Classical-Noise-Free Measurement by High-Order Quantum Correlations

One main challenge for realistic quantum computing and quantum sensing is to combat noise.Three formal strategies including quantum error correction,[1,2]decoherence-free subspace[3,4]and dynamical decoupling[5]have been developed for suppressing the noise.Quantum systems can lose their coherence when subjected to fluctuations of the local fields from their surrounding environments.Such decoherence phenomena are a fundamental effect in quantum physics and sometimes are referred to as a back-action from the measured system.As one typical dynamical decoupling technique,spin echo originated from magnetic resonance spectroscopy can average out the noise by flipping the target qubit.     

Quantum Gravity and the Problem of Measurement

We discuss the proposal that the quantumcorrelations contained in the pure statevector evolvingaccording to Schrodinger equation can be eliminated bythe action of multiply connected wormholes during measurement. A procedure is devised to obtaina proper master equation which governs the changes ofthe reduced density matrix of matter fields interactingwith doubly connected wormholes. It is shown that this master equation predicts an appropriatedamping of the off-diagonal correlations contained inthe state vector.     

Quantum Correlations Induced by Local von Neumann Measurement

We study the total quantum correlation, semiquantum correlation and joint quantum correlation induced by local von Neumann measurement in bipartite system. We analyze the properties of these quantum correlations and obtain analytical formula for pure states. The experimental witness for these quantum correlations is further provided and the significance of these quantum correlations is discussed in the context of local distinguishability of quantum states.     

The Quantum Measurement Problem and Cluster Separability

A modified Beltrametti-Cassinelli-Lahti model of the measurement apparatus that satisfies both the probability reproducibility condition and the objectification requirement is constructed. Only measurements on microsystems are considered. The cluster separability forms a basis for the first working hypothesis: the current version of quantum mechanics leaves open what happens to systems when they change their separation status. New rules that close this gap can therefore be added without disturbing the logic of quantum mechanics. The second working hypothesis is that registration apparatuses for microsystems must contain detectors and that their readings are signals from detectors. This implies that the separation status of a microsystem changes during both preparation and registration. A new rule that specifies what happens when these changes occur and that guarantees the objectification is formulated and discussed. A part of our result has certain similarities with ‘collapse of the wave function’.     

On the Measurement Problem for a Two-level Quantum System

A geometric approach to quantum mechanics with unitary evolution and non-unitary collapse processes is developed. In this approach the Schrödinger evolution of a quantum system is a geodesic motion on the space of states of the system furnished with an appropriate Riemannian metric. The measuring device is modeled by a perturbation of the metric. The process of measurement is identified with a geodesic motion of state of the system in the perturbed metric. Under the assumption of random fluctuations of the perturbed metric, the Born rule for probabilities of collapse is derived. The approach is applied to a two-level quantum system to obtain a simple geometric interpretation of quantum commutators, the uncertainty principle and Planck’s constant. In light of this, a lucid analysis of the double-slit experiment with collapse and an experiment on a pair of entangled particles is presented.     

The Quantum Measurement Problem and the Possible Role of the Gravitational Field

The quantum measurement problem and various unsuccessful attempts to resolve it are reviewed. A suggestion by Diosi and Penrose for the half-life of the quantum superposition of two Newtonian gravitational fields is generalized to an arbitrary quantum superposition of relativistic, but weak, gravitational fields. The nature of the collapse process of the wave function is examined.     

Emergent Quantum Mechanics and the Origin of Quantum Non-local Correlations

International Journal of Theoretical Physics - A geometric interpretation for quantum correlations and entanglement according to a particular framework of emergent quantum mechanics is developed....     

Separating Classical and Quantum Correlations

We examine to what extent the correlation between two quantum observables at a mixed state can be separated into a classical and a quantum term. The nonunique decomposition of quantum mixed states into pure states makes such a separation ambiguous. We outline this fact by a simple example, which also shows that classical and quantum correlations may cancel each other out.     

Dynamics of Super Quantum Correlations and Quantum Correlations for a System of Three Qubits

The dynamics of quantum discord for two qubits independently interacting with dephasing reservoirs have been studied recently.The authors [Phys.Rev.A 88(2013) 034304] found that for some Bell-diagonal states(BDS)which interact with their environments the calculation of quantum discord could experience a sudden transition in its dynamics,this phenomenon is known as the sudden change.Here in the present paper,we analyze the dynamics of normal quantum discord and super quantum discord for tripartite Bell-diagonal states independently interacting with dephasing reservoirs.Then,we find that basis change does not necessary mean sudden change of quantum correlations.     

Can ‘Unsharp Objectification’ Solve the Quantum Measurement Problem?

The quantum measurement problem is formulated inthe form of an insolubility theorem that states theimpossibility of obtaining, for all available objectpreparations, a mixture of states of the compound object and apparatus system that wouldrepresent definite pointer positions. A proof is giventhat comprises arbitrary object observables, whethersharp or unsharp, and besides sharp pointer observables a certain class of unsharp pointers, namely,those allowing for the property of pointer valuedefiniteness. A recent result of H. Stein is applied toallow for the possibility that a given measurement may not be applicable to all possible objectstates, but only to a subset of them. The question israised whether the statement of the insolubility theoremremains true for genuinely unsharp observables. This gives rise to a precise notion of unsharpobjectification.     

Quantum Correlations Evolution Asymmetry in Quantum Channels

It was demonstrated that the entanglement evolution of a specially designed quantum state in the bistochastic channel is asymmetric. In this work, we generalize the study of the quantum correlations, including entanglement and quantum discord, evolution asymmetry to various quantum channels. We found that the asymmetry of entanglement and quantum discord only occurs in some special quantum channels, and the behavior of the entanglement evolution may be quite different from the behavior of the quantum discord evolution. To quantum entanglement, in some channels it decreases monotonously with the increase of the quantum channel intensity. In some other channels, when we increase the intensity of the quantum channel, it decreases at first, then keeps zero for some time, and then rises up. To quantum discord, the evolution becomes more complex and you may find that it evolutes unsmoothly at some points. These results illustrate the strong dependence of the quantum correlations evolution on the property of the quantum channels.     

Quantum Correlations Reduce Classical Correlations with Ancillary Systems

We illustrate the dichotomy of classical/quantum correlations by virtue of monogamy. More precisely, we show that correlations in a bipartite state are classical if and only it each party ot the state can be perfectly correlated with other ancillary systems. In particular, this means that if there are quantum correlations between two parties, then the classical （as well as quantum） correlating capabilities of the two parties with other systems have to be strictly reduced.     

On Quantum Correlations and Positive Maps

We present a discussion on local quantum correlations and their relations with entanglement. We prove that a vanishing coefficient of quantum correlations implies separability. The new results on locally decomposable maps which we obtain in the course of the proof also seem to be of independent interest.     

Entanglement and Classical Correlations in the Quantum Frame

The frame of classical probability theory can be generalized by enlarging the usual family of random variables in order to encompass nondeterministic ones. This leads to a frame in which two kinds of correlations emerge: the classical correlation that is coded in the mixed state of the physical system and a new correlation, to be called probabilistic entanglement, which may occur also at pure states. We examine to what extent this characterization of correlations can be applied to quantum mechanics. Explicit calculations on simple examples outline that a same quantum state can show only classical correlations or only entanglement depending on its statistical content; situations may also arise in which the two kinds of correlations compensate each other.     

Quantum Correlations in Werner Derivatives

Quantum correlations in Werner derivatives are studied with two different approaches, i.e., measurement-induced disturbance (MID) [Phys. Rev. A 77 (2008) 022301] and ameliorated MID (AMID) [J. Phys. A 44 (2011) 352002]. They are derived via strict deductions with MID while numerically calculated via the measurement optimization with AMID. Interestingly, quantum correlations captured with both approaches are completely coincident. Moreover, some distinct features of the quantum correlations and their underlying physics are exposed via analyses and discussions.     

Exploring Tripartite Quantum Correlations: Entanglement Witness and Quantum Discord

In this study, we explore the tripartite quantum correlations by employing the quantum relative entropy as a distance measure. First, we evaluate the explicit expression for nonlinear entanglement witness (EW) of tripartite systems in the four dimensional space that lends itself to a straightforward algorithm for finding closest separable state (CSS) to the generic state. Then using nonlinear EW with specific feasible regions (FRs), quantum discord is derived analytically for the three-qubit and tripartite systems in the four dimensional space. Furthermore, we explicitly figure out the additivity relation of quantum correlations in tripartite systems.     

Analytically Computable Symmetric Quantum Correlations

One of the greatest challenges in developing the resource theory of a quantum feature is to establish an analytically computable quantifier, which directly limits the practicability of such quantifiers. Here, analytic quantifiers of both the symmetric quantum discord (SQD) and the symmetric measurement‐induced nonlocality (SMIN) in a bipartite system of qubits are studied on the basis of the quantum skew information. It is shown that the SMIN of any two‐qubit system and the SQD of bipartite “X”‐type states and block‐diagonal states can be analytically determined. In addition, the SQD and the SMIN are invariant with an attached quantum state. The validity of our analytical expressions is further illustrated numerically on the basis of several randomly generated density matrices.