Source code for pennylane.templates.subroutines.interferometer

# Copyright 2018-2021 Xanadu Quantum Technologies Inc.

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# you may not use this file except in compliance with the License.
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r"""
Contains the ``Interferometer`` template.
"""
from itertools import product

import pennylane as qml
from pennylane.operation import AnyWires, CVOperation

# pylint: disable-msg=too-many-branches,too-many-arguments,protected-access
from pennylane.ops import Beamsplitter, Rotation
from pennylane.wires import Wires


class Interferometer(CVOperation):
    r"""General linear interferometer, an array of beamsplitters and phase shifters.

    For :math:`M` wires, the general interferometer is specified by
    providing :math:`M(M-1)/2` transmittivity angles :math:`\theta` and the same number of
    phase angles :math:`\phi`, as well as :math:`M-1` additional rotation
    parameters :math:`\varphi`.

    By specifying the keyword argument ``mesh``, the scheme used to implement the interferometer
    may be adjusted:

    * ``mesh='rectangular'`` (default): uses the scheme described in
      `Clements et al. <https://dx.doi.org/10.1364/OPTICA.3.001460>`__, resulting in a *rectangular* array of
      :math:`M(M-1)/2` beamsplitters arranged in :math:`M` slices and ordered from left
      to right and top to bottom in each slice. The first beamsplitter acts on
      wires :math:`0` and :math:`1`:

      .. figure:: ../../_static/clements.png
          :align: center
          :width: 30%
          :target: javascript:void(0);


    * ``mesh='triangular'``: uses the scheme described in `Reck et al. <https://dx.doi.org/10.1103/PhysRevLett.73.58>`__,
      resulting in a *triangular* array of :math:`M(M-1)/2` beamsplitters arranged in
      :math:`2M-3` slices and ordered from left to right and top to bottom. The
      first and fourth beamsplitters act on wires :math:`M-1` and :math:`M`, the second
      on :math:`M-2` and :math:`M-1`, and the third on :math:`M-3` and :math:`M-2`, and
      so on.

      .. figure:: ../../_static/reck.png
          :align: center
          :width: 30%
          :target: javascript:void(0);

    In both schemes, the network of :class:`~pennylane.ops.Beamsplitter` operations is followed by
    :math:`M` local :class:`~pennylane.ops.Rotation` Operations.

    The rectangular decomposition is generally advantageous, as it has a lower
    circuit depth (:math:`M` vs :math:`2M-3`) and optical depth than the triangular
    decomposition, resulting in reduced optical loss.

    This is an example of a 4-mode interferometer with beamsplitters :math:`B` and rotations :math:`R`,
    using ``mesh='rectangular'``:

    .. figure:: ../../_static/layer_interferometer.png
        :align: center
        :width: 60%
        :target: javascript:void(0);

    .. note::

        The decomposition as formulated in `Clements et al. <https://dx.doi.org/10.1364/OPTICA.3.001460>`__ uses a different
        convention for a beamsplitter :math:`T(\theta, \phi)` than PennyLane, namely:

        .. math:: T(\theta, \phi) = BS(\theta, 0) R(\phi)

        For the universality of the decomposition, the used convention is irrelevant, but
        for a given set of angles the resulting interferometers will be different.

        If an interferometer consistent with the convention from `Clements et al. <https://dx.doi.org/10.1364/OPTICA.3.001460>`__
        is needed, the optional keyword argument ``beamsplitter='clements'`` can be specified. This
        will result in each :class:`~pennylane.ops.Beamsplitter` being preceded by a :class:`~pennylane.ops.Rotation` and
        thus increase the number of elementary operations in the circuit.

    Args:
        theta (tensor_like): size :math:`(M(M-1)/2,)` tensor of transmittivity angles :math:`\theta`
        phi (tensor_like): size :math:`(M(M-1)/2,)` tensor of phase angles :math:`\phi`
        varphi (tensor_like): size :math:`(M,)` tensor of rotation angles :math:`\varphi`
        wires (Iterable or Wires): Wires that the template acts on. Accepts an iterable of numbers or strings, or
            a Wires object.
        mesh (string): the type of mesh to use
        beamsplitter (str): if ``clements``, the beamsplitter convention from
          Clements et al. 2016 (https://dx.doi.org/10.1364/OPTICA.3.001460) is used; if ``pennylane``, the
          beamsplitter is implemented via PennyLane's ``Beamsplitter`` operation.

    Raises:
        ValueError: if inputs do not have the correct format

    Example:

        The template requires :math:`3` sets of parameters. The ``mesh`` and ``beamsplitter`` keyword arguments are optional and
        have ``'rectangular'`` and ``'pennylane'`` as default values.

        .. code-block:: python

            dev = qml.device('default.gaussian', wires=4)

            @qml.qnode(dev)
            def circuit(params):
                qml.Interferometer(*params, wires=range(4))
                return qml.expval(qml.Identity(0))

            shapes = [[6, ], [6, ], [4, ]]
            params = []
            for shape in shapes:
                params.append(np.random.random(shape))

        Using these random parameters, the resulting circuit is:

        >>> print(qml.draw(circuit, level="device")(params))
        0: ─╭BS(0.97,0.09)────────────────╭BS(0.89,0.33)──R(0.83)────────────────┤  <I>
        1: ─╰BS(0.97,0.09)─╭BS(0.94,0.05)─╰BS(0.89,0.33)─╭BS(0.92,0.27)──R(0.36)─┤
        2: ─╭BS(0.78,0.20)─╰BS(0.94,0.05)─╭BS(0.60,0.39)─╰BS(0.92,0.27)──R(0.28)─┤
        3: ─╰BS(0.78,0.20)────────────────╰BS(0.60,0.39)──R(0.54)────────────────┤

        Using different values for optional arguments:

        .. code-block:: python

            @qml.qnode(dev)
            def circuit(params):
                qml.Interferometer(*params, wires=range(4), mesh='triangular', beamsplitter='clements')
                return qml.expval(qml.Identity(0))

            shapes = [[6, ], [6, ], [4, ]]
            params = []
            for shape in shapes:
                params.append(np.random.random(shape))

            print(qml.draw(circuit, level="device")(params))

        .. code-block::

            0: ──R(0.71)───────────────────────────────╭BS(0.07,0.00)──R(0.36)──────────────────────────────
            1: ──R(0.82)────────────────╭BS(0.80,0.00)─╰BS(0.07,0.00)──R(0.77)───────╭BS(0.77,0.00)──R(0.12)
            2: ──R(0.01)─╭BS(0.14,0.00)─╰BS(0.80,0.00)──R(0.73)───────╭BS(0.99,0.00)─╰BS(0.77,0.00)──R(0.07)
            3: ──────────╰BS(0.14,0.00)───────────────────────────────╰BS(0.99,0.00)────────────────────────

            ──────────────────────────┤  <I>
            ──────────────────────────┤
            ──╭BS(0.20,0.00)──R(0.86)─┤
            ──╰BS(0.20,0.00)──R(0.62)─┤
    """

    num_wires = AnyWires
    grad_method = None

    def __init__(
        self,
        theta,
        phi,
        varphi,
        wires,
        mesh="rectangular",
        beamsplitter="pennylane",
        id=None,
    ):
        wires = Wires(wires)

        n_wires = len(wires)
        shape_theta_phi = n_wires * (n_wires - 1) // 2

        shape = qml.math.shape(theta)
        if shape != (shape_theta_phi,):
            raise ValueError(f"Theta must be of shape {(shape_theta_phi,)}; got {shape}.")

        shape = qml.math.shape(phi)
        if shape != (shape_theta_phi,):
            raise ValueError(f"Phi must be of shape {(shape_theta_phi,)}; got {shape}.")

        shape_varphi = qml.math.shape(varphi)
        if shape_varphi != (n_wires,):
            raise ValueError(f"Varphi must be of shape {(n_wires,)}; got {shape_varphi}.")

        self._hyperparameters = {
            "mesh": mesh,
            "beamsplitter": beamsplitter,
        }
        super().__init__(theta, phi, varphi, wires=wires, id=id)

    @staticmethod
    def compute_decomposition(
        theta, phi, varphi, wires, mesh, beamsplitter
    ):  # pylint: disable=arguments-differ
        r"""Representation of the operator as a product of other operators.

        .. math:: O = O_1 O_2 \dots O_n.



        .. seealso:: :meth:`~.Interferometer.decomposition`.

        Args:
            theta (tensor_like): size :math:`(M(M-1)/2,)` tensor of transmittivity angles
            phi (tensor_like): size :math:`(M(M-1)/2,)` tensor of phase angles
            varphi (tensor_like): size :math:`(M,)` tensor of rotation angles
            wires (Any or Iterable[Any]): wires that the operator acts on
            mesh (string): the type of mesh to use
            beamsplitter (str): beamsplitter convention

        Returns:
            list[.Operator]: decomposition of the operator
        """

        wires = Wires(wires)
        M = len(wires)
        op_list = []

        if M == 1:
            # the interferometer is a single rotation
            op_list.append(Rotation(varphi[0], wires=wires[0]))
        else:
            n = 0  # keep track of free parameters

            if mesh == "rectangular":
                # Apply the Clements beamsplitter array
                # The array depth is N
                for m, (k, (w1, w2)) in product(range(M), enumerate(zip(wires[:-1], wires[1:]))):
                    # skip even or odd pairs depending on layer
                    if (m + k) % 2 != 1:
                        if beamsplitter == "clements":
                            op_list.append(Rotation(phi[n], wires=Wires(w1)))
                            op_list.append(Beamsplitter(theta[n], 0, wires=Wires([w1, w2])))
                        elif beamsplitter == "pennylane":
                            op_list.append(Beamsplitter(theta[n], phi[n], wires=Wires([w1, w2])))
                        else:
                            raise ValueError(f"did not recognize beamsplitter {beamsplitter}")
                        n += 1

            elif mesh == "triangular":
                # apply the Reck beamsplitter array
                # The array depth is 2*N-3
                for m in range(2 * M - 3):
                    for k in range(abs(m + 1 - (M - 1)), M - 1, 2):
                        if beamsplitter == "clements":
                            op_list.append(Rotation(phi[n], wires=wires[k]))
                            op_list.append(
                                Beamsplitter(theta[n], 0, wires=wires.subset([k, k + 1]))
                            )
                        elif beamsplitter == "pennylane":
                            op_list.append(
                                Beamsplitter(theta[n], phi[n], wires=wires.subset([k, k + 1]))
                            )
                        else:
                            raise ValueError(f"did not recognize beamsplitter {beamsplitter} ")
                        n += 1
            else:
                raise ValueError(f"did not recognize mesh {mesh}")

            # apply the final local phase shifts to all modes
            for i in range(qml.math.shape(varphi)[0]):
                act_on = wires[i]
                op_list.append(Rotation(varphi[i], wires=act_on))

        return op_list

    @staticmethod
    def shape(n_wires):
        r"""Returns a list of shapes for the 3 parameter tensors.

        Args:
            n_wires (int): number of wires

        Returns:
            list[tuple[int]]: list of shapes
        """
        shape_theta_phi = n_wires * (n_wires - 1) // 2

        shapes = [(shape_theta_phi,)] * 2 + [(n_wires,)]

        return shapes

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