2,653 research outputs found
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
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
Quantum computers can search arbitrarily large databases by a single query
This paper shows that a quantum mechanical algorithm that can query
information relating to multiple items of the database, can search a database
in a single query (a query is defined as any question to the database to which
the database has to return a (YES/NO) answer). A classical algorithm will be
limited to the information theoretic bound of at least O(log N) queries (which
it would achieve by using a binary search).Comment: Several enhancements to the original pape
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
Entangled Quantum States Generated by Shor's Factoring Algorithm
The intermediate quantum states of multiple qubits, generated during the
operation of Shor's factoring algorithm are analyzed. Their entanglement is
evaluated using the Groverian measure. It is found that the entanglement is
generated during the pre-processing stage of the algorithm and remains nearly
constant during the quantum Fourier transform stage. The entanglement is found
to be correlated with the speedup achieved by the quantum algorithm compared to
classical algorithms.Comment: 7 pages, 4 figures submitted to Phys. Rev.
High-pressure x-ray diffraction study of bulk and nanocrystalline PbMoO4
We studied the effects of high-pressure on the crystalline structure of bulk
and nanocrystalline scheelite-type PbMoO4. We found that in both cases the
compressibility of the materials is highly non-isotropic, being the c-axis the
most compressible one. We also observed that the volume compressibility of
nanocrystals becomes higher that the bulk one at 5 GPa. In addition, at 10.7(8)
GPa we observed the onset of an structural phase transition in bulk PbMoO4. The
high-pressure phase has a monoclinic structure similar to M-fergusonite. The
transition is reversible and not volume change is detected between the low- and
high-pressure phases. No additional structural changes or evidence of
decomposition are found up to 21.1 GPa. In contrast nanocrystalline PbMoO4
remains in the scheelite structure at least up to 16.1 GPa. Finally, the
equation of state for bulk and nanocrystalline PbMoO4 are also determined.Comment: 18 pages, 4 figure
Energy and Efficiency of Adiabatic Quantum Search Algorithms
We present the results of a detailed analysis of a general, unstructured
adiabatic quantum search of a data base of items. In particular we examine
the effects on the computation time of adding energy to the system. We find
that by increasing the lowest eigenvalue of the time dependent Hamiltonian {\it
temporarily} to a maximum of , it is possible to do the
calculation in constant time. This leads us to derive the general theorem which
provides the adiabatic analogue of the bound of conventional quantum
searches. The result suggests that the action associated with the oracle term
in the time dependent Hamiltonian is a direct measure of the resources required
by the adiabatic quantum search.Comment: 6 pages, Revtex, 1 figure. Theorem modified, references and comments
added, sections introduced, typos corrected. Version to appear in J. Phys.
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
Quantum Analogue Computing
We briefly review what a quantum computer is, what it promises to do for us,
and why it is so hard to build one. Among the first applications anticipated to
bear fruit is quantum simulation of quantum systems. While most quantum
computation is an extension of classical digital computation, quantum
simulation differs fundamentally in how the data is encoded in the quantum
computer. To perform a quantum simulation, the Hilbert space of the system to
be simulated is mapped directly onto the Hilbert space of the (logical) qubits
in the quantum computer. This type of direct correspondence is how data is
encoded in a classical analogue computer. There is no binary encoding, and
increasing precision becomes exponentially costly: an extra bit of precision
doubles the size of the computer. This has important consequences for both the
precision and error correction requirements of quantum simulation, and
significant open questions remain about its practicality. It also means that
the quantum version of analogue computers, continuous variable quantum
computers (CVQC) becomes an equally efficient architecture for quantum
simulation. Lessons from past use of classical analogue computers can help us
to build better quantum simulators in future.Comment: 10 pages, to appear in the Visions 2010 issue of Phil. Trans. Roy.
Soc.
The Precise Formula in a Sine Function Form of the norm of the Amplitude and the Necessary and Sufficient Phase Condition for Any Quantum Algorithm with Arbitrary Phase Rotations
In this paper we derived the precise formula in a sine function form of the
norm of the amplitude in the desired state, and by means of he precise formula
we presented the necessary and sufficient phase condition for any quantum
algorithm with arbitrary phase rotations. We also showed that the phase
condition: identical rotation angles, is a sufficient but not a necessary phase
condition.Comment: 16 pages. Modified some English sentences and some proofs. Removed a
table. Corrected the formula for kol on page 10. No figure
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