Compilation increases length of chains of conditional single-qubit gates

[cqc-alec]

If we start with the circuit

c = Circuit(2, 1).CZ(0, 1, condition=Bit(0))

and compile it at optimization level 1 or 2, we obtain a circuit containing 7 Rz, 8 PhasedX and 1 ZZPhase gate (all conditional).

If we repeat the compilation, we get 12 Rz, 16 PhasedX and 1 ZZPhase.

If we repeat the compilation again, we get 17 Rz, 24 PhasedX and 1 ZZPhase ...

Optimization levels 0 and 3 both produce the expected counts (2 Rz, 2 PhasedX and 1 ZZPhase).

[cqc-alec]

Apparently caused by CQCL/tket#1723 .