Introduction to Quantum Key Distribution

By Louise E. Turner

Almost all traffic across the internet relies on encryption. Standard encryption, which is used every time someone hops on the internet, is made up of complex mathematical equations that scramble communications. Encryption ensures that an individual’s data is kept secure, private, and that other parties are not listening in on their online conversations. (NIST)

As computers have grown in power, computer scientists have had to increase encryption complexity to keep up. With the newfound potential of AI, the all-around increase in computing power, and the rise of quantum computing, researchers are starting to think outside the box and away from math to create a brand-new secure encryption method.

Enter: Quantum Key Distribution.

To fully understand Quantum Key Distribution (QKD) it is important to first understand how standard encryption works. As previously mentioned, standard encryption relies on complex math equations. There are two main kinds of standard encryption: symmetric key (private key) cryptography, and asymmetric key (public key) cryptography.

Symmetric key cryptography is when the sender and receiver of data share a mutual decryption key, that only they know, before the data is transferred. This key is used by the sender to encrypt the data and by the receiver to decrypt the data. This can be thought of as two friends giving each other a cheat sheet to their new secret language! One friend uses the sheet to convert their message into the secret language, and the other friend uses the same sheet to decode the message. (ISO)

Asymmetric key cryptography functions a bit differently. Instead of both parties using the same key, the sender uses a public key to encrypt their data which can then only be decrypted by the receiver’s private key. This is not unlike having a mailbox. Anyone can put letters safely into someone’s mailbox, but only the owner has the key to get in and read those letters! (ISO)

Now that those have been covered, it’s time to talk about QKD and how it differs from standard encryption:

QKD, rather than being based in math, is based in quantum mechanics, making it entirely unique! QKD works by using fiber optic cables or satellite connections that have photons transmitted along them. Each photon has a random quantum state which is then read by using beam splitters. Because the receiver of the photons does not know which beam splitters the sender used to send the message, they must guess which splitters they use to read it. After this, the receiver tells the sender which beam splitter they used to read each photon after which the sender will compare with the splitters they actually used to send the message. All photons that were read by the receiver using the wrong beam splitter will then be discarded, creating a unique sequence of read photons that are used as a cryptographic key. The key is created once and then never seen again, making it extremely secure. Since both parties share their one-time code with each other, QKD is considered symmetric. (IEEE) (QuantumXC)

This figure shows how the received photons are sifted, creating a genuinely random combination each time.

But what makes QKD so important? One of the greatest benefits of QKD is that it can detect eavesdroppers. Due to quantum mechanics, if these photons are observed by an outside party, it is instantly noticeable, indicating to the communicators that they should shut down their channel.

This fascinating and strange quantum property is a game changer when it comes to encryption, guaranteeing no eavesdroppers! Additionally, due to the rise of quantum computers with the potential ability to break current encryption standards, having a quantum-secure way to encrypt critical data like government or security communications is incredibly important. (NSA)

It’s easy for someone to brush off the importance of encryption since they have “nothing to hide”, but this mindset is dangerous as critical infrastructure such as health care, the supply chain, government, and the military could all grind to a halt if their encryption is broken. Research into topics like QKD also shine a light on just how much humans are capable of. If we can create systems like QKD, the possibilities for future technology are endless!