# Experiments of the Nobel Prize in Physics 2022, quantum entanglement & more

The Nobel Prize in Physics 2022 was given to Alain Aspect, John F. Clauser, and Anton Zeilinger “for experiments with entangled photons, establishing the violation of Bell inequalities and pioneering quantum information science”. Want to try to demonstrate entanglement yourself? Or perhaps violate Bell inequalities yourself? Or have a hands-on experience with some of the other groundbreaking quantum experiments? In our lab, you will make such experiments with photons!

## Hvad går øvelsen ud på?

The Nobel Prize in Physics 2022 was given to Alain Aspect, John F. Clauser, and Anton Zeilinger “for experiments with entangled photons, establishing the violation of Bell inequalities and pioneering quantum information science”.

Want to try to demonstrate entanglement yourself? Or perhaps violate Bell inequalities yourself? Or have a hands-on experience with some of the other groundbreaking quantum experiments?

In our lab, you will make such experiments with photons:

- quantum superposition, implying that an object can be at two different places at the same time;
- quantum entanglement, that two objects can stay in sync without talking, without delay, and even when very far from each other.

On the way you will also figure out how to distinguish indistinguishable photons, and you will learn how these basic experiments make a technology of tomorrow.

## Hvordan foregår øvelsen?

All the experiments will be “hands-on” – you will be conducting them, taking the data and analyzing the results.

#### Quantum entanglement

Entanglement is a peculiar phenomenon in physics — it keeps particles “connected” even when they are very far from each other (e.g., one here, another on the Moon).

Here you will generate two photons that are polarization-entangled. In easy words, it means that the polarizations of two photons are correlated. For instance, if the first photon is horizontally polarized, then the second one will be vertically polarized. And the opposite, if the first photon is vertically polarized, then the second one will be horizontally polarized.

But it is not only this. The intriguing part is that the polarization of the photons is not known before you measure them. Your measurement will, in fact, define the polarization of the photon.

And you need to measure only one photon to define the polarization of both of them. How does the second photon know which polarization “to take” when the first photon was measured? How fast does this happen? And what if the two photons are very far from each other (for example on different planets)? Does the process still work and how fast?

To find out – try to answer the questions below.

#### Entanglement

Entanglement is an indispensable resource in forthcoming quantum technology. For instance, it lies in the core of the forthcoming Quantum Internet

#### Two-photon interference

A beam-splitter has two input ports and two output ports. If a single photon enters a beam-splitter from one of the input ports it can exit from any of the two output ports. We do not know which one, and there is a 50% chance for each of the outputs. But what if two identical (indistinguishable) photons enter the beam-splitter at the same time from the different input ports? Will they still have each 50% chance of exiting from any of the two output ports? Or will they make a “joint decision” regarding of which port to exit?

To find out – try to answer the questions below.

#### Indistinguishability

Indistinguishability is one of the key properties of single photons required for the forthcoming quantum technologies, and this experiment is the way to test it!

#### Preparation

Try to answer, or at least to think about the questions below before the exercise. You can use Wikipedia to learn about the topics and find the answers. We will discuss the questions during the exercise

##### Questions

- What is a single photon?
- What is a single photon source?
- How does a beam-splitter work with single photons?
- What is quantum entanglement?
- What is the difference between classical and quantum correlations?b.
- Why quantum entanglement looks like a paradox?

- What are the Bell inequalities?
- What is a polarizer?
- How does a waveplate work?
- What is the Hong–Ou–Mandel effect?

## Hvilke faciliteter kommer du til at bruge?

The students will see a state-of-the-art quantum optical lab, where quantum phenomena with photons and many new physics can be explored.

The experiments that you will work with are the very foundation of many quantum technologies, such as quantum communication and quantum cryptography.

Photons are very suitable for transferring quantum information since they fly at the speed of light, and photons have many attributes that can be used to carry quantum information for instance their polarization.

So the theory behind these experiments can be applied to the whole emerging field of quantum technologies.

09:00-10:00 Introduction to the lab and to the experiments

10:00-11:30 Experiment 1: Quantum entanglement

11:30-12:30 Lunch (bring your own or have some money for the canteen)

12:30-13:30 Experiment 2: Two-photon interference

13:30-14:30 Experiment 3: Quantum random numbers (bonus experiment)

14:30-15:00 Final discussions

Note: This is a tentative program, we might deviate from it if needed.

In the Quantum Entanglement experiment, you will work with a setup (from quTools) to generate entangled photon pairs. Using photodetectors and coincidence counters, coincidence events are recorded and accumulated. The count rates can be used to test if Bell’s inequality is obeyed or broken (the Nobel prize experiment).

In the Two-photon Interference experiment, you will work with an optical beamsplitter together with the entangled photon pair source used in the first experiment.

By changing the arrival time of one photon relative to another, we can decide whether the entangled photon pair will meet up at the beamsplitter. When they do, they begin to interfere in a very counter-intuitive way (the quantum way), but only if they are identical photons, that is, if they have the same energy and polarization.

You will collect and analyze the coincidence count rate in different scenarios: when the photons are completely identical, partially identical or completely distinguishable.

Forslag til litteratur, du kan bruge i din opgave og som baggrund for øvelsen:

- Program baggrundsinformation og forberedelse
- Experiment 1: Entanglement demonstration, baggrund og beskrivelse af eksperimentet
- Sample Experiments – Particle Nature of Photons, yderligere baggrundsinformation og teori

#### Entanglement:

#### Bell's Inequality:

#### Wikipedia is a good starting point:

- https://en.wikipedia.org/wiki/Single-photon_source
- https://en.wikipedia.org/wiki/Beam_splitter
- https://en.wikipedia.org/wiki/Polarizer
- https://en.wikipedia.org/wiki/Waveplate
- https://en.wikipedia.org/wiki/Hong-Ou-Mandel_effect
- https://en.wikipedia.org/wiki/Quantum_entanglement
- https://en.wikipedia.org/wiki/Bell's_theorem
- https://en.wikipedia.org/wiki/CHSH_inequality
- https://en.wikipedia.org/wiki/Bell_test_experiments

**NB:** Når du har tilmeldt dig, vil du få besked, hvis der er litteratur, du skal læse som forberedelse til øvelsen.

### Relevante gymnasiefag

**Fysik****Matematik**

### Faglige nøgleord

- Quantum mechanics
- Bell’s inequality
- Entanglement
- Superposition
- Photon
- Quantum optics