Student handout: Working with Representations on the Ring

Central Forces 2021
  • How to form a state as a column vector in matrix representation.
  • How to do probability calculations on all three representations used for quantum systems in PH426.
  • How to find probabilities for and the resultant state after measuring degenerate eigenvalues.
What students learn
  • How to form a state as a column vector in matrix representation.
  • How to do probability calculations on all three representations used for quantum systems in PH426.
  • How to find probabilities for and the resultant state after measuring degenerate eigenvalues.

The following are 2 different representations for the same state on a quantum ring \begin{align} \left|{\Phi}\right\rangle = \sqrt{\frac{1}{2}}\left|{2}\right\rangle -\sqrt{\frac{1}{4}}\left|{0}\right\rangle +i\sqrt{\frac{1}{4}}\left|{-2}\right\rangle \end{align} \begin{equation} \Phi(\phi) \doteq \sqrt{\frac{1}{8 \pi r_0}} \left(\sqrt{2}e^{i 2 \phi} -1 + ie^{-i 2 \phi} \right) \end{equation}

  1. Write down the matrix representation for the same state.

  2. With all 3 representations, calculate the probability that a measurement of \(L_z\) will yield \(0\hbar\), \(-2\hbar\), \(2\hbar\).

  3. If you measured the \(z\)-component of angular momentum to be \(2\hbar\), write down the full resultant state immediately after the measurement.

  4. If an energy measurement is performed on the state \(\Phi(\phi)\), what is the probability that the energy measurement will yield each of the following values: \(0\frac{\hbar^2}{I}\)?,\(2\frac{\hbar^2}{I}\)?,\(4\frac{\hbar^2}{I}\)?

  5. If you measured the energy of the state to be \(2\frac{\hbar^2}{I}\), write down the full resultant state immediately after the measurement.


Author Information
Dustin Treece and Corinne Manogue
Keywords
Quantum Ring Probability Ket BraKet Dirac Matrix Wavefunction representations
Learning Outcomes