Evolution of quantum fidelity for two-level systems is studied in the context of periodic echo. From a general treatment for time independent case, we obtain a simple condition on the governing Hamiltonians under which the systems display periodic quantum echo. For a specific time dependent problem the quantum fidelity is shown to exhibit Rabi oscillation. This may be considered as a simple mechanism to generate periodic echo, except for a specific initial superpositional state in which case the fidelity remains invariant.
Evolution of quantum fidelity for two-level systems is studied in the context of periodic echo. From a general treatment for time independent case, we obtain a simple condition on the governing Hamiltonians under which the systems display periodic quantum echo. For a specific time dependent problem the quantum fidelity is shown to exhibit Rabi oscillation. This may be considered as a simple mechanism to generate periodic echo, except for a specific initial superpositional state in which case the fidelity remains invariant.
In this paper we perform an exact study of “quantum fidelity” (also called Loschmidt echo) for the time-periodic quantum harmonic oscillator of the following Hamiltonian: Ĥg(t)≔(P2∕2)+f(t)(Q2∕2)+(g2∕Q2), when compared with the quantum evolution induced by Ĥ0(t) (g=0), in the case where f is a T-periodic function and g a real constant. The reference (initial) state is taken to be an arbitrary “generalized coherent state” in the sense of Perelomov. We show that, starting with a quadratic decrease in time in the neighborhood of t=0, this quantum fidelity may recur to its initial value 1 at an infinite sequence of times tk. We discuss the result when the classical motion induced by Hamiltonian Ĥ0(t) is assumed to be stable versus unstable.
Abstract This chapter begins the discussion of the time evolution of an active quantum system. From the earlier chapters, time dependent physics has been observed through the presence of the time evolution of the phase of each eigenstate. The Hamiltonian itself is time independent. This represents the same kind of evolution of a classical system like the vibration of a tuning fork when it has been struck or the oscillation of an LC circuit if the capacitor is charged to some voltage and then the switch is closed. In the quantum case, the Hamiltonian has also been time independent. The time evolution evolves according the full-time dependent Schrödinger equation, depending only on a single initial condition of the state vector or wave function and the corresponding time evolution of the phase factor for each eigenstate. However in this chapter, we consider the case of when there is a time dependent Hamiltonian such as a sinewave generator or laser. As in the case of resonant tunneling, we see the importance in dynamics of resonant coupling. With an oscillating potential energy term, we see the presence of Rabi oscillations in the probability amplitude of a two-state system on resonance, which can be viewed as a quantum flip-flop between two states of a quantum bit (qubit).
In this paper,the Tavis-Cummings(TC) model is generalized to simultaneously consider atomic motions and fields via intensity dependent coupling.Evolution of the fidelity of quantum states of entangled two-atom with motion interacting with light field via intensity-dependence coupling is investigated by means of the full quantum theory.The effects on the evolution of the fidelity of quantum states from the initial entanglement factor of the two-atom and the average photon number n-and field-mode structure parameter p of the field are researched by numerical calculations.The formation rule of quantum echoes of two-atoms is analyzed,and its physical essence is revealed.The results show that period quantum echo of the two-atom is always produced in the case of field-mode structure parameter p=1,independent of light intensity n-and the initial entanglement factor of the two-atom.Bell-state atoms can produce quantum echo with 2π/p cycle on the conditions of strongly coherent field,via changing the p of from 1 to any value less than 800.When p≥800,no matter what value of takes,the light field is a vacuum field,weakly coherent field or strong coherent field,the fidelity of quantum states of the two-atom is always 1,that is the first class Bell state-atoms is in the fidelity state continuously.But when the initial entanglement factor of two-atom is 3π/4,no matter how both values of n-,p are,the fidelity of quantum states of the two-atom maintains 1,namely the two-atom is in the fidelity state of the second class Bell-state continuously.The conclusions show that the extension model has good properties of periodic quantum echoes of two-atom,and provides the theoretical parameters and physical carrier for high fidelity output of entangled two-atoms information and experimental implementation of quantum information processing in the noise environment.
Abstract: The general quantum-mechanical properties of two-level systems are very relevant for the study of different physical subjects of great interest, as, for instance, magnetic resonance, nuclear physics, molecular dynamics, masers, neutrino oscillations, quantum computation, etc. In this work we present a nonperturbative approach to the study of the oscillations among two quantum states when an interaction term is considered. First, we show that using a simple parametrization for the superposition of the two states, it is possible to derive easily the Rabi equation. Next, we show that the Pauli matrices can be conveniently used to write the Hamiltonian operator and to derive the Rabi equation, directly applying the time evolution operator to the initial state of the system.
Using the Jaynes-Cummings model,the dynamical properties of the fidelity of quantum states in the system that consists of a two-level atom interacting with a single-mode field are studied.The frequency of the field varying with time in the form of sine is considered.The results obtained by numerical calculations show that the time-evolution behavior of the fidelities of field and system are not influenced by the frequency variation of field,but the time-evolution behavior of the fidelity of atom is modulated by the frequency variation of field.The larger the amplitude of the field frequency variance u is,the stronger modulation interaction of the field frequency variance is.When the amplitude u is larger than a fixed value,the time-evolution behavior of the fidelity of atom will display periodic oscillation behavior.Its oscillation period is in agreement with the period of frequency varying.
Using the multiphoton Jaynes-Cummings model,the dynamical properties of the fidelity of quantum states in the system that consists of a two-level atom interacting with a single-mode field via multiphoton transition are studied.The frequency of the field varying with time in the form of sine is considered.The results obtained by numerical calculations show that the time-evolution behavior of the fidelities of field and system is not influenced by the frequency variation of field,but the time-evolution behavior of the fidelity of atom is modulated by the frequency variation of field.The larger the amplitude of the field frequency variance u is,the stronger modulation interaction of the field frequency variance is.When the amplitude u is larger than a fixed value,the time-evolution behavior of the fidelity of atom will display quasiperiodic oscillation behavior.Its oscillation period is in agreement with the period of frequency varying.The initial state of atom influences the time-evolution behavior of the fidelity of atom.