Some weeks ago I stumbled upon a nice paper by Garcia-Escartin and Chamorro-Posada (2013). There, they show the equivalence between a widely used circuit in quantum information, called the swap test, and a phenomena that goes under the name of Hong-Ou-Mandel effect. In their work, they rewrote the circuit of the swap test using less gates and with no ancilla qubit. I find this fact pretty interesting, and I think it’s worth sharing it with you. It gives us a new interpretation of the swap test, that is possible to prove with very simple gate’s manipulations. More specifically here we show the equivalence of the swap test and the circuit we use to measure in the Bell’s bases (an Hadamard on the first qubit and a CNOT) - the same used to create an EPR pair. Here we will work with single qubit register, but the result can be extended to register with multiple qubits. Assuming to work with one qubit register, let’s recall the original circuit of the swap test:

The probability of reading $1$ is $ \frac{1 - \braket{a | b} }{2} $ and the probability of reading $0$ is $ \frac{1 + \braket{a | b} }{2} $.

The probability of reading $1$ in the ancilla qubit of the original swap test is equal to the probability of reading $11$ in the modified version of the swap test in Figure 7.

Here is the proof. We start by rewriting the swap test as a series of controlled not operations. Note that $x \oplus x = 0 $ and that $x \oplus 0 = x$. It’s very simple to show that the swap gate can be replaced with a series of CNOTs:

We know that A NOT gate is just a $Z$ gate on another rotation axis. The rotation axis can easily be changed by two surrounding Hadamard’s gate.

The CCZ gate is pretty agnostic with respect to the target or control qubit, so we can put the $Z$ rotation on any of the control qubit.

In this circuit we note that some of the gates we are applying are actually useless for the measurement on the ancilla qubit, and we can remove them from the circuit.

Again, we use the equivalence between the $X$ gate $HZH$ gate.

We note that we could remove the ancilla qubit, and measure the other two qubits instead. This is possible thanks to the principle of deffered measurement. The probability of reading $1$ in the ancilla qubit is equivalent to the probability of reading $1$ in both the qubit $\ket{a}$ and $\ket{b}$. We don’t mind measuring the two qubit since after measuring the ancilla qubit we cannot use $\ket{a}$ and $\ket{b}$ nevertheless. So here we get the final circuit.

This equivalence might be useful when we need to optimize a circuit and we have to reduce both the number of gates and the number of ancilla qubit. This result gives us a nice intuition on the behavior of the swap test when the two qubits are entangled. But beware of remote hacking! Be sure to run the swap test only on non entangled data, otherwise you might get unexpected results! Take for example the Bell’s sates. $1$ of the four Bell’s basis for 2 qubit gates will pass the test ($\frac{\ket{01} - \ket{10}}{\sqrt{2}}$), but it’s pretty counterintuitive, since the first and the second qubit are always different. Therefore, we should use the swap test only with non-entangled data! Entanglement, along with its usefulness in quantum protocols and computation brings much troubles. Indeed it’s because of entanglement that bit commitment is not possible using quantum resources, and it’s because of entanglement that we can attack position based encryption schemes. And that’s it. Hope you enjoyed it as much as I did. To extend this equivalence to multi-qubit register, look at the paper!

Garcia-Escartin, Juan Carlos, and Pedro Chamorro-Posada. 2013. “Swap
Test and Hong-Ou-Mandel Effect Are Equivalent.” *Physical Review A* 87
(5). APS: 052330.