[unitaryHACK] Strawberry Fields: Gaussian Merge (#591)

* Added gaussian merge compiler, which merges gaussian operations in
hybrid circuits

Github Issue: #574

* gaussian_merge: add more comments and documentation

* gaussian_merge: fix codefactor issues

* gaussian_merge: fix last codefactor issues

* gaussian_merge: reformat files with to pass formatting check / black tests

* Apply suggestions from code review

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

* gaussian_merge: Add unit tests and fix bugs

* gaussian_merge: add actual tests and fix Code Factor issues bbrought up

* Add Gaussian Transform to primatives

* gaussian_merge: edited unit test to increase code coverage

* gaussian_merge: removed unnecessary method

* gaussian_merge: removed deprecated functions and if statements and updated test cases to get 100% code coverage

* Edited CHANGELOG.md to explain Gaussian Merge

* Fix contributors alphabetical order

* Update .github/CHANGELOG.md

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

* Update .github/CHANGELOG.md

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

* Update .github/CHANGELOG.md

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

* Update CHANGELOG.md

* Update tests/frontend/compilers/test_gaussianmerge.py

Co-authored-by: Nicolas Quesada <nicolas@xanadu.ai>

* Added more gaussian merge test cases for in test_gaussianunitary and fixed primatives

* test_gaussianmerge.py: added init parametrization for initial ket states.

* test_gaussianmerge.py: added ket vacuum state

* fixed black formatting issue

* Update strawberryfields/compilers/gaussian_merge.py

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

* Update strawberryfields/compilers/gaussian_merge.py

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

* Update strawberryfields/compilers/gaussian_merge.py

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

* Update strawberryfields/compilers/gaussian_merge.py

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

* Update strawberryfields/compilers/gaussian_merge.py

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

* Update strawberryfields/compilers/gaussian_merge.py

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

* Fixed docstrings and added info on logic flow.

* Apply suggestions from code review

* Update strawberryfields/compilers/gaussian_merge.py

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

* Update strawberryfields/compilers/gaussian_merge.py

* Fixed inheritance for gaussian merge class

* Fixed inheritance issue with Compiler by defining interactive variable

* Update .github/CHANGELOG.md

Co-authored-by: Nicolas Quesada <nicolas@xanadu.ai>

* Update strawberryfields/compilers/gaussian_merge.py

Co-authored-by: Nicolas Quesada <nicolas@xanadu.ai>

* Update strawberryfields/compilers/gaussian_merge.py

Co-authored-by: Nicolas Quesada <nicolas@xanadu.ai>

* Apply suggestions from code review

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

* Added check for Gaussian Unitary returning a single Displacement gate

Co-authored-by: Josh Izaac <josh146@gmail.com>
Co-authored-by: Nicolas Quesada <nicolas@xanadu.ai>
Co-authored-by: antalszava <antalszava@gmail.com>

.github/CHANGELOG.md

@@ -39,6 +39,47 @@
   print(np.allclose(U, Uout))
  ```
 
+* A new compiler, ``GaussianMerge``, has been added. It is aimed at reducing calculation
+  overhead for non-Gaussian circuits by minimizing the amount of Gaussian operations
+  in a circuit, while retaining the same functionality.
+  [(#591)](https://github.com/XanaduAI/strawberryfields/pull/591)
+
+  ``GaussianMerge`` merges Gaussian operations, where allowed, into ``GaussianTransform``
+  and ``Dgate`` operations. It utilizes the existing ``GaussianUnitary`` compiler to
+  merge operations and Directed Acyclic Graphs to determine which operations can be merged.
+
+  ```python 
+  modes = 4
+  cutoff_dim = 6
+
+  # prepare an intial state with 4 photons in as many modes
+  initial_state = np.zeros([cutoff_dim] * modes, dtype=complex)
+  initial_state[1, 1, 1, 1] = 1
+
+  prog = sf.Program(4)
+
+  with prog.context as q:
+      ops.Ket(initial_state) | q  # Initial state preparation
+      # Gaussian Layer
+      ops.S2gate(0.01, 0.01) | (q[0], q[1])
+      ops.BSgate(1.9, 1.7) | (q[1], q[2])
+      ops.BSgate(0.9, 0.2) | (q[0], q[1])
+      # Non-Gaussian Layer
+      ops.Kgate(0.5) | q[3]
+      ops.CKgate(0.7) | (q[2], q[3])
+      # Gaussian Layer
+      ops.BSgate(1.0, 0.4) | (q[0], q[1])
+      ops.BSgate(2.0, 1.5) | (q[1], q[2])
+      ops.Dgate(0.01) | q[0]
+      ops.Dgate(0.01) | q[0]
+      ops.Sgate(0.01, 0.01) | q[1]
+      # Non-Gaussian Layer
+      ops.Vgate(0.5) | q[2]
+
+  prog_merged = prog.compile(compiler="gaussian_merge")
+  ```
+
+
 <h3>Improvements</h3>
 
 * Cleanup `backends/tfbackend/ops.py` to reduce line count, clarify function
@@ -66,7 +107,7 @@
 
 This release contains contributions from (in alphabetical order):
 
-Jake Bulmer, Aaron Robertson, Jeremy Swinarton, Antal Száva, Yuan Yao.
+Jake Bulmer, Aaron Robertson, Jeremy Swinarton, Antal Száva, Federico Rueda, Yuan Yao.
 
 # Release 0.18.0 (current release)
 

strawberryfields/compilers/__init__.py

@@ -45,17 +45,9 @@
 from .bosonic import Bosonic
 from .gbs import GBS
 from .gaussian_unitary import GaussianUnitary
+from .gaussian_merge import GaussianMerge
 
-compilers = (
-    Fock,
-    Gaussian,
-    Bosonic,
-    GBS,
-    GaussianUnitary,
-    Xcov,
-    Xstrict,
-    Xunitary,
-)
+compilers = (Fock, Gaussian, Bosonic, GBS, GaussianUnitary, Xcov, Xstrict, Xunitary, GaussianMerge)
 
 compiler_db = {c.short_name: c for c in compilers}
 """dict[str, ~strawberryfields.compilers.Compiler]: Map from compiler name to the corresponding