Incorrect Optimization of Swap and Measure Instructions #11195
Description
Environment
- Qiskit Terra version: 0.45.0
- Python version: 3.10.9
- Operating system: MacOS 13.5 Ventura
What is happening?
Hello, The qiskit.transpile function at optimization_level=3 incorrectly optimizes circuits involving swap and measure instructions. This leads to incorrect possible outcomes when the circuit is executed.
How can we reproduce the issue?
The issue can be replicated using the following Python code:
import qiskit
from qiskit import QuantumCircuit, transpile, Aer
bug_qasm = """OPENQASM 2.0;
include "qelib1.inc";
qreg q[3];
creg c[3];
z q[0];
sx q[0];
swap q[0],q[1];
measure q[1] -> c[2];
measure q -> c;
"""
simulator = Aer.get_backend("aer_simulator")
circ = QuantumCircuit.from_qasm_str(bug_qasm)
original = transpile(circ, backend=simulator, optimization_level=0)
optimized = transpile(circ, backend=simulator, optimization_level=3)
orig_count = simulator.run(original, shots=100000).result().get_counts()
trans_count = simulator.run(optimized, shots=100000).result().get_counts()
keys = set(orig_count.keys()).union(trans_count.keys())
for k in sorted(list(keys)):
print(f"{k}: {orig_count.get(k, 0):>5} {trans_count.get(k, 0):>5}")The possible outcomes differ between the original and optimized circuits:
000: 50006 49660
010: 49994 0
100: 0 50340
What should happen?
The optimized circuit should yield only 000 and 010 outcomes, consistent with the original circuit.
Any suggestions?
The potential source of the bug could be in
qiskit/transpiler/passes/optimization/optimize_swap_before_measure