Fixes for version 0.16.0 (#73)

* draft fix and test

* add more cases

* comments

* tests for bit2dec

* changelog and version number

* Update pennylane_forest/wavefunction.py

Co-authored-by: Theodor <theodor@xanadu.ai>

* change test name and docstring

* Update CHANGELOG.md

Co-authored-by: Theodor <theodor@xanadu.ai>

* dtype

Co-authored-by: Theodor <theodor@xanadu.ai>

CHANGELOG.md

@@ -1,19 +1,18 @@
-# Release 0.16.0-dev
-
-### New features
-
-### Improvements
+# Release 0.16.0
 
 ### Bug fixes
 
-### Breaking changes
+* Fixed a bug caused by the `expand_state` method always assuming that
 
-### Documentation
+  inactive wires are the least significant bits.
+  [(#73)](https://github.com/PennyLaneAI/pennylane-forest/pull/73)
 
 ### Contributors
 
 This release contains contributions from (in alphabetical order):
 
+Antal Száva.
+
 ---
 
 # Release 0.15.0

pennylane_forest/_version.py

@@ -2,4 +2,4 @@
    Version number (convention major.minor.patch[-label])
 """
 
-__version__ = "0.16.0-dev"
+__version__ = "0.16.0"

pennylane_forest/wavefunction.py

@@ -90,6 +90,22 @@ def apply(self, operations, **kwargs):
         self._state = self._state.reshape([2] * len(self._active_wires)).T.flatten()
         self.expand_state()
 
+    @staticmethod
+    def bit2dec(x):
+        """Auxiliary method that converts a bitstring to a decimal integer
+        using the PennyLane convention of bit ordering.
+
+        Args:
+            x (Iterable): bit string
+
+        Returns:
+            int: decimal value of the bitstring
+        """
+        y = 0
+        for i, j in enumerate(x[::-1]):
+            y += j << i
+        return y
+
     def expand_state(self):
         """The pyQuil wavefunction simulator initializes qubits dymnically as they are requested.
         This method expands the state to the full number of wires in the device."""
@@ -98,20 +114,28 @@ def expand_state(self):
             # all wires in the device have been initialised
             return
 
-        num_inactive_wires = len(self.wires) - len(self._active_wires)
-
         # translate active wires to the device's labels
         device_active_wires = self.map_wires(self._active_wires)
 
-        # place the inactive subsystems in the vacuum state
-        other_subsystems = np.zeros([2 ** num_inactive_wires])
-        other_subsystems[0] = 1
+        inactive_wires = [x for x in range(len(self.wires)) if x not in device_active_wires]
+
+        # initialize the entire new expanded state to zeros
+        expanded_state = np.zeros([2 ** len(self.wires)], dtype=self.C_DTYPE)
 
-        # expand the state of the device into a length-num_wire state vector
-        expanded_state = np.kron(self._state, other_subsystems).reshape([2] * self.num_wires)
-        expanded_state = np.moveaxis(
-            expanded_state, range(len(device_active_wires)), device_active_wires.labels
+        # gather the bit strings for the subsystem made up of the active qubits
+        subsystem_bit_strings = self.states_to_binary(
+            np.arange(2 ** len(self._active_wires)), len(self._active_wires)
         )
-        expanded_state = expanded_state.flatten()
+
+        for string, amplitude in zip(subsystem_bit_strings, self._state):
+            for w in inactive_wires:
+
+                # expand the bitstring by inserting a zero bit for each inactive qubit
+                string = np.insert(string, w, 0)
+
+            # calculate the decimal value of the bit string, that gives the
+            # index of the amplitude in the state vector
+            decimal_val = self.bit2dec(string)
+            expanded_state[decimal_val] = amplitude
 
         self._state = expanded_state

tests/test_wavefunction.py

@@ -21,22 +21,6 @@
 class TestWavefunctionBasic(BaseTest):
     """Unit tests for the wavefunction simulator."""
 
-    def test_expand_state(self):
-        """Test that a multi-qubit state is correctly expanded for a N-qubit device"""
-        dev = plf.WavefunctionDevice(wires=3)
-
-        # expand a two qubit state to the 3 qubit device
-        dev._state = np.array([0, 1, 1, 0]) / np.sqrt(2)
-        dev._active_wires = Wires([0, 2])
-        dev.expand_state()
-        self.assertAllEqual(dev._state, np.array([0, 1, 0, 0, 1, 0, 0, 0]) / np.sqrt(2))
-
-        # expand a three qubit state to the 3 qubit device
-        dev._state = np.array([0, 1, 1, 0, 0, 1, 1, 0]) / 2
-        dev._active_wires = Wires([0, 1, 2])
-        dev.expand_state()
-        self.assertAllEqual(dev._state, np.array([0, 1, 1, 0, 0, 1, 1, 0]) / 2)
-
     def test_var(self, tol, qvm):
         """Tests for variance calculation"""
         dev = plf.WavefunctionDevice(wires=2)
@@ -411,3 +395,48 @@ def circuit(x, y, z):
 
         expected_var = np.sqrt(1 / shots)
         self.assertAlmostEqual(np.mean(runs), np.cos(a) * np.sin(b), delta=expected_var)
+
+class TestExpandState(BaseTest):
+    """Test the expand_state method"""
+
+    def test_expand_state(self):
+        """Test that a multi-qubit state is correctly expanded for a N-qubit device"""
+        dev = plf.WavefunctionDevice(wires=3)
+
+        # expand a two qubit state to the 3 qubit device
+        dev._state = np.array([0, 1, 1, 0]) / np.sqrt(2)
+        dev._active_wires = Wires([0, 2])
+        dev.expand_state()
+        self.assertAllEqual(dev._state, np.array([0, 1, 0, 0, 1, 0, 0, 0]) / np.sqrt(2))
+
+        # expand a three qubit state to the 3 qubit device
+        dev._state = np.array([0, 1, 1, 0, 0, 1, 1, 0]) / 2
+        dev._active_wires = Wires([0, 1, 2])
+        dev.expand_state()
+        self.assertAllEqual(dev._state, np.array([0, 1, 1, 0, 0, 1, 1, 0]) / 2)
+
+
+    @pytest.mark.parametrize("bitstring, dec", [([1], 1), ([0,0,1], 1), ([0,1,0], 2), ([1,1,1], 7)])
+    def test_bit2dec(self, bitstring, dec):
+        """Test that the bit2dec method produces the correct decimal values for
+        bitstrings"""
+        assert plf.WavefunctionDevice.bit2dec(bitstring) == dec
+
+    @pytest.mark.parametrize("num_wires", [3,4] )
+    @pytest.mark.parametrize("wire_idx1, wire_idx2", [(0, 0), (1, 0), (1, 1)] )
+    def test_expanded_matches_default_qubit(self, wire_idx1, wire_idx2, num_wires):
+        """Test that the statevector is expanded correctly by checking the
+        output of a QNode with default.qubit."""
+        dev_default_qubit = qml.device('default.qubit', wires=num_wires)
+        dev_wavefunction = qml.device('forest.wavefunction', wires=num_wires)
+
+        def circuit():
+            qml.RY(np.pi/2, wires=[wire_idx1 + 1])
+            qml.RY(np.pi, wires=[wire_idx2 + 0])
+            qml.CNOT(wires=[wire_idx1 + 1, wire_idx2 + 0])
+            qml.CNOT(wires=[wire_idx1 + 1, wire_idx2 + 0])
+            return qml.probs(range(num_wires))
+
+        qnode1 = qml.QNode(circuit, dev_wavefunction)
+        qnode2 = qml.QNode(circuit, dev_default_qubit)
+        assert np.allclose(qnode1(), qnode2())