4,352 research outputs found
New Trends in Quantum Computing
Classical and quantum information are very different. Together they can
perform feats that neither could achieve alone, such as quantum computing,
quantum cryptography and quantum teleportation. Some of the applications range
from helping to preventing spies from reading private communications. Among the
tools that will facilitate their implementation, we note quantum purification
and quantum error correction. Although some of these ideas are still beyond the
grasp of current technology, quantum cryptography has been implemented and the
prospects are encouraging for small-scale prototypes of quantum computation
devices before the end of the millennium.Comment: 8 pages. Presented at the 13th Symposium on Theoretical Aspects of
Computer Science, Grenoble, 22 February 1996. Will appear in the proceedings,
Lecture Notes in Computer Science, Springer-Verlag. Standard LaTeX. Requires
llncs.sty (included
Teleportation as a quantum computation
An explicit quantum circuit is given to implement quantum teleportation. This
circuit makes teleportation straightforward to anyone who believes that quantum
computation is a reasonable proposition. It could also be genuinely used inside
a quantum computer if teleportation is needed to move quantum information
around. An unusual feature of this circuit is that there are points in the
computation at which the quantum information can be completely disrupted by a
measurement (or some types of interaction with the environment) without ill
effects: the same final result is obtained whether or not these measurements
takes place.Comment: 3 pages, LaTeX2e, PhysComp 96 submissio
Brief History of Quantum Cryptography: A Personal Perspective
Quantum cryptography is the only approach to privacy ever proposed that
allows two parties (who do not share a long secret key ahead of time) to
communicate with provably perfect secrecy under the nose of an eavesdropper
endowed with unlimited computational power and whose technology is limited by
nothing but the fundamental laws of nature. This essay provides a personal
historical perspective on the field. For the sake of liveliness, the style is
purposely that of a spontaneous after-dinner speech.Comment: 14 pages, no figure
Might Carbon-Atmosphere White Dwarfs Harbour a New Type of Pulsating Star?
In the light of the recent and unexpected discovery of a brand new type of
white dwarfs, those with carbon-dominated atmospheres, we examine the
asteroseismological potential of such stars. The motivation behind this is
based on the observation that past models of carbon-atmosphere white dwarfs
have partially ionized outer layers that bear strong resemblance with those
responsible for mode excitation in models of pulsating DB (helium-atmosphere)
and pulsating DA (hydrogen-atmosphere) white dwarfs. Our exciting main result
is that, given the right location in parameter space, some carbon-atmosphere
white dwarfs are predicted to show pulsational instability against gravity
modes. We are eagerly waiting the results of observational searches for
luminosity variations in these stars.Comment: 4-page letter + 4 figure
On local-hidden-variable no-go theorems
The strongest attack against quantum mechanics came in 1935 in the form of a
paper by Einstein, Podolsky and Rosen. It was argued that the theory of quantum
mechanics could not be called a complete theory of Nature, for every element of
reality is not represented in the formalism as such. The authors then put forth
a proposition: we must search for a theory where, upon knowing everything about
the system, including possible hidden variables, one could make precise
predictions concerning elements of reality. This project was ultimatly doomed
in 1964 with the work of Bell Bell, who showed that the most general local
hidden variable theory could not reproduce correlations that arise in quantum
mechanics. There exist mainly three forms of no-go theorems for local hidden
variable theories. Although almost every physicist knows the consequences of
these no-go theorems, not every physicist is aware of the distinctions between
the three or even their exact definitions. Thus we will discuss here the three
principal forms of no-go theorems for local hidden variable theories of Nature.
We will define Bell inequalities, Bell inequalities without inequalities and
pseudo-telepathy. A discussion of the similarities and differences will follow.Comment: 7 pages, no figure, replaced "Bell inequalities" with "Bell theorems"
and updated the reference
On The Power of Exact Quantum Polynomial Time
We investigate the power of quantum computers when they are required to
return an answer that is guaranteed correct after a time that is upper-bounded
by a polynomial in the worst case. In an oracle setting, it is shown that such
machines can solve problems that would take exponential time on any classical
bounded-error probabilistic computer.Comment: 10 pages, LaTeX2e, no figure
An Exact Quantum Polynomial-Time Algorithm for Simon's Problem
We investigate the power of quantum computers when they are required to
return an answer that is guaranteed to be correct after a time that is
upper-bounded by a polynomial in the worst case. We show that a natural
generalization of Simon's problem can be solved in this way, whereas previous
algorithms required quantum polynomial time in the expected sense only, without
upper bounds on the worst-case running time. This is achieved by generalizing
both Simon's and Grover's algorithms and combining them in a novel way. It
follows that there is a decision problem that can be solved in exact quantum
polynomial time, which would require expected exponential time on any classical
bounded-error probabilistic computer if the data is supplied as a black box.Comment: 12 pages, LaTeX2e, no figures. To appear in Proceedings of the Fifth
Israeli Symposium on Theory of Computing and Systems (ISTCS'97
On the power of non-local boxes
A non-local box is a virtual device that has the following property: given
that Alice inputs a bit at her end of the device and that Bob does likewise, it
produces two bits, one at Alice's end and one at Bob's end, such that the XOR
of the outputs is equal to the AND of the inputs. This box, inspired from the
CHSH inequality, was first proposed by Popescu and Rohrlich to examine the
question: given that a maximally entangled pair of qubits is non-local, why is
it not maximally non-local? We believe that understanding the power of this box
will yield insight into the non-locality of quantum mechanics. It was shown
recently by Cerf, Gisin, Massar and Popescu, that this imaginary device is able
to simulate correlations from any measurement on a singlet state. Here, we show
that the non-local box can in fact do much more: through the simulation of the
magic square pseudo-telepathy game and the Mermin-GHZ pseudo-telepathy game, we
show that the non-local box can simulate quantum correlations that no entangled
pair of qubits can in a bipartite scenario and even in a multi-party scenario.
Finally we show that a single non-local box cannot simulate all quantum
correlations and propose a generalization for a multi-party non-local box. In
particular, we show quantum correlations whose simulation requires an
exponential amount of non-local boxes, in the number of maximally entangled
qubit pairs.Comment: 14 pages, 1 figur
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