set the default state prep label to a psi ket (#4340)

* set the default state prep label to a psi ket

* Update doc/releases/changelog-dev.md

Co-authored-by: Mudit Pandey <mudit.pandey@xanadu.ai>

* fix tests

* do not test matrix for stateprep op labels

* lint test_operation.py

* fix pylint disable

* Update doc/releases/changelog-dev.md

Co-authored-by: Christina Lee <christina@xanadu.ai>

---------

Co-authored-by: Mudit Pandey <mudit.pandey@xanadu.ai>
Co-authored-by: Christina Lee <christina@xanadu.ai>

doc/releases/changelog-dev.md

@@ -30,6 +30,9 @@
 * QNode transforms in `qml.qinfo` now support custom wire labels.
   [#4331](https://github.com/PennyLaneAI/pennylane/pull/4331)
 
+* The default label for a `StatePrep` operator is now `|Ψ⟩`.
+  [(#4340)](https://github.com/PennyLaneAI/pennylane/pull/4340)
+
 <h3>Breaking changes 💔</h3>
 
 * The `do_queue` keyword argument in `qml.operation.Operator` has been removed. Instead of

pennylane/operation.py

@@ -2706,6 +2706,9 @@ def state_vector(self, wire_order=None):
             array: A state vector for all wires in a circuit
         """
 
+    def label(self, decimals=None, base_label=None, cache=None):
+        return "|Ψ⟩"
+
 
 def operation_derivative(operation) -> np.ndarray:
     r"""Calculate the derivative of an operation.

tests/drawer/test_draw.py

@@ -166,18 +166,14 @@ def matrices_circuit():
             qml.QubitUnitary(np.eye(2), wires=0)
             return qml.expval(qml.Hermitian(np.eye(2), wires=0))
 
-        expected1 = (
-            "0: ─╭QubitStateVector(M0)──U(M1)─┤  <𝓗(M1)>\n"
-            "1: ─╰QubitStateVector(M0)────────┤         "
-        )
+        expected1 = "0: ─╭|Ψ⟩──U(M0)─┤  <𝓗(M0)>\n1: ─╰|Ψ⟩────────┤         "
 
         assert draw(matrices_circuit, show_matrices=False)() == expected1
 
         expected2 = (
-            "0: ─╭QubitStateVector(M0)──U(M1)─┤  <𝓗(M1)>\n"
-            "1: ─╰QubitStateVector(M0)────────┤         \n"
-            "M0 = \n[1. 0. 0. 0.]\n"
-            "M1 = \n[[1. 0.]\n [0. 1.]]"
+            "0: ─╭|Ψ⟩──U(M0)─┤  <𝓗(M0)>\n"
+            "1: ─╰|Ψ⟩────────┤         \n"
+            "M0 = \n[[1. 0.]\n [0. 1.]]"
         )
 
         assert draw(matrices_circuit)() == expected2
@@ -194,21 +190,20 @@ def matrices_circuit(x):
             return qml.expval(qml.Hermitian(np.eye(2, requires_grad=False), wires=1))
 
         expected1 = (
-            "0: ─╭QubitStateVector(M0)──U(M1)────┤         \n"
-            "1: ─╰QubitStateVector(M0)──RX(1.20)─┤  <𝓗(M1)>\n\n"
-            "0: ─╭QubitStateVector(M0)──U(M1)────┤         \n"
-            "1: ─╰QubitStateVector(M0)──RX(0.80)─┤  <𝓗(M1)>\n\n"
-            "M0 = \n[1. 0. 0. 0.]\n"
-            "M1 = \n[[1. 0.]\n [0. 1.]]"
+            "0: ─╭|Ψ⟩──U(M0)────┤         \n"
+            "1: ─╰|Ψ⟩──RX(1.20)─┤  <𝓗(M0)>\n\n"
+            "0: ─╭|Ψ⟩──U(M0)────┤         \n"
+            "1: ─╰|Ψ⟩──RX(0.80)─┤  <𝓗(M0)>\n\n"
+            "M0 = \n[[1. 0.]\n [0. 1.]]"
         )
         output = draw(matrices_circuit)(np.array(1.0, requires_grad=True))
         assert output == expected1
 
         expected2 = (
-            "0: ─╭QubitStateVector(M0)──U(M1)────┤         \n"
-            "1: ─╰QubitStateVector(M0)──RX(1.20)─┤  <𝓗(M1)>\n\n"
-            "0: ─╭QubitStateVector(M0)──U(M1)────┤         \n"
-            "1: ─╰QubitStateVector(M0)──RX(0.80)─┤  <𝓗(M1)>"
+            "0: ─╭|Ψ⟩──U(M0)────┤         \n"
+            "1: ─╰|Ψ⟩──RX(1.20)─┤  <𝓗(M0)>\n\n"
+            "0: ─╭|Ψ⟩──U(M0)────┤         \n"
+            "1: ─╰|Ψ⟩──RX(0.80)─┤  <𝓗(M0)>"
         )
         output = draw(matrices_circuit, show_matrices=False)(np.array(1.0, requires_grad=True))
         assert output == expected2

tests/drawer/test_tape_text.py

@@ -282,7 +282,7 @@ def test_setting_max_length(self, ml):
     (qml.AmplitudeDamping(0.98, wires=0), "0: ──AmplitudeDamping(0.98)─┤  "),
     (
         qml.QubitStateVector([0, 1, 0, 0], wires=(0, 1)),
-        "0: ─╭QubitStateVector(M0)─┤  \n1: ─╰QubitStateVector(M0)─┤  ",
+        "0: ─╭|Ψ⟩─┤  \n1: ─╰|Ψ⟩─┤  ",
     ),
     (qml.Kerr(1.234, wires=0), "0: ──Kerr(1.23)─┤  "),
     (
@@ -396,21 +396,17 @@ def test_default_shows_matrix_parameters(self):
         """Test matrices numbered but not included by default."""
 
         expected = (
-            "0: ─╭QubitStateVector(M0)──U(M1)─┤  <𝓗(M1)>\n"
-            "1: ─╰QubitStateVector(M0)────────┤         \n"
-            "M0 = \n[1. 0. 0. 0.]\n"
-            "M1 = \n[[1. 0.]\n [0. 1.]]"
+            "0: ─╭|Ψ⟩──U(M0)─┤  <𝓗(M0)>\n"
+            "1: ─╰|Ψ⟩────────┤         \n"
+            "M0 = \n[[1. 0.]\n [0. 1.]]"
         )
 
         assert tape_text(tape_matrices) == expected
 
     def test_do_not_show_matrices(self):
         """Test matrices included when requested."""
 
-        expected = (
-            "0: ─╭QubitStateVector(M0)──U(M1)─┤  <𝓗(M1)>\n"
-            "1: ─╰QubitStateVector(M0)────────┤         "
-        )
+        expected = "0: ─╭|Ψ⟩──U(M0)─┤  <𝓗(M0)>\n1: ─╰|Ψ⟩────────┤         "
 
         assert tape_text(tape_matrices, show_matrices=False) == expected
 
@@ -421,15 +417,13 @@ def test_matrix_parameters_provided_cache(self):
         cache = {"matrices": [np.eye(2), -np.eye(3)]}
 
         expected = (
-            "0: ─╭QubitStateVector(M2)──U(M0)─┤  <𝓗(M0)>\n"
-            "1: ─╰QubitStateVector(M2)────────┤         \n"
+            "0: ─╭|Ψ⟩──U(M0)─┤  <𝓗(M0)>\n"
+            "1: ─╰|Ψ⟩────────┤         \n"
             "M0 = \n[[1. 0.]\n [0. 1.]]\n"
-            "M1 = \n[[-1. -0. -0.]\n [-0. -1. -0.]\n [-0. -0. -1.]]\n"
-            "M2 = \n[1. 0. 0. 0.]"
+            "M1 = \n[[-1. -0. -0.]\n [-0. -1. -0.]\n [-0. -0. -1.]]"
         )
 
         assert tape_text(tape_matrices, show_matrices=True, cache=cache) == expected
-        assert qml.math.allequal(cache["matrices"][2], [1.0, 0.0, 0.0, 0.0])
 
 
 # @pytest.mark.skip("Nested tapes are being deprecated")

tests/ops/qubit/test_state_prep.py

@@ -40,8 +40,6 @@ def test_adjoint_error_exception(op):
 @pytest.mark.parametrize(
     "op, mat, base",
     [
-        (qml.BasisState(np.array([0, 1]), wires=0), [0, 1], "BasisState"),
-        (qml.QubitStateVector(np.array([1.0, 0.0]), wires=0), [1.0, 0.0], "QubitStateVector"),
         (qml.QubitDensityMatrix(densitymat0, wires=0), densitymat0, "QubitDensityMatrix"),
     ],
 )

tests/test_operation.py

@@ -251,7 +251,7 @@ class MyOp(qml.operation.Operation):
         class MyOpOverwriteInit(qml.operation.Operation):
             num_wires = 1
 
-            def __init__(self, wires):
+            def __init__(self, wires):  # pylint:disable=super-init-not-called
                 pass
 
         op = MyOp(wires=0)
@@ -266,7 +266,7 @@ def test_custom_hyperparams(self):
         class MyOp(qml.operation.Operation):
             num_wires = 1
 
-            def __init__(self, wires, basis_state=None):
+            def __init__(self, wires, basis_state=None):  # pylint:disable=super-init-not-called
                 self._hyperparameters = {"basis_state": basis_state}
 
         state = [0, 1, 0]
@@ -417,7 +417,7 @@ class MyOp(qml.operation.Operator):
             num_wires = 1
 
             @staticmethod
-            def compute_matrix():
+            def compute_matrix(*params, **hyperparams):
                 return np.eye(2)
 
         assert MyOp.has_matrix is True
@@ -469,7 +469,7 @@ class MyOp(qml.operation.Operator):
             num_params = 1
 
             @staticmethod
-            def compute_decomposition(x, wires=None):
+            def compute_decomposition(x, wires=None):  # pylint:disable=arguments-differ
                 return [qml.RX(x, wires=wires)]
 
         assert MyOp.has_decomposition is True
@@ -507,7 +507,7 @@ class MyOp(qml.operation.Operator):
             num_params = 1
 
             @staticmethod
-            def compute_diagonalizing_gates(x, wires=None):
+            def compute_diagonalizing_gates(x, wires=None):  # pylint:disable=arguments-differ
                 return []
 
         assert MyOp.has_diagonalizing_gates is True
@@ -2082,7 +2082,7 @@ class MyOpWithMat(Operator):
     num_wires = 1
 
     @staticmethod
-    def compute_matrix(theta):
+    def compute_matrix(theta):  # pylint:disable=arguments-differ
         return np.tensordot(theta, np.array([[0.4, 1.2], [1.2, 0.4]]), axes=0)
 
 
@@ -2117,7 +2117,7 @@ class DummyOp(qml.operation.Channel):
             grad_method = "F"
 
             @staticmethod
-            def compute_kraus_matrices(p):
+            def compute_kraus_matrices(p):  # pylint:disable=arguments-differ
                 K1 = np.sqrt(p) * X
                 K2 = np.sqrt(1 - p) * I
                 return [K1, K2]
@@ -2258,19 +2258,17 @@ class DummyOp(qml.operation.CVOperation):
 class TestStatePrep:
     """Test the StatePrep interface."""
 
+    class DefaultPrep(StatePrep):
+        """A dummy class that assumes it was given a state vector."""
+
+        # pylint:disable=unused-argument,too-few-public-methods
+        def state_vector(self, wire_order=None):
+            return self.parameters[0]
+
     # pylint:disable=unused-argument,too-few-public-methods
     def test_basic_stateprep(self):
         """Tests a basic implementation of the StatePrep interface."""
-
-        class DefaultPrep(StatePrep):
-            """A dummy class that assumes it was given a state vector."""
-
-            num_wires = qml.operation.AllWires
-
-            def state_vector(self, wire_order=None):
-                return self.parameters[0]
-
-        prep_op = DefaultPrep([1, 0], wires=[0])
+        prep_op = self.DefaultPrep([1, 0], wires=[0])
         assert np.array_equal(prep_op.state_vector(), [1, 0])
 
     def test_child_must_implement_state_vector(self):
@@ -2279,11 +2277,15 @@ def test_child_must_implement_state_vector(self):
         class NoStatePrepOp(StatePrep):
             """A class that is missing the state_vector implementation."""
 
-            num_wires = qml.operation.AllWires
+            # pylint:disable=abstract-class-instantiated
 
         with pytest.raises(TypeError, match="Can't instantiate abstract class"):
             NoStatePrepOp(wires=[0])
 
+    def test_StatePrep_label(self):
+        """Tests that StatePrep classes by default have a psi ket label"""
+        assert self.DefaultPrep([1], 0).label() == "|Ψ⟩"
+
 
 class TestCriteria:
     doubleExcitation = qml.DoubleExcitation(0.1, wires=[0, 1, 2, 3])