# What Is the Probability of Finding the Perturbed Oscillator in Its Ground State?

• Demon117
In summary, the eigenvalues for the Harmonic oscillator are shifted by \frac{F^{2}}{2m\omega^{2}} from the "unperturbed" case. It was also discussed that x\rightarrow x-\frac{F}{m\omega^{2}}. An oscillator is initially in its ground state (n=0). At t=0, a perturbation V is suddenly applied. What is the probability of finding the system in its (new) ground state for t>0, i.e. find |a_{o}|^{2}.The Attempt at a SolutionFor t>0, the state of
Demon117

## Homework Statement

I showed earlier this semester that in the presence of a "constant force", $F_{o}$, i.e. $V=-Fx$, that the eigenvalues for the Harmonic oscillator are shifted by

$\frac{F^{2}}{2m\omega^{2}}$

from the "unperturbed" case. It was also discussed that $x\rightarrow x-\frac{F}{m\omega^{2}}$.

An oscillator is initially in its ground state (n=0). At t=0, a perturbation V is suddenly applied. What is the probability of finding the system in its (new) ground state for t>0, i.e. find $|a_{o}|^{2}$.

## Homework Equations

For this $|a_{n}|^{2}=|\int \Phi^{*}_{n}(x)\Psi_{o}(x)dx|^{2}$ over all space.

## The Attempt at a Solution

For t>0, the state of the system is $\Psi(x,t)=\sum a_{n}exp(-i(\frac{E_{n}}{\hbar})t)\Phi_{n}(x)$. Here $\Phi_{n}(x)$ is an eigenvector of H. And the coefficients $a_{n}$ are obtained by expanding $\Psi_{o}(x)$, the ground state of $H_{o}$, in terms of $\Phi_{n}(x)$.

I also know that the basis states $\Phi_{n}(x)$ as well as $\Psi_{o}(x)$ are Hermite polynomials.

With that in mind my assumption would simply be to integrate the following:

$|a_{o}|^{2}=|\int \Phi_{o}(x) \Psi_{o}(x) dx|^{2} =|\int 1*1 dx|^{2}$

If I integrate this over all space I end up with a probability that goes to infinity. . . .Maybe I am missing something as far as Hermite polynomials go. . . or maybe I have the wrong idea about this problem. Any suggestions would be helpful.

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Hi matumich, sorry I'm not sure if I can help but if you could so kindly explain to me where you got the relavant equation for |a_n|^2 and for psi(x,t) I would greatly appreciate it. Also, could you explain to me why the last line is equal to |integral 1*1 dx|^2?

Hi matumich, sorry I'm not sure if I can help but if you could so kindly explain to me where you got the relavant equation for |a_n|^2 and for psi(x,t)
.

Well I made several mistakes. The equation $|a_{n}|^{2}=|∫Φ^{∗}_{n}(x)Ψ_{o}(x)dx|^{2}$ was given in class. The coefficients $a_{n}$ are obtained by expanding $Ψ_{o}(x)$, the ground state of $H_{o}$, in terms of $Φ_{n}(x)$. From this it follows that the probability of finding the system in some state (t>0) is given by that integral.

I would greatly appreciate it. Also, could you explain to me why the last line is equal to |integral 1*1 dx|^2?

This was just purely as mistake and since the eigenfunctions $Φ_{n}(x)$ correspond to the state once the perturbation is applied they can be expanded in terms of their basis elements, which in this case are the Hermite polynomials. Similarly, the unperturbed wave function $Ψ_{o}(x)$ can be expressed in terms of the Hermite polynomials. Therefore we have:

$Φ_{n}(x) = C exp(-\frac{1}{2}\alpha^{2}(x-\frac{F}{m \omega^{2})^{2})H_{n}(x)$

&

$Ψ_{o}(x) = Cexp(-\frac{1}{2}\alpha^{2} x^{2})H_{o}(x) = exp(-\frac{1}{2}\alpha^{2} x^{2})$

The probability would be along the lines of

$|a_{o}|^{2}=|∫C exp(-\frac{1}{2}\alpha^{2}(x-\frac{F}{m \omega^{2})^{2})exp(-\frac{1}{2}\alpha^{2} x^{2})dx|^{2}$

After normalizing both wave functions you can integrate this and find the appropriate probability of finding the system in the ground state for t>0.

## 1. What is a perturbed harmonic oscillator?

A perturbed harmonic oscillator is a system in which a harmonic oscillator is subjected to an external force or disturbance. This disturbance can cause the oscillator to deviate from its regular motion and exhibit more complex behavior.

## 2. How is the behavior of a perturbed harmonic oscillator different from a regular harmonic oscillator?

The behavior of a perturbed harmonic oscillator is often more complex or chaotic compared to a regular harmonic oscillator. This is because the external force or disturbance can introduce new frequencies and affect the amplitude and phase of the oscillator's motion.

## 3. What are some examples of perturbed harmonic oscillators?

A pendulum with a swinging weight attached to it, a mass on a spring experiencing external vibrations, and an electron in an atom subjected to an external electromagnetic field are all examples of perturbed harmonic oscillators.

## 4. How is the behavior of a perturbed harmonic oscillator studied?

The behavior of a perturbed harmonic oscillator can be studied using mathematical models, such as differential equations, to describe the motion of the oscillator under the influence of the external force. Experimental methods, such as measuring the displacement or velocity of the oscillator, can also be used to study its behavior.

## 5. What are the applications of perturbed harmonic oscillators?

Perturbed harmonic oscillators have applications in various fields, including physics, chemistry, and engineering. They can be used to study the behavior of complex systems, such as molecules and atoms, and to design and analyze mechanical and electrical systems that are subjected to external disturbances.

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