I'm trying to understand how Y-axis rotations are represented in ZX Calculus. In the paper, wikipedia, everywhere I look, it's as if there is no such thing as Y-axis rotations, only X and Z.
I understand that I can translate a Y-axis rotation to X and Z, for example (using qiskit):
circuit = QuantumCircuit(1) circuit.ry(pi/2, 0)
is equivalent to
circuit = QuantumCircuit(1) circuit.rx(pi/2, 0) circuit.rz(pi/2, 0) circuit.rx(-pi/2, 0)
and the ZX-diagram looks like this
But I don't understand, if ZX calculus doesn't allow Y-axis rotations, is it stated anywhere? What is the logic. Thanks.