Secure communication with modern computers relies on mathematics so intense, it is effectively impossible for a hacker to break it without the key. How the encryption system works is to multiply the data by a large prime number. There is no known way to predict prime numbers, so cracking the encryption would require testing every smaller number. For modern computers this would take more time than the age of the Universe, but for quantum computers, this is not the case. Already an algorithm exists to efficiently crack encryption with a quantum computer, so how would a quantum network transmit information securely?
Quantum key distribution (QKD) is one possible method and takes advantage of observation destroying quantum states. The states, in this case, are the orientations of a photon. For now, you can picture the photon as a compass; the needle is either vertical (↑) or horizontal (→), or at a diagonal (â†— or â†˜). If Alice and Bob (told you they’d get back together) want to transmit information to each other, Alice, starts by randomly sending a bit to Bob, with one of those orientations. Now Bob has to decide what orientations he is measuring, because ↑ and → are only recorded correctly if he measures rectilinearly, and the diagonal bits (â†— or â†˜) would be randomly 0s or 1s, and similarly ↑ and → would be random when Bob measures on a diagonal. Alice continues to send random bits like this, changing the orientation and the reference for what is 0 and 1 each time.
With all the random bits sent and measured, Bob tells Alice, without any quantum mechanics involved, what he measured each time; a 0 or 1. Alice replies with if he is right or wrong for each bit, and Bob, knowing how he measured it each bit, then knows how each bit was sent. (The only way he would be right is if he measured the same way she sent it.) They both now throw out whenever he was incorrect, and what’s left makes the shared key between them, for secure communication.
This method is secure because every time a photon is measured, the photon is disturbed in such a way that either Alice or Bob can notice it. Any would-be eavesdroppers are immediately discovered.
Now researchers know this method can work in real life. From March 2009 to January 2011, a QKD network was used by Swiss scientists at ID Quantique and the scientists at CERN. The only times the network was disrupted, an external event like a power outage caused it, and not a fault of the network or QKD system itself.