No qudit Y gate (quantumlib#5814)

Author: viathor

cirq-core/cirq/ops/common_gates.py

@@ -389,6 +389,16 @@ class YPowGate(eigen_gate.EigenGate):
     parameter when initializing.
 
     `cirq.Y`, the Pauli Y gate, is an instance of this gate at `exponent=1`.
+
+    Unlike `cirq.XPowGate` and `cirq.ZPowGate`, this gate has no generalization
+    to qudits and hence does not take the dimension argument. Ignoring the
+    global phase all generalized Pauli operators on a d-level system may be
+    written as X**a Z**b for a,b=0,1,...,d-1. For a qubit, there is only one
+    "mixed" operator: XZ, conventionally denoted -iY. However, when d > 2 there
+    are (d-1)*(d-1) > 1 such "mixed" operators (still ignoring the global phase).
+    Due to this ambiguity, qudit Y gate is not well defined. The "mixed" operators
+    for qudits are generally not referred to by name, but instead are specified in
+    terms of X and Z.
     """
 
     def _num_qubits_(self) -> int: