qml.transforms.combine_global_phases

combine_global_phases(tape)

Combine all qml.GlobalPhase gates into a single qml.GlobalPhase operation.

This transform returns a new circuit where all qml.GlobalPhase gates in the original circuit (if exists) are removed, and a new qml.GlobalPhase is added at the end of the list of operations with its phase being a total global phase computed as the algebraic sum of all global phases in the original circuit.

Parameters

tape (QNode or QuantumScript or Callable) – the input circuit to be transformed.

Returns

the transformed circuit as described in qml.transform.

Return type

qnode (QNode) or quantum function (Callable) or tuple[List[QuantumScript], function]

Example

Suppose we want to combine all the global phase gates in a given quantum circuit. The combine_global_phases transform can be used to do this as follows:

dev = qml.device("default.qubit", wires=3)

@qml.transforms.combine_global_phases
@qml.qnode(dev)
def circuit():
    qml.GlobalPhase(0.3, wires=0)
    qml.PauliY(wires=0)
    qml.Hadamard(wires=1)
    qml.CNOT(wires=(1,2))
    qml.GlobalPhase(0.46, wires=2)
    return qml.expval(qml.X(0) @ qml.Z(1))

To check the result, let’s print out the circuit:

>>> print(qml.draw(circuit)())
0: ──Y─────GlobalPhase(0.76)─┤ ╭<X@Z>
1: ──H─╭●──GlobalPhase(0.76)─┤ ╰<X@Z>
2: ────╰X──GlobalPhase(0.76)─┤

code/api/pennylane.transforms.combine_global_phases

 

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