The problem of sending messages securely has troubled humankind since the dawn of civilisation and probably before.
In recent years, however, physicists have raised expectations that this problem has been solved by the invention of quantum key distribution. This exploits the strange quantum property of entanglement to guarantee the secrecy of a message.
Entanglement is so fragile that any eavesdropper cannot help but break it, revealing the ruse. So cryptographers can use it to send a secure key called a one time pad that can then be used to encrypt a message. If the key is intercepted, the sender simply sends another and repeats this until one gets through.
So-called quantum key distribution is unconditionally secure--it offers perfect secrecy guaranteed by the laws of physics.
Or at least that's what everyone thought. More recently, various groups have begun to focus on a fly in the ointment: the practical implementation of this process. While quantum key distribution offers perfect security in practice, the devices used to send quantum messages are inevitably imperfect.
For example, lasers that are supposed to send one photon at a time can sometimes send several and this allows information to leak to an eavesdropper.
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