Switch cvxopt usage to cvxpy

This commit switches the usage of cvxopt to use cvxpy instead.

qiskit/providers/aer/noise/utils/noise_transformation.py

@@ -30,6 +30,7 @@
 
 import itertools
 import numpy
+import cvxpy
 
 from qiskit.providers.aer.noise.errors import QuantumError
 from qiskit.providers.aer.noise import NoiseModel
@@ -691,22 +692,13 @@ def solve_quadratic_program(self, P, q):
         Returns:
             list: The solution of the quadratic program (represents probabilites)
 
-        Raises:
-            ImportError: If cvxopt external module is not installed
-
         Additional information
         ======================
-        This method is the only place in the code where we rely on the cvxopt library
+        This method is the only place in the code where we rely on the cvpy library
         should we consider another library, only this method needs to change
         """
-        try:
-            import cvxopt
-        except ImportError:
-            raise ImportError(
-                "The CVXOPT library is required to use this module")
-
-        P = cvxopt.matrix(numpy.array(P).astype(float))
-        q = cvxopt.matrix(numpy.array(q).astype(float)).T
+        P = numpy.array(P).astype(float)
+        q = numpy.array(q).astype(float).T
         n = len(q)
         # G and h constrain:
         #   1) sum of probs is less then 1
@@ -718,7 +710,11 @@ def solve_quadratic_program(self, P, q):
         if self.fidelity_data is not None:
             G_data.append(self.fidelity_data['coefficients'])
             h_data.append(self.fidelity_data['goal'])
-        G = cvxopt.matrix(numpy.array(G_data).astype(float))
-        h = cvxopt.matrix(numpy.array(h_data).astype(float))
-        cvxopt.solvers.options['show_progress'] = False
-        return cvxopt.solvers.qp(P, q, G, h)['x']
+        G = numpy.array(G_data).astype(float)
+        h = numpy.array(h_data).astype(float)
+        x = cvxpy.Variable(n)
+        prob = cvxpy.Problem(
+            cvxpy.Minimize((1 / 2) * cvxpy.quad_form(x, P) + q.T@x),
+            [G@x <= h])
+        prob.solve()
+        return x.value

requirements-dev.txt

@@ -3,7 +3,7 @@ cmake
 scikit-build
 cython
 asv
-cvxopt
+cvxpy
 pylint
 pycodestyle
 Sphinx>=1.8.3

test/terra/noise/test_noise_transformation.py

@@ -25,14 +25,7 @@
 from qiskit.providers.aer.noise.errors.standard_errors import pauli_error
 from qiskit.providers.aer.noise.errors.quantum_error import QuantumError
 
-try:
-    import cvxopt
-    has_cvxopt = True
-except ImportError:
-    has_cvxopt = False
 
-
-@unittest.skipUnless(has_cvxopt, "Needs cvxopt to test")
 class TestNoiseTransformer(unittest.TestCase):
     def setUp(self):
         self.ops = {
@@ -296,15 +289,6 @@ def test_errors(self):
                                     "No information about noise type seven"):
             approximate_quantum_error(error, operator_string="seven")
 
-        # let's pretend cvxopt does not exist; the script should raise ImportError with proper message
-        import unittest.mock
-        import sys
-        with unittest.mock.patch.dict(sys.modules, {'cvxopt': None}):
-            with self.assertRaisesRegex(
-                    ImportError,
-                    "The CVXOPT library is required to use this module"):
-                approximate_quantum_error(error, operator_string="reset")
-
 
 if __name__ == '__main__':
     unittest.main()