Support parsing QASM `gphase` gates

[natestemen]

Is your feature request related to a use case or problem?

Other quantum SDKs output gphase gates as part of QASM output, and converting those to cirq is a critical step in mitiq's functionality. As of the 0.44.0 release of pennylane, gphase gates are added to qasm output (PennyLaneAI/pennylane#8809), which break mitiq functionality due to the inability to convert to cirq.

Describe the solution you would prefer

circuit_from_qasm supports qasm strings with gphase gates. E.g. to be able to run the following code:

from cirq.contrib.qasm_import import circuit_from_qasm

qasm = """
OPENQASM 2.0;
include "qelib1.inc";
qreg q[1];

x q[0];
gphase(pi/4) q[0];
"""

circuit = circuit_from_qasm(qasm)

Currently I get

QasmException: Unknown gate "gphase" at line 7

How urgent is this for you? Is it blocking important work?

P1 – I need this no later than the next release
(this feels very demanding, but P2 was too lax. We will find workarounds if this is feature is not possible or desired.)

[ACE07-Sev]

I get the following error with qiskit as well

from qiskit import QuantumCircuit

qasm = """
OPENQASM 2.0;
include "qelib1.inc";
qreg q[1];

x q[0];
gphase(pi/4) q[0];
"""

qc = QuantumCircuit.from_qasm_str(qasm)
qc.draw('mpl')

---------------------------------------------------------------------------
QASM2ParseError                           Traceback (most recent call last)
Cell In[1], [line 12](vscode-notebook-cell:?execution_count=1&line=12)
      1 from qiskit import QuantumCircuit
      3 qasm = """
      4 OPENQASM 2.0;
      5 include "qelib1.inc";
   (...)      9 gphase(pi/4) q[0];
     10 """
---> [12](vscode-notebook-cell:?execution_count=1&line=12) qc = QuantumCircuit.from_qasm_str(qasm)
     13 qc.draw('mpl')

File c:\Users\User\Documents\GitHub\.venv\Lib\site-packages\qiskit\circuit\quantumcircuit.py:4764, in QuantumCircuit.from_qasm_str(qasm_str)
   4761 # pylint: disable=cyclic-import
   4762 from qiskit import qasm2
-> [4764](file:///C:/Users/User/Documents/GitHub/.venv/Lib/site-packages/qiskit/circuit/quantumcircuit.py:4764) return qasm2.loads(
   4765     qasm_str,
   4766     include_path=qasm2.LEGACY_INCLUDE_PATH,
   4767     custom_instructions=qasm2.LEGACY_CUSTOM_INSTRUCTIONS,
   4768     custom_classical=qasm2.LEGACY_CUSTOM_CLASSICAL,
   4769     strict=False,
   4770 )

File c:\Users\User\Documents\GitHub\.venv\Lib\site-packages\qiskit\qasm2\__init__.py:602, in loads(string, include_path, custom_instructions, custom_classical, strict)
    586 """Parse an OpenQASM 2 program from a string into a :class:`.QuantumCircuit`.
    587 
    588 Args:
   (...)    599     A circuit object representing the same OpenQASM 2 program.
    600 """
    601 custom_instructions = list(custom_instructions)
--> [602](file:///C:/Users/User/Documents/GitHub/.venv/Lib/site-packages/qiskit/qasm2/__init__.py:602) return _parse.from_bytecode(
    603     _qasm2.bytecode_from_string(
    604         string,
    605         [_normalize_path(path) for path in include_path],
    606         [
    607             _qasm2.CustomInstruction(x.name, x.num_params, x.num_qubits, x.builtin)
    608             for x in custom_instructions
    609         ],
    610         tuple(custom_classical),
    611         strict,
    612     ),
    613     custom_instructions,
    614 )

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

QASM2ParseError: "<input>:7,0: 'gphase' is not defined in this scope"

[ACE07-Sev]

https://openqasm.com/language/gates.html

[ACE07-Sev]

Thank you, I'll work on adding this ASAP @natestemen .