I Writing the wave function solutions for a particle in a 2-D box

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The wave function solutions for a particle in a 2-D box are expressed as a double summation involving coefficients that represent the probability of the system's state. The correct form of the wave function is given by the equation that includes both spatial variables, x and y, and incorporates time dependence through an exponential term. The probability of finding the system in a specific state is proportional to the square of the coefficient associated with each unique combination of quantum numbers n and m, denoted as |C_{nm}|^2, rather than the product of individual coefficients |C_n C_m|^2. This distinction is crucial for accurately describing the quantum state of the particle in the box. Understanding these wave function solutions is essential for analyzing quantum systems in confined geometries.
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The final wave function solutions for a particle trapped in an infinite square well is written as:

$$\Psi(x,t) = \Sigma_{n=1}^{\infty} C_n\sqrt{\frac{2}{L_x}}sin(\frac{n\pi}{L_x}x)e^{-\frac{in^2{\pi}^2\hbar t}{2m{L_x}^2}}$$

The square of the coefficient ##C_n## i.e. ##{|C_n|}^2## is proportional to the probability of the system being in that state on measurement.

The wave function solution of a particle in a 2-D box
$$\Psi_{n,m}(x,y,t) = \frac{2}{\sqrt{L_x L_y}} sin(\frac{n\pi}{L_x}x)sin(\frac{m\pi}{L_y}y)e^{-\frac{i\pi^2 \hbar^2 t}{2m}[\frac{n^2}{{L_x}^2}+\frac{m^2}{{L_y}^2}]}$$

is it correct to write the final solution as:
$$\Psi (x,y,t) = \Sigma_{n=1}^{\infty} \Sigma_{m=1}^{\infty}C_n C_m\frac{2}{\sqrt{L_x L_y}} sin(\frac{n\pi}{L_x}x)sin(\frac{m\pi}{L_y}y)e^{-\frac{i\pi^2 \hbar^2 t}{2m}[\frac{n^2}{{L_x}^2}+\frac{m^2}{{L_y}^2}]}$$

will the probability of the system being in a particular state be proportional to ##|C_n C_m|^2##?
 
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Not quite. Each combination of ##n, m## has its own coefficient: so, it should be ##C_{nm}##
 
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For the quantum state ##|l,m\rangle= |2,0\rangle## the z-component of angular momentum is zero and ##|L^2|=6 \hbar^2##. According to uncertainty it is impossible to determine the values of ##L_x, L_y, L_z## simultaneously. However, we know that ##L_x## and ## L_y##, like ##L_z##, get the values ##(-2,-1,0,1,2) \hbar##. In other words, for the state ##|2,0\rangle## we have ##\vec{L}=(L_x, L_y,0)## with ##L_x## and ## L_y## one of the values ##(-2,-1,0,1,2) \hbar##. But none of these...

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