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a) What was the purpose of the Einstein-Podolsky-Rosen paper (1935) and what is the (possibly simplified) argument? b) Explain the difference between superposition and mixture

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Exam Advanced Quantum Mechanics 25 August 2017

Name:...

• Please write your answers on numbered pages. Write your name on each page. Start a separate page for each new question. Additional pages with your draft work, rough calculations or incomplete answers are handed in separately but are not considered.

• The exam is oral, closed book

Oral: Give clear and short answers to the following questions. Use drawings and formulae to explain better your words.

1. a) What was the purpose of the Einstein-Podolsky-Rosen paper (1935) and what is the (possibly simplified) argument?

b) Explain the difference between superposition and mixture.

c) What is an entangled state or system? Give a specific example to illustrate your point.

d) Recent and not so recent experiments have shown that the Bell inequalities are violated. What does that mean?

2. Show that the Schr¨odinger equation is time-reversal invariant when the potential V is real.

3. Explain how the first Born approximation for the cross section can be obtained from the Lippmann-Schwinger equation:

ψ~k(~r) = ei~k·~r− 1 4π

Z exp[ik|~r − ~u|]

|~r − ~u| U (~u) ψ~k(~u) d~u Explain all the notation in that equation.

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Written: write clearly.

3. Calculate for a complex number z,

ezae−za|0i

where, respectively, a and a are the annihilation and creation operator for the harmonic oscillator with ground state |0i. Call |ni the eigenstate with n particles/photons.

4. The most general density matrix for a spin 1/2 system is ρ = 1

2(1 + a · σ)

where a is a vector whose length is not greater than 1, and σ is the vector of the three Pauli matrices.

If the system has a magnetic moment µ = γ~ σ/2 and is in a constant magnetic field B, calculate the time-dependent density matrix ρ(t) in terms of the polarization vector at in

ρ(t) = 1

2(1 + at· σ) .

5. Determine the Clebsch-Gordan coefficients associated with the addition of one spin 1/2 and one spin 1.

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