QASM Exporter Misplaces Consecutive Operations (Breaking Data Flow)

[MattePalte]

System Information

• Operating System: Linux
• Operating System Version: Ubuntu 22.04.5 LTS
• BQSKit Version: 1.2.0

Expected behavior

I expect the BQSKit QASM exporter to correctly translate the input QASM circuit to an equivalent QASM, preserving the original circuit's semantics and structure.

Actual behavior

The actual behavior is that the BQSKit QASM exporter incorrectly translates the input QASM circuit and produces a circuit where the operations are misplaced. Specifically, given this circuit:

OPENQASM 2.0;
include "qelib1.inc";
qreg reg_1_q[5];
qreg reg_2_q[4];
creg reg_2_c[4];
measure reg_2_q[0] -> reg_2_c[0];
h reg_2_q[0];

# Circuit Diagram

reg_1_q_0: ────────

reg_1_q_1: ────────

reg_1_q_2: ────────

reg_1_q_3: ────────

reg_1_q_4: ────────
           ┌─┐┌───┐
reg_2_q_0: ┤M├┤ H ├
           └╥┘└───┘
reg_2_q_1: ─╫──────
            ║
reg_2_q_2: ─╫──────
            ║
reg_2_q_3: ─╫──────
            ║
reg_2_c: 4/═╩══════
            0

where the measurement operation (measure reg_2_q[0] -> reg_2_c[0]) is followed by a Hadamard gate (h reg_2_q[0]), the BQSKit QASM exporter produces the following output:

# PRODUCED BY THE BQSKIT QASM EXPORTER
OPENQASM 2.0;
include "qelib1.inc";
qreg q[9];
creg reg_2_c[4];
measure q[0] -> reg_2_c[0];
h q[5];

# Circuit Diagram
           ┌─┐
      q_0: ┤M├─────
           └╥┘
      q_1: ─╫──────
            ║
      q_2: ─╫──────
            ║
      q_3: ─╫──────
            ║
      q_4: ─╫──────
            ║ ┌───┐
      q_5: ─╫─┤ H ├
            ║ └───┘
      q_6: ─╫──────
            ║
      q_7: ─╫──────
            ║
      q_8: ─╫──────
            ║
reg_2_c: 4/═╩══════
            0

Where the measurement operation and the Hadamard gate are applied to different qubits.

Steps to reproduce

The problem can be reproduced with the provided code snippet using only BQSKit. The issue occurs during the QASM export process, where the operations seem to be misplaced.

• The initial QASM file operates on the same qubits, but the operations are misplaced in the output QASM produced by BQSKit.
• The percentage of reproducibility is 100%, as the issue occurs consistently in the provided code snippet.

The source code snippet that reproduces the issue is:

from bqskit import Circuit
import tempfile

qasm_content = """
OPENQASM 2.0;
include "qelib1.inc";
qreg reg_1_q[5];
qreg reg_2_q[4];
creg reg_2_c[4];
measure reg_2_q[0] -> reg_2_c[0];
h reg_2_q[0];
"""

with tempfile.NamedTemporaryFile(delete=False, suffix=".qasm") as temp_file:
    temp_file.write(qasm_content.encode())
    temp_file.flush()  # Ensure the content is written to the file
    file_path = temp_file.name
    bqskit_circuit = Circuit.from_file(file_path)
    print(bqskit_circuit)
    # OUTPUT:
    # Circuit(9)[measurement@(5,), HGate@(5,)]

# QASM export
bqskit_circuit.save("test_exported_qc_bqskit.qasm")
# OUTPUT:
# OPENQASM 2.0;
# include "qelib1.inc";
# qreg q[9];
# creg reg_2_c[4];
# measure q[0] -> reg_2_c[0];
# h q[5];

The problem could be related to the way the BQSKit QASM exporter processes the input QASM file, either creating a corrupted internal representation (not visible when printing), or it could be a bug in the QASM export process.

Let me know if you need any additional information or if you have any questions.
Thanks in advance.