Last time we left off with the tantalizing question: how do you do a quantum “AND” operation on two qubits? In this post we’ll see why the tensor product is the natural mathematical way to represent the joint state of multiple qubits. Then we’ll define some basic quantum gates, and present the definition of a quantum circuit. Working with Multiple Qubits In a classical system, if you have two bits with values $ b_1, b_2$, then the “joint state” of the two bits is given by the concatenated string...