Qubit Conspiracy Theories

Chapters 2 and 3 explained measuring individual qubits. You can send a qubit along a wire, measure the qubit, and add this measurement to your tally of results. That’s fine for individual qubits. But in computing and life, things are more interesting when you start combining elements. So, in this chapter, we’ll see what happens when one qubit interacts with another qubit.

We start by examining gates in which the state of one qubit depends on the state of another. It’s like writing the following informal, English-language code:

if q0 is 0    set q1 to 0
else
    set q1 to 1

Since the state of q0 can be somewhere between |0⟩ and |1⟩, the end result for q1 may be between |0⟩ and |1⟩. What’s more, q0 and q1 may become mysteriously linked across time and space. They’re like identical twins who are separated at birth. When one twin stubs her toe, the other...