Search results for "Quantum physic"
showing 10 items of 1596 documents
On the probability of finding marked connected components using quantum walks
2017
Finding a marked vertex in a graph can be a complicated task when using quantum walks. Recent results show that for two or more adjacent marked vertices search by quantum walk with Grover's coin may have no speed-up over classical exhaustive search. In this paper, we analyze the probability of finding a marked vertex for a set of connected components of marked vertices. We prove two upper bounds on the probability of finding a marked vertex and sketch further research directions.
Adjacent vertices can be hard to find by quantum walks
2016
Quantum walks have been useful for designing quantum algorithms that outperform their classical versions for a variety of search problems. Most of the papers, however, consider a search space containing a single marked element only. We show that if the search space contains more than one marked element, their placement may drastically affect the performance of the search. More specifically, we study search by quantum walks on general graphs and show a wide class of configurations of marked vertices, for which search by quantum walk needs $\Omega(N)$ steps, that is, it has no speed-up over the classical exhaustive search. The demonstrated configurations occur for certain placements of two or…
Metrology and multipartite entanglement in measurement-induced phase transition
2023
Measurement-induced phase transition arises from the competition between a deterministic quantum evolution and a repeated measurement process. We explore the measurement-induced phase transition through the Quantum Fisher Information in two different metrological scenarios. We demonstrate through the scaling behavior of the quantum Fisher information the transition of the multi-partite entanglement across the phases. In analogy with standard quantum phase transition, we reveal signature of a measurement-induced phase transition in the non-analytic behaviour of the quantum Fisher information as the measurement strength approaches the critical value. Our results offer novel insights into the …
Determinism and Quantum Mechanics with time Bell inequalities
2010
We propose a definition of determinism for a physical system that includes, besides the measurement device, the surrounding environment. This enlarged system is assumed to follow a predefined trajectory starting from some (unknown) initial conditions that play the role of hidden variables for the experiment. These assumptions, which are different from realism, allow us to derive a type of time Bell inequalities, which are violated by Quantum Mechanics. In order to illustrate this violation, we discuss the particular case of measurements on a spin 1/2 particle.
Fundamentals of Relativity Providing Time, Distance and Velocity Standards
1998
The paper presents an attempt to suggest an alternative way of considering the fundamentals of Einstein's special relativity theory. Its formulation based on introducing an inertial reference system by rigid linking of clocks to a free entity to define proper time is self-contradictory since the operation of rigid linking involves interaction which is excluded by definition for a free entity. The way to overcome this inconsistency is proposed by postulating that each free entity (object, particle) has to be ideal clocks by itself, providing ideal internal time standards defined by the only intrinsic feature of a free entity, namely its proper mass which means that de Broglie's periodic phen…
Localization for Dirac fermions
2016
This work is devoted to incorporating into QFT the notion that particles and hence the particle states should be localizable in space. It focuses on the case of the Dirac field in 1+1 dimensional flat spacetime, generalizing a recently developed formalism for scalar fields. This is achieved exploiting again the non-uniqueness of quantization process. Instead of elementary excitations carrying definite amounts of energy and momentum, we construct the elementary excitations of the field localized (at some instant) in a definite region of space. This construction not only leads to a natural notion of localized quanta, but also provides a local algebra of operators. Once constructed, the new re…
Quantum Control of Many-Body Systems by the Density
2014
In this work we focus on a recently introduced method [1] to construct the external potential $v$ that, for a given initial state, produces a prescribed time-dependent density in an interacting quantum many-body system. We show how this method can also be used to perform flexible and efficient quantum control. The simple interpretation of the density (the amount of electrons per volume) allows us to use our physical intuition to consider interesting control problems and to easily restrict the search space in optimization problems. The method's origin in time-dependent density-functional theory makes studies of large systems possible. We further discuss the generalization of the method to hi…
Macrodeterminism without non-invasive measurability
2013
We propose a definition of determinism for a physical system that includes, besides the measured system, the measurement device and the surrounding environment. This enlarged system is assumed to follow a predefined trajectory starting from some (unknown) initial conditions that play the role of hidden variables for the experiment. These assumptions, which are different from realism, allow us to derive some Leggett-Garg inequalities, which are violated by Quantum Mechanics in the particular case of consecutive measurements on individual photon polarizations.
Information dynamics in quantum theory
2006
Shannon entropy and Fisher information functionals are known to quantify certain information-theoretic properties of continuous probability distributions of various origins. We carry out a systematic study of these functionals, while assuming that the pertinent probability density has a quantum mechanical appearance $\rho \doteq |\psi |^2$, with $\psi \in L^2(R)$. Their behavior in time, due to the quantum Schr\"{o}dinger picture evolution-induced dynamics of $\rho (x,t)$ is investigated as well, with an emphasis on thermodynamical features of quantum motion.
An experimental proposal for a Gaussian amendable quantum channel
2013
We propose a quantum optics experiment where a single two-mode Gaussian entangled state is used for realizing the paradigm of an amendable Gaussian channel recently presented in Phys. Rev. A, \textbf{87}, 062307 (2013). Depending on the choice of the experimental parameters the entanglement of the probe state is preserved or not and the relative map belongs or not to the class of entanglement breaking channels. The scheme has been optimized to be as simple as possible: it requires only a single active non-linear operation followed by four passive beam-splitters. The effects of losses, detection inefficiencies and statistical errors are also taken into account, proving the feasibility of the…