Allow Generator object for seed in QuantumVolume circuit (#4867)

* Fix RNG in QuantumVolume circuit

* Add seed back to circuit name

Note that if seed is a generator object the name contain [num_qubits, depth, Generator(PCG64)at<memory>]

* Add reno

* Add labels back to unitaries

This means we are not using the shared Generator object, but instead instantiating new generator objects with fixed integer seeds for each call to random_unitary.

* fixup

* Update releasenotes/notes/quantum_volume_rng-2e6f46e3821aebeb.yaml

Co-authored-by: Matthew Treinish <mtreinish@kortar.org>
Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

qiskit/circuit/library/quantum_volume.py

@@ -14,7 +14,7 @@
 
 """Quantum Volume model circuit."""
 
-from typing import Optional
+from typing import Optional, Union
 
 import numpy as np
 from qiskit.quantum_info.random import random_unitary
@@ -62,30 +62,37 @@ class QuantumVolume(QuantumCircuit):
     def __init__(self,
                  num_qubits: int,
                  depth: Optional[int] = None,
-                 seed: Optional[int] = None,
+                 seed: Optional[Union[int, np.random.Generator]] = None,
                  classical_permutation: bool = True) -> None:
         """Create quantum volume model circuit of size num_qubits x depth.
 
         Args:
             num_qubits: number of active qubits in model circuit.
             depth: layers of SU(4) operations in model circuit.
-            seed: randomization seed.
+            seed: Random number generator or generator seed.
             classical_permutation: use classical permutations at every layer,
                 rather than quantum.
         """
-        depth = depth or num_qubits  # how many layers of SU(4)
-        width = int(np.floor(num_qubits/2))  # how many SU(4)s fit in each layer
-
+        # Initialize RNG
         if seed is None:
             rng_set = np.random.default_rng()
             seed = rng_set.integers(low=1, high=1000)
+        if isinstance(seed, np.random.Generator):
+            rng = seed
+        else:
+            rng = np.random.default_rng(seed)
 
+        # Parameters
+        depth = depth or num_qubits  # how many layers of SU(4)
+        width = int(np.floor(num_qubits/2))  # how many SU(4)s fit in each layer
         name = "quantum_volume_" + str([num_qubits, depth, seed]).replace(' ', '')
-
         super().__init__(num_qubits, name=name)
 
-        rng = np.random.default_rng(seed)
-
+        # Generator random unitary seeds in advance.
+        # Note that this means we are constructing multiple new generator
+        # objects from low-entropy integer seeds rather than pass the shared
+        # generator object to the random_unitary function. This is done so
+        # that we can use the integer seed as a label for the generated gates.
         unitary_seeds = rng.integers(low=1, high=1000, size=[depth, width])
 
         # For each layer, generate a permutation of qubits

releasenotes/notes/quantum_volume_rng-2e6f46e3821aebeb.yaml

@@ -0,0 +1,5 @@
+---
+features:
+  - |
+    Allow passing in a Numpy random ``Generator`` object for the ``seed`` kwarg
+    in :class:`qiskit.circuit.library.QuantumVolume`.

test/python/circuit/test_library.py

@@ -274,11 +274,20 @@ class TestQuantumVolumeLibrary(QiskitTestCase):
 
     def test_qv(self):
         """Test qv circuit."""
-        circuit = QuantumVolume(2, 2, seed=2, classical_permutation=False)
+        seed = 10203
+        rng1 = np.random.default_rng(seed)
+        rng2 = np.random.default_rng(seed)
+
+        depth = 2
+        width = 1
+        circuit = QuantumVolume(2, depth, seed=rng1, classical_permutation=False)
+
         expected = QuantumCircuit(2)
-        expected.swap(0, 1)
-        expected.append(random_unitary(4, seed=837), [0, 1])
-        expected.append(random_unitary(4, seed=262), [0, 1])
+        unitary_seeds = rng2.integers(low=1, high=1000, size=[depth, width])
+        for d in range(depth):
+            if rng2.permutation([0, 1]).tolist() == [1, 0]:
+                expected.swap(0, 1)
+            expected.append(random_unitary(4, seed=unitary_seeds[d][0]), [0, 1])
         expected = Operator(expected)
         simulated = Operator(circuit)
         self.assertTrue(expected.equiv(simulated))