25.2 Quantum Key Distribution

Now that we have set up some of the ideas behind quantum mechanics, we can use them to describe a technique for distributing bits through a quantum channel. These bits can be used to establish a key that can be used for communicating across a classical channel, or any other shared secret.

We begin by describing a quantum bit. Start with a two-dimensional complex vector space. Choose a pair of orthogonal vectors of length 1; call them |0 and |1. For example, these two vectors could be either of the two pairs of orthogonal vectors used in the previous section. A quantum bit, also known as a qubit, is a unit vector in this vector space. For the purposes of the present discussion, we can think of a qubit as a polarized ...

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