31,305 research outputs found
A bird's eye view of quantum computers
Quantum computers are discussed in the general framework of computation, the
laws of physics and the foundations of quantum mechanics.Comment: 6 pages, 1 figur
Computing the distance between quantum channels: Usefulness of the Fano representation
The diamond norm measures the distance between two quantum channels. From an
operational vewpoint, this norm measures how well we can distinguish between
two channels by applying them to input states of arbitrarily large dimensions.
In this paper, we show that the diamond norm can be conveniently and in a
physically transparent way computed by means of a Monte-Carlo algorithm based
on the Fano representation of quantum states and quantum operations. The
effectiveness of this algorithm is illustrated for several single-qubit quantum
channels.Comment: 8 pages, 7 figure
Non-perturbative interpretation of the Bloch vector's path beyond rotating wave approximation
The Bloch vector's path of a two-level system exposed to a monochromatic
field exhibits, in the regime of strong coupling, complex corkscrew
trajectories. By considering the infinitesimal evolution of the two-level
system when the field is treated as a classical object, we show that the Bloch
vector's rotation speed oscillates between zero and twice the rotation speed
predicted by the rotating wave approximation. Cusps appear when the rotation
speed vanishes. We prove analytically that in correspondence to cusps the
curvature of the Bloch vector's path diverges. On the other hand, numerical
data show that the curvature is very large even for a quantum field in the deep
quantum regime with mean number of photons . We finally
compute numerically the typical error size in a quantum gate when the terms
beyond rotating wave approximation are neglected.Comment: 9 pages, 8 figure
Exotic States in the Dynamical Casimir Effect
We consider the interaction of a qubit with a single mode of the quantized
electromagnetic field and show that, in the ultrastrong coupling regime and
when the qubit-field interaction is switched on abruptly, the dynamical Casimir
effect leads to the generation of a variety of exotic states of the field,
which cannot be simply described as squeezed states. Such effect also appears
when initially both the qubit and the field are in their ground state. The
non-classicality of the obtained exotic states is characterized by means of a
parameter based on the volume of the negative part of the Wigner function. A
transition to non-classical states is observed by changing either the
interaction strength or the interaction time. The observed phenomena appear as
a general feature of nonadiabatic quantum gates, so that the dynamical Casimir
effect can be the origin of a fundamental upper limit to the maximum speed of
quantum computation and communication protocols.Comment: 5 pages, 4 figure
Dynamical Casimir Effect in Quantum Information Processing
We demonstrate, in the regime of ultrastrong matter-field coupling, the
strong connection between the dynamical Casimir effect (DCE) and the
performance of quantum information protocols. Our results are illustrated by
means of a realistic quantum communication channel and show that the DCE is a
fundamental limit for quantum computation and communication and that novel
schemes are required to implement ultrafast and reliable quantum gates.
Strategies to partially counteract the DCE are also discussed.Comment: 7 pages, 5 figure
Competition Between National Economies and Competition Between Businesses--A Response to Judge Pescatore
This Article challenges the validity of Judge Pescatore’s theory [that Member States of the ECC are prevented by Community law from intervening in the marketplace by legislation]
Fixed point theorems for metric spaces with a conical geodesic bicombing
We derive two fixed point theorems for a class of metric spaces that includes
all Banach spaces and all complete Busemann spaces. We obtain our results by
the use of a 1-Lipschitz barycenter construction and an existence result for
invariant Radon probability measures. Furthermore, we construct a bounded
complete Busemann space that admits an isometry without fixed points.Comment: 19 page
Entanglement, randomness and chaos
Entanglement is not only the most intriguing feature of quantum mechanics,
but also a key resource in quantum information science. The entanglement
content of random pure quantum states is almost maximal; such states find
applications in various quantum information protocols. The preparation of a
random state or, equivalently, the implementation of a random unitary operator,
requires a number of elementary one- and two-qubit gates that is exponential in
the number n_q of qubits, thus becoming rapidly unfeasible when increasing n_q.
On the other hand, pseudo-random states approximating to the desired accuracy
the entanglement properties of true random states may be generated efficiently,
that is, polynomially in n_q. In particular, quantum chaotic maps are efficient
generators of multipartite entanglement among the qubits, close to that
expected for random states. This review discusses several aspects of the
relationship between entanglement, randomness and chaos. In particular, I will
focus on the following items: (i) the robustness of the entanglement generated
by quantum chaotic maps when taking into account the unavoidable noise sources
affecting a quantum computer; (ii) the detection of the entanglement of
high-dimensional (mixtures of) random states, an issue also related to the
question of the emergence of classicality in coarse grained quantum chaotic
dynamics; (iii) the decoherence induced by the coupling of a system to a
chaotic environment, that is, by the entanglement established between the
system and the environment.Comment: Review paper, 40 pages, 7 figures, added reference
For a history of effects: Hume and German anti-rationalism
L'articolo analizza la ricezione del pensiero di Hume da parte di quello che I. Berlin ha definito l'anti-razionalismo tedesco, in pensatori come Hamann e, in particolare, H.F. Jacobi. E' anche attraverso tale ricezione infatti che si è annunciato lo spirito del grande movimento romantico. Di qui l'importanza di questi autori che sono stati protagonisti di una vera svolta d'epoca
- …