Bugfix in _strat_has_stabilizer_effect_from_decompose

[tanujkhattar]

Fixes quantumlib/Qualtran#665

@NoureldinYosri Do you remember why you added the following check?

    qid_shape = qid_shape_protocol.qid_shape(val, default=None)
    if qid_shape is None or len(qid_shape) <= 3:
        return None

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (6a40da5) 97.82% compared to head (fb3c062) 97.82%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6467   +/-   ##
=======================================
  Coverage   97.82%   97.82%           
=======================================
  Files        1115     1115           
  Lines       97448    97452    +4     
=======================================
+ Hits        95327    95331    +4     
  Misses       2121     2121           

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[NoureldinYosri]

to stop the recursion. if it has 3 or less qubits then the from_unitary strategy should have caught it

Cirq/cirq-core/cirq/protocols/has_stabilizer_effect_protocol.py

Lines 78 to 85 in 6a40da5

	if (
	qid_shape is None
	or len(qid_shape) > 3
	or qid_shape != (2,) * len(qid_shape)
	or not has_unitary_protocol.has_unitary(val)
	):
	return None
	unitary = unitary_protocol.unitary(val)

if it outputs the wrong result then the bug will be there not here.

is there an example of this producing the wrong result?

[tanujkhattar]

is there an example of this producing the wrong result?

The bug was in decompose_protocol.decompose_once(val). That works only when val knows the qubits it acts upon (so operations, moments, circuits) but fails for gates. _try_decompose_into_operations_and_qubits works in the general case. I've added a tests that was failing earlier but passes now

to stop the recursion

I think the check is redundant, because the control flow would reach the decomposition strategy only when it couldn't figure out the answer using the first 3 strategies and that would imply a scenario where, for example, the operation decomposes into non unitary operations (eg: measurements + cliffords) and therefore we should continue to check decomposition. An example in the test shows a scenario where _unitary_ is not defined but decomposition and _has_stabilizer_effect_ is.

[tanujkhattar]

Let's wait and not merge right now, I need to run some more tests