What is quantum cryptography?

If the concept of ‘quantum cryptography' sounds complicated to you, you're right. That’s why this ‘encryption tutorial for dummies’ shall demystify the concept and provide an explanation in layman’s terms.

Quantum cryptography, which has been around for a few decades, is becoming more and more important to our daily lives because of its ability to protect essential data in a manner that conventional encryption techniques cannot.

What is it?

Cryptography, as we all know, is a technique that aims to encrypt data by scrambling plain text so that only those with the appropriate ‘key’ can read it. By extension, quantum cryptography encrypts data and transmits it in an unhackable manner using the principles of quantum mechanics.

While such a concept seems straightforward, the intricacy resides in the quantum mechanics that underpin quantum cryptography. For example:

• The particles that make up the cosmos are fundamentally unpredictable, and they may exist in several places or states of existence at the same time;
• A quantum attribute cannot be measured without causing it to change or be disturbed;
• Some quantum attributes of a particle can be cloned, but not the whole particle.

How does it work?

Theoretically, quantum cryptography operates by following a model that was first published in 1984.

Assume there are two people called Alice and Bob who want to communicate a message in a safe manner, according to the model of quantum cryptography. Alice sends Bob a key, which serves as the signal for the communication to begin. One of the most important components is a stream of photons that go in just one direction. Each photon corresponds to a single bit of data — either a 0 or a 1 — in the computer's memory. However, in addition to traveling in a straight path, these photons are oscillating, or vibrating, in a certain fashion as they move.

The photons pass via a polarizer before reaching Alice, the sender, who then commences the transmission. When some photons pass through a polarizer with the same vibrations as before, and when others pass through with different vibrations, the filter is said to be ‘polarized’. There are many polarization states to choose from, including vertical (1 bit), horizontal (0 bit), 45 degrees right (1 bit) and 45 degrees left (0 bit). In whatever system she employs, the broadcast has one of two polarizations, each encoding a single bit, which is either 0 or 1.

From the polarizer to the receiver, the photons are now traveling via optical fiber to Bob. Each photon is analyzed using a beam splitter, which determines the polarization of each photon. After receiving the photon key, Bob does not recognize the right polarization of the photons, so he chooses one polarization at random from a pool of available options. Alice now compares the polarizers Bob used to polarize the key and informs Bob of the polarizer she used to deliver each photon to the receiver. Bob checks to see whether he used the right polarizer at this point. The photons that were read with the incorrect splitter are then eliminated, and the sequence that is left is deemed the key sequence.

Let's pretend there is an eavesdropper present, who goes by the name of Eve. Eve seeks to listen in and has the same tools as Bob in order to do so successfully. However, Bob has the benefit of being able to converse with Alice in order to check which polarizer type was used for each photon, but Eve does not. Eve is ultimately responsible for rendering the final key.

Alice and Bob would also be aware if Eve was listening in on their conversation. After Eve observes the flow of photons, the photon locations that Alice and Bob anticipate to see will be altered as a result of her observations.

Well, that’s all pretty mind-blowing, but for us, the general public, the biggest question is…

Is it really used?

Although the model described above has not yet been fully developed, there have been successful implementations of it, including the following:

• The University of Cambridge and the Toshiba Corporation collaborated to develop a high-bit-rate quantum key distribution system based on the BB84 quantum cryptography protocol;
• DARPA's Quantum Network, which operated from 2002 to 2007, was a 10-node QKD (Quantum Key Distribution) network constructed by Boston University, Harvard University, and IBM Research. It was operated by the Defense Advanced Research Projects Agency;
• Quantum Xchange created the first quantum network in the United States, which is comprised of over 1,000 kilometers of optical fiber;
• The development of commercial QKD systems was also carried out by commercial businesses such as ID Quantique, Toshiba, Quintessence Labs, and MagiQ Technologies Inc.

As you can see, these rare implementations are pretty far from what you’d expect to use every day. But hopefully, that will change in the near future.

The pros and cons of quantum cryptography

As with any developing technology, the state of it now (2022), may be very different to its state in the future. Thus, the following table may change dramatically. We do believe, however, that we’ll see fewer points in the ‘Limitations’ column as the years go on.

The need for unbreakable encryption is right there staring us down. The development of quantum computers is on the horizon, and the security of encrypted data is now in jeopardy due to the threat of quantum computing. We are fortunate in that quantum cryptography, in the form of QKD, provides us with the answer we need to protect our information long into the future — all while adhering to the difficult laws of quantum physics.