In this age of information, where our data is precious, encryption is a crucial tool in the fight for our privacy. An encryption key converts plaintext which anyone can read into ciphertext, a secret code. The same encryption key can be used to descramble this code. Most messaging services use encryption to protect your messages. Your phone, and whoever you are texting’s phone have the same encryption key. Once you have typed your message, it is encrypted before being sent to the recipient phone, which decrypts your message so that it can be read. However, encryption is not entirely secure, as hackers and government agencies can, and have been known to, take a peek at your private conversations. This is where quantum key decryption comes in. It promises to be an extremely fast, un-hackable method of transferring information from place to place. But to talk about quantum key distribution, we must first talk about light.
 
In linear optical quantum computing, or LOQC, photons, referred to as qubits (quantum bits) are used as information carriers. The reason this is possible is simply because single photons are polarised, having either a vertical polarisation or a horizontal polarisation which replaces the 1s and 0s that store our information today. LOQC mainly uses optical instruments (including reciprocal mirrors and waveplates) to process quantum information and uses photon detectors and quantum memories to detect and store quantum information.
 
The key quality of photons which allows all of this to happen is superposition. For now, let us suppose that these photons are in fact cats in boxes, with a radioactive substance inside the box which has a 50% percent chance of killing the cat. In this analogy, the vertical polarisation of the photon is the cat being alive, and the horizontal polarisation of the photon is the cat being dead. You have most likely heard about this: the famous “Schrödinger's cat” thought experiment. Until we open the box, each cat is both alive and dead at the same time. This is called superposition. Only once we observe the cat, we can know whether the cat is alive or dead.
 
Now suppose we have two cats in boxes. Simple mathematics will tell you that there are four possible outcomes of this: both cats are alive; both cats are dead; the first cat is alive and the second is dead and vice versa. Each of these outcomes has an equal, so 25%, chance of occurring. However, if these cats are “entangled,” they can only have opposite polarisation – either the first is alive and the second is dead or vice versa. What is even more amazing about quantum entanglement is that it occurs no matter the distance between the cats. With two entangled cats in boxes, if I observe the first one to be dead, I know that the second one is alive, even if it is light years away. This was referred to as “spooky action” by Einstein, and this phenomenon has been proven in multiple different lab experiments.
 
As the technology of quantum supercomputers matures and develops, entangled particles could be used to link certain devices so that they can transfer data between each other instantaneously regardless of distance and in a completely secure fashion.