Initialize DefaultQubit2 local rng from global rng if no seed provi…

…ded (#4394)

* initialize dev rng from global state

* change default value to global str

* Apply suggestions from code review

Co-authored-by: Frederik Wilde <[email protected]>

* Update tests/devices/experimental/test_default_qubit_2.py

* fix indentation

---------

Co-authored-by: Frederik Wilde <[email protected]>

doc/releases/changelog-dev.md

@@ -88,6 +88,10 @@
 * When given a callable, `qml.ctrl` now does its custom pre-processing on all queued operators from the callable.
   [(#4370)](https://github.com/PennyLaneAI/pennylane/pull/4370)
 
+* If no seed is specified on initialization with `DefaultQubit2`, the local random number generator will be
+  seeded from on the NumPy's global random number generator.
+  [(#4394)](https://github.com/PennyLaneAI/pennylane/pull/4394)
+
 * The experimental `DefaultQubit2` device now supports computing VJPs and JVPs using the adjoint method.
   [(#4374)](https://github.com/PennyLaneAI/pennylane/pull/4374)
 

pennylane/devices/experimental/default_qubit_2.py

@@ -44,12 +44,14 @@
 class DefaultQubit2(Device):
     """A PennyLane device written in Python and capable of backpropagation derivatives.
 
-    Args:
-        seed (Union[None, int, array_like[int], SeedSequence, BitGenerator, Generator]): A
-            seed-like parameter matching that of ``seed`` for ``numpy.random.default_rng``.
-            If no value is provided, a default RNG will be used.
-
-        max_workers (int): A ``ProcessPoolExecutor`` executes tapes asynchronously
+    Keyword Args:
+        seed="global" (Union[str, None, int, array_like[int], SeedSequence, BitGenerator, Generator]): A
+            seed-like parameter matching that of ``seed`` for ``numpy.random.default_rng`` or
+            a request to seed from numpy's global random number generator.
+            The default, ``seed="global"`` pulls a seed from NumPy's global generator. ``seed=None``
+            will pull a seed from the OS entropy.
+
+        max_workers=None (int): A ``ProcessPoolExecutor`` executes tapes asynchronously
             using a pool of at most ``max_workers`` processes. If ``max_workers`` is ``None``,
             only the current process executes tapes. If you experience any
             issue, say using JAX, TensorFlow, Torch, try setting ``max_workers`` to ``None``.
@@ -135,9 +137,10 @@ def name(self):
         """The name of the device."""
         return "default.qubit.2"
 
-    def __init__(self, seed=None, max_workers=None) -> None:
+    def __init__(self, seed="global", max_workers=None) -> None:
         super().__init__()
         self._max_workers = max_workers
+        seed = np.random.randint(0, high=10000000) if seed == "global" else seed
         self._rng = np.random.default_rng(seed)
         self._debugger = None
 

tests/devices/experimental/test_default_qubit_2.py

@@ -12,7 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 """Tests for default qubit 2."""
-# pylint: disable=import-outside-toplevel
+# pylint: disable=import-outside-toplevel, no-member
 
 import pytest
 
@@ -1456,7 +1456,7 @@ def test_different_executions(self, measurements, max_workers):
             [qml.sample(wires=0), qml.expval(qml.PauliZ(0)), qml.probs(wires=0)],
         ],
     )
-    def test_global_seed(self, measurements, max_workers):
+    def test_global_seed_and_device_seed(self, measurements, max_workers):
         """Test that a global seed does not affect the result of devices
         provided with a seed"""
         qs = qml.tape.QuantumScript([qml.Hadamard(0)], measurements, shots=1000)
@@ -1477,6 +1477,47 @@ def test_global_seed(self, measurements, max_workers):
 
         assert all(np.all(res1 == res2) for res1, res2 in zip(result1, result2))
 
+    def test_global_seed_no_device_seed_by_default(self):
+        """Test that the global numpy seed initializes the rng if device seed is none."""
+        np.random.seed(42)
+        dev = DefaultQubit2()
+        first_num = dev._rng.random()  # pylint: disable=protected-access
+
+        np.random.seed(42)
+        dev2 = DefaultQubit2()
+        second_num = dev2._rng.random()  # pylint: disable=protected-access
+
+        assert qml.math.allclose(first_num, second_num)
+
+        np.random.seed(42)
+        dev2 = DefaultQubit2(seed="global")
+        third_num = dev2._rng.random()  # pylint: disable=protected-access
+
+        assert qml.math.allclose(third_num, first_num)
+
+    def test_None_seed_not_using_global_rng(self):
+        """Test that if the seed is None, it is uncorrelated with the global rng."""
+        np.random.seed(42)
+        dev = DefaultQubit2(seed=None)
+        first_nums = dev._rng.random(10)  # pylint: disable=protected-access
+
+        np.random.seed(42)
+        dev2 = DefaultQubit2(seed=None)
+        second_nums = dev2._rng.random(10)  # pylint: disable=protected-access
+
+        assert not qml.math.allclose(first_nums, second_nums)
+
+    def test_rng_as_seed(self):
+        """Test that a PRNG can be passed as a seed."""
+        rng1 = np.random.default_rng(42)
+        first_num = rng1.random()
+
+        rng = np.random.default_rng(42)
+        dev = DefaultQubit2(seed=rng)
+        second_num = dev._rng.random()  # pylint: disable=protected-access
+
+        assert qml.math.allclose(first_num, second_num)
+
 
 class TestHamiltonianSamples:
     """Test that the measure_with_samples function works as expected for