Create projectors in any basis (#4192)

* First version of StateVectorProjector * Outer product fix * Support projector in default qubit * change inheritance * consistent diagonalizing gates * Update docstring * Proper bra-ket in label * Tests for StateVectorProjector * fix tests * Projector prototype * Explicit signature and pow method * discard hacky prototype * New hack (thanks Tymmy <3) * Exception tests * Docstring for __new__ * Label method adds matrix to cache * tests for label method * Update docstrings * Update changelog * enhanced docstring for label method * `Projector.__new__` docstring rephrased * Fix docstring Co-authored-by: Matthew Silverman <matthews@xanadu.ai> * Remove boolean kwarg * Remove `basis_representation` from changelog * Fix wire length issue * Update Projector example in docstring Co-authored-by: Matthew Silverman <matthews@xanadu.ai> * Update changelog description Co-authored-by: Tom Bromley <49409390+trbromley@users.noreply.github.com> * Update projector description Co-authored-by: Tom Bromley <49409390+trbromley@users.noreply.github.com> * change argument names to be `state` * Remove shape indication in docstring Co-authored-by: Tom Bromley <49409390+trbromley@users.noreply.github.com> * fix expval with state vector projector * Code example and remove hidden class docstring * minor docstring corrections * add expval test (it was failing) * projector bind new parameters dispatcher * Update error string Co-authored-by: Matthew Silverman <matthews@xanadu.ai> * fix projector and qubit device * extensive projector testing * remove outdated test * fix tests typo * Additional indications on input shape Co-authored-by: Mudit Pandey <mudit.pandey@xanadu.ai> * revision feedback * update copy --------- Co-authored-by: = <=> Co-authored-by: Matthew Silverman <matthews@xanadu.ai> Co-authored-by: Tom Bromley <49409390+trbromley@users.noreply.github.com> Co-authored-by: Mudit Pandey <mudit.pandey@xanadu.ai>
PennyLaneAI · Jun 16, 2023 · e102fe2 · e102fe2
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diff --git a/doc/releases/changelog-dev.md b/doc/releases/changelog-dev.md
@@ -63,6 +63,26 @@
 
 <h3>Improvements 🛠</h3>
 
+* `Projector` now accepts a state vector representation, which enables the creation of projectors
+  in any basis.
+  [(#4192)](https://github.com/PennyLaneAI/pennylane/pull/4192)
+
+  ```python
+  dev = qml.device("default.qubit", wires=2)
+  @qml.qnode(dev)
+  def circuit(state):
+      return qml.expval(qml.Projector(state, wires=[0, 1]))
+  zero_state = [0, 0]
+  plusplus_state = np.array([1, 1, 1, 1]) / 2
+  ```
+  ```pycon
+  >>> circuit(zero_state)
+  1.
+  >>> 
+  >>> circuit(plusplus_state)
+  0.25
+  ```
+
 * The pulse differentiation methods, `pulse_generator` and `stoch_pulse_grad` now raise an error when they
   are applied to a `QNode` directly. Instead, use differentiation via a JAX entry point (`jax.grad`, `jax.jacobian`, ...).
   [(4241)](https://github.com/PennyLaneAI/pennylane/pull/4241)

diff --git a/pennylane/_qubit_device.py b/pennylane/_qubit_device.py
@@ -298,7 +298,7 @@ def execute(self, circuit, **kwargs):
 
         * :meth:`~.probability`
 
-        Additional keyword arguments may be passed to the this method
+        Additional keyword arguments may be passed to this method
         that can be utilised by :meth:`apply`. An example would be passing
         the ``QNode`` hash that can be used later for parametric compilation.
 
@@ -1649,8 +1649,10 @@ def marginal_prob(self, prob, wires=None):
         return self._reshape(prob, flat_shape)
 
     def expval(self, observable, shot_range=None, bin_size=None):
-        if observable.name == "Projector":
-            # branch specifically to handle the projector observable
+        if observable.name == "Projector" and len(observable.parameters[0]) == len(
+            observable.wires
+        ):
+            # branch specifically to handle the basis state projector observable
             idx = int("".join(str(i) for i in observable.parameters[0]), 2)
             probs = self.probability(
                 wires=observable.wires, shot_range=shot_range, bin_size=bin_size
@@ -1679,8 +1681,10 @@ def expval(self, observable, shot_range=None, bin_size=None):
         return np.squeeze(np.mean(samples, axis=axis))
 
     def var(self, observable, shot_range=None, bin_size=None):
-        if observable.name == "Projector":
-            # branch specifically to handle the projector observable
+        if observable.name == "Projector" and len(observable.parameters[0]) == len(
+            observable.wires
+        ):
+            # branch specifically to handle the basis state projector observable
             idx = int("".join(str(i) for i in observable.parameters[0]), 2)
             probs = self.probability(
                 wires=observable.wires, shot_range=shot_range, bin_size=bin_size

diff --git a/pennylane/devices/tests/test_compare_default_qubit.py b/pennylane/devices/tests/test_compare_default_qubit.py
@@ -69,7 +69,23 @@ def circuit(theta, phi):
         assert np.allclose(qnode(theta, phi), qnode_def(theta, phi), atol=tol(dev.shots))
         assert np.allclose(grad(theta, phi), grad_def(theta, phi), atol=tol(dev.shots))
 
-    @pytest.mark.parametrize("state", [[0, 0], [0, 1], [1, 0], [1, 1]])
+    @pytest.mark.parametrize(
+        "state",
+        [
+            [0, 0],
+            [0, 1],
+            [1, 0],
+            [1, 1],
+            [1, 0, 0, 0],
+            [0, 1, 0, 0],
+            [0, 0, 1, 0],
+            [0, 0, 0, 1],
+            np.array([1, 1, 0, 0]) / np.sqrt(2),
+            np.array([0, 1, 0, 1]) / np.sqrt(2),
+            np.array([1, 1, 1, 0]) / np.sqrt(3),
+            np.array([1, 1, 1, 1]) / 2,
+        ],
+    )
     def test_projector_expectation(self, device, state, tol):
         """Test that arbitrary multi-mode Projector expectation values are correct"""
         n_wires = 2
@@ -88,11 +104,11 @@ def test_projector_expectation(self, device, state, tol):
         theta = 0.432
         phi = 0.123
 
-        def circuit(theta, phi, basis_state):
+        def circuit(theta, phi, state):
             qml.RX(theta, wires=[0])
             qml.RX(phi, wires=[1])
             qml.CNOT(wires=[0, 1])
-            return qml.expval(qml.Projector(basis_state, wires=[0, 1]))
+            return qml.expval(qml.Projector(state, wires=[0, 1]))
 
         qnode_def = qml.QNode(circuit, dev_def)
         qnode = qml.QNode(circuit, dev)