page
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