How to add 1 + 1 using a quantum computer?

The first two qubits are the input registers, whose values may be either 0 or 1. The third qubit represents the Carry-in bit value. The fourth qubit will hold the sum result (modulo 2) and the fifth qubit holds the Carry-out bit value.

You can play around by removing some of the Pauli-X gates in the first layer and this way modifying the input value for the adder circuit.

DO NOT FORGET to uncheck the Big Endian ordering checkbox because casting the measured state to a number works when using Little Endian convention for qubit and bit ordering. You might also want to uncheck the base-2 checkbox in order to see decimal results.

If this circuit looks to you rather involved for such a simple task, you are right. The truth is that performing arithmetic is not the kind of task where a quantum computer truly shines. Classical computers are much better for doing arithmetic operations.