Many of the encryption methods that keep our online data safe rely on a digital key which is very hard for computers to crack – for instance, requiring the identification of two very large prime numbers, which standard computers are very poor at. But if a powerful quantum computer were to be built, it could crack these types of code with ease and jeopardize the safety of our digital communications.
The only encryption method that has been proven to be completely secure if applied correctly – quantum computers or not – is the so-called "one-time pad." Here's how it works: first, a secret digital key is created consisting of a completely random sequence of bits. The key is then securely sent to the receiver, and kept private. Now, the sender can encrypt his message by adding the message's bits to the random bits of the key. Under these conditions, the code is deemed truly uncrackable.
Keeping the key secret can be a challenge, but quantum physics can come to the rescue. That's because the weird and wonderful properties of quantum mechanics make it so that if someone were to try and intercept it, the key itself would change and both the sender and the receiver would immediately notice.