Inverse QFT - Implementation and Test (#15)

Implement and test inverse QFT

grove/qft/fourier.py

@@ -32,6 +32,28 @@ def bit_reversal(qubits):
         p.inst(SWAP(qubits[i], qubits[-i - 1]))
     return p
 
+def _core_qft(qubits, coeff):
+    """
+    Generates the core program to perform the quantum Fourier transform
+    
+    :param qubits: A list of qubit indexes.
+    :param coeff: A modifier for the angle used in rotations (-1 for inverse 
+                  QFT, 1 for QFT)
+    :return: A Quil program to compute the core (inverse) QFT of the qubits.
+    """
+
+    q = qubits[0]
+    qs = qubits[1:]
+    if 1 == len(qubits):
+        return [H(q)]
+    else:
+        n = 1 + len(qs)
+        cR = []
+        for idx, i in enumerate(xrange(n - 1, 0, -1)):
+            q_idx = qs[idx]
+            angle = math.pi / 2 ** (n - i)
+            cR.append(CPHASE(coeff * angle)(q, q_idx))
+        return _core_qft(qs, coeff) + list(reversed(cR)) + [H(q)]
 
 def qft(qubits):
     """
@@ -42,23 +64,27 @@ def qft(qubits):
     :return: A Quil program to compute the Fourier transform of the qubits.
     """
 
-    def core_qft(qubits):
-        q = qubits[0]
-        qs = qubits[1:]
-        if 1 == len(qubits):
-            return [H(q)]
-        else:
-            n = 1 + len(qs)
-            cR = []
-            for idx, i in enumerate(xrange(n - 1, 0, -1)):
-                q_idx = qs[idx]
-                angle = math.pi / 2 ** (n - i)
-                cR.append(CPHASE(angle)(q, q_idx))
-            return core_qft(qs) + list(reversed(cR)) + [H(q)]
-
-    p = pq.Program().inst(core_qft(qubits))
+    p = pq.Program().inst(_core_qft(qubits, 1))
     return p + bit_reversal(qubits)
 
 
+def inverse_qft(qubits):
+    """
+    Generate a program to compute the inverse quantum Fourier transform on
+    a set of qubits.
+
+    :param qubits: A list of qubit indexes.
+    :return: A Quil program to compute the inverse Fourier transform of the 
+             qubits.
+    """
+
+    qft_result = pq.Program().inst(_core_qft(qubits, -1))
+    qft_result += bit_reversal(qubits)
+    inverse_qft = pq.Program()
+    while len(qft_result.actions) > 0:
+        new_inst = qft_result.actions.pop()[1]
+        inverse_qft.inst(new_inst)
+    return inverse_qft
+
 if __name__ == '__main__':
     print qft([0, 1, 2, 3])

grove/qft/tests/test_qft.py

@@ -0,0 +1,46 @@
+##############################################################################
+# Copyright 2016-2017 Rigetti Computing
+#
+#    Licensed under the Apache License, Version 2.0 (the "License");
+#    you may not use this file except in compliance with the License.
+#    You may obtain a copy of the License at
+#
+#        http://www.apache.org/licenses/LICENSE-2.0
+#
+#    Unless required by applicable law or agreed to in writing, software
+#    distributed under the License is distributed on an "AS IS" BASIS,
+#    WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+#    See the License for the specific language governing permissions and
+#    limitations under the License.
+##############################################################################
+
+from pyquil.gates import *
+from grove.qft.fourier import inverse_qft
+import pyquil.quil as pq
+from grove.pyqaoa.utils import compare_progs # This would be nice to have in
+                                             # general purpose Util module
+
+def test_simple_inverse_qft():
+
+    trial_prog = pq.Program()
+    trial_prog.inst(X(0))
+    trial_prog = trial_prog + inverse_qft([0])
+
+    result_prog = pq.Program().inst([X(0), H(0)])
+
+    compare_progs(trial_prog, result_prog)
+
+def test_multi_qubit_qft():
+
+    trial_prog = pq.Program()
+    trial_prog.inst(X(0), X(1), X(2))
+    trial_prog = trial_prog + inverse_qft([0, 1, 2])
+
+    result_prog = pq.Program().inst([X(0), X(1), X(2),
+                                     SWAP(0, 2), H(0),
+                                     CPHASE(-1.5707963267948966, 0, 1),
+                                     CPHASE(-0.7853981633974483, 0, 2),
+                                     H(1), CPHASE(-1.5707963267948966, 1, 2),
+                                     H(2)])
+
+    compare_progs(trial_prog, result_prog)