Density Matrix: Theorem & Normality Conditions

In summary, the conversation discussed the proof of a theorem that states that two vectors, | \psi_i^{~}\rangle and |\phi_{i}^{~}\rangle, will generate the same density matrix if |\psi_{i}^{~}\rangle = \sum_{j} u_{ij} |\phi_{j}^{~}\rangle, even if | \psi_i^{~}\rangle is not normalized. The question was raised about what would happen if | \psi_i^{~}\rangle is normalized and | \phi_i^{~}\rangle is not independent. It was concluded that in this case, the necessary condition for equality between p = | \psi_i \rangle \langle \psi_i |=
  • #1
Nusc
760
2
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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  • #2
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]
 

Related to Density Matrix: Theorem & Normality Conditions

1. What is a density matrix?

A density matrix, also known as a density operator, is a mathematical representation of a quantum system that contains information about the quantum states of the system. It is a matrix that describes the probabilities of a system being in different quantum states.

2. What is the Density Matrix Theorem?

The Density Matrix Theorem is a fundamental concept in quantum mechanics that states that the density matrix of a mixed quantum state can be obtained by taking a statistical average of the density matrices for each individual quantum state in the mixture.

3. What are the normality conditions for a density matrix?

The normality conditions for a density matrix refer to the requirements that must be met for the matrix to accurately represent a physical system. These include the matrix being Hermitian (equal to its own conjugate transpose) and having a trace value of 1.

4. How is the density matrix used in quantum mechanics?

The density matrix is a useful tool in quantum mechanics because it allows for the calculation of expectation values for observables in a mixed quantum state. It also allows for the study of the evolution of a system over time and the measurement of entanglement between particles.

5. Can the density matrix be used to describe classical systems?

No, the density matrix is a specific concept in quantum mechanics and does not have a classical analogue. It is used to describe the probabilistic nature of quantum systems, which is not present in classical systems. In classical mechanics, the state of a system can be fully described by its position and momentum, whereas in quantum mechanics the state is described by a wave function that contains probabilities for different states.

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