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Notes for Quantum Mechanics 3 Course, CMI Spring 2012 Govind S

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Notes for Quantum Mechanics 3 Course, CMI Spring 2012 Govind S Notes for Quantum Mechanics 3 course, CMI Spring 2012 Govind S. Krishnaswami, May 1, 2012 corrections: August 19, 2019 Contents 1 Time dependent quantum systems (continued) 1 1.1 Lifetime of an excited state due to spontaneous emission......................................1 1.2 Relation between spontaneous and stimulated emission (Einstein A & B coefficients)......................2 1.3 Rate of dipole transitions due to unpolarized polychromatic light incident from all directions in an incoherent superposition.3 1.4 Frequency-time relation for harmonic perturbations.........................................6 1.5 Adiabatic or quasi-static time dependence & perturbations.....................................8 1.5.1 Adiabatic `theorem'...................................................... 10 1.5.2 Criterion for validity of adiabatic approximation....................................... 12 1.5.3 Example: Spin in a slowly varying magnetic field...................................... 15 1.5.4 Dynamical and geometric phases for cyclic adiabatic processes............................... 17 1.5.5 Example of Berry's geometric phase: spin in an adiabatically rotated magnetic field................... 18 1.5.6 Some remarks on the geometric phase and adiabatic approximation............................ 19 2 Time evolution operator for Schr¨odingerequation 21 2.1 Uniqueness of Schr¨odingerevolution................................................. 21 2.2 Time evolution operator........................................................ 22 2.3 Separation of variables for a separable hamiltonian......................................... 23 2.4 Time-ordered exponential....................................................... 25 2.5 Failure of naive guess for time-evolution operator.......................................... 27 2.6 Finite evolution from infinitesimal evolution: infinite product form of U(t; t0).......................... 28 2.7 Fundamental solution of the Schr¨odingerequation and the time evolution operator........................ 29 2.8 Example: time evolution operator for spin in a rotating B~ field.................................. 29 2.9 Reproducing property / composition law for U(t; t0)........................................ 30 2.10 Time evolution operator in the position basis............................................ 31 2.11 Free particle propagator........................................................ 31 2.12 Feynman path integral for a free particle............................................... 33 2.13 Path integral for a particle in a potential.............................................. 35 3 Aharonov-Bohm effect 37 3.1 Effect on energy spectrum....................................................... 39 3.2 Effect on time-evolution........................................................ 40 3.3 Path integral approach to Aharonov-Bohm effect.......................................... 42 4 A second look at the harmonic oscillator 43 4.1 Coherent states of the harmonic oscillator.............................................. 43 4.2 Harmonic oscillator propagator by path integral........................................... 49 4.3 Harmonic oscillator spectrum from path integral.......................................... 51 5 Variational principle for Schrodinger eigenvalue problem 52 6 Schr¨odinger-Pauli equation for spin half electron 54 7 Relativistic quantum mechanics 55 7.1 Klein-Gordon equation........................................................ 56 7.1.1 Plane wave solutions of KG.................................................. 57 7.1.2 Lorentz invariance....................................................... 58 7.1.3 Non-relativistic limit...................................................... 59 7.1.4 Coupling to electromagnetic field............................................... 59 1 7.1.5 Local conservation law and physical interpretation..................................... 60 7.2 Dirac equation............................................................. 61 7.2.1 Dirac's 4 × 4 representation of ~α,β matrices........................................ 63 7.2.2 Local conservation law and probability interpretation.................................... 63 7.2.3 Plane waves and energy spectrum............................................... 64 7.2.4 Non-relativistic limit of plane waves............................................. 66 7.2.5 Spin and helicity........................................................ 67 7.2.6 Dirac equation coupled to an EM field and non-relativistic limit.............................. 68 7.2.7 Lorentz covariance of the Dirac equation........................................... 69 7.2.8 Lorentz invariance of the continuity equation........................................ 72 7.2.9 Dirac equation in a Coulomb potential: relativistic corrections............................... 73 7.2.10 Fine structure of hydrogen spectrum............................................. 75 7.2.11 Negative energy states, holes and anti-particles....................................... 79 • These notes (http://www.cmi.ac.in/ ∼ govind/teaching/qm3-e12) are a continuation of the notes from QM1 and QM2, available on this website. They are very sketchy and aren't a substitute for text books or attendance and taking notes at lectures. Some suggested text books have been listed on the course webpage. Please let me know ([email protected]) of any comments or corrections. 1 Time dependent quantum systems (continued) 1.1 Lifetime of an excited state due to spontaneous emission • Even if an atom isn't exposed to an external electromagnetic wave, it can make a spontaneous transition from a higher to any lower energy level that is allowed by the selection rules. This `spontaneous' emission is in a sense induced by the electromagnetic radiation that is present even in the vacuum. This is the process by which atoms in excited states decay. • Suppose an atom is initially in the excited (upper energy) state u and has available to it several lower energy states l1; l2; : : : ln to which it can spontaneously decay. Each of these is called a decay channel or decay mode. Suppose the spontaneous transition probability from u ! li per unit time, i.e., the spontaneous decay rate is Rsp . This spontaneous decay rate will depend u!li on the electromagnetic radiation present in the vacuum, just as the rate for stimulated emission depends on the electromagnetic radiation incident on the atom. The spontaneous decay rate Rsp was called the A-coefficient by Einstein. u!li • If there are initially Nu atoms in upper state u, then in a time dt the increase in Nu dN = − Rsp + ··· + Rsp N dt (1) u u!l1 u!ln u Thus the expected number of atoms remaining in upper state u at time t is h i N (t) = N (t = 0) exp − Rsp + ··· + Rsp t (2) u u u!l1 u!ln The time constant for this exponential depletion of population is called the lifetime of the upper state 1 τ = (3) Rsp + ··· + Rsp u!l1 u!ln It is the time after which (on average) a fraction 1=e ≈ 1=2:718 ≈ 0:37 the population has decayed to any of the available lower states, and only about 63% remain in upper state u. If selection rules do not permit any lower state to decay to, then the rates are zero and the lifetime τ infinite. 2 • Note that spontaneous absorption is almost never seen to occur, an atom in its ground state in vacuum is rarely found to spontaneously get excited. A statistical mechanics argument for this may be offered, using the principle of maximization of entropy. Consider an atom in the presence of electromagnetic radiation present in the vacuum. Suppose the energy difference between the ground and first excited state of the atom is ∆E . There is only one way in which this quantum of energy can be possessed by the atom: by being in the first excited state. On the other hand, this energy can be kept in the radiation field in very many ways, essentially, since the electromagnetic field has very many degrees of freedom, the electric and magnetic fields at each point of space. Since a priori all these possibilities are equally probable, it is entropically favorable for the quantum of energy to be in the vacuum electromagnetic field than in the atom. This explains why atoms are typically found in their ground states and are rarely seen to spontaneously absorb radiation from the vacuum and get excited. 1.2 Relation between spontaneous and stimulated emission (Einstein A & B coefficients) sp • To estimate this life time, we need to know the rates for spontaneous emission Ru!l . To find this rate we could apply first order time-dependent perturbation theory as in the case of stimulated emission/absorption. However, to do so, we will need to know the electromagnetic field in the vacuum, which is not available to us. However, a clever argument of Einstein (1916) relates the spontaneous emission rate to the stimulated emission rate, which we can calculate. See Liboff or Griffiths for more details. • Suppose we have a box of atoms in thermal equilibrium with EM radiation at temperature T . We already know that EM radiation in equilibrium at temp T has a spectral distribution of energy density given by Planck's black body law. Now suppose the atoms are in one of two states, with Nu in the upper state and Nl in the lower state. In equilibrium these numbers do not change with time. However, this is a dynamic equilibrium where atoms in the upper state decay to l both via spontaneous and stimulated emission (due to the EM waves in the cavity). At the same time, atoms in the lower state get exited to u via stimulated absorption. The rates
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