diff --git a/cirq-core/cirq/ops/common_gates.py b/cirq-core/cirq/ops/common_gates.py index dfbe1716622..a8131613942 100644 --- a/cirq-core/cirq/ops/common_gates.py +++ b/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: