182 research outputs found
Causality and Cirel'son bounds
An EPR-Bell type experiment carried out on an entangled quantum system can
produce correlations stronger than allowed by local realistic theories. However
there are correlations that are no-signaling and are more non local than the
quantum correlations. Here we show that any correlations more non local than
those achievable in an EPR-Bell type experiment necessarily allow -in the
context of the quantum formalism- both for signaling and for generation of
entanglement. We use our approach to rederive Cirel'son bound for the CHSH
expression, and we derive a new Cirel'son type bound for qutrits. We discuss in
detail the interpretation of our approach.Comment: 5 page
Quantum Computing on Lattices using Global Two-Qubit Gate
We study the computation power of lattices composed of two dimensional
systems (qubits) on which translationally invariant global two-qubit gates can
be performed. We show that if a specific set of 6 global two qubit gates can be
performed, and if the initial state of the lattice can be suitably chosen, then
a quantum computer can be efficiently simulatedComment: 9 page
Optimal strategies for sending information through a quantum channel
Quantum states can be used to encode the information contained in a
direction, i.e., in a unit vector. We present the best encoding procedure when
the quantum state is made up of spins (qubits). We find that the quality of
this optimal procedure, which we quantify in terms of the fidelity, depends
solely on the dimension of the encoding space. We also investigate the use of
spatial rotations on a quantum state, which provide a natural and less
demanding encoding. In this case we prove that the fidelity is directly related
to the largest zeros of the Legendre and Jacobi polynomials. We also discuss
our results in terms of the information gain.Comment: 4 pages, RevTex, final version to appear in Phys.Rev.Let
Improved Quantum Communication Complexity Bounds for Disjointness and Equality
We prove new bounds on the quantum communication complexity of the
disjointness and equality problems. For the case of exact and non-deterministic
protocols we show that these complexities are all equal to n+1, the previous
best lower bound being n/2. We show this by improving a general bound for
non-deterministic protocols of de Wolf. We also give an O(sqrt{n}c^{log^*
n})-qubit bounded-error protocol for disjointness, modifying and improving the
earlier O(sqrt{n}log n) protocol of Buhrman, Cleve, and Wigderson, and prove an
Omega(sqrt{n}) lower bound for a large class of protocols that includes the
BCW-protocol as well as our new protocol.Comment: 11 pages LaTe
Compression of quantum measurement operations
We generalize recent work of Massar and Popescu dealing with the amount of
classical data that is produced by a quantum measurement on a quantum state
ensemble. In the previous work it was shown how spurious randomness generally
contained in the outcomes can be eliminated without decreasing the amount of
knowledge, to achieve an amount of data equal to the von Neumann entropy of the
ensemble. Here we extend this result by giving a more refined description of
what constitute equivalent measurements (that is measurements which provide the
same knowledge about the quantum state) and also by considering incomplete
measurements. In particular we show that one can always associate to a POVM
with elements a_j, an equivalent POVM acting on many independent copies of the
system which produces an amount of data asymptotically equal to the entropy
defect of an ensemble canonically associated to the ensemble average state and
the initial measurement (a_j). In the case where the measurement is not
maximally refined this amount of data is strictly less than the von Neumann
entropy, as obtained in the previous work. We also show that this is the best
achievable, i.e. it is impossible to devise a measurement equivalent to the
initial measurement (a_j) that produces less data. We discuss the
interpretation of these results. In particular we show how they can be used to
provide a precise and model independent measure of the amount of knowledge that
is obtained about a quantum state by a quantum measurement. We also discuss in
detail the relation between our results and Holevo's bound, at the same time
providing a new proof of this fundamental inequality.Comment: RevTeX, 13 page
Two-dimensional black holes in accelerated frames: quantum aspects
By considering charged black hole solutions of a one parameter family of two
dimensional dilaton gravity theories, one finds the existence of quantum
mechanically stable gravitational kinks with a simple mass to charge relation.
Unlike their Einsteinian counterpart (i.e. extreme Reissner-Nordstr\"om), these
have nonvanishing horizon surface gravity.Comment: 18 pages, harvmac, 2 figure
Entanglement and non-locality are different resources
Bell's theorem states that, to simulate the correlations created by
measurement on pure entangled quantum states, shared randomness is not enough:
some "non-local" resources are required. It has been demonstrated recently that
all projective measurements on the maximally entangled state of two qubits can
be simulated with a single use of a "non-local machine". We prove that a
strictly larger amount of this non-local resource is required for the
simulation of pure non-maximally entangled states of two qubits
with
.Comment: 8 pages, 3 figure
Universal Quantum Information Compression
Suppose that a quantum source is known to have von Neumann entropy less than
or equal to S but is otherwise completely unspecified. We describe a method of
universal quantum data compression which will faithfully compress the quantum
information of any such source to S qubits per signal (in the limit of large
block lengths).Comment: RevTex 4 page
New Insights into Uniformly Accelerated Detector in a Quantum Field
We obtained an exact solution for a uniformly accelerated Unruh-DeWitt
detector interacting with a massless scalar field in (3+1) dimensions which
enables us to study the entire evolution of the total system, from the initial
transient to late-time steady state. We find that the Unruh effect as derived
from time-dependent perturbation theory is valid only in the transient stage
and is totally invalid for cases with proper acceleration smaller than the
damping constant. We also found that, unlike in (1+1)D results, the (3+1)D
uniformly accelerated Unruh-DeWitt detector in a steady state does emit a
positive radiated power of quantum nature at late-times, but it is not
connected to the thermal radiance experienced by the detector in the Unruh
effect proper.Comment: 6 pages, invited talk given by SYL at the conference of International
Association for Relativistic Dynamics (IARD), June 2006, Storrs, Connecticut,
US
Optimal minimal measurements of mixed states
The optimal and minimal measuring strategy is obtained for a two-state system
prepared in a mixed state with a probability given by any isotropic a priori
distribution. We explicitly construct the specific optimal and minimal
generalized measurements, which turn out to be independent of the a priori
probability distribution, obtaining the best guesses for the unknown state as
well as a closed expression for the maximal mean averaged fidelity. We do this
for up to three copies of the unknown state in a way which leads to the
generalization to any number of copies, which we then present and prove.Comment: 20 pages, no figure
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