Influence Of Quantum Excitations Of Pointer State On Postselected Weak Measurements

Postselected weak measurement is a quantum measurement protocol first in-vented by Aharonov,Albert,and Vaidman in 1988.It involves weak coupling between the system and the pointer,but the postselection on the system leads to a surprisingly counterintauitive effect:the average shift of the final pointer state can go far beyond the eigenvalue spectrum of the system observable(multiplied by the coupling constant)in sharp contrast to the projective quanttum measure-ment.The mechanism behind this effect is the coherence between the pointer states translated by different eigenvalues of the system observable.Postselected weak measurement has aroused enormous research interest in different fields,due to its ability to amplify tiny physical effects.Thanks to technical progress in re-cent years,the weak value has been measured in experiments,and postselected weak measurements have been applied to measuring small parameters in various systems,including optical systems and atomic systems.Recently,the use of quantum excitation technique to improve nonclasscality of quantum states and to develop quantum enhanced technology has made rapid progress,which provides a novel idea for the study of precision measurement.In this thesis,we study the influence of quantum excitations of pointer states on the post-selection weak measurement of a two-level system is studied.The preselected state of the measurement-device pointer is an excited coherent state with multi-quantum excitations.The main research results of this thesis are described as follows.In Chapter 2,we investigate nonclassicality of the initial pointer state,excited coherent state with n quantum excitations,in terms of the Gluaber-Sudarshan P function and the Wigner function in the phase space of quantum states.We analytically calculate and numerically simulate the Gluaber-Sudarshan P function and the Wigner function of the multi-quantum excited coherent state.It is shown that the nonclassical areas of the quantum state are enlarged while the classical areas are shrunk with the increase of the quantum excited number.In Chapter 3,we study the post-selected weak measurement protocol of a two-level system when the initial pointer state is in a excited coherent state with n quantum excitations.We analytically calculate and numerically simulate the average shift of the measurement pointer and the signal-noise ratio(SNR)in the post-selected weak scheme.It is found that quantum excitations can increase the average shift of the measurement pointer and the SNR in the post-selected weak measurement protocol.In Chapter 4,we study quantum entanglement between the measured quan-tum system and the measurement device(pointer)in the process of the post-selected weak measurement scheme by the use of the quantum concurrence as the measure of quantum entanglement.It is shown that quantum excitations can enhance quantum entanglement between the measured quantum system and the measurement pointer in certain parameter regimes.In Chapter 5,we present a summary and outlook.