330 research outputs found
Quantum state engineering, purification, and number resolved photon detection with high finesse optical cavities
We propose and analyze a multi-functional setup consisting of high finesse
optical cavities, beam splitters, and phase shifters. The basic scheme projects
arbitrary photonic two-mode input states onto the subspace spanned by the
product of Fock states |n>|n> with n=0,1,2,.... This protocol does not only
provide the possibility to conditionally generate highly entangled photon
number states as resource for quantum information protocols but also allows one
to test and hence purify this type of quantum states in a communication
scenario, which is of great practical importance. The scheme is especially
attractive as a generalization to many modes allows for distribution and
purification of entanglement in networks. In an alternative working mode, the
setup allows of quantum non demolition number resolved photodetection in the
optical domain.Comment: 14 pages, 10 figure
Ensemble Quantum Computation with atoms in periodic potentials
We show how to perform universal quantum computation with atoms confined in
optical lattices which works both in the presence of defects and without
individual addressing. The method is based on using the defects in the lattice,
wherever they are, both to ``mark'' different copies on which ensemble quantum
computation is carried out and to define pointer atoms which perform the
quantum gates. We also show how to overcome the problem of scalability on this
system
All Teleportation and Dense Coding Schemes
We establish a one-to-one correspondence between (1) quantum teleportation
schemes, (2) dense coding schemes, (3) orthonormal bases of maximally entangled
vectors, (4) orthonormal bases of unitary operators with respect to the
Hilbert-Schmidt scalar product, and (5) depolarizing operations, whose Kraus
operators can be chosen to be unitary. The teleportation and dense coding
schemes are assumed to be ``tight'' in the sense that all Hilbert spaces
involved have the same finite dimension d, and the classical channel involved
distinguishes d^2 signals. A general construction procedure for orthonormal
bases of unitaries, involving Latin Squares and complex Hadamard Matrices is
also presented.Comment: 21 pages, LaTe
Tripartite entanglement and quantum relative entropy
We establish relations between tripartite pure state entanglement and
additivity properties of the bipartite relative entropy of entanglement. Our
results pertain to the asymptotic limit of local manipulations on a large
number of copies of the state. We show that additivity of the relative entropy
would imply that there are at least two inequivalent types of asymptotic
tripartite entanglement. The methods used include the application of some
useful lemmas that enable us to analytically calculate the relative entropy for
some classes of bipartite states.Comment: 7 pages, revtex, no figures. v2: discussion about recent results, 2
refs. added. Published versio
On 1-qubit channels
The entropy H_T(rho) of a state rho with respect to a channel T and the
Holevo capacity of the channel require the solution of difficult variational
problems. For a class of 1-qubit channels, which contains all the extremal
ones, the problem can be significantly simplified by associating an Hermitian
antilinear operator theta to every channel of the considered class. The
concurrence of the channel can be expressed by theta and turns out to be a flat
roof. This allows to write down an explicit expression for H_T. Its maximum
would give the Holevo (1-shot) capacity.Comment: 12 pages, several printing or latex errors correcte
"Squashed Entanglement" - An Additive Entanglement Measure
In this paper, we present a new entanglement monotone for bipartite quantum
states. Its definition is inspired by the so-called intrinsic information of
classical cryptography and is given by the halved minimum quantum conditional
mutual information over all tripartite state extensions. We derive certain
properties of the new measure which we call "squashed entanglement": it is a
lower bound on entanglement of formation and an upper bound on distillable
entanglement. Furthermore, it is convex, additive on tensor products, and
superadditive in general.
Continuity in the state is the only property of our entanglement measure
which we cannot provide a proof for. We present some evidence, however, that
our quantity has this property, the strongest indication being a conjectured
Fannes type inequality for the conditional von Neumann entropy. This inequality
is proved in the classical case.Comment: 8 pages, revtex4. v2 has some more references and a bit more
discussion, v3 continuity discussion extended, typos correcte
Conversion of a general quantum stabilizer code to an entanglement distillation protocol
We show how to convert a quantum stabilizer code to a one-way or two-way
entanglement distillation protocol. The proposed conversion method is a
generalization of those of Shor-Preskill and Nielsen-Chuang. The recurrence
protocol and the quantum privacy amplification protocol are equivalent to the
protocols converted from [[2,1]] stabilizer codes. We also give an example of a
two-way protocol converted from a stabilizer better than the recurrence
protocol and the quantum privacy amplification protocol. The distillable
entanglement by the class of one-way protocols converted from stabilizer codes
for a certain class of states is equal to or greater than the achievable rate
of stabilizer codes over the channel corresponding to the distilled state, and
they can distill asymptotically more entanglement from a very noisy Werner
state than the hashing protocol.Comment: LaTeX2e, 18 pages, 1 figure. Version 4 added an example of two-way
protocols better than the recurrence protocol and the quantum privacy
amplification protocol. Version 2 added the quantum privacy amplification
protocol as an example converted from a stabilizer code, and corrected many
errors. Results unchanged from V
Quantum computation in optical lattices via global laser addressing
A scheme for globally addressing a quantum computer is presented along with
its realisation in an optical lattice setup of one, two or three dimensions.
The required resources are mainly those necessary for performing quantum
simulations of spin systems with optical lattices, circumventing the necessity
for single qubit addressing. We present the control procedures, in terms of
laser manipulations, required to realise universal quantum computation. Error
avoidance with the help of the quantum Zeno effect is presented and a scheme
for globally addressed error correction is given. The latter does not require
measurements during the computation, facilitating its experimental
implementation. As an illustrative example, the pulse sequence for the
factorisation of the number fifteen is given.Comment: 11 pages, 10 figures, REVTEX. Initialisation and measurement
procedures are adde
Teleportation and entanglement distillation in the presence of correlation among bipartite mixed states
The teleportation channel associated with an arbitrary bipartite state
denotes the map that represents the change suffered by a teleported state when
the bipartite state is used instead of the ideal maximally entangled state for
teleportation. This work presents and proves an explicit expression of the
teleportation channel for the teleportation using Weyl's projective unitary
representation of the space of 2n-tuples of numbers from Z/dZ for integers d>1,
n>0, which has been known for n=1. This formula allows any correlation among
the n bipartite mixed states, and an application shows the existence of
reliable schemes for distillation of entanglement from a sequence of mixed
states with correlation.Comment: 12 pages, 1 figur
Cooling toolbox for atoms in optical lattices
We propose and analyze several schemes for cooling bosonic and fermionic
atoms in an optical lattice potential close to the ground state of the
no-tunnelling regime. Some of the protocols rely on the concept of algorithmic
cooling, which combines occupation number filtering with ideas from ensemble
quantum computation. We also design algorithms that create an ensemble of
defect-free quantum registers. We study the efficiency of our protocols for
realistic temperatures and in the presence of a harmonic confinement. We also
propose an incoherent physical implementation of filtering which can be
operated in a continuous way.Comment: 14 pages, 13 figure
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