In this day and age, we have a lot of secrets, and we constantly have to give them away to the Internet. For example, we voluntarily share our credit card number with Amazon to order something we need. But how do we know where this information is kept? How can we trust that Amazon does not accidently give it away? This challenge of keeping information secure is an important problem for companies and governments. That is why encryption (or translating information into a code only the right people can read) is extremely important.

Modern asymmetric key cryptography uses mathematical operations that are fairly easy to do in one direction, but extremely hard to do in reverse. The standard example used (indeed, the one that is almost synonymous with public key encryption) is that of prime factorization. (1) This method is based on the fact that the process of factorization of numbers requires a huge amount of time. Nowadays, however, as computers are getting smarter and faster, these codes are becoming easier to unscramble. This has resulted in a focus on improving encryption. Experts warn that with new technologies like quantum computing, which lets computers try many more solutions at once, codes that currently take hundreds of years to crack could be solved within minutes. (2)

For this reason, scientists have begun focussing on quantum cryptography, a technology that hides information in photons. It applies principles of quantum mechanics to encrypt messages in a way that they are never read by anyone outside of the intended recipient. Quantum cryptography takes advantage of quantum’s multiple states, coupled with its “no change theory,” which means it cannot be unknowingly interrupted. (3)

Essentially, quantum cryptography is based on the usage of individual particles/waves of light (photons) and their intrinsic quantum properties to develop an unbreakable cryptosystem—because it is impossible to measure the quantum state of any system without disturbing that system. It is theoretically possible that other particles could be used, but photons offer all the qualities needed—their behaviour is comparatively well-understood, and they are the information carriers in optical fibre cables, the most promising medium for extremely high-bandwidth communications. (4)

Quantum cryptography is based on two fundamental and unchanging principles of quantum mechanics- *Heisenberg’s Uncertainty principle *and *the principle of photon polarization. *The Uncertainty principle states that it is not possible to know simultaneously the accurate position and momentum of a particle. Thus, the polarization of a photon can only be known at the point when it is measured. The photon polarization principle describes how light photons can be oriented or polarized in specific directions. A photon filter with the correct polarization can only detect a polarized photon or else the photon will be destroyed. The photons can be polarized at various orientations, and these orientations can be used to represent bits encompassing ones and zeros. (5) For example, 11100100110 could correspond with h-e-l-l-o. So a binary code can be assigned to each photon– a photon that has a vertical spin ( | ) can be assigned a 1. (6) The representation of bits through polarized photons is the foundation of quantum cryptography that serves as the underlying principle of quantum key distribution. (5)

As an example, let us assume that two people Alice and Bob want to exchange a message. When Alice sends Bob her photons using a Light Emitting Diode(LED), she will randomly polarize them through either the vertical or the diagonal filters of the LED, recording the polarization of each photon. After the entire transmission, Bob and Alice have a non-encrypted discussion about the transmission. As Bob receives these photons, he decides whether to measure each with either of the filters, telling Alice each time which filter he is using. Alice then tells Bob whether it was the correct filter or not. Since Bob isn’t saying what the measurements are – only the type of filter used – a third party listening to their conversation cannot determine what the actual photon sequence is. (6)

Thus, while the strength of modern digital cryptography is dependent on the computational difficulty of factoring large numbers, quantum cryptography is completely dependent on the rules of physics and is also independent of the processing power of current computing systems. Since the principle of physics will always hold true, quantum cryptography provides an answer to the uncertainty problem that current cryptography suffers from; it is no longer necessary to make assumptions about the computing power of malicious attackers or the development of a theorem to quickly solve the large integer Factorization problem. (5)

Quantum cryptography holds both promises and threats for our current cryptographic infrastructure. The most obvious threat is quantum computers could decrypt data that’s been encrypted using many of our current systems. But it also holds the promise of secure communications channels for key(typically large, random, numbers that can be used to encrypt or decrypt data) distribution. Eventually, using quantum technology, it may even be possible to build entire encryption systems that are considered unbreakable. (7)

**References:**

- Review of Methods for Integer Factorization Applied to Cryptography. (n.d.). Retrieved July 23, 2020, from https://scialert.net/fulltext/?doi=jas.2006.458.481
- Girl, Physics, director.
*Quantum Cryptography Explained*.*YouTube*, 2016, www.youtube.com/watch?v=UiJiXNEm-Go. - DougDrinkwater, Maria Korolov and. “What Is Quantum Cryptography? It's No Silver Bullet, but Could Improve Security.”
*CSO Online*, CSO, 12 Mar. 2019, www.csoonline.com/article/3235970/what-is-quantum-cryptography-it-s-no-silver-bullet-but-could-improve-security.html. - Rouse, Margaret. “What Is Quantum Cryptography? - Definition from WhatIs.com.”
*SearchSecurity*, TechTarget, 21 Sept. 2005, searchsecurity.techtarget.com/definition/quantum-cryptography. - Aditya, J., and P. Shankar Rao.
*Quantum Cryptography*. cs.stanford.edu/people/adityaj/QuantumCryptography.pdf. - Clark, J. (2020, June 23). How Quantum Cryptology Works. Retrieved July 16, 2020, from https://science.howstuffworks.com/science-vs-myth/everyday-myths/quantum-cryptology4.htm
- David Cardinal on March 11, 2019 at 12:08 pm Comment. “Quantum Cryptography Demystified: How It Works in Plain Language.”
*ExtremeTech*, 28 May 2019, www.extremetech.com/extreme/287094-quantum-cryptography.