Class to add as a tag onto an Operation to denote a Physical Z operation.

By default, all Z rotations on Google devices are considered to be virtual.
When performing the Z operation, the device will update its internal phase
tracking mechanisms, essentially commuting it forwards through the circuit
until it hits a non-commuting operation (Such as a sqrt(iSwap)).
When applied to a Z rotation operation, this tag indicates to the hardware
that an actual physical operation should be done instead. This class can
only be applied to instances of cirq.ZPowGate. If applied to other gates
(such as PhasedXZGate), this class will have no effect.

Initialize self. See help(type(self)) for accurate signature.