optimize_for_target_gateset returns a non-optimal circuit #6422
Description
Using cirq.optimize_for_target_gateset(circuit, gateset=cirq.CZTargetGateset()) returns a circuit with more moments. First, it doesn't combine the multiple single qubit rotations into a single PhasedXZGate. Second, it distributes the operations of an optimal moment to make it non-optimal
How to reproduce the issue
circ = cirq.Circuit([cirq.Moment(
cirq.X(cirq.LineQubit(1)),
cirq.X(cirq.LineQubit(2)),
cirq.X(cirq.LineQubit(3)),
cirq.X(cirq.LineQubit(6)),
),
cirq.Moment(
cirq.H(cirq.LineQubit(0)),
cirq.H(cirq.LineQubit(1)),
cirq.H(cirq.LineQubit(2)),
cirq.H(cirq.LineQubit(3)),
cirq.H(cirq.LineQubit(4)),
cirq.H(cirq.LineQubit(5)),
cirq.H(cirq.LineQubit(6)),
),
cirq.Moment(
cirq.H(cirq.LineQubit(1)),
cirq.H(cirq.LineQubit(3)),
cirq.H(cirq.LineQubit(5)),
),
cirq.Moment(
cirq.CZ(cirq.LineQubit(0), cirq.LineQubit(1)),
cirq.CZ(cirq.LineQubit(2), cirq.LineQubit(3)),
cirq.CZ(cirq.LineQubit(4), cirq.LineQubit(5)),
),
cirq.Moment(
cirq.CZ(cirq.LineQubit(2), cirq.LineQubit(1)),
cirq.CZ(cirq.LineQubit(4), cirq.LineQubit(3)),
cirq.CZ(cirq.LineQubit(6), cirq.LineQubit(5)))])
circ
print(circ)
print("number of moments = ", len(circ),"\n")
circ_opt = cirq.optimize_for_target_gateset(circuit=circ,gateset=gateset)
print(circ_opt)
print("number of moments = ", len(circ_opt))
0: ───────H───────@───────
│
1: ───X───H───H───@───@───
│
2: ───X───H───────@───@───
│
3: ───X───H───H───@───@───
│
4: ───────H───────@───@───
│
5: ───────H───H───@───@───
│
6: ───X───H───────────@───
number of moments = 5
0: ───PhXZ(a=-0.5,x=0.5,z=-1)────────────────────────────@────────────────────────────────────────────────────
│
1: ───PhXZ(a=-0.5,x=0.5,z=0)────PhXZ(a=0.5,x=-0.5,z=1)───@───@────────────────────────────────────────────────
│
2: ───PhXZ(a=-0.5,x=0.5,z=0)─────────────────────────────@───@────────────────────────────────────────────────
│
3: ───PhXZ(a=-0.5,x=0.5,z=0)────PhXZ(a=0.5,x=-0.5,z=1)───@───@────────────────────────────────────────────────
│
4: ───PhXZ(a=-0.5,x=0.5,z=-1)───@────────────────────────────@────────────────────────────────────────────────
│
5: ─────────────────────────────@─────────────────────────────────────────────────────────────────────────@───
│
6: ──────────────────────────────────────────────────────────PhXZ(a=0,x=1,z=0)───PhXZ(a=0.5,x=-0.5,z=1)───@───
number of moments = 6
I find that using cirq.merge_single_qubit_moments_to_phxz(circ) first fixes the issue.
Cirq version
1.2.0.dev20230628175157