Bugfix: Cannot initialize ParticleConservingU1/2 with default init_state

doc/releases/changelog-dev.md

@@ -377,6 +377,9 @@
 
 <h3>Bug fixes 🐛</h3>
 
+* `qml.ParticleConservingU1` and `qml.ParticleConservingU2` no longer raise an error when the initial state is not specified.
+  [(#5535)](https://github.com/PennyLaneAI/pennylane/pull/5535)
+
 * The `dynamic_one_shot` transform now works with broadcasting.
   [(#5473)](https://github.com/PennyLaneAI/pennylane/pull/5473)
 

pennylane/templates/layers/particle_conserving_u1.py

@@ -183,9 +183,9 @@ class ParticleConservingU1(Operation):
         weights (tensor_like): Array of weights of shape ``(D, M, 2)``.
             ``D`` is the number of entangler block layers and :math:`M=N-1`
             is the number of exchange gates :math:`U_{1,\mathrm{ex}}` per layer.
-        wires (Iterable): wires that the template acts on
+        wires (Iterable): wires that the template acts on.
         init_state (tensor_like): iterable or shape ``(len(wires),)`` tensor representing the Hartree-Fock state
-            used to initialize the wires
+            used to initialize the wires. If ``None``, a tuple of zeros is selected as initial state.
 
     .. details::
         :title: Usage Details
@@ -266,6 +266,8 @@ def __init__(self, weights, wires, init_state=None, id=None):
                 f"Weights tensor must have third dimension of length 2; got {shape[2]}"
             )
 
+        init_state = tuple(0 for _ in wires) if init_state is None else init_state
+
         self._hyperparameters = {"init_state": tuple(init_state)}
 
         super().__init__(weights, wires=wires, id=id)

pennylane/templates/layers/particle_conserving_u2.py

@@ -93,9 +93,9 @@ class ParticleConservingU2(Operation):
         weights (tensor_like): Weight tensor of shape ``(D, M)`` where ``D`` is the number of
             layers and ``M`` = ``2N-1`` is the total number of rotation ``(N)`` and exchange
             ``(N-1)`` gates per layer.
-        wires (Iterable): wires that the template acts on
+        wires (Iterable): wires that the template acts on.
         init_state (tensor_like): iterable or shape ``(len(wires),)`` tensor representing the Hartree-Fock state
-            used to initialize the wires.
+            used to initialize the wires. If ``None``, a tuple of zeros is selected as initial state.
 
     .. details::
         :title: Usage Details
@@ -171,6 +171,8 @@ def __init__(self, weights, wires, init_state=None, id=None):
                 f"Weights tensor must have a second dimension of length {2 * len(wires) - 1}; got {shape[1]}"
             )
 
+        init_state = tuple(0 for _ in wires) if init_state is None else init_state
+
         self._hyperparameters = {"init_state": tuple(init_state)}
 
         super().__init__(weights, wires=wires, id=id)

tests/templates/test_layers/test_particle_conserving_u1.py

@@ -20,11 +20,11 @@
 from pennylane import numpy as pnp
 
 
-def test_standard_validity():
+@pytest.mark.parametrize("init_state", [np.array([1, 1, 0]), None])
+def test_standard_validity(init_state):
     """Check the operation using the assert_valid function."""
 
     weights = np.random.random(size=(1, 2, 2))
-    init_state = np.array([1, 1, 0])
     op = qml.ParticleConservingU1(weights, wires=range(3), init_state=init_state)
 
     qml.ops.functions.assert_valid(op)

tests/templates/test_layers/test_particle_conserving_u2.py

@@ -20,14 +20,14 @@
 from pennylane import numpy as pnp
 
 
-def test_standard_validity():
+@pytest.mark.parametrize("init_state", [np.array([1, 1, 0, 0]), None])
+def test_standard_validity(init_state):
     """Run standard checks with the assert_valid function."""
 
     layers = 2
     qubits = 4
 
     weights = np.random.normal(0, 2 * np.pi, (layers, 2 * qubits - 1))
-    init_state = np.array([1, 1, 0, 0])
     op = qml.ParticleConservingU2(weights, wires=range(qubits), init_state=init_state)
     qml.ops.functions.assert_valid(op)