Support inverse gates (#89)

* Expose inverse

* Expose inverse to kernels

* Add inverses

* Add to inverses

* Add missing kernels

* Update pennylane_lightning/src/Apply.hpp

* Add inverse in general kernel:

* Update capability

* Remove .inv from string

* Add tests

* Fix phase shift

* Fix

* Add Rot kernel

* Add tests for arbitrary unitary

* Remove

* Rename

* Fix

* Add to changelog

* Try fix codefactor

* Probably fix codefactor

* Apply suggestions from code review

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

* Remove kernel operations

* Add to docstrings

* Update one-liners

* Use unnamed param

* Remove kernel_operations

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

CHANGELOG.md

@@ -6,6 +6,9 @@
 
 ### Improvements
 
+* Inverse gates are now supported.
+  [(#89)](https://github.com/PennyLaneAI/pennylane-lightning/pull/89)
+
 * Add new lightweight backend with performance improvements.
   [(#57)](https://github.com/PennyLaneAI/pennylane-lightning/pull/57)
 

pennylane_lightning/lightning_qubit.py

@@ -85,7 +85,7 @@ def capabilities(cls):
         capabilities.update(
             model="qubit",
             supports_reversible_diff=False,
-            supports_inverse_operations=False,
+            supports_inverse_operations=True,
             supports_analytic_computation=True,
             returns_state=True,
         )
@@ -133,10 +133,23 @@ def apply_lightning(self, state, operations):
         Returns:
             array[complex]: the output state tensor
         """
-        op_names = [o.name for o in operations]
+        op_names = [self._remove_inverse_string(o.name) for o in operations]
         op_wires = [self.wires.indices(o.wires) for o in operations]
         op_param = [o.parameters for o in operations]
+        op_inverse = [o.inverse for o in operations]
 
         state_vector = np.ravel(state)
-        apply(state_vector, op_names, op_wires, op_param, self.num_wires)
+        apply(state_vector, op_names, op_wires, op_param, op_inverse, self.num_wires)
         return np.reshape(state_vector, state.shape)
+
+    @staticmethod
+    def _remove_inverse_string(string):
+        """Removes the ``.inv`` appended to the end of inverse gates.
+
+        Args:
+            string (str): name of operation
+
+        Returns:
+            str: name of operation with ``.inv`` removed (if present)
+        """
+        return string.replace(".inv", "")

pennylane_lightning/src/Apply.cpp

@@ -53,6 +53,7 @@ void Pennylane::constructAndApplyOperation(
     const string& opLabel,
     const vector<unsigned int>& opWires,
     const vector<double>& opParams,
+    bool inverse,
     const unsigned int qubits
 ) {
     unique_ptr<AbstractGate> gate = constructGate(opLabel, opParams);
@@ -64,14 +65,15 @@ void Pennylane::constructAndApplyOperation(
     vector<unsigned int> externalWires = getIndicesAfterExclusion(opWires, qubits);
     vector<size_t> externalIndices = generateBitPatterns(externalWires, qubits);
 
-    gate->applyKernel(state, internalIndices, externalIndices);
+    gate->applyKernel(state, internalIndices, externalIndices, inverse);
 }
 
 void Pennylane::apply(
     StateVector& state,
     const vector<string>& ops,
     const vector<vector<unsigned int>>& wires,
     const vector<vector<double>>& params,
+    const vector<bool>& inverse,
     const unsigned int qubits
 ) { 
     if (qubits <= 0)
@@ -86,7 +88,7 @@ void Pennylane::apply(
         throw std::invalid_argument("Invalid arguments: number of operations, wires, and parameters must all be equal");
 
     for (int i = 0; i < numOperations; i++) {
-        constructAndApplyOperation(state, ops[i], wires[i], params[i], qubits);
+        constructAndApplyOperation(state, ops[i], wires[i], params[i], inverse[i], qubits);
     }
 
 }

pennylane_lightning/src/Apply.hpp

@@ -62,13 +62,15 @@ namespace Pennylane {
      * @param opLabel unique string corresponding to a gate type
      * @param opWires index of qubits on which the gate acts
      * @param opParams defines the gate parameterisation (may be zero-length for some gates)
+     * @param inverse boolean indicating whether to apply the gate or its inverse
      * @param qubits number of qubits
      */
     void constructAndApplyOperation(
         StateVector& state,
         const std::string& opLabel,
         const std::vector<unsigned int>& opWires,
         const std::vector<double>& opParams,
+        bool inverse,
         const unsigned int qubits
     );
 
@@ -79,12 +81,15 @@ namespace Pennylane {
      * @param ops list of unique string names corresponding to gate types, in the order they should be applied
      * @param wires list of wires on which each gate acts
      * @param params list of parameters that defines the gate parameterisation
+     * @param inverse list of booleans indicating whether a given gate or its inverse should be applied
+     * @param qubits number of qubits
      */
     void apply(
         StateVector& state,
         const std::vector<std::string>& ops,
         const std::vector<std::vector<unsigned int>>& wires,
         const std::vector<std::vector<double>>& params,
+        const std::vector<bool>& inverse,
         const unsigned int qubits
     );
 

pennylane_lightning/src/Bindings.cpp

@@ -39,10 +39,11 @@ void apply(
     vector<string> ops,
     vector<vector<unsigned int>> wires,
     vector<vector<double>> params,
+    vector<bool> inverse,
     const unsigned int qubits
 ) {
     StateVector state = create(&stateNumpyArray);
-    Pennylane::apply(state, ops, wires, params, qubits);
+    Pennylane::apply(state, ops, wires, params, inverse, qubits);
 }
 
 PYBIND11_MODULE(lightning_qubit_ops, m)