7.1 Introducing quantum bits

If you have seen descriptions of qubits elsewhere, you may have read something like “a qubit implements a two-state quantum mechanical system and is the quantum analog of a classical bit.” As we saw in section 2.1, a bit also has two states, 0 and 1.

Those other discussions usually include one or more of the following: light switches, spinning electrons, polarized light, and rotating coins or donuts. These approaches have merit and are the basis for teasing apart the difference between the quantum and classical situations. The electron and polarized light examples do depict quantum systems.

Otherwise, analogies tend to be imperfect and eventually may lead you into corners where their behavior and your understanding are not consistent with the actual situation. For this reason, we developed the essential mathematics and insight to reason accurately what happens in quantum computing.

Let’s begin by thinking...