Imagine you want to send a super-secret message to someone, but you’re worried someone might read it along the way.

Scientists have found a way to use the strange rules of quantum physics to make sure only the person you’re sending the message to can read it.

This is called Quantum Key Distribution (QKD), and it uses tiny particles of light called photons to lock up your information securely. Even if someone tries to snoop, they’ll get caught!

Before delving into the details, let's first talk about the concept of quantum superposition.

Quantum Superposition

Imagine a coin that’s spinning in the air. While it’s spinning, it’s not just heads or tails—it’s both at the same time. This is like quantum superposition, where a particle (like a photon) exists in multiple states at once, such as being in two directions at the same time.

Once you measure it—like catching the coin—it "chooses" a single state, such as heads or tails.

Polarization of light

Credit: Lance Hayashida for Caltech Science Exchange

How Does QKD Work?

At the ends of the communication channel there's always two people, A and B. Well, those are not very nice names. Let's call them Alice and Bob instead!

Alice sends photons to Bob. Each photon is tilted a certain way (like up-and-down or side-to-side) to carry information. Think of it like tossing a coin that lands heads or tails.

Bob then tries to “read” the photons. He has to guess the right way to measure them (either up-and-down or side-to-side). If he guesses right, he gets the correct information. If not, he gets a random answer.

Credit: Renato Renner

Basis reconciliation

Then Alice and Bob compare directions in which they measured every photon (vertical or horizontal for example). These directions are called basis. They do this publicly over a classical channel but mind you, they are not sharing their results (left/right, up/down) just which basis they used for each measurement.

Finally, they keep only the results where their bases matched. These matching results form the secret key.

This process is called basis reconciliation, and it ensures that Alice and Bob end up with the same key while anyone trying to eavesdrop gets caught.

Credit: crypto.stackexchange

But why does anyone spying get caught? Let's say that our eavesdropper (Eve for short), measures the photons sent from Alice before Bob receives them. She will also choose a basis, and when Alice and Bob announce their basis she will know which measurements she has to keep.

But, if she choses a basis different that Alice's she will alter the photons in the process, meaning that when Bob measures them later he will get different results from what Alice sent, even if their basis match.

In this process, Eve will have given herself away, because when Alice and Bob realize their results don't match, they will cut the communications leaving Eve in the dark.

Your Quantum Learning Journey Starts Here

Get started on learning quantum physics and quantum communication! Quantum might sound complex, but with the right approach, anyone can understand it. We've gathered a list of references to guide you from a beginner to someone who truly grasps the concepts.