How to understand the off-diagonal elements

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In summary: This is more meaningful for ##m \neq n##, because when ##m = n## we expect the overlap to be constant, and so the change in the overlap to be zero.In summary, the off-diagonal elements of an observable operator, such as momentum, have a physical meaning related to infinitesimal transformations on states. They can be interpreted as the change in overlap between two states when one of them is translated by a small distance. This understanding applies to both the off-diagonal elements in the density matrix and in the Hamilton matrix.
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phdphysics
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In quantum mechanics, observable variables are represented by operators, and thus can be replaced by matrix in a certain basis.
If we have H|n>=E(n)|n>, where |n> are eigenfunctions of Hamilton matrix. Here is the problem: what's the physical meaning of <m|P|n>, namely off-diagonal elements of another observable operator, such as momentum?
I have known that sometimes this has something to do with quantum transition. But I want to know is there any universal understanding of off-diagonal elements?
Thanks!
 
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same question forthe off diagonal elements in the density matrix. Those which disappear during decoherence.
 
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phdphysics said:
what's the physical meaning of <m|P|n>, namely off-diagonal elements of another observable operator, such as momentum?
I have known that sometimes this has something to do with quantum transition. But I want to know is there any universal understanding of off-diagonal elements?
Thanks!

Many operators can be interpreted as generators of infinitesimal transformations on states. For example consider the momentum operator ##P##. If I have a state ##| \psi \rangle## and ##\epsilon## is an infinitesimal distance, then ##(1 + i \frac{\epsilon}{\hbar} P ) | \psi \rangle## is a version of the state ##| \psi \rangle## that has been translated to the left by a distance ##\epsilon##.

Then ##\langle m | P | n \rangle## can be interpreted as something like "the change in the overlap of ##| m \rangle## and ##| n \rangle## when one of them is translated by a small distance."
 

FAQ: How to understand the off-diagonal elements

What are off-diagonal elements?

Off-diagonal elements refer to the elements in a matrix that are not on the main diagonal. In other words, they are the elements that are not in the same row and column in a square matrix.

Why is it important to understand off-diagonal elements?

Understanding off-diagonal elements is important because they can provide valuable information about the relationships between different variables in a matrix. They can also be used to perform certain calculations and analyses in various fields of science and mathematics.

How can off-diagonal elements be interpreted?

Off-diagonal elements can be interpreted as the strength of the relationship between the variables represented by the row and column in which they are located. A larger off-diagonal element indicates a stronger relationship, while a smaller one indicates a weaker relationship.

What techniques can be used to better understand off-diagonal elements?

One technique is to visualize the matrix as a heat map, with different colors representing different magnitudes of off-diagonal elements. Additionally, statistical techniques such as correlation and covariance analysis can also provide insights into the off-diagonal elements.

Are there any special considerations when dealing with off-diagonal elements?

Yes, off-diagonal elements may be affected by factors such as matrix size, data distribution, and the presence of outliers. Therefore, it is important to carefully select the appropriate techniques and methods for analyzing off-diagonal elements to ensure accurate interpretation and analysis.

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