Merge pull request qiskit-community/qiskit-aqua#794 from richardyoung…

…00/master

qiskit-community/qiskit-aqua#793 support Quantum Post-Selection for processing counts

qiskit/aqua/utils/subsystem.py

@@ -2,7 +2,7 @@
 
 # This code is part of Qiskit.
 #
-# (C) Copyright IBM 2018, 2019.
+# (C) Copyright IBM 2018, 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
@@ -64,7 +64,7 @@ def get_subsystem_fidelity(statevector, trace_systems, subsystem_state):
     return fidelity
 
 
-def get_subsystems_counts(complete_system_counts):
+def get_subsystems_counts(complete_system_counts, post_select_index=None, post_select_flag=None):
     """
     Extract all subsystems' counts from the single complete system count dictionary.
 
@@ -76,11 +76,19 @@ def get_subsystems_counts(complete_system_counts):
     (one 2-qubit, and the other 3-qubit) in order to get the counts for the 2-qubit
     partial measurement '11' or the 3-qubit partial measurement '011'.
 
+    If the post_select_index and post_select_flag parameter are specified, the counts are
+    returned subject to that specific post selection, that is, the counts for all subsystems where
+    the subsystem at index post_select_index is equal to post_select_flag.
+
 
     Args:
         complete_system_counts (dict): The measurement count dictionary of a complete system
             that contains multiple classical registers for measurements s.t. the dictionary's
             keys have space delimiters.
+        post_select_index (int): Optional, the index of the subsystem to apply the post selection
+            to.
+        post_select_flag (str): Optional, the post selection value to apply to the subsystem
+            at index post_select_index.
 
     Returns:
         list: A list of measurement count dictionaries corresponding to
@@ -90,6 +98,9 @@ def get_subsystems_counts(complete_system_counts):
     subsystems_counts = [defaultdict(int) for _ in mixed_measurements[0].split()]
     for mixed_measurement in mixed_measurements:
         count = complete_system_counts[mixed_measurement]
-        for k, d_l in zip(mixed_measurement.split(), subsystems_counts):
-            d_l[k] += count
+        subsystem_measurements = mixed_measurement.split()
+        for k, d_l in zip(subsystem_measurements, subsystems_counts):
+            if (post_select_index is None or
+                    subsystem_measurements[post_select_index] == post_select_flag):
+                d_l[k] += count
     return [dict(d) for d in subsystems_counts]