47 research outputs found
A Meaner King uses Biased Bases
The mean king problem is a quantum mechanical retrodiction problem, in which
Alice has to name the outcome of an ideal measurement on a d-dimensional
quantum system, made in one of (d+1) orthonormal bases, unknown to Alice at the
time of the measurement. Alice has to make this retrodiction on the basis of
the classical outcomes of a suitable control measurement including an entangled
copy. We show that the existence of a strategy for Alice is equivalent to the
existence of an overall joint probability distribution for (d+1) random
variables, whose marginal pair distributions are fixed as the transition
probability matrices of the given bases. In particular, for d=2 the problem is
decided by John Bell's classic inequality for three dichotomic variables. For
mutually unbiased bases in any dimension Alice has a strategy, but for randomly
chosen bases the probability for that goes rapidly to zero with increasing d.Comment: 5 pages, 1 figur
Lower bounds on entanglement measures from incomplete information
How can we quantify the entanglement in a quantum state, if only the
expectation value of a single observable is given? This question is of great
interest for the analysis of entanglement in experiments, since in many
multiparticle experiments the state is not completely known. We present several
results concerning this problem by considering the estimation of entanglement
measures via Legendre transforms. First, we present a simple algorithm for the
estimation of the concurrence and extensions thereof. Second, we derive an
analytical approach to estimate the geometric measure of entanglement, if the
diagonal elements of the quantum state in a certain basis are known. Finally,
we compare our bounds with exact values and other estimation methods for
entanglement measures.Comment: 9 pages, 4 figures, v2: final versio
Experimental entanglement verification and quantification via uncertainty relations
We report on experimental studies on entanglement quantification and
verification based on uncertainty relations for systems consisting of two
qubits. The new proposed measure is shown to be invariant under local unitary
transformations, by which entanglement quantification is implemented for
two-qubit pure states. The nonlocal uncertainty relations for two-qubit pure
states are also used for entanglement verification which serves as a basic
proposition and promise to be a good choice for verification of multipartite
entanglement.Comment: 5 pages, 3 figures and 2 table
Tema Con Variazioni: Quantum Channel Capacity
Channel capacity describes the size of the nearly ideal channels, which can
be obtained from many uses of a given channel, using an optimal error
correcting code. In this paper we collect and compare minor and major
variations in the mathematically precise statements of this idea which have
been put forward in the literature. We show that all the variations considered
lead to equivalent capacity definitions. In particular, it makes no difference
whether one requires mean or maximal errors to go to zero, and it makes no
difference whether errors are required to vanish for any sequence of block
sizes compatible with the rate, or only for one infinite sequence.Comment: 32 pages, uses iopart.cl
Permutationally invariant state reconstruction
Feasible tomography schemes for large particle numbers must possess, besides
an appropriate data acquisition protocol, also an efficient way to reconstruct
the density operator from the observed finite data set. Since state
reconstruction typically requires the solution of a non-linear large-scale
optimization problem, this is a major challenge in the design of scalable
tomography schemes. Here we present an efficient state reconstruction scheme
for permutationally invariant quantum state tomography. It works for all common
state-of-the-art reconstruction principles, including, in particular, maximum
likelihood and least squares methods, which are the preferred choices in
today's experiments. This high efficiency is achieved by greatly reducing the
dimensionality of the problem employing a particular representation of
permutationally invariant states known from spin coupling combined with convex
optimization, which has clear advantages regarding speed, control and accuracy
in comparison to commonly employed numerical routines. First prototype
implementations easily allow reconstruction of a state of 20 qubits in a few
minutes on a standard computer.Comment: 25 pages, 4 figues, 2 table
Conceptual Design of an Urban Electric Microcar
In the scope of this work, design steps of an L6e-class vehicle which is one of the most suitable alternatives for future urban transportation was introduced. Firstly, technical requirements have determined. Design features such as the vehicle mass, motor power and maximum speed were indicated, and the limitations that are determinative on them were pointed out. Effects of these parameters on driving capabilities and driving comfort were investigated. Proper types of suspension and steering linkage were employed based on the axle loads and the driving dynamics, as well as the major dimensions concerning the useful design volume. The suitable electric motor which satisfies the power requirements was chosen. Finally, the driveline scheme was established