Mixture Of Pure States Research Articles

Fisher information of accelerated two-qubit systems

In this paper, Fisher information for an accelerated system initially prepared in the X-state is discussed. An analytical solution, which consists of three parts: classical, the average over all pure states and a mixture of pure states, is derived for the general state and for Werner state. It is shown that the Unruh acceleration has a depleting effect on the Fisher information. This depletion depends on the degree of entanglement of the initial state settings. For the X-state, for some intervals of Unruh acceleration, the Fisher information remains constant, irrespective to the Unruh acceleration. In general, the possibility of estimating the state’s parameters decreases as the acceleration increases. However, the precision of estimation can be maximized for certain values of the Unruh acceleration. We also investigate the contribution of the different parts of the Fisher information on the dynamics of the total Fisher information.

Thermodynamics for Spatially Inhomogeneous Magnetization and Young-Gibbs Measures

We derive thermodynamic functionals for spatially inhomogeneous magnetization on a torus in the context of an Ising spin lattice model. We calculate the corresponding free energy and pressure (by applying an appropriate external field using a quadratic Kac potential) and show that they are related via a modified Legendre transform. The local properties of the infinite volume Gibbs measure, related to whether a macroscopic configuration is realized as a homogeneous state or as a mixture of pure states, are also studied by constructing the corresponding Young-Gibbs measures.

How measurement reversal could erroneously suggest the capability to discriminate the preparation basis of a quantum ensemble

Mixed states of a quantum system, represented by density operators, can be decomposed as a statistical mixture of pure states in a number of ways where each decomposition can be viewed as a different preparation recipe. However the fact that the density matrix contains full information about the ensemble makes it impossible to estimate the preparation basis for the quantum system. Here we present a measurement scheme to (seemingly) improve the performance of unsharp measurements. We argue that in some situations this scheme is capable of providing statistics from a single copy of the quantum system, thus making it possible to perform state tomography from a single copy. One of the byproduct of the scheme is a way to distinguish between different preparation methods used to prepare the state of the quantum system. However, our numerical simulations disagree with our intuitive predictions. We show that a counter-intuitive property of a biased classical random walk is responsible for the proposed mechanism not working.

Algorithm for simulation of quantum many-body dynamics using dynamical coarse-graining

An algorithm for simulation of quantum many-body dynamics having su(2) spectrum-generating algebra is developed. The algorithm is based on the idea of dynamical coarse-graining. The original unitary dynamics of the target observables--the elements of the spectrum-generating algebra--is simulated by a surrogate open-system dynamics, which can be interpreted as weak measurement of the target observables, performed on the evolving system. The open-system state can be represented by a mixture of pure states, localized in the phase space. The localization reduces the scaling of the computational resources with the Hilbert-space dimension n by factor n{sup 3/2}(ln n){sup -1} compared to conventional sparse-matrix methods. The guidelines for the choice of parameters for the simulation are presented and the scaling of the computational resources with the Hilbert-space dimension of the system is estimated. The algorithm is applied to the simulation of the dynamics of systems of 2x10{sup 4} and 2x10{sup 6} cold atoms in a double-well trap, described by the two-site Bose-Hubbard model.

Constraints on the mixing of bipartite states

A mixed state may be represented in many different ways as a mixture of pure states ρ=∑pi∣ψi⟩⟨ψi∣. The mixing problem in quantum mechanics asks the characterization of the probability distribution (pi) and the mixed states (ρi) such that ρ=∑piρi for any given mixed state ρ. Some constraints based on eigenvalues of the mixed states are established in uni-party case [see Nielsen, Phys. Rev. A. 63, 052308 (2000), 63, 022144 (2000), Nielsern and Vidal Quantum Inf. Comput. 1 76 (2001)]. We develop some new invariant sets for bipartite mixed states under local unitary operations, which are independent of eigenvalues, and prove some strong constraints based on these invariant sets for the mixing problem in bipartite case. This exhibits a remarkable difference from the uni-party case.

How to mix a density matrix

A given density matrix may be represented in many ways as a mixture of pure states. We show how any density matrix may be realized as a uniform ensemble. It has been conjectured that one may realize all probability distributions that are majorized by the vector of eigenvalues of the density matrix. We show that if the states in the ensemble are assumed to be distinct then this is not true, but a marginally weaker statement may still be true.

Probability distributions consistent with a mixed state

A density matrix $\rho$ may be represented in many different ways as a mixture of pure states, $\rho = \sum_i p_i |\psi_i\ra \la \psi_i|$. This paper characterizes the class of probability distributions $(p_i)$ that may appear in such a decomposition, for a fixed density matrix $\rho$. Several illustrative applications of this result to quantum mechanics and quantum information theory are given.

Do zero-frequency modes contribute to the entropy?

Unruh's mixed state of a field with a zero-frequency mode, which purportedly contributes an arbitrarily large entropy for a given energy, is actually a mixture of pure states belonging to different systems, since the mean value of the scalar field is different for each pure state in the mixture. Considerations related to superfluidity support the idea that vanishing entropy is to be ascribed to a zero-frequency mode.

A dynamical phase transition in an infinite particle system

The local equilibrium picture of the time evolution of a gas may have to be modified in the presence of shocks in order to admit statistical mixtures of pure states in the hydrodynamic description. An example drawn from a stochastic many-particle model (asymmetric zero-range model) is described.

The Ignorance Interpretation Defended

The “Ignorance Interpretation” of quantum mechanical mixtures holds, roughly, that whenever a system S belongs to an ensemble, which is represented by a mixed statistical operator U = Σpi P[ψi] (0 ≤ pi ≤ 1, Σipi = 1, P[ψi] is the projection operator for the state ψi), then S is in some pure state, although we are ignorant as to which one. It has been concluded, e.g. by van Fraassen, that “the ignorance interpretation is untenable,” and he presumably favors adopting “the position that mixtures of pure states are themselves new states ... to say that a system is in a proper mixture is to say that it is not in a pure state.” I wish to argue in this paper that there are no good grounds for rejecting the ignorance interpretation.

Experimental Consequences ofφ−ωMixing

In this paper a simple model is proposed in which the observed φ and ω resonant states are considered as mixtures of pure states |Y〉 and |B〉 corresponding to hypercharge and baryonic mesons. The implications of this model for the isoscalar nucleon form factor; the decays of the φ, ω, and π0 mesons; the role of the ω and φ mesons in nuclear forces; the mass distribution of Dalitz pairs in the decay π0→γ+e++e-; and the photoproduction of η mesons are briefly considered.