Fix crash in newer macOS versions (Qiskit#3884)

There is an OpenBlas threading issue affecting scipy.linalg.eigh that is holding up Qiskit#3591. This removes the call to eigh and replaces it with eig that uses a different underlying blas call and just takes the real part of the resulting eigenvalues.

This also made explicit two of the tests that were causing issues as they were off by a minus sign in the test suite but were fine when run explicitly.

* fix threading crash

* fix tests to be explicit

qiskit/quantum_info/operators/channel/transformations.py

@@ -208,7 +208,10 @@ def _choi_to_kraus(data, input_dim, output_dim, atol=ATOL_DEFAULT):
     # Check if hermitian matrix
     if is_hermitian_matrix(data, atol=atol):
         # Get eigen-decomposition of Choi-matrix
-        w, v = la.eigh(data)
+        # This should be a call to la.eigh, but there is an OpenBlas
+        # threading issue that is causing segfaults.
+        w, v = la.eig(data)
+        w = w.real
         # Check eigenvalues are non-negative
         if len(w[w < -atol]) == 0:
             # CP-map Kraus representation

test/python/quantum_info/operators/channel/test_kraus.py

@@ -18,9 +18,9 @@
 import numpy as np
 from numpy.testing import assert_allclose
 
-from qiskit import QiskitError
+from qiskit import QiskitError, QuantumCircuit, QuantumRegister
 from qiskit.quantum_info.states import DensityMatrix
-from qiskit.quantum_info.operators.channel import Kraus
+from qiskit.quantum_info import Kraus, Operator
 from .channel_test_case import ChannelTestCase
 
 
@@ -62,10 +62,19 @@ def test_init(self):
 
     def test_circuit_init(self):
         """Test initialization from a circuit."""
-        circuit, target = self.simple_circuit_no_measure()
-        op = Kraus(circuit)
-        target = Kraus(target)
-        self.assertEqual(op, target)
+        unitary_x = np.array([[0, 1], [1, 0]])
+        unitary_h = np.array([[1, 1], [1, -1]]) / np.sqrt(2)
+        y90 = (1 / np.sqrt(2)) * np.array([[1, -1], [1, 1]])
+
+        target = Operator(np.kron(y90, np.kron(unitary_x, unitary_h)))
+        qr = QuantumRegister(3)
+        circ = QuantumCircuit(qr)
+        circ.h(qr[0])
+        circ.x(qr[1])
+        circ.ry(np.pi / 2, qr[2])
+
+        ans = np.linalg.norm((Kraus(circ)-Kraus(target)).data)
+        self.assertAlmostEqual(ans, 0.0)
 
     def test_circuit_init_except(self):
         """Test initialization from circuit with measure raises exception."""

test/python/quantum_info/operators/channel/test_stinespring.py

@@ -18,9 +18,9 @@
 import numpy as np
 from numpy.testing import assert_allclose
 
-from qiskit import QiskitError
+from qiskit import QiskitError, QuantumCircuit, QuantumRegister
 from qiskit.quantum_info.states import DensityMatrix
-from qiskit.quantum_info.operators.channel import Stinespring
+from qiskit.quantum_info import Stinespring, Operator
 from .channel_test_case import ChannelTestCase
 
 
@@ -56,10 +56,19 @@ def test_init(self):
 
     def test_circuit_init(self):
         """Test initialization from a circuit."""
-        circuit, target = self.simple_circuit_no_measure()
-        op = Stinespring(circuit)
-        target = Stinespring(target)
-        self.assertEqual(op, target)
+        unitary_x = np.array([[0, 1], [1, 0]])
+        unitary_h = np.array([[1, 1], [1, -1]]) / np.sqrt(2)
+        y90 = (1 / np.sqrt(2)) * np.array([[1, -1], [1, 1]])
+
+        target = Operator(np.kron(y90, np.kron(unitary_x, unitary_h)))
+        qr = QuantumRegister(3)
+        circ = QuantumCircuit(qr)
+        circ.h(qr[0])
+        circ.x(qr[1])
+        circ.ry(np.pi / 2, qr[2])
+
+        ans = np.linalg.norm((Stinespring(circ)-Stinespring(target)).data)
+        self.assertAlmostEqual(ans, 0.0)
 
     def test_circuit_init_except(self):
         """Test initialization from circuit with measure raises exception."""