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Activities

Small Group Activity

60 min.

##### Quantum Calculations on the Hydrogen Atom

Students are asked to find eigenvalues, probabilities, and expectation values for $H$, $L^2$, and $L_z$ for a superposition of $\vert n \ell m \rangle$ states. This can be done on small whiteboards or with the students working in groups on large whiteboards.

Students then work together in small groups to find the matrices that correspond to $H$, $L^2$, and $L_z$ and to redo $\langle E\rangle$ in matrix notation.

• Found in: Central Forces course(s)

Small Group Activity

60 min.

##### Expectation Value and Uncertainty for the Difference of Dice

You have a system that consists of two identical (fair) six-sided dice. Imagine that you will perform an experiment where you roll the pair of dice together and record the observable: the norm of the difference between the values displayed by the two dice.

1. What are the possible results of the observable for each roll?

2. What is the theoretical probability of measuring each of those results? Assume the results are fair.

Plot a probability histogram. Use your histogram to make a guess about where the average value is and the standard deviation.

3. Use your theoretical probabilities to determine a theoretical average value of the observable (the expectation value)? Indicate the expectation value on your histogram.

4. Use your theoretical probabilities to determine the standard deviation (the uncertainty) of the distribution of possible results. Indicate the uncertainty on your histogram.

5. Challenge: Use

1. Dirac bra-ket notation
2. matrices

to represent:

• the possible states of the dice after a measurement is made;

• the state of the dice when you're shaking them up in your hand;

• an operator that represents the norm of the difference of the dice.

• Found in: Quantum Fundamentals course(s)

Problem

5 min.

##### Frequency
Consider a two-state quantum system with a Hamiltonian $$\hat{H}\doteq \begin{pmatrix} E_1&0\\ 0&E_2 \end{pmatrix}$$ Another physical observable $M$ is described by the operator $$\hat{M}\doteq \begin{pmatrix} 0&c\\ c&0 \end{pmatrix}$$ where $c$ is real and positive. Let the initial state of the system be $\left|{\psi(0)}\right\rangle =\left|{m_1}\right\rangle$, where $\left|{m_1}\right\rangle$ is the eigenstate corresponding to the larger of the two possible eigenvalues of $\hat{M}$. What is the expectation value of $M$ as a function of time? What is the frequency of oscillation of the expectation value of $M$?
• Found in: Quantum Fundamentals course(s)

Small Group Activity

30 min.

##### Expectation Values for a Particle on a Ring
Students calculate the expectation value of energy and angular momentum as a function of time for an initial state for a particle on a ring. This state is a linear combination of energy/angular momentum eigenstates written in bra-ket notation.
• Found in: Central Forces, Theoretical Mechanics course(s) Found in: Quantum Ring Sequence sequence(s)

Small Group Activity

30 min.

##### Quantum Expectation Values

Expectation Values and Uncertainty

You have a system that consists of quantum particles with spin. On this system, you will perform a Stern-Gerlach experiment with an analyzer oriented in the $z$-direction.

Consider one of the different initial spin states described below:

A spin 1/2 particle described by:

1. $\left|{+}\right\rangle$
2. $\frac{i}{2}\left|{+}\right\rangle -\frac{\sqrt{3}}{2}\left|{-}\right\rangle$
3. $\left|{+}\right\rangle _x$

A spin 1 particle described by:

4. $\left|{0}\right\rangle$
5. $\left|{-1}\right\rangle _x$
6. $\frac{2}{3}\left|{1}\right\rangle +\frac{i}{3}\left|{0}\right\rangle -\frac{2}{3}\left|{-1}\right\rangle$
• List the possible values of spin you could measure and determine the probability associated with each value of the z-component of spin.

• Plot a histogram of the probabilities.

• Find the expectation value of the z-component of spin.

• Find the uncertainty of the z-component of spin.

## Introduction

I like to break this activity into two parts:

(1) Calculating expectation values and relating them to the associated distributions of the probabilities of results, and

(2) Calculating the quantum uncertainty of the state and relating the uncertainty to distributions of the probabilities of results.

Therefore, I have my students do the first part of the activity before I introduce quantum uncertainty.

I introduce the activity by reminding students about two ways of calculating the expectation value. Given a quantum state $\left|{\psi}\right\rangle$, for a measurement of an observable represented by an operator $\hat{A}$ with eigenstates $\left|{a_i}\right\rangle$ and eigenvalues $a_i$: \begin{align*} \langle \hat{A} \rangle &= \sum_{i} a_i\mathcal{P}(a_i) \\ &= \left\langle {\psi}\right|\hat{A}\left|{\psi}\right\rangle \end{align*}

After the students calculate expectation values and we have a whole class discussion about 1 of the examples, then I do a lecture introducing the quantity of quantum uncertainty (relating it to the standard deviation of the distribution of probabilities by spin component value) and deriving the simplified equation:

$\Delta A = \sqrt{\langle A^2 \rangle - \langle A \rangle ^2}$

## Student Conversations

1. One could have each group report out, or the instructor could discuss a few key examples.

For expectation value, I like to talk about Case 2: $\frac{i}{2}\left|{+}\right\rangle -\frac{\sqrt{3}}{2}\left|{-}\right\rangle$, where the probabilities of the two outcomes are not equal to show how the weighting plays out. Also, the expectation value is not a possible measurement value, and I like to talk about that. “Expectation” value is a misleading name for this quantity - it characterizes the distribution and is not necessarily a result of an individual measurement.

I also like to discuss an example like Case 5: $\left|{1}\right\rangle _x$ where the distribution is symmetric around $0\hbar$.

2. I think it's important to encourage students to calculate expectation values both ways (with probabilities and as a bracket with matrix notation) while the teaching team is available to help them.

3. For quantum uncertainty, I like to talk about an example like Case 3: $\left|{+}\right\rangle _x$ where all the individual measurements are the same ”distance” away from the expectation value as a sensemaking exercise to connect to a conceptual interpretation of physics.

I also like to discuss an example like Case 5: $\left|{-1}\right\rangle _x$, where the fact that we're taking an rms average is apparent: half the measurements are $\hbar$ away from the expectation value and the other half are $0\hbar$ away, but the uncertainty is $\hbar/\sqrt{2}$.

• Found in: Quantum Fundamentals course(s)

Small Group Activity

30 min.

##### Hydrogen Probabilities in Matrix Notation
This activity reinforces the strategies students have been practicing on each system by letting them create their own matrix operators and columns on the hydrogen atom and do some calculations with them.
• Found in: Central Forces course(s)

Small Group Activity

60 min.

##### Visualizing Plane Waves

Each small group of 3-4 students is given a white board or piece of paper with a square grid of points on it.

Each group is given a different two-dimensional vector $\vec{k}$ and is asked to calculate the value of $\vec{k} \cdot \vec {r}$ for each point on the grid and to draw the set of points with constant value of $\vec{k} \cdot \vec{r}$ using rainbow colors to indicate increasing value.

Computer Simulation

30 min.

##### Visualizing Flux through a Cube
Students explore the effects of putting a point charge at various places inside, outside, and on the surface of a cubical Gaussian surface. The Mathematica worksheet or Sage activity shows the electric field due to the charge, then plots the the flux integrand on the top surface of the box, calculates the flux through the top of the box, and the value of the flux through the whole cube.
• Found in: Static Fields, AIMS Maxwell, Surfaces/Bridge Workshop, Problem-Solving course(s) Found in: Gauss/Ampere Sequence (Integral Form), Geometry of Vector Fields Sequence, Flux Sequence sequence(s)

Small Group Activity

30 min.

##### Wavefunctions on a Quantum Ring
This activity lets students explore translating a wavefunction that isn't obviously made up of eigenstates at first glance into ket and matrix form. Then students explore wave functions, probabilities in a region, expectation values, and what wavefunctions can tell you about measurements of $L_z$.
• Found in: Central Forces course(s)