QASM Exporter does not Define cs gate

[MattePalte]

Expected Behavior

The quantum circuit should be successfully converted from Qiskit to BQSKit, exported to QASM, and re-imported using Qiskit’s QASM parser without errors. The circuit should maintain its integrity throughout the process.

Actual Behavior

The exported QASM file fails to be re-imported into Qiskit, raising a QASM2ParseError with the message "<input>:4,0: 'cs' is not defined in this scope". This indicates that the custom gate cs defined in the original QASM file is not recognized during the re-import process.

Additional Information

The issue appears to be related to the handling of custom gate definitions during the export/import process. The original QASM file explicitly defines the cs gate, but the exported QASM file from BQSKit does not retain this definition, leading to a parse error during re-import. The issue reproduces 100% of the time when following the provided steps.

Source Code

#!/usr/bin/env python
# coding: utf-8

from bqskit.ext import qiskit_to_bqskit
from qiskit import qasm2

qasm_file = """
OPENQASM 2.0;
include "qelib1.inc";
gate cs q0,q1 { p(pi/4) q0; cx q0,q1; p(-pi/4) q1; cx q0,q1; p(pi/4) q1; }
qreg q[5];
cs q[4],q[0];
"""

# Load the QASM into a Qiskit circuit
qc = qasm2.loads(
    qasm_file, custom_instructions=qasm2.LEGACY_CUSTOM_INSTRUCTIONS)
qc.draw()

# OUTPUT:
#      ┌─────┐
# q_0: ┤1    ├
#      │     │
# q_1: ┤     ├
#      │     │
# q_2: ┤  Cs ├
#      │     │
# q_3: ┤     ├
#      │     │
# q_4: ┤0    ├
#      └─────┘

# Convert Qiskit circuit to BQSKit circuit
bqskit_circuit = qiskit_to_bqskit(qc)
print("BQSKit Circuit:")
print(bqskit_circuit)

# Export the BQSKit circuit to QASM
bqskit_qasm_path = "bqskit_circuit.qasm"
bqskit_circuit.save(bqskit_qasm_path)
print(f"BQSKit QASM file saved at: {bqskit_qasm_path}")

# OUTPUT:
# BQSKit Circuit:
# Circuit(5)[CSGate@(4, 0)]
# BQSKit QASM file saved at: bqskit_circuit.qasm
# OPENQASM 2.0;
# include "qelib1.inc";
# qreg q[5];
# cs q[4], q[0];


# Read the exported QASM file and attempt to re-import into Qiskit
with open(bqskit_qasm_path, 'r') as file:
    bqskit_qasm = file.read()
    qc_read = qasm2.loads(
        bqskit_qasm, custom_instructions=qasm2.LEGACY_CUSTOM_INSTRUCTIONS)

Tracebacks

# OUTPUT:
# QASM2ParseError                           Traceback (most recent call last)
# ...
# QASM2ParseError: "<input>:4,0: 'cs' is not defined in this scope"

This traceback suggests that the custom gate cs is not included or properly handled in the exported QASM file.

Complete trace:

QASM2ParseError                           Traceback (most recent call last)
Cell In[15], line 16
     14 with open(bqskit_qasm_path, 'r') as file:
     15     bqskit_qasm = file.read()
---> 16     qc_read = qasm2.loads(bqskit_qasm, custom_instructions=qasm2.LEGACY_CUSTOM_INSTRUCTIONS)

File ~/python3.10/site-packages/qiskit/qasm2/__init__.py:587, in loads(string, include_path, custom_instructions, custom_classical, strict)
    571 """Parse an OpenQASM 2 program from a string into a :class:`.QuantumCircuit`.
    572
    573 Args:
   (...)
    584     A circuit object representing the same OpenQASM 2 program.
    585 """
    586 custom_instructions = list(custom_instructions)
--> 587 return _parse.from_bytecode(
    588     _qasm2.bytecode_from_string(
    589         string,
    590         [_normalize_path(path) for path in include_path],
    591         [
    592             _qasm2.CustomInstruction(x.name, x.num_params, x.num_qubits, x.builtin)
    593             for x in custom_instructions
    594         ],
    595         tuple(custom_classical),
    596         strict,
    597     ),
    598     custom_instructions,
    599 )

File ~/python3.10/site-packages/qiskit/qasm2/parse.py:243, in from_bytecode(bytecode, custom_instructions)
    240 # Pull this out as an explicit iterator so we can manually advance the loop in `DeclareGate`
    241 # contexts easily.
    242 bc = iter(bytecode)
--> 243 for op in bc:
    244     # We have to check `op.opcode` so many times, it's worth pulling out the extra attribute
    245     # access.  We should check the opcodes in order of their likelihood to be in the OQ2 program
    246     # for speed.  Gate applications are by far the most common for long programs.  This function
    247     # is deliberately long and does not use hashmaps or function lookups for speed in
    248     # Python-space.
    249     opcode = op.opcode
    250     # `OpCode` is an `enum` in Rust, but its instances don't have the same singleton property as
    251     # Python `enum.Enum` objects.

QASM2ParseError: "<input>:4,0: 'cs' is not defined in this scope"

Let me know if you need any further help or information from my side.
Thanks in advance

[edyounis]

BQSKit should handle this fine. Can you try loading the qasm directly into BQSKit?