cirq.MergeInteractions not canceling out single qubit gates for a circuit with two cirq.CZ layers

[vtomole]

How to reproduce the issue

import cirq
import sys

print(f"Python version {sys.version}")
print(f"Cirq version {cirq.__version__}")
qubits = cirq.LineQubit.range(2)
circuit = cirq.Circuit(cirq.CZ(*qubits), cirq.CZ(*qubits))
print(circuit)
print("\n--------------------\n")
cirq.MergeInteractions().optimize_circuit(circuit)
print(circuit)

Python version 3.8.10 (default, Nov 26 2021, 20:14:08) 
[GCC 9.3.0]
Cirq version 0.14.0.dev20220105215702
0: ───@───@───
      │   │
1: ───@───@───

--------------------

0: ───S───S^-1───

S and S^-1 should cancel out; leading to an empty circuit.

[MichaelBroughton]

cc: @tanujkhattar to xref with new Transformers implementation.

[tanujkhattar]

You can achieve your desired behavior via

 def post_clean_up(ops):
     c = cirq.Circuit(ops)
     cirq.merge_single_qubit_gates_into_phxz(c)
     return c.all_operations()

 cirq.MergeInteractions(post_clean_up=post_clean_up).optimize_circuit(circuit)

[vtomole]

@tanujkhattar Thanks! Do we want to keep this open for your transformer work or should we open a new issue? Maybe you already have a note of this so we don't need an issue about it.

[tanujkhattar]

Opened #4831 to track the specific issue and drive a broader discussion on differences in the decomposer APIs.

[vtomole]

👍