[QInfo broadcasting (1/3)] Support for qml.math density matrix functions

doc/development/deprecations.rst

@@ -11,6 +11,11 @@ Pending deprecations
   - Still accessible in v0.31
   - Removed in v0.32
 
+* ``qml.math.reduced_dm`` has been deprecated. Please use ``qml.math.reduce_dm`` or ``qml.math.reduce_statevector`` instead.
+
+  - Still accessible in v0.31
+  - Removed in v0.32
+
 * The ``qml.specs`` dictionary will no longer support direct key access to certain keys. Instead
   these quantities can be accessed as fields of the new ``Resources`` object saved under
   ``specs_dict["resources"]``:

doc/releases/changelog-dev.md

@@ -141,6 +141,8 @@
   and now inherits from `qml.ops.op_math.ControlledOp`.
   [(#4116)](https://github.com/PennyLaneAI/pennylane/pull/4116/)
 
+* Added support for broadcasting in `qml.math.reduce_dm` and `qml.math.reduce_statevector`.
+  [(#4173)](https://github.com/PennyLaneAI/pennylane/pull/4173)
 
 <h3>Breaking changes 💔</h3>
 
@@ -176,6 +178,9 @@
 
 * `qml.grouping` module is removed. The functionality has been reorganized in the `qml.pauli` module.
 
+* `qml.math.reduced_dm` has been deprecated. Please use `qml.math.reduce_dm` or `qml.math.reduce_statevector` instead.
+  [(#4173)](https://github.com/PennyLaneAI/pennylane/pull/4173)
+
 <h3>Documentation 📝</h3>
 
 * The description of `mult` in the `qchem.Molecule` docstring now correctly states the value

pennylane/_qubit_device.py

@@ -1281,7 +1281,7 @@ def density_matrix(self, wires):
         """
         state = getattr(self, "state", None)
         wires = self.map_wires(wires)
-        return qml.math.reduced_dm(state, indices=wires, c_dtype=self.C_DTYPE)
+        return qml.math.reduce_statevector(state, indices=wires, c_dtype=self.C_DTYPE)
 
     def vn_entropy(self, wires, log_base):
         r"""Returns the Von Neumann entropy prior to measurement.

pennylane/devices/default_mixed.py

@@ -241,6 +241,20 @@ def state(self):
         # User obtains state as a matrix
         return qnp.reshape(self._pre_rotated_state, (dim, dim))
 
+    def density_matrix(self, wires):
+        """Returns the reduced density matrix over the given wires.
+
+        Args:
+            wires (Wires): wires of the reduced system
+
+        Returns:
+            array[complex]: complex array of shape ``(2 ** len(wires), 2 ** len(wires))``
+            representing the reduced density matrix of the state prior to measurement.
+        """
+        state = getattr(self, "state", None)
+        wires = self.map_wires(wires)
+        return qml.math.reduce_dm(state, indices=wires, c_dtype=self.C_DTYPE)
+
     def reset(self):
         """Resets the device"""
         super().reset()

pennylane/math/__init__.py

@@ -71,6 +71,8 @@
     mutual_info,
     purity,
     reduced_dm,
+    reduce_dm,
+    reduce_statevector,
     relative_entropy,
     sqrt_matrix,
     vn_entropy,