What Are the Possible Values of L_x in a Quantum System with l=1?

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SUMMARY

The discussion focuses on calculating the possible values of \(L_x\) in a quantum system with angular momentum quantum number \(l=1\). The values obtainable from measuring \(L_x\) are \(m=0, \pm 1\). The relationship \(L_x = \frac{L_+ + L_-}{2}\) is utilized, and the eigenstate representation of \(|1, 1\rangle_x\) in terms of \(L_z\) eigenstates is clarified. The correct representation is \(|1, 1\rangle_x = \frac{1}{2}[|1, 1\rangle + \sqrt{2}|1, 0\rangle + |1, -1\rangle]\), highlighting the importance of normalization in quantum states.

PREREQUISITES
  • Understanding of angular momentum in quantum mechanics
  • Familiarity with quantum state notation and ket vectors
  • Knowledge of the operators \(L_+\) and \(L_-\)
  • Concept of eigenstates and eigenvalues in quantum systems
NEXT STEPS
  • Study the derivation of eigenstates for angular momentum operators
  • Learn about the normalization of quantum states
  • Explore the relationship between \(L_x\) and \(L_z\) eigenstates
  • Investigate the role of the ladder operators \(L_+\) and \(L_-\) in quantum mechanics
USEFUL FOR

Students and researchers in quantum mechanics, particularly those studying angular momentum and quantum state representations.

bznm
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Homework Statement


I've a physical system with ##l=1## and I have to calculate the values I can obtain if I measure ##L_x## and their probability.

Homework Equations


I know that:

- the values I can obtain are ##\ m=0, \pm 1##
- ##\displaystyle L_x=\frac{L_+ + L_-}{2}##
- ##L_x|1, m>_x=\hbar m |1, m>_x##

The Attempt at a Solution



But I can't understand, for example, why I should obtain
##|1, 1>_x=\frac{1}{2}[|1,1>+\sqrt{2}|1,0>+|1,-1>] ##

(I have obtained ##|1, 1>_x=\frac{\sqrt{2}}{2} |1,0>##)

Can I have some hints?
 
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bznm said:
But I can't understand, for example, why I should obtain
##|1, 1>_x=\frac{1}{2}[|1,1>+\sqrt{2}|1,0>+|1,-1>] ##
The three kets on the RHS are the eigenstates of ##L_z##. You are asked to represent the eigenstate of ##L_x##, ##|1,1\rangle_x## in terms of eigenstates of ##L_z##.

bznm said:
I've a physical system with l=1l=1 and I have to calculate the values I can obtain if I measure LxL_x and their probability.
You have to know precisely the state associated to your system. From what you are asked to do above, it seems like your system is already in one of the eigenstates of ##L_x## and you are asked to calculate the probability of finding the state in the eigenstates of ##L_z##.

bznm said:
(I have obtained ##|1, 1>_x=\frac{\sqrt{2}}{2} |1,0>##)

Can I have some hints?
It's obvious that the right handside is not normalized while the LHS is. How did you obtain this?
 
Last edited:

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