46 research outputs found
Evaluating the Applicability of Advanced Techniques for Practical Real-time Train Scheduling
AbstractThis paper reports on the practical applicability of published techniques for real-time train scheduling. The final goal is the development of an advanced decision support system for supporting dispatchersâ work and for guiding them toward near-optimal real-time re-timing, re-ordering and re-routing decisions. The paper focuses on the optimization system AGLIBRARY that manages trains at the microscopic level of block sections and block signals and at a precision of seconds. The system outcome is a detailed conflict-free train schedule, being able to avoid deadlocks and to minimize train delays. Experiments on a British railway nearby London demonstrate that AGLIBRARY can quickly compute near-optimal solutions
Optimal covariant quantum networks
A sequential network of quantum operations is efficiently described by its
quantum comb, a non-negative operator with suitable normalization constraints.
Here we analyze the case of networks enjoying symmetry with respect to the
action of a given group of physical transformations, introducing the notion of
covariant combs and testers, and proving the basic structure theorems for these
objects. As an application, we discuss the optimal alignment of reference
frames (without pre-established common references) with multiple rounds of
quantum communication, showing that i) allowing an arbitrary amount of
classical communication does not improve the alignment, and ii) a single round
of quantum communication is sufficient.Comment: 10 pages, 3 figure
No-signaling, entanglement-breaking, and localizability in bipartite channels
A bipartite quantum channel represents the interaction between systems,
generally allowing for exchange of information. A special class of bipartite
channels are the no-signaling ones, which do not allow communication. In Ref.
[1] it has been conjectured that all no-signaling channels are mixtures of
entanglement-breaking and localizable channels, which require only local
operations and entanglement. Here we provide the general realization scheme,
giving a counterexample to the conjecture.Comment: 4 pages, revtex
Efficient universal programmable quantum measurements
A universal programmable detector is a device that can be tuned to perform
any desired measurement on a given quantum system, by changing the state of an
ancilla. With a finite dimension d for the ancilla only approximate universal
programmability is possible, with "size" d=f(1/e) increasing function of the
"accuracy" 1/e. In this letter we show that, much better than the exponential
size known in the literature, one can achieve polynomial size. An explicit
example with linear size is also presented. Finally, we show that for covariant
measurements exact programmability is feasible.Comment: 4 pages, RevTex
Superbroadcasting of mixed states
We derive the optimal universal broadcasting for mixed states of qubits. We
show that the nobroadcasting theorem cannot be generalized to more than a
single input copy. Moreover, for four or more input copies it is even possible
to purify the input states while broadcasting. We name such purifying
broadcasting superbroadcasting.Comment: 4 pages, 4 figures, to appear on Phys. Rev. Let
Quantum error correction with degenerate codes for correlated noise
We introduce a quantum packing bound on the minimal resources required by
nondegenerate error correction codes for any kind of noise. We prove that
degenerate codes can outperform nondegenerate ones in the presence of
correlated noise, by exhibiting examples where the quantum packing bound is
violated.Comment: 5 pages, published versio
Discord and non-classicality in probabilistic theories
Quantum discord quantifies non-classical correlations in quantum states. We
introduce discord for states in causal probabilistic theories, inspired by the
original definition proposed in Ref. [17]. We show that the only probabilistic
theory in which all states have null discord is classical probability theory.
Non-null discord is then not just a quantum feature, but a generic signature of
non-classicality.Comment: 5 pages, revtex styl
Superbroadcasting and classical information
We address the problem of broadcasting N copies of a generic qubit state to
M>N copies by estimating its direction and preparing a suitable output state
according to the outcome of the estimate. This semiclassical broadcasting
protocol is more restrictive than a general one, since it requires an
intermediate step where classical information is extracted and processed.
However, we prove that a suboptimal superbroadcasting, namely broadcasting with
simultaneous purification of the local output states with respect to the input
ones, is possible. We show that in the asymptotic limit of the
purification rate converges to the optimal one, proving the conjecture that
optimal broadcasting and state estimation are asymptotically equivalent. We
also show that it is possible to achieve superbroadcasting with simultaneous
inversion of the Bloch vector direction (universal NOT). We prove that in this
case the semiclassical procedure of state estimation and preparation turns out
to be optimal. We finally analyse semiclassical superbroadcasting in the
phase-covariant case.Comment: 9 pages, 2 figure
Optimal estimation of quantum observables
We consider the problem of estimating the ensemble average of an observable
on an ensemble of equally prepared identical quantum systems. We show that,
among all kinds of measurements performed jointly on the copies, the optimal
unbiased estimation is achieved by the usual procedure that consists in
performing independent measurements of the observable on each system and
averaging the measurement outcomes.Comment: Submitted to J. Math Phy
Optimal phase estimation for qubit mixed states
We address the problem of optimal estimation of the relative phase for
two-dimensional quantum systems in mixed states. In particular, we derive the
optimal measurement procedures for an arbitrary number of qubits prepared in
the same mixed state.Comment: revised version accepted for publicatio