The harmonic oscillator potential is V(x) = ½kx² = ½mω²x². The hamiltonian is time independent. The initial state vector is Ψ(t=0) = A⎛❙0❭ + 2exp(ι͟π͟)❙1❭⎞ ⎝ 2 ⎠ Finding the normalization constant A is simple enough, since 1 = A² + (2Aexp(ι͟π͟))² = A²(1 + (2exp(ι͟π͟))²). 2 2 A = √⎛ ______͟1͟______ ⎞ = √⎛ ________͟1͟_______ ⎞ ⎜ 1 + (2exp(ι͟π͟))² ⎟ ⎜ 1 + 4exp(ι͟π͟ - ι͟π͟ ) ⎟, ⎝ 2 ⎠ ⎝ 2 2 ⎠ (𝐚) A = √⎛ ____͟1͟____ ⎞ = √⅕. ⎝ 1 + 4exp(0) ⎠ The time evolution prescription for a time-independent hamiltonian is Ψ(t) = exp(-ι͟E͟ₙ͟t) Ψ(t=0); ħ Ψ(t) = exp(-ι͟E͟ₙ͟t) √⅕⎛❙0❭ + 2exp(ι͟π͟)❙1❭⎞. ħ ⎝ 2 ⎠ (𝐛) Ψ(t) = √⅕⎛exp⎛-ι͟E͟₀͟t⎞❙0❭ + 2 exp⎛ι⎛_͟π͟ - E͟₁͟t⎞⎞❙1❭⎞. ⎝ ⎝ ħ ⎠ ⎝ ⎝ 2 ħ ⎠⎠ ⎠ The expecation values ❬x̂❭ and ❬p̂❭ are of interest. ❬x̂❭ = ❬Ψ⃰(t)❙x̂❙Ψ(t)❭. Ψ⃰(t) = √⅕⎛exp⎛ι͟E͟₀͟t⎞❬0❙ + 2 exp⎛ι⎛E͟₁͟t - _͟π͟⎞⎞❬1❙⎞. ⎝ ⎝ ħ ⎠ ⎝ ⎝ ħ 2⎠⎠ ⎠ ❬x̂❭ = ⅕⎛exp⎛ι͟E͟₀͟t⎞❬0❙ + 2 exp⎛ι⎛E͟₁͟t - _͟π͟⎞⎞❬1❙⎞ x̂ ⎛exp⎛-ι͟E͟₀͟t⎞❙0❭ + 2 exp⎛ι⎛_͟π͟ - E͟₁͟t⎞⎞❙1❭⎞. ⎝ ⎝ ħ ⎠ ⎝ ⎝ ħ 2⎠⎠ ⎠ ⎝ ⎝ ħ ⎠ ⎝ ⎝ 2 ħ ⎠⎠ ⎠ The matrix representations of the position operator x̂ and momentum operator p̂ have been developed from the definition of the increment/decrement operators. The matrix elements may be ascertained by inspection. x̂ ≐ √⎛_͟ħ͟ ⎞⎛ 0 √1 ⎞ p̂ ≐ √⎛͟ħ͟m͟ω͟⎞⎛ 0 -ι√1 ⎞ ⎝2mω⎠⎝ √1 0 ⎠, ⎝ 2 ⎠⎝ ι√1 0 ⎠. ω is a characteristic parameter of the system. It is related to the steepness of the parabolic potential curve. m is the particle's mass. ❬x̂❭ = ⅕⎛exp⎛ι͟E͟₀͟t⎞ exp⎛-ι͟E͟₀͟t⎞❬0❙x̂❙0❭ + ⎞ ⎜ ⎝ ħ ⎠ ⎝ ħ ⎠ ⎟ ⎜ ⎟ ⎜ exp⎛ι͟E͟₀͟t⎞ 2 exp⎛ι⎛_͟π͟ - E͟₁͟t⎞⎞ ❬0❙x̂❙1❭ + ⎟ ⎜ ⎝ ħ ⎠ ⎝ ⎝ 2 ħ ⎠⎠ ⎟ ⎜ ⎟ ⎜ exp⎛-ι͟E͟₀͟t⎞ 2 exp⎛ι⎛E͟₁͟t - _͟π͟⎞⎞ ❬1❙x̂❙0❭ + ⎟ ⎜ ⎝ ħ ⎠ ⎝ ⎝ ħ 2⎠⎠ ⎟ ⎜ ⎟ ⎜ 2 exp⎛ι⎛E͟₁͟t - _͟π͟⎞⎞2 exp⎛ι⎛_͟π͟ - E͟₁͟t⎞⎞❬1❙x̂❙1❭⎞⎟ ⎝ ⎝ ⎝ ħ 2⎠⎠ ⎝ ⎝ 2 ħ ⎠⎠ ⎠⎠. ❬0❙x̂❙0❭ = x₀₀ = ❬1❙x̂❙1❭ = x₁₁ = 0. ❬0❙x̂❙1❭ = x₀₁ = ❬1❙x̂❙0❭ = x₁₀ = √⎛_͟ħ͟ ⎞. ⎝2mω⎠ Substituting the matrix elements: ❬x̂❭ = ⅕ √⎛͟2͟͟ħ͟⎞⎛ exp⎛ι͟E͟₀͟t + ι⎛_͟π͟ - E͟₁͟t⎞⎞ + exp⎛-ι͟E͟₀͟t + ι⎛E͟₁͟t - _͟π͟⎞⎞ ⎞ ⎝mω⎠⎝ ⎝ ħ ⎝ 2 ħ ⎠⎠ ⎝ ħ ⎝ ħ 2⎠⎠ ⎠; ❬x̂❭ = ⅕ √⎛͟2͟͟ħ͟⎞⎛ exp⎛ι⎛⎛E͟₀͟−͟E͟₁͟⎞t + _͟π͟⎞⎞ + exp⎛ι⎛⎛E͟₁͟−͟E͟₀͟⎞t - _͟π͟⎞⎞ ⎞ ⎝mω⎠⎝ ⎝ ⎝⎝ ħ ⎠ 2⎠⎠ ⎝ ⎝⎝ ħ ⎠ 2⎠⎠ ⎠. For the Harmonic Oscillator, Eₙ = ħω(n + ½). Eₙ−Eₙ′ = ħω(n - n′): E₀−E₁ = ħω(0 - 1) = -ħω; E₁−E₀ = ħω. ❬x̂❭ = ⅕ √⎛͟2͟͟ħ͟⎞⎛ exp⎛ι⎛⎛−͟ħ͟ω͟⎞t + _͟π͟⎞⎞ + exp⎛ι⎛⎛ħ͟ω͟⎞t - _͟π͟⎞⎞ ⎞ ⎝mω⎠⎝ ⎝ ⎝⎝ ħ ⎠ 2⎠⎠ ⎝ ⎝⎝ ħ⎠ 2⎠⎠ ⎠. ❬x̂❭ = ⅕ √⎛͟2͟͟ħ͟⎞ exp(ι͟π͟)⎛ exp⎛ι⎛−͟ħ͟ω͟⎞t⎞ + exp⎛ι⎛⎛ħ͟ω͟⎞t - π⎞⎞ ⎞ ⎝mω⎠ 2 ⎝ ⎝ ⎝ ħ ⎠ ⎠ ⎝ ⎝⎝ ħ⎠ ⎠⎠ ⎠. ❬x̂❭ = ⅕ √⎛͟2͟͟ħ͟⎞ exp(ι͟π͟)⎛ exp⎛ι⎛−͟ħ͟ω͟⎞t⎞ - exp⎛ι⎛ħ͟ω͟⎞t⎞ ⎞ ⎝mω⎠ 2 ⎝ ⎝ ⎝ ħ ⎠ ⎠ ⎝ ⎝ ħ⎠ ⎠ ⎠. This is a sine function. ❬x̂❭ = ⅕ √⎛͟2͟͟ħ͟⎞ exp(ι͟π͟) 2ι sin⎛−͟ħ͟ω͟t͟⎞ = ⅕ √⎛͟8͟ħ͟⎞ ι² -sin(ωt). ⎝mω⎠ 2 ⎝ ħ ⎠ ⎝mω⎠ (𝐜,x̂) ❬x̂❭ = ⅕ √⎛͟8͟ħ͟⎞ sin(ωt). ⎝mω⎠ The expectation value progresses periodically with the time parameter t. A very similar argument can be made for the momentum operator. ❬p̂❭ = ⅕⎛exp⎛ι͟E͟₀͟t⎞ exp⎛-ι͟E͟₀͟t⎞❬0❙p̂❙0❭ + ⎞ ⎜ ⎝ ħ ⎠ ⎝ ħ ⎠ ⎟ ⎜ ⎟ ⎜ exp⎛ι͟E͟₀͟t⎞ 2 exp⎛ι⎛_͟π͟ - E͟₁͟t⎞⎞ ❬0❙p̂❙1❭ + ⎟ ⎜ ⎝ ħ ⎠ ⎝ ⎝ 2 ħ ⎠⎠ ⎟ ⎜ ⎟ ⎜ exp⎛-ι͟E͟₀͟t⎞ 2 exp⎛ι⎛E͟₁͟t - _͟π͟⎞⎞ ❬1❙p̂❙0❭ + ⎟ ⎜ ⎝ ħ ⎠ ⎝ ⎝ ħ 2⎠⎠ ⎟ ⎜ ⎟ ⎜ 2 exp⎛ι⎛E͟₁͟t - _͟π͟⎞⎞2 exp⎛ι⎛_͟π͟ - E͟₁͟t⎞⎞❬1❙p̂❙1❭⎞⎟ ⎝ ⎝ ⎝ ħ 2⎠⎠ ⎝ ⎝ 2 ħ ⎠⎠ ⎠⎠. ❬0❙p̂❙0❭ = p₀₀ = ❬1❙p̂❙1❭ = p₁₁ = 0. ❬0❙p̂❙1❭ = p₀₁ = -ι√⎛͟ħ͟m͟ω͟⎞. ⎝ 2 ⎠ ❬1❙p̂❙0❭ = p₁₀ = ι√⎛͟ħ͟m͟ω͟⎞. ⎝ 2 ⎠ ❬p̂❭ = ⅖ ι √⎛͟ħ͟m͟ω͟⎞⎛-exp⎛ι͟E͟₀͟t + ι⎛_͟π͟ - E͟₁͟t⎞⎞ + exp⎛-ι͟E͟₀͟t + ι⎛E͟₁͟t - _͟π͟⎞⎞ ⎝ 2 ⎠⎜ ⎝ ħ ⎝ 2 ħ ⎠⎠ ⎝ ħ ⎝ ħ 2⎠⎠. ❬p̂❭ = ⅖ ι √⎛͟ħ͟m͟ω͟⎞⎛exp⎛ι⎛⎛−͟ħ͟ω͟⎞t - _͟π͟⎞⎞ + exp⎛ι⎛⎛ħ͟ω͟⎞t - _͟π͟⎞⎞⎞ ⎝ 2 ⎠⎝ ⎝ ⎝⎝ ħ⎠ 2⎠⎠ ⎝ ⎝⎝ ħ⎠ 2⎠⎠⎠. ❬p̂❭ = ⅖ ι √⎛͟ħ͟m͟ω͟⎞ exp⎛−͟ι͟π͟⎞ ⎛ exp⎛ι⎛−͟ħ͟ω͟⎞t⎞ + exp⎛ι⎛ħ͟ω͟⎞t⎞ ⎞ ⎝ 2 ⎠ ⎝ 2 ⎠ ⎝ ⎝ ⎝ ħ⎠ ⎠ ⎝ ⎝ ħ⎠ ⎠ ⎠. This is a cosine. (𝐜,p̂) ❬p̂❭ = ⅘ ι √⎛͟ħ͟m͟ω͟⎞ exp⎛−͟ι͟π͟⎞ cos(ωt) = ⅕ √(8ħmω) cos(ωt). ⎝ 2 ⎠ ⎝ 2 ⎠ Ehrenfest's theorem states ❬p̂❭ = m d͟❬͟x̂͟❭͟. dt m d͟❬͟x̂͟❭͟ = ⅕ mω √⎛͟8͟ħ͟⎞ cos(ωt) = ⅕ √(8mωħ) cos(ωt) = ❬p̂❭. dt ⎝mω⎠ So, the theorem holds for this case.