Shuangshuang Fu

Global effects of quantum states induced by locally invariant measurements

In quantum mechanics, general measurements often cause disturbance which may be exploited to quantify entanglement, nonlocality or quantumness. Imagine a bipartite state ρab shared by two parties a and b, and a von Neumann measurement performed locally on party a which does not disturb the local state ρa:=tru2009bu2009ρab, but nevertheless may disturb the global state ρab. This disturbance is an indication of some kind of correlations or global effect in ρab which cannot be accounted for locally. We propose to use the maximum disturbance on ρab caused by locally non-disturbing measurements as a figure of merit quantifying the global effect (nonlocality), and investigate its fundamental properties. For general two-qubit states and some higher-dimensional symmetric states, we present analytic formulas for their global effects.

Decorrelating capabilities of operations with application to decoherence

Decoherence, interpreted broadly, is essentially the leakage of system information into the environment and is often accompanied by dissipation. The basic questions arise: how to quantify decoherence induced by an operation and how to quantitatively compare decoherence induced by different operations. In this paper, based on a joint ancilla-system-environment tripartite purification for the initial system state and the operation, and by exploiting the intrinsic relations between the loss of correlations in the ancilla-system and the correlations established in the system-environment, we characterize and quantify decoherence from a decorrelating perspective. For this purpose, we first address the issue of separating and quantifying the classical and quantum parts of decorrelation. By use of the canonical isomorphism between operations and bipartite states, we propose two intrinsic decorrelation measures: One is the classical decorrelation based on the loss of classical correlations, and the other is the quantum decorrelation based on the loss of quantum correlations. With the help of quantum decorrelation, we introduce an intuitive measure of (quantum) decoherence. We further employ these informational quantities to analyze some widely used channels such as the complete decoherent channel, the depolarizing channel, the bit-flip channel, the transpose depolarizing channel, the amplitude damping channel, and the phasemorexa0» damping channel. Our analysis illustrates the intriguing interplay between classical and quantum decorrelations and sheds some light on the informational nature of decoherence.«xa0less

MAXIMUM NONLOCAL EFFECTS OF QUANTUM STATES

A fundamental feature of quantum mechanics radically different from classical theory lies in the role and consequence of quantum measurements, which usually cause disturbance to quantum states. For a bipartite state, the minimum disturbance caused by local measurements has been used to define quantum correlations from a measurement perspective. In contrast to this minimum approach, we investigate the maximum disturbance of local measurements, and define the nonlocal effect of a bipartite state as the maximum discrepancy between the global and local disturbances caused by local quantum measurements. Some analytical results are obtained and the significance of the maximum nonlocal effect is briefly discussed.

Determination of the Si-Si bond energy from the temperature dependence of elastic modulus and surface tension

We report a theoretical method to obtain the single-bond energy derivative from temperature-dependent Youngs modulus and surface tension based on the extension of the recently developed bond-order-length-strength correlation mechanism to the temperature domain. Reproducing simultaneously the measured physical quantities of the Si specimen could reveal the information on the Si-Si single-bond energy. It is shown that the single-bond energy and the response of the stimulus of temperature change can be connected to the bulk properties. Analytical solutions also provide a consistent understanding of the interdependence of these quantities and their commonly atomistic origin as arising from thermally driven bond expansion and bond weakening, which is beyond the scope of conventional approaches.