Use the circuit library's QFT (#909)

* use QFT from circuit lib

* make algos accept circuits + QFTs

* deprecate QFT circ and components

* allow QFT to be passed as circuit

* add deprecation warnings if QFT component used

* fix missing import

* fix cyclic import

* remove todos, fixed in Qiskit/qiskit#4209

* filter deprecations warnings from QFT

* add checks on qft size

* only filter warnings locally

* re-enable tests in tearDown to be sure

Author: Cryoris

qiskit/aqua/algorithms/amplitude_estimators/ae.py

@@ -11,21 +11,24 @@
 # Any modifications or derivative works of this code must retain this
 # copyright notice, and modified files need to carry a notice indicating
 # that they have been altered from the originals.
+
 """The Quantum Phase Estimation-based Amplitude Estimation algorithm."""
 
 from typing import Optional, Union, List, Tuple
+import warnings
 import logging
 from collections import OrderedDict
 import numpy as np
 from scipy.stats import chi2, norm
 from scipy.optimize import bisect
 
 from qiskit import QuantumCircuit
+from qiskit.circuit.library import QFT
 from qiskit.providers import BaseBackend
 from qiskit.aqua import QuantumInstance, AquaError
 from qiskit.aqua.utils import CircuitFactory
 from qiskit.aqua.circuits import PhaseEstimationCircuit
-from qiskit.aqua.components.iqfts import IQFT, Standard
+from qiskit.aqua.components.iqfts import IQFT
 from qiskit.aqua.utils.validation import validate_min
 from .ae_algorithm import AmplitudeEstimationAlgorithm
 from .ae_utils import pdf_a, derivative_log_pdf_a, bisect_max
@@ -54,7 +57,7 @@ def __init__(self, num_eval_qubits: int,
                  a_factory: Optional[CircuitFactory] = None,
                  q_factory: Optional[CircuitFactory] = None,
                  i_objective: Optional[int] = None,
-                 iqft: Optional[IQFT] = None,
+                 iqft: Optional[Union[QuantumCircuit, IQFT]] = None,
                  quantum_instance: Optional[Union[QuantumInstance, BaseBackend]] = None) -> None:
         r"""
         Args:
@@ -75,10 +78,13 @@ def __init__(self, num_eval_qubits: int,
         self._m = num_eval_qubits
         self._M = 2 ** num_eval_qubits
 
-        if iqft is None:
-            iqft = Standard(self._m)
-
-        self._iqft = iqft
+        if isinstance(iqft, IQFT):
+            warnings.warn('The qiskit.aqua.components.iqfts.IQFT module is deprecated as of 0.7.0 '
+                          'and will be removed no earlier than 3 months after the release. '
+                          'You should pass a QuantumCircuit instead, see '
+                          'qiskit.circuit.library.QFT and the .inverse() method.',
+                          DeprecationWarning, stacklevel=2)
+        self._iqft = iqft or QFT(self._m).inverse()
         self._circuit = None
         self._ret = {}
 

qiskit/aqua/algorithms/linear_solvers/hhl.py

@@ -11,9 +11,8 @@
 # Any modifications or derivative works of this code must retain this
 # copyright notice, and modified files need to carry a notice indicating
 # that they have been altered from the originals.
-"""
-The HHL algorithm.
-"""
+
+"""The HHL algorithm."""
 
 from typing import Optional, Union
 import logging
@@ -37,9 +36,7 @@
 
 
 class HHL(QuantumAlgorithm):
-
-    r"""
-    The HHL algorithm.
+    r"""The HHL algorithm.
 
     The HHL algorithm (after the author's surnames Harrow-Hassidim-Lloyd) is a quantum algorithm
     to solve systems of linear equations :math:`A\overrightarrow{x}=\overrightarrow{b}`.

qiskit/aqua/algorithms/minimum_eigen_solvers/qpe.py

@@ -11,9 +11,8 @@
 # Any modifications or derivative works of this code must retain this
 # copyright notice, and modified files need to carry a notice indicating
 # that they have been altered from the originals.
-"""
-The Quantum Phase Estimation Algorithm.
-"""
+
+"""The Quantum Phase Estimation Algorithm."""
 
 import logging
 from typing import Optional, List, Dict, Union
@@ -60,7 +59,7 @@ class QPEMinimumEigensolver(QuantumAlgorithm, MinimumEigensolver):
     def __init__(self,
                  operator: Optional[BaseOperator] = None,
                  state_in: Optional[InitialState] = None,
-                 iqft: Optional[IQFT] = None,
+                 iqft: Optional[Union[QuantumCircuit, IQFT]] = None,
                  num_time_slices: int = 1,
                  num_ancillae: int = 1,
                  expansion_mode: str = 'trotter',
@@ -90,7 +89,15 @@ def __init__(self,
         validate_min('expansion_order', expansion_order, 1)
         super().__init__(quantum_instance)
         self._state_in = state_in
+
+        if isinstance(iqft, IQFT):
+            warnings.warn('The qiskit.aqua.components.iqfts.IQFT module is deprecated as of 0.7.0 '
+                          'and will be removed no earlier than 3 months after the release. '
+                          'You should pass a QuantumCircuit instead, see '
+                          'qiskit.circuit.library.QFT and the .inverse() method.',
+                          DeprecationWarning, stacklevel=2)
         self._iqft = iqft
+
         self._num_time_slices = num_time_slices
         self._num_ancillae = num_ancillae
         self._expansion_mode = expansion_mode

qiskit/aqua/circuits/fourier_transform_circuits.py

@@ -2,7 +2,7 @@
 
 # This code is part of Qiskit.
 #
-# (C) Copyright IBM 2019.
+# (C) Copyright IBM 2019, 2020.
 #
 # This code is licensed under the Apache License, Version 2.0. You may
 # obtain a copy of this license in the LICENSE.txt file in the root directory
@@ -11,28 +11,17 @@
 # Any modifications or derivative works of this code must retain this
 # copyright notice, and modified files need to carry a notice indicating
 # that they have been altered from the originals.
-"""
-Quantum Fourier Transform Circuit.
-"""
 
-import numpy as np
+"""DEPRECATED. Quantum Fourier Transform Circuit."""
 
-from qiskit.circuit import QuantumRegister, QuantumCircuit, Qubit  # pylint: disable=unused-import
+import warnings
 
+from qiskit.circuit.library import QFT
 from qiskit.aqua import AquaError
 
 
 class FourierTransformCircuits:
-    """
-    Quantum Fourier Transform Circuit.
-    """
-    @staticmethod
-    def _do_swaps(circuit, qubits):
-        num_qubits = len(qubits)
-        for i in range(num_qubits // 2):
-            circuit.cx(qubits[i], qubits[num_qubits - i - 1])
-            circuit.cx(qubits[num_qubits - i - 1], qubits[i])
-            circuit.cx(qubits[i], qubits[num_qubits - i - 1])
+    """DEPRECATED. Quantum Fourier Transform Circuit."""
 
     @staticmethod
     def construct_circuit(
@@ -42,8 +31,7 @@ def construct_circuit(
             approximation_degree=0,
             do_swaps=True
     ):
-        """
-        Construct the circuit representing the desired state vector.
+        """Construct the circuit representing the desired state vector.
 
         Args:
             circuit (QuantumCircuit): The optional circuit to extend from.
@@ -60,50 +48,21 @@ def construct_circuit(
         Raises:
             AquaError: invalid input
         """
+        warnings.warn('The class FourierTransformCircuits is deprecated and will be removed '
+                      'no earlier than 3 months after the release 0.7.0. You should use the '
+                      'qiskit.circuit.library.QFT class instead.',
+                      DeprecationWarning, stacklevel=2)
 
         if circuit is None:
             raise AquaError('Missing input QuantumCircuit.')
 
         if qubits is None:
             raise AquaError('Missing input qubits.')
 
-        if isinstance(qubits, QuantumRegister):
-            if not circuit.has_register(qubits):
-                circuit.add_register(qubits)
-        elif isinstance(qubits, list):
-            for qubit in qubits:
-                if isinstance(qubit, Qubit):
-                    if not circuit.has_register(qubit.register):
-                        circuit.add_register(qubit.register)
-                else:
-                    raise AquaError('A QuantumRegister or a list of qubits '
-                                    'is expected for the input qubits.')
-        else:
-            raise AquaError('A QuantumRegister or a list of qubits '
-                            'is expected for the input qubits.')
-
-        if do_swaps and not inverse:
-            FourierTransformCircuits._do_swaps(circuit, qubits)
-
-        qubit_range = reversed(range(len(qubits))) if inverse else range(len(qubits))
-        for j in qubit_range:
-            neighbor_range = range(np.max([0, j - len(qubits) + approximation_degree + 1]), j)
-            if inverse:
-                neighbor_range = reversed(neighbor_range)
-                circuit.u2(0, np.pi, qubits[j])
-            for k in neighbor_range:
-                lam = 1.0 * np.pi / float(2 ** (j - k))
-                if inverse:
-                    lam *= -1
-                circuit.u1(lam / 2, qubits[j])
-                circuit.cx(qubits[j], qubits[k])
-                circuit.u1(-lam / 2, qubits[k])
-                circuit.cx(qubits[j], qubits[k])
-                circuit.u1(lam / 2, qubits[k])
-            if not inverse:
-                circuit.u2(0, np.pi, qubits[j])
+        qft = QFT(len(qubits), approximation_degree=approximation_degree, do_swaps=do_swaps)
+        if inverse:
+            qft = qft.inverse()
 
-        if do_swaps and inverse:
-            FourierTransformCircuits._do_swaps(circuit, qubits)
+        circuit.append(qft.to_instruction(), qubits)
 
         return circuit

qiskit/aqua/circuits/phase_estimation_circuit.py

@@ -11,10 +11,10 @@
 # Any modifications or derivative works of this code must retain this
 # copyright notice, and modified files need to carry a notice indicating
 # that they have been altered from the originals.
-"""
-Quantum Phase Estimation Circuit.
-"""
 
+"""Quantum Phase Estimation Circuit."""
+
+import warnings
 import numpy as np
 
 from qiskit import QuantumRegister, QuantumCircuit, ClassicalRegister
@@ -26,9 +26,7 @@
 
 
 class PhaseEstimationCircuit:
-    """
-    Quantum Phase Estimation Circuit.
-    """
+    """Quantum Phase Estimation Circuit."""
 
     def __init__(
             self,
@@ -46,13 +44,12 @@ def __init__(
             pauli_list=None
     ):
         """
-        Constructor.
-
         Args:
             operator (WeightedPauliOperator): the hamiltonian Operator object
             state_in (InitialState): the InitialState component
             representing the initial quantum state
-            iqft (IQFT): the Inverse Quantum Fourier Transform component
+            iqft (Union[QuantumCircuit, IQFT]): the Inverse Quantum Fourier Transform as circuit or
+                Aqua component
             num_time_slices (int): the number of time slices
             num_ancillae (int): the number of ancillary qubits to use for the measurement
             expansion_mode (str): the expansion mode (trotter|suzuki)
@@ -65,6 +62,7 @@ def __init__(
             shallow_circuit_concat (bool): indicate whether to use shallow (cheap) mode
             for circuit concatenation
             pauli_list (list[Pauli]): the flat list of paulis for the operator
+
         Raises:
             AquaError: Missing input
         """
@@ -80,7 +78,16 @@ def __init__(
         self._unitary_circuit_factory = unitary_circuit_factory
         self._state_in = state_in
         self._state_in_circuit_factory = state_in_circuit_factory
+
+        # cannot check for IQFT type due to circular import
+        if not isinstance(iqft, QuantumCircuit):
+            warnings.warn('The qiskit.aqua.components.iqfts.IQFT module is deprecated as of 0.7.0 '
+                          'and will be removed no earlier than 3 months after the release. '
+                          'You should pass a QuantumCircuit instead, see '
+                          'qiskit.circuit.library.QFT and the .inverse() method.',
+                          DeprecationWarning, stacklevel=2)
         self._iqft = iqft
+
         self._num_time_slices = num_time_slices
         self._num_ancillae = num_ancillae
         self._expansion_mode = expansion_mode
@@ -100,8 +107,7 @@ def construct_circuit(
             auxiliary_register=None,
             measurement=False,
     ):
-        """
-        Construct the Phase Estimation circuit
+        """Construct the Phase Estimation circuit
 
         Args:
             state_register (QuantumRegister): the optional register to use for the quantum state
@@ -113,6 +119,7 @@ def construct_circuit(
 
         Returns:
             QuantumCircuit: the QuantumCircuit object for the constructed circuit
+
         Raises:
             RuntimeError: Multiple identity pauli terms are present
             ValueError: invalid mode
@@ -209,7 +216,20 @@ def construct_circuit(
                     self._unitary_circuit_factory.build_controlled_power(qc, q, a[i], 2 ** i, aux)
 
             # inverse qft on ancillae
-            self._iqft.construct_circuit(mode='circuit', qubits=a, circuit=qc, do_swaps=False)
+            if isinstance(self._iqft, QuantumCircuit):
+                # check if QFT has the right size
+                if self._iqft.num_qubits != len(a):
+                    try:  # try resizing
+                        self._iqft.num_qubits = len(a)
+                    except AttributeError:
+                        raise ValueError('The IQFT cannot be resized and does not have the '
+                                         'required size of {}'.format(len(a)))
+
+                if hasattr(self._iqft, 'do_swaps'):
+                    self._iqft.do_swaps = False
+                qc.append(self._iqft.to_instruction(), a)
+            else:
+                self._iqft.construct_circuit(mode='circuit', qubits=a, circuit=qc, do_swaps=False)
 
             if measurement:
                 c_ancilla = ClassicalRegister(self._num_ancillae, name='ca')