qp.transforms.combine_global_phases

combine_global_phases(tape)

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

This transform returns a new circuit where all qp.GlobalPhase gates in the original circuit (if exists) are removed, and a new qp.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 qp.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 = qp.device("default.qubit", wires=3)

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

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

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

Usage with qjit

There are two key differences to note when using combine_global_phases with qjit:

• combine_global_phases must be applied to a QNode. Quantum functions are not supported as input.

• When used with qjit, the combine_global_phases compilation pass will merge operations surrounding control flow together, while those within the control flow are merged together separately (i.e., no formal loop-boundary optimizations).

Consider the following example:

import pennylane as qp

n = 3
dev = qp.device('null.qubit', wires=n)

@qp.qjit(keep_intermediate=True, capture=True)
@qp.transforms.combine_global_phases
@qp.qnode(dev)
def circuit():
    qp.GlobalPhase(0.1, wires = 2)
    qp.X(n-1)
    qp.GlobalPhase(0.1, wires = 1)
    qp.H(n-2)

    @qp.for_loop(0, 2)
    def loop(i):
        qp.GlobalPhase(0.1967, wires=i)
        qp.GlobalPhase(0.7691, wires=i)

    loop()

    qp.GlobalPhase(0.1, wires=0)
    qp.GlobalPhase(0.1, wires=0)

    return qp.expval(qp.Z(0))

The two GlobalPhase operations within the for_loop context will be merged together. However, they will not be merged together with the GlobalPhase operations that occur before and after the for_loop.

This behaviour is shown in the image below, where the application of combine_global_phases results in two GlobalPhase instances (one inside of a for_loop and the other from the GlobalPhase instances outside of the for_loop).

>>> qp.specs(circuit, level="device")().resources.gate_counts
{'GlobalPhase': 3, 'Hadamard': 1, 'PauliX': 1}
>>> print(qp.draw_graph(circuit)()) 

code/api/pennylane.transforms.combine_global_phases

 

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