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Quantum computing may be controversial, but there's no doubt about the success of the related field of quantum cryptography. This uses quantum techniques to distribute cryptography keys that can guarantee secure communications, and will alert the user if anyone tries to intercept them.

Unlike quantum computing, quantum cryptography been definitely demonstrated over a distance of more than 140km.

The problem with codes is in making sure that the people who are communicating have the keys, and no-one else. To make an unbreakable code, you simply combine your message with a unique string of characters that you use only once. Only someone who knows that string can read the message. But an unbreakable code is no use if an eavesdropper has the key.

There are two kinds of quantum cryptography. One uses Heisenberg's uncertainty principle and polarised photons, and the other uses quantum entanglement.

The polarised photon approach relies on the fact that, depending how you measure the polarisation of a photon of light, you get the right answer or a random answer. Light can be polarised in any direction, but this system uses just two pairs of states - rectilinear (horizontal or vertical) and diagonal (in two directions). The person sending the code creates a random string of 0s and 1s. For each bit, the sender makes a random choice between using the rectilinear or the diagonal scheme, and then sets the actual direction of polarisation according to a pre-agreed rule - so in the rectilinear scheme, for example, vertical represents 0 and horizontal represents 1. It doesn't matter if the eavesdropper knows this.

The receiver looks at each photon using polarising filters, either rectilinear or diagonal, which he picks at random. If he picks the right one, he can see which bit was encoded; if not, he inadvertently polarises the photon and gets a random digit, 0 or 1. At this point he doesn't know which digits are correct.

Once he has a string of digits, he transmits the polarisations he used (not the digits) and the sender does the same. It doesn't matter if an eavesdropper sees this, because the stream of bits is still private.

The two people throw out the bits where the receiver used the wrong polarisation, which leaves a string of digits they both know. There are refinements, but essentially, they know that no one has eavesdropped on them, because anyone observing the state of a photon will affect it.

Quantum entanglement is different. When two entangled photons are created, their polarisation must be opposite - but until it is measured, it's not determined (because of the principles of quantum mechanics). So if the photons are sent to two different people, when one is measured, the other will immediately take the opposite value. Again, there are refinements, but basically they end up with a string of bits that they can both agree on.