Quantum Key Distribution
The appeal of quantum key distribution is largely due to the fact that information shared on the internet is not secure. Hackers can access your information without you even knowing. Due to the rise of quantum computers in recent years many algorithms and innovations have grown in popularity, leading to further development. One of these algorithms being quantum key distribution. Quantum key distribution almost entirely ensures security on the internet. Essentially, quantum key distribution’s goal is to administer an undisclosed key between two users (throughout this article, I will be referring to them as Alice and Bob).
Classical Encryption
Before we jump into the quantum world of cryptography, I’m going to give a brief overview of how encryption works right now. One basic way of encryption is to set a number of the alphabet to a number, but obviously, this would be pretty easy to guess.
A more secure method of encryption is where a third party gives a key to both the messenger and the receiver. This key would contain the procedure to decrypt and encrypt. A common way of representing this key is in the form of a string of bits. Unfortunately, to actually obtain this key they need to have an encrypted channel (which is what we’re trying to get).
BB84 Protocol
The BB84 Protocol was proposed in 1984 by Charles Bennett and Gilles Brassard. The name is derived from the creators and the date of creation. The main concept is to encode a secret key — made up of a sequence of bits — into the polarization state of a single photon. The idea is that a photon cannot be measured without changing its state, therefore, the information is delicate and cannot be accessed by eavesdroppers. Any act to access this photon will cause the eavesdropper (named Eve) to reveal themselves and the information to be lost. If Eve tries to duplicate this information and send it as normal, the information will be corrupted and cause noticeable errors in the information.
Now, the procedure includes the following. Alice will send an arrangement of pulses in femtoseconds or picoseconds (that’s a range of one quadrillionth of a second to one trillionth of a second!). Each of these pulses will hopefully contain a single photon polarized differently. Half of the 0’s will be encoded into H-polarized (horizontally polarized) photons and unities will be encoded into V-polarized (vertical polarized) photons. The other half will be encoded using a diagonal polarization basis.
Bob will now measure the polarization with two single-photon detectors. Using this setup, Bob is able to distinguish between the H and V polarizations. However, in half of the cases, Bob will randomly change his bases to the other (AD).
Once a sufficient number of bits have been transferred, Bob will publicly declare the basis he used for each bit — that’s one of the benefits of this protocol, it’s safe to broadcast the bases used publicly since the interceptor would’ve already tried to guess with their own bases. Alice will then compare their bases, and inform Bob of the bits where they used the same bases. Then, they destroy the bits with different bases as they are now rendered useless.
To ensure that their key is the same, they will take a part of the key (5–10% of it) and compare it. This part of the procedure is public as well, but it doesn’t matter since once they are done comparing this part of the key is scrapped. If the bits compared are correct, then it’s likely the rest is correct as well and the procedure is complete.
However, if Eve had intercepted, how exactly would that look? Eve would intercept when Alice is sending the qubits to Bob. She is unable to copy the qubits because of the No-Cloning Theorem of Quantum mechanics, if she does then Bob will never receive the qubits. To make this work, she needs to send fake qubits, and generate random bases. Everything is normal until the final step of comparing the key. When they compare their keys it will contain many errors. Then, the entire key would be scrapped and the process would have restarted.
There are other ways of executing quantum key distribution, but this is one of the most efficient, developed, and most popular ways.
Obstacles
The BB84 Protocol is theoretically possible, however, we do not have the materials required to create it. Quantum computers are not yet advanced enough to complete even the simplest of tasks, this is due to a property known as decoherence (which refers to the fragility of qubits). Qubits are affected by many environmental factors from temperature to the altering of magnetic and electrical fields.
Another material that we don’t have available right now is a communication channel capable of transmitting qubits. A potential option is a fibre-optic cable that has the capability to move polarized photons.
Conclusion
Overall, quantum key distribution is an effective algorithm that has the potential to protect the sensitive data of its users. Once quantum computers are further developed, this will be one of the first algorithms utilized in mainstream society.
Thanks for reading! I hope you learned something new and valuable today. If you want to pursue this topic further, I encourage you to check out my video on Quantum Key Distribution! If you want to reach out to me, feel free to check out my:
Twitter: @EthanSelvarajah || Substack: Ethan’s Expeditions
