Bosonic backend - backend.py and states.py (#539)

* Adds __init__ and backend.py for bosonic backend

- Contains all the gaussian methods for the bosonic backend

Co-Authored-By: ilan-tz <57886357+ilan-tz@users.noreply.github.com>
Co-Authored-By: Nicolas Quesada <991946+nquesada@users.noreply.github.com>
Co-Authored-By: GDauphinais <48553380+GDauphinais@users.noreply.github.com>

* Modifies the backend __init__ and base.py files

- Includes BaseBosonic object
- Includes references to bosonic backend in init file

Co-Authored-By: ilan-tz <57886357+ilan-tz@users.noreply.github.com>
Co-Authored-By: Nicolas Quesada <991946+nquesada@users.noreply.github.com>

* Update states.py file

Adds BaseBosonicState and its tailored methods

Co-Authored-By: ilan-tz <57886357+ilan-tz@users.noreply.github.com>
Co-Authored-By: Nicolas Quesada <991946+nquesada@users.noreply.github.com>

* Updates conftest.py

Adds bosonic backend to backends for testing

Co-Authored-By: ilan-tz <57886357+ilan-tz@users.noreply.github.com>

* Tests for bosonic backend

- Adds/removes bosonic marking to/from some existing tests
- New tests for gaussian_cptp, mbsqueezing, and BaseBosonicState methods

* Tidying code based on codefactor

* More codefactor tidying

* More codefactor

* ran black

* Mark integration tests

so that they don't apply to the bosonic backend yet.
We will mark them for the bosonic backend once the PR with the bosonic engine is added.

* Ensure peaks in add_mode are integers

* Apply suggestions from code review

Mostly minor docstring/formatting edits

Co-authored-by: Josh Izaac <josh146@gmail.com>
Co-authored-by: Nicolas Quesada <zeitus@gmail.com>

* Update BaseBosonic inheritance

Now inherits from BaseGaussian

Co-Authored-By: Josh Izaac <josh@iza.ac>

* Update strawberryfields/backends/base.py

Co-authored-by: antalszava <antalszava@gmail.com>

* Adding reference to bosonic in docstrings

* Applies Antal and Theo's suggestions

- Tidies or simply deletes inheritable docstrings
- Changes is_vacuum method

* Ran black

* Minor changes

- Add back in state method for BaseBosonic since the output is different than BaseGaussian
- Add shortcut to BosonicModes for fidelity_coherent if all modes have been deleted
- Tidy docstring of state purity function

* Ran black

* Tidying code + test

- wrote new test that checks fidelity_vacuum in states.py, since the is_vacuum method of the backend changed, and consequently this was no longer being tested

* Apply suggestions from code review

Co-authored-by: antalszava <antalszava@gmail.com>
Co-authored-by: Theodor <theodor@xanadu.ai>

* ran black

* Apply suggestions from code review

* Fix photon number variance for non-gaussian states

* Deletes TODO note

* Apply suggestions from code review

Co-authored-by: antalszava <antalszava@gmail.com>

* Ran black

* Tidying docstrings

* Breaks the test test_multimode_gaussian_random_state

Co-authored-by: ilan-tz <57886357+ilan-tz@users.noreply.github.com>
Co-authored-by: Nicolas Quesada <991946+nquesada@users.noreply.github.com>
Co-authored-by: GDauphinais <48553380+GDauphinais@users.noreply.github.com>
Co-authored-by: Josh Izaac <josh146@gmail.com>
Co-authored-by: Nicolas Quesada <zeitus@gmail.com>
Co-authored-by: Josh Izaac <josh@iza.ac>
Co-authored-by: antalszava <antalszava@gmail.com>
Co-authored-by: Theodor <theodor@xanadu.ai>

strawberryfields/backends/__init__.py

@@ -31,6 +31,7 @@
     FockBackend
     GaussianBackend
     ~tfbackend.TFBackend
+    BosonicBackend
 
 .. raw:: html
 
@@ -43,6 +44,7 @@
     BaseFockState
     BaseGaussianState
     ~tfbackend.states.FockStateTF
+    BaseBosonicState
 
 .. raw:: html
 
@@ -62,6 +64,7 @@
     BaseBackend
     BaseFock
     BaseGaussian
+    BaseBosonic
 
 Utility modules
 ---------------
@@ -76,10 +79,11 @@
     shared_ops
 """
 
-from .base import BaseBackend, BaseFock, BaseGaussian, ModeMap
+from .base import BaseBackend, BaseFock, BaseGaussian, BaseBosonic, ModeMap
 from .gaussianbackend import GaussianBackend
 from .fockbackend import FockBackend
-from .states import BaseState, BaseGaussianState, BaseFockState
+from .bosonicbackend import BosonicBackend
+from .states import BaseState, BaseGaussianState, BaseFockState, BaseBosonicState
 
 # There is no import for the TFBackend to avoid TensorFlow being a direct
 # requirement of SF through a chain of imports
@@ -88,18 +92,23 @@
     "BaseBackend",
     "BaseFock",
     "BaseGaussian",
+    "BaseBosonic",
     "FockBackend",
     "GaussianBackend",
+    "BosonicBackend",
     "TFBackend",
     "BaseState",
     "BaseFockState",
     "BaseGaussianState",
+    "BaseBosonicState",
 ]
 
 
 virtual_backends = ["X8_01"]
 
-local_backends = {b.short_name: b for b in (BaseBackend, GaussianBackend, FockBackend)}
+local_backends = {
+    b.short_name: b for b in (BaseBackend, GaussianBackend, FockBackend, BosonicBackend)
+}
 
 
 def load_backend(name):

strawberryfields/backends/base.py

@@ -139,6 +139,8 @@ def supports(self, name):
         * "gaussian": for manipulations in the Gaussian representation using the
           displacements and covariance matrices
         * "fock_basis": for manipulations in the Fock representation
+        * "bosonic": for manipulations of states represented as linear combinations
+          of Gaussian functions in phase space
         * "mixed_states": for representations where the quantum state is mixed
         * "batched": allows for a multiple circuits to be simulated in parallel
 
@@ -170,7 +172,7 @@ def begin_circuit(self, num_subsystems, **kwargs):
         """
         # BaseBackend can be instantiated for testing purposes, even though it does not do anything.
 
-    def add_mode(self, n=1):
+    def add_mode(self, n=1, **kwargs):
         """Add modes to the circuit.
 
         The new modes are initialized to the vacuum state.
@@ -181,6 +183,9 @@ def add_mode(self, n=1):
 
         Returns:
             list[int]: indices of the newly added modes
+
+        Keyword Args:
+            peaks (list): number of Gaussian peaks for each new mode (for bosonic backend only)
         """
         raise NotImplementedError
 
@@ -677,3 +682,50 @@ def state(self, modes=None, **kwargs):
             BaseGaussianState: state description
         """
         raise NotImplementedError
+
+
+# ==============================
+# Base bosonic backend
+# ==============================
+
+
+class BaseBosonic(BaseGaussian):
+    """Abstract base class for backends that are only capable of manipulating states
+    represented as linear combinations of Gaussian functions in phase space."""
+
+    compiler = None
+
+    def __init__(self):
+        super().__init__()
+        self._supported["gaussian_linear_combo"] = True
+
+    def prepare_gaussian_state(self, r, V, modes):
+        r"""Prepare a Gaussian state.
+
+        Note the different basis-ordering from the :class:`~.GaussianBackend`.
+
+        The specified modes are traced out and replaced with a Gaussian state
+        provided via a vector of means and a covariance matrix.
+
+        Args:
+            r (array): vector of means in :math:`(x_1,p_1,x_2,p_2,\dots)` ordering
+            V (array): covariance matrix in :math:`(x_1,p_1,x_2,p_2,\dots)` ordering
+            modes (int or Sequence[int]): Which modes to prepare the state in.
+                If the modes are not sorted, this is taken into account when preparing the state.
+                I.e., when a two mode state is prepared with ``modes=[3,1]``, the first
+                mode of the given state goes into mode 3 and the second mode goes into mode 1.
+
+        Raises:
+            ValueError: if the shapes of r or V do not match the number of modes.
+        """
+        raise NotImplementedError
+
+    def state(self, modes=None, **kwargs):
+        """Returns the state of the quantum simulation.
+
+        See :meth:`.BaseBackend.state`.
+
+        Returns:
+            BaseBosonicState: state description
+        """
+        raise NotImplementedError

strawberryfields/backends/bosonicbackend/__init__.py

@@ -0,0 +1,16 @@
+# Copyright 2021 Xanadu Quantum Technologies Inc.
+
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+
+#     http://www.apache.org/licenses/LICENSE-2.0
+
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+r"""Bosonic qubit simulator backend"""
+
+from .backend import BosonicBackend