Density Matrix: Theorem & Normality Conditions

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SUMMARY

The discussion centers on the theorem presented in the lecture slides from the University of Helsinki, specifically addressing the conditions under which two states, |ψ_i⟩ and |φ_i⟩, generate the same density matrix. It is established that |ψ_i⟩ generates the same density matrix as |φ_j⟩ if |ψ_i⟩ can be expressed as a linear combination of |φ_j⟩, represented by the equation |ψ_i⟩ = ∑_j u_ij |φ_j⟩. The query raised pertains to the implications when |ψ_i⟩ is normalized while |φ_i⟩ is not independent, questioning whether the necessary condition still holds true under these circumstances.

PREREQUISITES
  • Understanding of quantum states and density matrices
  • Familiarity with linear combinations in quantum mechanics
  • Knowledge of normalization conditions in quantum states
  • Basic grasp of the mathematical representation of quantum states
NEXT STEPS
  • Study the proof of the theorem on pages 18-21 of the provided lecture slides
  • Explore the implications of normalization in quantum mechanics
  • Research the concept of linear independence in quantum states
  • Learn about the mathematical properties of density matrices in quantum theory
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Quantum physicists, students of quantum mechanics, and researchers interested in the mathematical foundations of quantum state representation and density matrices.

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I have a question regarding the slide:

http://theory.physics.helsinki.fi/~kvanttilaskenta/Lecture3.pdf

On page 18-21 it gives the proof of the theorem that [tex]| \psi_i^{~} \rangle[/tex] and [tex]|\phi_{i}^{~}\rangle[/tex] generate the same density matrix iff [tex]|\psi_{i}^{~}\rangle = \sum_{j} u_{ij} |\phi_{j}^{~}\rangle[/tex] assuming that [tex]| \psi_i^{~}\rangle[/tex] is not necessarily normalized.

What if [tex]| \psi_i^{~}\rangle[/tex] is normalized and [tex]| \phi_i^{~}\rangle[/tex] not independent?

Would the necessary condition for which [tex]p = | \psi_i \rangle \langle \psi_i |= q = | \phi_j \rangle \langle \phi_j |[/tex] require that you have [tex]|\psi_{i}^{~}\rangle = \sum_{j} u_{ij} |\phi_{j}^{~}\rangle[/tex] ?
 
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We know for normalized states psi and phi that [tex]p = | \psi_i \rangle \langle \psi_i |= q = | \phi_j \rangle \langle \phi_j |[/tex] iff [tex]\sqrt{p_{i}} | \psi_i \rangle = \sum_j u_{ij} \sqrt{q_j} | \phi_j \rangle[/tex]
 

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