Quantum cryptography is a way of securing information based on the principle of uncertainty in quantum physics.
An encryption based on such physics would in theory be 'unbreakable', since intercepting a message causes the encryption process to shatter like the Universe's most delicate lock.
Prior to being measured, a particle's states can only be described as a probability, which is defined by a specific equation.
This is called Schrödinger's equation, and like the thought experiment involving a cat, measuring a particle interferes with it in a way that alters the specifics of this equation, affecting the result.
This rule of quantum physics can be applied to security methods that make it all but impossible for a person to intercept a private message.
How is it any different to normal encryption?
If you wanted to send a million dollars to your cousin without it being stolen, you might put it into a personal post box to which only he has the key.
In the digital world, an equivalent process would be described as an asymmetric system. The post box serves as what's known as a public key - a digital process for storing information that any member of the public can slide data into, but not open.
Once inside the post box, the million dollars can only be accessed by a person who has the key. Their private key is generated using an algorithm that includes known elements from the public key and secret ones known only to them.
A second method, called a symmetric system, would require you and your cousin to both have already worked out a private key. Much like a classic form of encoding using a cipher, the key can be a lot more complicated, and the transfer of data much more efficient, but the system can't be swiftly adapted for the general public to use.
There are also combinations of each, relying on a mix of keys that add sequences of locks to really make sure nobody can peek inside.
Even with numerous layers of traditional encryption, the rise of quantum computing means that it will take less effort to mathematically calculate a private key, putting even the most complex key-generation methods at risk of being broken.
How does quantum encryption work?
One existing method for securing a message using quantum physics is referred to as Quantum Key Distribution. It isn't an encryption in the strictest sense since the information itself isn't encoded using a cipher, but rather is more like a lock-and-key system.
Instead a message is secured using individual particles, such as photons, to create a binary code that forms the basis of a digital key.
A property of each particle, such as its spin, can't be predicted with certainty. It is determined by its quantum equation, which relies on a specific set of known conditions.
Those conditions can be entangled with other particles, which means changes to one particle will immediately affect the equation describing the other.
Interfering with an entangled photon in any way, such as by catching it and reading it, would immediately affect the rest of the system (such as other entangled particles held in a secure location) at the same time. This in effect makes entangled particles a perfect set of matching keys.
Should anybody try to make a copy of a key by replacing the photon they caught with a new one, their phony particle wouldn't be entangled with the key, alerting everybody to the deceit.
Researchers from China demonstrated the method in a video link in 2017, providing a sound proof of concept. It still requires some hardcore hardware to operate, but ever-shrinking technology means one day quantum key distribution could be a common way to hide our secrets.
Is quantum encryption really unbreakable?
While the method is in principle tamper-proof, in practice it relies on fallible technology and human activity.
Security is only ever as good as the hardware it's made of, and the trustworthiness of the people operating it. In other words, physics might be unhackable, but we sure aren't.
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