Results 1 - 10 of 70
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[en] A new series of measurement of the speed of the neutrino between the CERN (Switzerland) and the Gran Sasso laboratory (Italy) confirms the previous result: neutrinos go faster than light and in this case with a mean lead of 62.1 nanoseconds. This measurement is based on 20 events and a more individual follow-up of the neutrino since its formation during the impact between a proton and a target at CERN and to its observation in the Gran Sasso. (A.C.)
[en] Recently, Chau (2011 Quantum Inform. Comp. 11 721) found a family of metrics and pseudo-metrics on n-dimensional unitary operators that can be interpreted as the minimum resources (given by certain tight quantum speed limit bounds) needed to transform one unitary operator to another. This result is closely related to the weighted ℓ1-norm on Rn. Here we generalize this finding by showing that every weighted ℓp-norm on Rn with 1 ⩽ p ⩽ π/2 induces a metric and a pseudo-metric on n-dimensional unitary operators with quantum information-theoretic meanings related to certain tight quantum speed limit bounds. Besides, we investigate how far the correspondence between the existence of metrics and pseudo-metrics of this type and the quantum speed limits can go. (paper)
[en] We cast observable measure of quantum coherence as a resource to control the quantum speed limit (QSL) for unitary evolutions. For non-unitary evolutions, QSL depends on that of the state of the system and environment together. We show that the product of the time bound and the coherence (asymmetry) or the quantum part of the uncertainty behaves in a geometric way under partial elimination and classical mixing of states. These relations give a new insight into the quantum speed limit. We also show that our bound is experimentally measurable and is tighter than various existing bounds in the literature. - Highlights: • QSL for any general processes is proposed. • Quantum coherence (asymmetry) is portrayed as a resource for QSL. • A tighter quantum speed limit is proposed. • QSL under mixing and partial elimination of states is studied.
[en] The generic bound of quantum speed limit time (the minimal evolution time) for a qubit system interacting with a structural environment is investigated. We define a new bound for the quantum speed limits. It is shown that the non-Markovianity and the population of the excited state can fail to signal the quantum evolution acceleration, but the initial-state dependence is an important factor. In particular, we find that different quantum speed limits could produce contradictory predictions on the quantum evolution acceleration. (paper)
[en] We show how the driving suppresses the decoherence, triggers the non-Markovian dynamics, and further compresses the quantum speed limit time in the damping channel. For the open composite system consisting of a qubit and a single-mode cavity, we find that solely driving the qubit is most effective against the decoherence, but simultaneously driving the qubit and the cavity with high enough frequency will bring sufficient non-Markovianity, implying a better performance in reducing the quantum speed limit time. Graphical abstract: .
[en] Memory effects play a fundamental role in the dynamics of open quantum systems. There exist two different views on memory for quantum noises. In the first view, the quantum channel has memory when there exist correlations between successive uses of the channels on a sequence of quantum systems. These types of channels are also known as correlated quantum channels. In the second view, memory effects result from correlations which are created during the quantum evolution. In this work, we will consider the first view and study the quantum speed limit time for a correlated quantum channel. Quantum speed limit time is the bound on the minimal time which is needed for a quantum system to evolve from an initial state to desired states. The quantum evolution is fast if the quantum speed limit time is short. In this work, we will study the quantum speed limit time for some correlated unital and correlated non-unital channels. As an example for unital channels, we choose correlated dephasing colored noise. We also consider the correlated amplitude damping and correlated squeezed generalized amplitude damping channels as the examples for non-unital channels. It will be shown that the quantum speed limit time for correlated pure dephasing colored noise is increased by increasing correlation strength, while for correlated amplitude damping and correlated squeezed generalized amplitude damping channels quantum speed limit time is decreased by increasing correlation strength.
[en] Since chaos control has found its way into many applications, the development of fast, easy-to-implement and universally applicable chaos control methods is of crucial importance. Predictive feedback control has been widely applied but suffers from a speed limit imposed by highly unstable periodic orbits. We show that this limit can be overcome by stalling the control, thereby taking advantage of the stable directions of the uncontrolled chaotic map. This analytical finding is confirmed by numerical simulations, giving a chaos-control method that is capable of successfully stabilizing periodic orbits of high period. (paper)
[en] We study quantum information processing by means of optimal control theory. To this end, we analyze the damped Jaynes–Cummings model, and derive optimal control protocols that minimize the heating or energy dispersion rates, and controls that drive the system at the quantum speed limit. Special emphasis is put on analyzing the subtleties of optimal control theory for our system. In particular, it is shown how two fundamentally different approaches to the quantum speed limit can be reconciled by carefully formulating the problem. (paper)
[en] The quantum speed limit (QSL) sets a bound on the minimum time required for a quantum system to evolve between two states. For open quantum systems this quantity depends on the dynamical map describing the time evolution in presence of the environment, on the evolution time τ, and on the initial state of the system. We consider a general single qubit open dynamics and show that there is no simple relationship between memory effects and the tightness of the QSL bound. We prove that only for specific classes of dynamical evolutions and initial states, there exists a link between non-Markovianity and the QSL. Our results shed light on the connection between information back-flow between system and environment and the speed of quantum evolution. (paper)
[en] In this Letter, we give new constraints on planet migration. They were obtained under the assumption that Saturn's current obliquity is due to a capture in resonance with Neptune's ascending node. If planet migration is too fast, then Saturn crosses the resonance without being captured and it keeps a small obliquity. This scenario thus gives a lower limit on the migration timescale τ. We found that this boundary depends strongly on Neptune's initial inclination. For two different migration types, we found that τ should be at least greater than 7 Myr. This limit increases rapidly as Neptune's initial inclination decreases from 10 deg. to 1 deg. We also give an algorithm to know if Saturn can be tilted for any migration law.