# A simple circuit

Here is a simple quantum computing circuit: There are two qubits (quantum bits), q and q, and two classical bits, c and c. The latter will be used to store results of measuring the former.

Read the circuit left to right.

∣0⟩ is a qubit that will always have a measurement outcome of 0 (in the computational basis).

H is a Hadamard gate that puts that ∣0⟩ into a “superposition” (a sum) of both the “basis states” ∣0⟩ and ∣1⟩. The resulting superposition will collapse to either ∣0⟩ or ∣1⟩ with equal probability when measured (again, assuming the computational basis is used).

The next items on the circuit that look like little dials with cables attached denote measurement. Qubit q is measured first and the result saved into c, then q is measured and the result is saved into c. The two qubits are unentangled, which means that measuring one has no effect on the other. (See this post for an example with entanglement.)

So basically this circuit is a fancy way to flip two coins, using quantum objects in superposition rather than metal discs. You can run it on a real quantum computer for free at IBM Quantum. I used such a circuit to decide what to do at the weekend, choosing randomly from four options. With $$n$$ qubits you can do this for $$2^n$$ options. It took about an hour to get the answer. There may be better things to do with quantum computers…