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catalyst.passes.diagonalize_measurements

diagonalize_measurements(qnode=None, supported_base_obs: tuple[str, ...] = ('PauliZ', 'Identity'), to_eigvals: bool = False)[source]

Specify that the diagonalize-final-measurements compiler pass will be applied, which diagonalizes measurements into the standard basis.

Parameters:

  • qnode (QNode) – The QNode to apply the diagonalize_final_measurement compiler pass to.

  • supported_base_obs (tuple[str, ...]) – A list of supported base observable names. Allowed observables are PauliX, PauliY, PauliZ, Hadamard and Identity. PauliZ and Identity are always treated as supported, regardless of input. Defaults to (PauliZ, Identity).

  • to_eigvals (bool) – Whether the diagonalization should create measurements using eigenvalues and wires rather than observables. Defaults to False.

Returns:

QNode

Note

Unlike the PennyLane tape transform, pennylane.transforms.diagonalize_measurements(), the QNode itself will not be changed or transformed by applying this decorator.

Unlike the PennyLane tape transform, supported_base_obs here only accepts a tuple of supported base observable names, instead of the corresponding classes. The reason is that xDSL does not accept class types as values of option-elements. For more details, please refer to the xDSL repo.

Unlike the PennyLane tape transform, only to_eigvals = False is supported. Setting to_eigvals as True will raise an error.

An error will be raised if non-commuting terms are encountered.

Example

The diagonalize-final-measurements compilation pass can be applied as a decorator on a QNode:

import pennylane as qml
from catalyst import qjit
from catalyst.passes import diagonalize_measurements

@qjit
@diagonalize_measurements(supported_base_obs=("PauliX",))
@qml.qnode(qml.device("lightning.qubit", wires=1))
def circuit():
    qml.Hadamard(0)
    qml.RZ(1.1, 0)
    qml.PhaseShift(0.22, 0)
    return qml.expval(qml.Y(0))

expected_substr = 'transform.apply_registered_pass "diagonalize-final-measurements" with options = {"supported-base-obs" = ["PauliX"], "to-eigvals" = false}'

>>> expected_substr in circuit.mlir
True
>>> circuit()
0.9687151001182651

An error is raised if to_eigvals=True is passed as an option:

import pennylane as qml
from catalyst import qjit
from catalyst.passes import diagonalize_measurements

@diagonalize_measurements(to_eigvals=True)
@qml.qnode(qml.device("lightning.qubit", wires=1))
def circuit():
    qml.Hadamard(0)
    qml.PhaseShift(0.22, 0)
    return qml.expval(qml.Y(0))

error_msg = None

try:
    qjit(circuit)
except ValueError as e:
    error_msg = str(e)

>>> print(error_msg)
Only to_eigvals = False is supported.

A compile error is raised if non-commuting terms are encountered:

import pennylane as qml
from pennylane.exceptions import CompileError
from catalyst import qjit
from catalyst.passes import diagonalize_measurements

@diagonalize_measurements
@qml.qnode(qml.device("lightning.qubit", wires=1))
def circuit():
    qml.Hadamard(0)
    return qml.expval(qml.Y(0) + qml.X(0))

error_msg = None

try:
    qjit(circuit)
except CompileError as e:
    error_msg = str(e)

>>> print(error_msg)
Observables are not qubit-wise commuting. Please apply the `split-non-commuting` pass first.
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