Quantum Fourier Transform | Turn lessons into working experiments

Starter code

module qft_two;

fn main() {
  let q: QReg<2> = qreg[2];

  H(q[1]);
  CP(q[0], q[1], PI / 2);
  H(q[0]);
  SWAP(q[0], q[1]);

  assert probability(|00>) > 0.2;
  assert probability(|11>) > 0.2;

  let a: Bit = measure(q[0]);
  let b: Bit = measure(q[1]);

  return (a, b);
}

Expected output

The QFT maps |00> to a uniform superposition

Each basis state lands near 25% probability

The Circuit tab shows H, CP, and SWAP

Debugging checklist

Run the circuit and open the Circuit tab

Change the CP angle and watch the assertions

Compare the state probabilities to 25% each

Replace the manual gates with use std.qft2; and qft2(q)

Explain the role of the final SWAP

Extension challenge

Compare the manual QFT with use std.qft2; then remove the SWAP and describe how the output qubit order changes.