Add Projector Obs to LGPU (#894)

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------------------------------------------------------------------------------------------------------------

**Context:**
Add support for `Projector` observable via diagonalization to LGPU. 

**Description of the Change:**

**Benefits:**

**Possible Drawbacks:**

**Related GitHub Issues:**

Fixes #883 
[sc-72652]

---------

Co-authored-by: ringo-but-quantum <[email protected]>

.github/CHANGELOG.md

@@ -2,6 +2,9 @@
 
 ### New features since last release
 
+* Add `Projector` observable support via diagonalization to Lightning-GPU.
+  [(#894)](https://github.com/PennyLaneAI/pennylane-lightning/pull/894)
+
 * Add 1-target wire controlled gate support to `lightning.tensor`. Note that `cutensornet` only supports 1-target wire controlled gate as of `v24.08`. A controlled gate with more than 1 target wire should be converted to dense matrix.
   [(#880)](https://github.com/PennyLaneAI/pennylane-lightning/pull/880)
 

doc/lightning_gpu/device.rst

@@ -104,6 +104,7 @@ Supported operations and observables
     ~pennylane.PauliX
     ~pennylane.PauliY
     ~pennylane.PauliZ
+    ~pennylane.Projector
     ~pennylane.Hermitian
     ~pennylane.Hamiltonian
     ~pennylane.SparseHamiltonian

pennylane_lightning/core/_version.py

@@ -16,4 +16,4 @@
    Version number (major.minor.patch[-label])
 """
 
-__version__ = "0.39.0-dev10"
+__version__ = "0.39.0-dev11"

pennylane_lightning/lightning_gpu/lightning_gpu.py

@@ -187,6 +187,7 @@ def _mebibytesToBytes(mebibytes):
     "LinearCombination",
     "Hermitian",
     "Identity",
+    "Projector",
     "Sum",
     "Prod",
     "SProd",
@@ -804,6 +805,15 @@ def expval(self, observable, shot_range=None, bin_size=None):
         Returns:
             Expectation value of the observable
         """
+        if isinstance(observable, qml.Projector):
+            diagonalizing_gates = observable.diagonalizing_gates()
+            if self.shots is None and diagonalizing_gates:
+                self.apply(diagonalizing_gates)
+            results = super().expval(observable, shot_range=shot_range, bin_size=bin_size)
+            if self.shots is None and diagonalizing_gates:
+                self.apply([qml.adjoint(g, lazy=False) for g in reversed(diagonalizing_gates)])
+            return results
+
         if self.shots is not None:
             # estimate the expectation value
             samples = self.sample(observable, shot_range=shot_range, bin_size=bin_size)
@@ -887,6 +897,15 @@ def var(self, observable, shot_range=None, bin_size=None):
         Returns:
             Variance of the observable
         """
+        if isinstance(observable, qml.Projector):
+            diagonalizing_gates = observable.diagonalizing_gates()
+            if self.shots is None and diagonalizing_gates:
+                self.apply(diagonalizing_gates)
+            results = super().var(observable, shot_range=shot_range, bin_size=bin_size)
+            if self.shots is None and diagonalizing_gates:
+                self.apply([qml.adjoint(g, lazy=False) for g in reversed(diagonalizing_gates)])
+            return results
+
         if self.shots is not None:
             # estimate the var
             # Lightning doesn't support sampling yet

tests/test_expval.py

@@ -106,9 +106,6 @@ def test_projector_expectation(self, theta, phi, qubit_device, tol):
         dev_def = qml.device("default.qubit", wires=n_qubits)
         dev = qubit_device(wires=n_qubits)
 
-        if "Projector" not in dev.observables:
-            pytest.skip("Device does not support the Projector observable.")
-
         init_state = np.random.rand(2**n_qubits) + 1j * np.random.rand(2**n_qubits)
         init_state /= np.linalg.norm(init_state)
         obs = qml.Projector(np.array([0, 1, 0, 0]) / np.sqrt(2), wires=[0, 1])

tests/test_var.py

@@ -50,19 +50,15 @@ def test_var(self, theta, phi, qubit_device, tol):
 
         assert np.allclose(var, expected, tol)
 
-    pytest.mark.skipif(
+    @pytest.mark.skipif(
         device_name == "lightning.tensor", reason="lightning.tensor doesn't support projector."
     )
-
     def test_projector_var(self, theta, phi, qubit_device, tol):
         """Test that Projector variance value is correct"""
         n_qubits = 2
         dev_def = qml.device("default.qubit", wires=n_qubits)
         dev = qubit_device(wires=n_qubits)
 
-        if "Projector" not in dev.observables:
-            pytest.skip("Device does not support the Projector observable.")
-
         init_state = np.random.rand(2**n_qubits) + 1j * np.random.rand(2**n_qubits)
         init_state /= np.linalg.norm(init_state)
         obs = qml.Projector(np.array([0, 1, 0, 0]) / np.sqrt(2), wires=[0, 1])