Migrate Quantum Volume circuits to use circuit library

This commit starts the process of using the terra circuits library by
refactoring the quantum volume circuit generator function to internally
use the quantum volume circuit from the library instead of constructing
it manually. It also tries to rationalize the input parameters since
several didn't have any effect and others were not properly listed as
required.

Partially addresses qiskit-community#395

qiskit/ignis/verification/quantum_volume/circuits.py

@@ -16,13 +16,18 @@
 Generates quantum volume circuits
 """
 
+import copy
+import itertools
+import warnings
+
 import numpy as np
-import qiskit
-from qiskit.quantum_info.random import random_unitary
+
+from qiskit.circuit.library import QuantumVolume
+from qiskit.circuit.quantumcircuit import QuantumCircuit
 
 
-def qv_circuits(qubit_lists=None, ntrials=1,
-                qr=None, cr=None):
+def qv_circuits(qubit_lists, ntrials=1,
+                qr=None, cr=None, num_qubits=None, seed=None):
     """
     Return a list of square quantum volume circuits (depth=width)
 
@@ -36,64 +41,58 @@ def qv_circuits(qubit_lists=None, ntrials=1,
         ntrials (int): number of random iterations
         qr (QuantumRegister): quantum register to act on (if None one is created)
         cr (ClassicalRegister): classical register to measure to (if None one is created)
+        seed (int): An optional RNG seed to use for the generated circuit
 
     Returns:
         tuple: A tuple of the type (``circuits``, ``circuits_nomeas``) wheere:
             ``circuits`` is a list of lists of circuits for the qv sequences
             (separate list for each trial) and `` circuitss_nomeas`` is the
             same circuits but with no measurements for the ideal simulation
     """
+    if qr is not None:
+        warnings.warn("Passing in a custom quantum register is deprecated and "
+                      "will be removed in a future release. This argument "
+                      "never had any effect.",
+                      DeprecationWarning)
+
+    if cr is not None:
+        warnings.warn("Passing in a custom classical register is deprecated "
+                      "and will be removed in a future release. This argument "
+                      "never had any effect.",
+                      DeprecationWarning)
+    if qubit_lists:
+        for qubit_list in qubit_lists:
+            count = itertools.count(qubit_list[0])
+            for qubit in qubit_list:
+                if qubit != next(count):
+                    warnings.warn("Using a qubit list to map a virtual circuit to "
+                                  "a physical layout is deprecated and will be "
+                                  "removed in a future release. Instead use "
+                                  "''qiskit.transpile' with the "
+                                  "'initial_layout' parameter")
+        depth_list = [len(qubit_list) for qubit_list in qubit_lists]
 
     circuits = [[] for e in range(ntrials)]
     circuits_nomeas = [[] for e in range(ntrials)]
 
-    # get the largest qubit number out of all the lists (for setting the
-    # register)
-
-    depth_list = [len(qubit_list) for qubit_list in qubit_lists]
-
-    # go through for each trial
     for trial in range(ntrials):
-
-        # go through for each depth in the depth list
         for depthidx, depth in enumerate(depth_list):
-
             n_q_max = np.max(qubit_lists[depthidx])
-
-            qr = qiskit.QuantumRegister(int(n_q_max+1), 'qr')
-            qr2 = qiskit.QuantumRegister(int(depth), 'qr')
-            cr = qiskit.ClassicalRegister(int(depth), 'cr')
-
-            qc = qiskit.QuantumCircuit(qr, cr)
-            qc2 = qiskit.QuantumCircuit(qr2, cr)
-
+            qv_circ = QuantumVolume(depth, depth, seed=seed)
+            # TODO: Remove this when we remove support for doing pseudo-layout
+            # via qubit lists
+            if n_q_max != depth:
+                qc = QuantumCircuit(int(n_q_max + 1))
+                qc2 = copy.copy(qv_circ)
+                qc.compose(qv_circ, qubit_lists[depthidx], inplace=True)
+            else:
+                qc = qv_circ
+                qc2 = copy.copy(qc)
+            qc.measure_active()
             qc.name = 'qv_depth_%d_trial_%d' % (depth, trial)
             qc2.name = qc.name
 
-            # build the circuit
-            for _ in range(depth):
-                # Generate uniformly random permutation Pj of [0...n-1]
-                perm = np.random.permutation(depth)
-                # For each pair p in Pj, generate Haar random SU(4)
-                for k in range(int(np.floor(depth/2))):
-                    unitary = random_unitary(4)
-                    pair = int(perm[2*k]), int(perm[2*k+1])
-                    qc.append(unitary, [qr[qubit_lists[depthidx][pair[0]]],
-                                        qr[qubit_lists[depthidx][pair[1]]]])
-                    qc2.append(unitary, [qr2[pair[0]],
-                                         qr2[pair[1]]])
-
-            # append an id to all the qubits in the ideal circuits
-            # to prevent a truncation error in the statevector
-            # simulators
-            qc2.u1(0, qr2)
-
             circuits_nomeas[trial].append(qc2)
-
-            # add measurement
-            for qind, qubit in enumerate(qubit_lists[depthidx]):
-                qc.measure(qr[qubit], cr[qind])
-
             circuits[trial].append(qc)
 
     return circuits, circuits_nomeas