课程名称︰量子力学一 课程性质︰物理系系定选修 课程教师︰蒋正伟 开课学院：理学院 开课系所︰物理学系 考试日期（年月日）︰2019年12月4日 考试时限（分钟）：未知 是否需发放奖励金：是 （如未明确表示，则不予发放） 试题 : 注：为明确表示数学式，部分较难以图像表示之部分以\LaTeX码表示 ex. \vec{a} 表 a向量, a^{\dagger} 表a dagger, \sart{a}表根号a, \hbar 表 hbar, \int^{b}_{a}表自a积分至b 12/04/2019 PHYS7014 Quantum Mechanics I ─ Second Midterm Exam INSTRUCTIONS This is an open-book, 120-minute exam. Your are only allowed to use the main textbook (by Sakurai and Napolitano) and your own handwritten notes. Only derived results in the main text of Sakurai and Napolitano up to the range of this exam (i.e., Chapter 2) can be used. The score of each sub-problem is indicated by the number in square brackets. To avoid any misunderstanding, ask if you have any queations about the problems or notations. In your answers, define clearly your notations if they differ from those in the main textbook. Note that we also do not distinguish the notation of an operator from that of its corresponding variable. You may find some of the following formulas useful: \int^{x}\sqrt{1-u^2}du=1/2(x\sqrt(1-x^2}+arcsin(x))+C \int_{-∞}^{∞}exp(-(ax^2+bx+c))=(π/α)^(1/2)exp((b^2-4ac)/4a)) (a>0) PROBLEMS 1.[70 points] Consider a one-dimensional simple harmocis oscillator (SHO) with the potential given by V(x)=1/2mω^2x^2. The n-th energy level is denoted by |n>. Recall that a } = 1/\sqrt{2}(x/x_0π±ix_0/\hbar p) a^{\dagger} } a } = {\sqrt{n}|n-1> a^{\dagger} } {\sqrt{n+1}|n+1> , where x_0≡\sqrt{\hbar/(mω)}. Also, recall that the position and momentum operators in the Heisenberg pircture for the SHO are solved to be {x(t)=x(0)cos(ωt)+p(0)/(mω)sin(ωt), {p(t)=-mωx(0)sin(ωt)+p(0)cos(ωt). (a)[10] Work out the matrix forms of the following operators: a, a^{\dagger} , x and p in the Schrodinger picture. Show at least the upper left 4×4 sub-matrix. (b)[10] Calculate (Δx)(Δp) for the n-th level state, where for your convenience Δa≡\sqrt{<A^2>-<A>^2}. (c)[10] Calculate the expectation values of kinetic and potential energy of the n-th level state, and show that they satisfy the virial theorem. (d)[10] Compute the expectation values of x(t) and p(t) for the n-th level state. Explain physically what your results mean. In particular, are they consistent with the classical picture of an oscillator? (e)[10] Use the WKB approximation method to work out the eigenstate energies. Define the coherent state, denotes by |α>, satisfying a|α>=α|α> with the normalization <α|α>=1 (f)[10] Derive the normalized coherent state in the basis of {|n>}. (g)[10] Explain what kind of quantity the eigenvalue α can be, and whether or not one can set it to be real at all times. 2.[10 points] Consider a one-dimensional quantum mechanical problem of a particle with a time-independent Hamiltonian H. In the path integral calculation, we have shown in class that for a particle with a general Hamiltonian H(p,x): <x_{i+1}|H|x_i>=\int^{∞}^{-∞}(dp/(2π\hbar))H(p,\bar{x}_i)× exp(i/\hbar p(x_{x+1}-x_i)) where x_i and x_{i+1} denote respectively the positions of the particle at two infinitesimally separated moments t_i and t_{i+1}. Explain why one should use \bar{x}_i=(x_i+x_{i+1})/2 instead of x_i or x_{i+1}. 3.[20 points] Consider a particle of electrin charge Qe and mass m in an electromagnetic potential, described by the Hamiltonian H=Π^2/(2m)+Qeφ with Π=\vec{p}-Qe/c\vec{A}, where φ(\vec{x}) and \vec{A}(\vec{x}) are time-independent scalar and vector potentials, respectibely. Use the Heisenberg pircture throughout this problem. (a)[10] Explain whether d\vec{x}/dt and \vec{x} can commute with each other. If so, prove it. If not, give a simple example. (b)[10] Derive the quantum mechanical version of the Lorentz force: m(d^\vec{x}/dt^2)=dΠ/dt=Qe[\vec{E}+1/(2c)× (d\vec{x}/dt×\vec{B}-\vec{B}×d\vec{x}/dt)]. --

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