[BUG] Unsupported Controlled QASM operations

[MTakahashi-KWH]

Environment

• PyQASM version: 0.3.2
• Python version: 3.12.8
• Operating system: Windows

What happened?

This may be a feature request, but it looks like a partially stubbed implementation based on #32, #121, and #125.

When attempting to define and run a custom gate with multiple controls:

gate Z_on_two3 aa, ab, ac {
    x ac;
    ctrl(2) @ z aa, ab, ac;
    x ac;
}
ctrl@ gate Z_on_two3 qb[0], qb[1], qb[2];

I encounter the following error:

ValidationError: Unsupported controlled QASM operation: z with 3 controls

Source

The error originates from map_qasm_ctrl_op_to_callable, which currently contains the following comment:

# TODO: decompose controls if not built in

Expected Behavior(?)

Support for decomposing multi-controlled operations (e.g., z with 3+ controls), or clarification on whether this is intentionally unsupported.

Suggestions (Optional)

IDK if this is a semantic decomposition or needs full function decomposition to base gate ie through Solavay Kiteav or Cartan. Id assume it can be left to a semantic decomp as openqasm parsing itself accepts gate modifiers directly. LMK future development intent in this category

[TheGupta2012]

I think this is not supported at the moment as multi-controlled gate decomposition is not natively available in pyqasm.

As far as I understand, we do not support decompositions from arbitrary controlled circuits on our basic gates such as c(n) <gate>.

It is okay as long as the final decomposition contains gates from the CTRL_GATE_MAP eg -

In [38]: import pyqasm

In [39]: qasm = '''
    ...: OPENQASM 3;
    ...: include "stdgates.inc";
    ...: 
    ...: qubit[5] qb;
    ...: gate Z_on_two3 aa, ab, ac {
    ...:     x ac;
    ...:     z aa;
    ...:     x ac;
    ...: }
    ...: ctrl(1) @ Z_on_two3 qb[0], qb[1], qb[2], qb[3];
    ...: '''

In [40]: mod = pyqasm.loads(qasm)

In [41]: mod.unroll()

In [42]: print(mod)
OPENQASM 3.0;
include "stdgates.inc";
qubit[5] qb;
cx qb[0], qb[3];
cz qb[0], qb[1];
cx qb[0], qb[3];

[TheGupta2012]

But I think we can definitely have a feature to support the CX(n) decompositions