Fix possibility of falsy zero logic for work_wires in templates (#6720

)

**Context:**

Follow up to #6713. 

Previously, `work_wires=0` was incorrectly being mapped to an empty
list,
```python
>>> qml.Adder(1, x_wires=(), mod=1, work_wires=0)
TypeError: object of type 'int' has no len()
```

Now, we have it properly handled,
```python
>>> qml.Adder(1, x_wires=(), mod=1, work_wires=0)
Adder(wires=[0])
>>> qml.Adder(1, x_wires=(), mod=1, work_wires=None)
ValueError: Adder: wrong number of wires. At least one wire has to be given.
```

**Description of the Change:**

**Source Code**

Change the faulty logic of `work_wires = work_wires or ()` which will
fail if `work_wires=0`.

**Test Suite**

Added tests to templates that allow single work wires, ensuring
`work_wires=0` can be passed.

**Benefits:** Fixes possibility of falsy zero logic in `work_wires`.

**Possible Drawbacks:** None.

[[sc-80454](https://app.shortcut.com/xanaduai/story/80454)]

---------

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

doc/releases/changelog-dev.md

@@ -494,6 +494,7 @@ same information.
 
 * The `Wires` object throws a `TypeError` if `wires=None`. 
   [(#6713)](https://github.com/PennyLaneAI/pennylane/pull/6713)
+  [(#6720)](https://github.com/PennyLaneAI/pennylane/pull/6720)
 
 * The `qml.Hermitian` class no longer checks that the provided matrix is hermitian.
   The reason for this removal is to allow for faster execution and avoid incompatibilities with `jax.jit`.

pennylane/ops/qubit/state_preparation.py

@@ -71,8 +71,9 @@ class BasisState(StatePrepBase):
     [0.+0.j 0.+0.j 0.+0.j 1.+0.j]
     """
 
-    def __init__(self, state, wires, id=None):
+    def __init__(self, state, wires: WiresLike, id=None):
 
+        wires = Wires(wires)
         if isinstance(state, list):
             state = qml.math.stack(state)
 
@@ -86,7 +87,6 @@ def __init__(self, state, wires, id=None):
             bin = 2 ** math.arange(len(wires))[::-1]
             state = qml.math.where((state & bin) > 0, 1, 0)
 
-        wires = Wires(wires)
         shape = qml.math.shape(state)
 
         if len(shape) != 1:

pennylane/templates/subroutines/adder.py

@@ -105,8 +105,7 @@ def __init__(
     ):  # pylint: disable=too-many-arguments
 
         x_wires = qml.wires.Wires(x_wires)
-        work_wires = work_wires or ()
-        work_wires = qml.wires.Wires(work_wires)
+        work_wires = qml.wires.Wires(() if work_wires is None else work_wires)
 
         num_works_wires = len(work_wires)
 
@@ -125,15 +124,11 @@ def __init__(
                 f"with len(x_wires)={len(x_wires)} is {2 ** len(x_wires)}, but received {mod}."
             )
 
-        all_wires = (
-            qml.wires.Wires(x_wires) + qml.wires.Wires(work_wires)
-            if work_wires
-            else qml.wires.Wires(x_wires)
-        )
+        all_wires = x_wires + work_wires
 
         self.hyperparameters["k"] = k
         self.hyperparameters["mod"] = mod
-        self.hyperparameters["work_wires"] = qml.wires.Wires(work_wires)
+        self.hyperparameters["work_wires"] = work_wires
         self.hyperparameters["x_wires"] = x_wires
 
         super().__init__(wires=all_wires, id=id)

pennylane/templates/subroutines/mod_exp.py

@@ -117,8 +117,8 @@ def __init__(
     ):  # pylint: disable=too-many-arguments
 
         output_wires = qml.wires.Wires(output_wires)
-        work_wires = work_wires or ()
-        work_wires = qml.wires.Wires(work_wires)
+        work_wires = qml.wires.Wires(() if work_wires is None else work_wires)
+
         if len(work_wires) == 0:
             raise ValueError("Work wires must be specified for ModExp")
 

pennylane/templates/subroutines/multiplier.py

@@ -122,8 +122,7 @@ def __init__(
     ):  # pylint: disable=too-many-arguments
 
         x_wires = qml.wires.Wires(x_wires)
-        work_wires = work_wires or ()
-        work_wires = qml.wires.Wires(work_wires)
+        work_wires = qml.wires.Wires(() if work_wires is None else work_wires)
         if len(work_wires) == 0:
             raise ValueError("Work wires must be specified for Multiplier")
 
@@ -148,9 +147,9 @@ def __init__(
 
         self.hyperparameters["k"] = k
         self.hyperparameters["mod"] = mod
-        self.hyperparameters["work_wires"] = qml.wires.Wires(work_wires)
-        self.hyperparameters["x_wires"] = qml.wires.Wires(x_wires)
-        all_wires = qml.wires.Wires(x_wires) + qml.wires.Wires(work_wires)
+        self.hyperparameters["work_wires"] = work_wires
+        self.hyperparameters["x_wires"] = x_wires
+        all_wires = x_wires + work_wires
         super().__init__(wires=all_wires, id=id)
 
     @property

pennylane/templates/subroutines/out_adder.py

@@ -151,8 +151,7 @@ def __init__(
         x_wires = qml.wires.Wires(x_wires)
         y_wires = qml.wires.Wires(y_wires)
         output_wires = qml.wires.Wires(output_wires)
-        work_wires = work_wires or ()
-        work_wires = qml.wires.Wires(work_wires)
+        work_wires = qml.wires.Wires(() if work_wires is None else work_wires)
 
         num_work_wires = len(work_wires)
 

pennylane/templates/subroutines/out_multiplier.py

@@ -152,8 +152,7 @@ def __init__(
         x_wires = qml.wires.Wires(x_wires)
         y_wires = qml.wires.Wires(y_wires)
         output_wires = qml.wires.Wires(output_wires)
-        work_wires = work_wires or ()
-        work_wires = qml.wires.Wires(work_wires)
+        work_wires = qml.wires.Wires(() if work_wires is None else work_wires)
 
         num_work_wires = len(work_wires)
 

pennylane/templates/subroutines/out_poly.py

@@ -273,8 +273,7 @@ def __init__(
 
         registers_wires = [*input_registers, output_wires]
 
-        work_wires = work_wires or ()
-        work_wires = qml.wires.Wires(work_wires)
+        work_wires = qml.wires.Wires(() if work_wires is None else work_wires)
         num_work_wires = len(work_wires)
         if mod is None:
             mod = 2 ** len(registers_wires[-1])
@@ -302,7 +301,7 @@ def __init__(
 
         self.hyperparameters["polynomial_function"] = polynomial_function
         self.hyperparameters["mod"] = mod
-        self.hyperparameters["work_wires"] = qml.wires.Wires(work_wires)
+        self.hyperparameters["work_wires"] = work_wires
 
         wires_vars = [len(w) for w in registers_wires[:-1]]
 

pennylane/templates/subroutines/phase_adder.py

@@ -127,8 +127,7 @@ def __init__(
         self, k, x_wires: WiresLike, mod=None, work_wire: WiresLike = (), id=None
     ):  # pylint: disable=too-many-arguments
 
-        work_wire = work_wire or ()
-        work_wire = qml.wires.Wires(work_wire)
+        work_wire = qml.wires.Wires(() if work_wire is None else work_wire)
         x_wires = qml.wires.Wires(x_wires)
 
         num_work_wires = len(work_wire)

pennylane/templates/subroutines/qrom.py

@@ -117,8 +117,7 @@ def __init__(
         control_wires = qml.wires.Wires(control_wires)
         target_wires = qml.wires.Wires(target_wires)
 
-        work_wires = work_wires or ()
-        work_wires = qml.wires.Wires(work_wires)
+        work_wires = qml.wires.Wires(() if work_wires is None else work_wires)
 
         self.hyperparameters["bitstrings"] = tuple(bitstrings)
         self.hyperparameters["control_wires"] = control_wires

tests/templates/test_subroutines/test_phase_adder.py

@@ -32,6 +32,16 @@ def test_standard_validity_Phase_Adder():
     qml.ops.functions.assert_valid(op)
 
 
+def test_falsy_zero_as_work_wire():
+    """Test that work wire is not treated as a falsy zero."""
+    k = 6
+    mod = 11
+    x_wires = [1, 2, 3, 4]
+    work_wire = 0
+    op = qml.PhaseAdder(k, x_wires=x_wires, mod=mod, work_wire=work_wire)
+    qml.ops.functions.assert_valid(op)
+
+
 def test_add_k_fourier():
     """Test the private _add_k_fourier function."""
 

tests/templates/test_subroutines/test_qrom.py

@@ -29,6 +29,12 @@ def test_assert_valid_qrom():
     qml.ops.functions.assert_valid(op)
 
 
+def test_falsy_zero_as_work_wire():
+    """Test that work wire is not treated as a falsy zero."""
+    op = qml.QROM(["1", "0", "0", "1"], control_wires=[1, 2], target_wires=[3], work_wires=0)
+    qml.ops.functions.assert_valid(op)
+
+
 class TestQROM:
     """Test the qml.QROM template."""