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<h2>Full example</h2>
<pre class="language-cpp"><code>// https://github.com/Qiskit/openqasm
/*
* Repeat-until-success circuit for Rz(theta),
* cos(theta-pi)=3/5, from Nielsen and Chuang, Chapter 4.
*/
OPENQASM 3;
include "stdgates.inc";
/*
* Applies identity if out is 01, 10, or 11 and a Z-rotation by
* theta + pi where cos(theta)=3/5 if out is 00.
* The 00 outcome occurs with probability 5/8.
*/
def segment qubit[2]:anc, qubit:psi -> bit[2] {
bit[2] b;
reset anc;
h anc;
ccx anc[0], anc[1], psi;
s psi;
ccx anc[0], anc[1], psi;
z psi;
h anc;
measure anc -> b;
return b;
}
qubit input;
qubit ancilla[2];
bit flags[2] = "11";
bit output;
reset input;
h input;
// braces are optional in this case
while(int(flags) != 0) {
flags = segment ancilla, input;
}
rz(pi - arccos(3 / 5)) input;
h input;
output = measure input; // should get zero
</code></pre>
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