The idea of interacting via the quantum realm is as appealing as it is strange. Essentially, it involves communication by pure energy. Photons—light particles—can take on one of four polarizations or states. Such relatively simple changes of state can be exploited to convey one of four pieces of information (e.g., one state signifies “1,” another “0,” etc.) If you string a bunch of photons together, a relatively complicated message, like the key to a code, can be conveyed.

What makes quantum communication perfect for cryptography is Heisenberg’s Uncertainty Principle. The principle states that the observation of a quantum system necessarily changes that system. It is an extremely odd, but nevertheless fundamental property of photons that when unobserved they exist in a state of undecidedness. When they are observed, however, the very act of observation forces them to assume a well-defined state, which in this case could be a specific polarization. This means that the spied-upon photons will carry an indelible record of any third-party observation. By comparing a small sample of the received information with the sender, the intended recipient of the key can determine if the state of the photons has been significantly altered. They may also find that the structure of the message has been changed, because it’s not possible for an eavesdropper to intercept a quantum transmission channel without absorbing some of the photons.

In fact, the mark left by an eavesdropper is proportional to the amount they have eavesdropped. So, even though quantum cryptography can’t ensure that someone won’t try to spy on you, it does mean that the recipient of your key can always tell how much a third party has been listening in. And once you know that someone has been eavesdropping enough to determine what the key is, you can pick another key and try again.