qml.grouping.optimize_measurements

optimize_measurements(observables, coefficients=None, grouping='qwc', colouring_method='rlf')[source]

Partitions then diagonalizes a list of Pauli words, facilitating simultaneous measurement of all observables within a partition.

The input list of observables are partitioned into mutually qubit-wise commuting (QWC) or mutually commuting partitions by approximately solving minimum clique cover on a graph where each observable represents a vertex. The unitaries which diagonalize the partitions are then found. See arXiv:1907.03358 and arXiv:1907.09386 for technical details of the QWC and fully-commuting measurement-partitioning approaches respectively.

Parameters
  • observables (list[Observable]) – a list of Pauli words (Pauli operation instances and Tensor instances thereof)

  • coefficients (list[float]) – a list of float coefficients, for instance the weights of the Pauli words comprising a Hamiltonian

  • grouping (str) – the binary symmetric relation to use for operator partitioning

  • colouring_method (str) – the graph-colouring heuristic to use in obtaining the operator partitions

Returns

  • list[callable]: a list of the post-rotation templates, one for each partition

  • list[list[Observable]]: A list of the obtained groupings. Each grouping is itself a list of Pauli words diagonal in the measurement basis.

  • list[list[float]]: A list of coefficient groupings. Each coefficient grouping is itself a list of the partitions corresponding coefficients. Only output if coefficients are specified.

Return type

tuple

Example

>>> obs = [qml.PauliY(0), qml.PauliX(0) @ qml.PauliX(1), qml.PauliZ(1)]
>>> coeffs = [1.43, 4.21, 0.97]
>>> rotations, groupings, grouped_coeffs = optimize_measurements(obs, coeffs, 'qwc', 'rlf')
>>> print(rotations)
[[RY(-1.5707963267948966, wires=[0]), RY(-1.5707963267948966, wires=[1])],
 [RX(1.5707963267948966, wires=[0])]]
>>> print(groupings)
[[PauliZ(wires=[0]) @ PauliZ(wires=[1])], [PauliZ(wires=[0]), PauliZ(wires=[1])]]
>>> print(grouped_coeffs)
[[4.21], [1.43, 0.97]]