Advanced Quantum Mechanics Rajdeep Sensarma
[email protected] Quantum Dynamics Lecture #9 Schrodinger and Heisenberg Picture Time Independent Hamiltonian Schrodinger Picture: A time evolving state in the Hilbert space with time independent operators iHtˆ i@t (t) = Hˆ (t) (t) = e− (0) | i | i | i | i i✏nt Eigenbasis of Hamiltonian: Hˆ n = ✏ n (t) = cn(t) n c (t)=c (0)e− n | i | i n n | i | i n X iHtˆ iHtˆ Operators and Expectation: A(t)= (t) Aˆ (t) = (0) e Aeˆ − (0) = (0) Aˆ(t) (0) h | | i h | | i h | | i Heisenberg Picture: A static initial state and time dependent operators iHtˆ iHtˆ Aˆ(t)=e Aeˆ − Schrodinger Picture Heisenberg Picture (0) (t) (0) (0) | i!| i | i!| i Equivalent description iHtˆ iHtˆ Aˆ Aˆ Aˆ Aˆ(t)=e Aeˆ − ! ! of a quantum system i@ (t) = Hˆ (t) i@ Aˆ(t)=[Aˆ(t), Hˆ ] t| i | i t Time Evolution and Propagator iHtˆ (t) = e− (0) Time Evolution Operator | i | i (x, t)= x (t) = x Uˆ(t) (0) = dx0 x Uˆ(t) x0 x0 (0) h | i h | | i h | | ih | i Z Propagator: Example : Free Particle The propagator satisfies and hence is often called the Green’s function Retarded and Advanced Propagator The following propagators are useful in different contexts Retarded or Causal Propagator: This propagates states forward in time for t > t’ Advanced or Anti-Causal Propagator: This propagates states backward in time for t < t’ Both the retarded and the advanced propagator satisfies the same diff. eqn., but with different boundary conditions Propagators in Frequency Space Energy Eigenbasis |n> The integral is ill defined due to oscillatory nature