Fix the AmplitudeEstimator algorithms for primitive usage (#9394)

* Fix the Amplitude Estimation algorithms with primitives

* fix bernoulli tests

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

qiskit/algorithms/amplitude_estimators/ae.py

@@ -387,15 +387,12 @@ def estimate(self, estimation_problem: EstimationProblem) -> "AmplitudeEstimatio
 
                 shots = ret.metadata[0].get("shots")
                 if shots is None:
-                    result.circuit_results = {
-                        np.binary_repr(k, circuit.num_qubits): v
-                        for k, v in ret.quasi_dists[0].items()
-                    }
+                    result.circuit_results = ret.quasi_dists[0].binary_probabilities()
                     shots = 1
                 else:
                     result.circuit_results = {
-                        np.binary_repr(k, circuit.num_qubits): round(v * shots)
-                        for k, v in ret.quasi_dists[0].items()
+                        k: round(v * shots)
+                        for k, v in ret.quasi_dists[0].binary_probabilities().items()
                     }
 
         # store shots

qiskit/algorithms/amplitude_estimators/ae_utils.py

@@ -20,22 +20,6 @@
 # pylint: disable=invalid-name
 
 
-def _probabilities_from_sampler_result(num_qubits, result, estimation_problem):
-    """calculate probabilities from sampler result"""
-    prob = 0
-    for bit, probabilities in result.quasi_dists[0].items():
-        i = int(bit)
-        # get bitstring of objective qubits
-        full_state = bin(i)[2:].zfill(num_qubits)[::-1]
-        state = "".join([full_state[i] for i in estimation_problem.objective_qubits])
-
-        # check if it is a good state
-        if estimation_problem.is_good_state(state[::-1]):
-            prob += probabilities
-
-    return prob
-
-
 def bisect_max(f, a, b, steps=50, minwidth=1e-12, retval=False):
     """Find the maximum of the real-valued function f in the interval [a, b] using bisection.
 

qiskit/algorithms/amplitude_estimators/fae.py

@@ -25,7 +25,6 @@
 
 from .amplitude_estimator import AmplitudeEstimator, AmplitudeEstimatorResult
 from .estimation_problem import EstimationProblem
-from .ae_utils import _probabilities_from_sampler_result
 
 
 class FasterAmplitudeEstimation(AmplitudeEstimator):
@@ -156,10 +155,14 @@ def _cos_estimate(self, estimation_problem, k, shots):
             if shots is None:
                 shots = 1
             self._num_oracle_calls += (2 * k + 1) * shots
+
             # sum over all probabilities where the objective qubits are 1
-            prob = _probabilities_from_sampler_result(
-                circuit.num_qubits, result, estimation_problem
-            )
+            prob = 0
+            for bit, probabilities in result.quasi_dists[0].binary_probabilities().items():
+                # check if it is a good state
+                if estimation_problem.is_good_state(bit):
+                    prob += probabilities
+
             cos_estimate = 1 - 2 * prob
         elif self._quantum_instance.is_statevector:
             circuit = self.construct_circuit(estimation_problem, k, measurement=False)

qiskit/algorithms/amplitude_estimators/iae.py

@@ -26,7 +26,6 @@
 
 from .amplitude_estimator import AmplitudeEstimator, AmplitudeEstimatorResult
 from .estimation_problem import EstimationProblem
-from .ae_utils import _probabilities_from_sampler_result
 from ..exceptions import AlgorithmError
 
 
@@ -426,12 +425,11 @@ def estimate(
                             ) from exc
 
                         # calculate the probability of measuring '1'
-                        prob = _probabilities_from_sampler_result(
-                            circuit.num_qubits, ret, estimation_problem
-                        )
-                        prob = cast(
-                            float, prob
-                        )  # tell MyPy it's a float and not Tuple[int, float ]
+                        prob = 0.0
+                        for bit, probabilities in ret.quasi_dists[0].binary_probabilities().items():
+                            # check if it is a good state
+                            if estimation_problem.is_good_state(bit):
+                                prob += probabilities
 
                         a_confidence_interval = [prob, prob]  # type: list[float]
                         a_intervals.append(a_confidence_interval)
@@ -444,8 +442,8 @@ def estimate(
                         break
 
                     counts = {
-                        np.binary_repr(k, circuit.num_qubits): round(v * shots)
-                        for k, v in ret.quasi_dists[0].items()
+                        k: round(v * shots)
+                        for k, v in ret.quasi_dists[0].binary_probabilities().items()
                     }
 
                 # calculate the probability of measuring '1', 'prob' is a_i in the paper

qiskit/algorithms/amplitude_estimators/mlae.py

@@ -364,19 +364,16 @@ def estimate(
                 result.circuit_results = []
                 shots = ret.metadata[0].get("shots")
                 if shots is None:
-                    for i, quasi_dist in enumerate(ret.quasi_dists):
-                        circuit_result = {
-                            np.binary_repr(k, circuits[i].num_qubits): v
-                            for k, v in quasi_dist.items()
-                        }
+                    for quasi_dist in ret.quasi_dists:
+                        circuit_result = quasi_dist.binary_probabilities()
                         result.circuit_results.append(circuit_result)
                     shots = 1
                 else:
                     # get counts and construct MLE input
-                    for circuit in circuits:
+                    for quasi_dist in ret.quasi_dists:
                         counts = {
-                            np.binary_repr(k, circuit.num_qubits): round(v * shots)
-                            for k, v in ret.quasi_dists[0].items()
+                            k: round(v * shots)
+                            for k, v in quasi_dist.binary_probabilities().items()
                         }
                         result.circuit_results.append(counts)
 

releasenotes/notes/fix-ae-algorithms-1c0a43c596766cb3.yaml

@@ -0,0 +1,7 @@
+---
+fixes:
+  - |
+    Fix the amplitude estimation algorithms in :mod:`.amplitude_estimators` for the usage
+    with the :class:`.BaseSampler` primitive. Previously, the measurements results were 
+    expanded to more bits than actually measured which, for oracles with more than one qubit,
+    led to potential errors in the detection of the "good" quantum states for oracles.

test/python/algorithms/test_amplitude_estimators.py

@@ -196,21 +196,21 @@ def test_qasm(self, prob, shots, qae, expect):
 
     @idata(
         [
-            [0.2, 100, AmplitudeEstimation(4), {"estimation": 0.500000, "mle": 0.562783}],
+            [0.2, 100, AmplitudeEstimation(4), {"estimation": 0.14644, "mle": 0.198783}],
             [0.0, 1000, AmplitudeEstimation(2), {"estimation": 0.0, "mle": 0.0}],
             [
                 0.2,
                 100,
                 MaximumLikelihoodAmplitudeEstimation([0, 1, 2, 4, 8]),
-                {"estimation": 0.474790},
+                {"estimation": 0.200308},
             ],
             [0.8, 10, IterativeAmplitudeEstimation(0.1, 0.05), {"estimation": 0.811711}],
-            [0.2, 1000, FasterAmplitudeEstimation(0.1, 3, rescale=False), {"estimation": 0.199073}],
+            [0.2, 1000, FasterAmplitudeEstimation(0.1, 3, rescale=False), {"estimation": 0.198640}],
             [
                 0.12,
                 100,
                 FasterAmplitudeEstimation(0.01, 3, rescale=False),
-                {"estimation": 0.120016},
+                {"estimation": 0.120017},
             ],
         ]
     )
@@ -422,10 +422,10 @@ def test_statevector(self, n, qae, expect):
 
     @idata(
         [
-            [2, AmplitudeEstimation(2), {"estimation": 0.5, "mle": 0.270290}],
-            [4, MaximumLikelihoodAmplitudeEstimation(4), {"estimation": 0.0}],
-            [3, IterativeAmplitudeEstimation(0.1, 0.1), {"estimation": 0.0}],
-            [3, FasterAmplitudeEstimation(0.01, 1), {"estimation": 0.017687}],
+            [2, AmplitudeEstimation(2), {"estimation": 0.5, "mle": 0.2702}],
+            [4, MaximumLikelihoodAmplitudeEstimation(4), {"estimation": 0.2725}],
+            [3, IterativeAmplitudeEstimation(0.1, 0.1), {"estimation": 0.2721}],
+            [3, FasterAmplitudeEstimation(0.01, 1), {"estimation": 0.2792}],
         ]
     )
     @unpack
@@ -444,7 +444,7 @@ def test_sampler(self, n, qae, expect):
 
     @idata(
         [
-            [4, 10, AmplitudeEstimation(2), {"estimation": 0.5, "mle": 0.333333}],
+            [4, 100, AmplitudeEstimation(2), {"estimation": 0.5, "mle": 0.281196}],
             [3, 10, MaximumLikelihoodAmplitudeEstimation(2), {"estimation": 0.256878}],
             [3, 1000, IterativeAmplitudeEstimation(0.01, 0.01), {"estimation": 0.271790}],
             [3, 1000, FasterAmplitudeEstimation(0.1, 4), {"estimation": 0.274168}],
@@ -465,10 +465,10 @@ def test_qasm(self, n, shots, qae, expect):
 
     @idata(
         [
-            [4, 10, AmplitudeEstimation(2), {"estimation": 0.0, "mle": 0.0}],
-            [3, 10, MaximumLikelihoodAmplitudeEstimation(2), {"estimation": 0.0}],
-            [3, 1000, IterativeAmplitudeEstimation(0.01, 0.01), {"estimation": 0.0}],
-            [3, 1000, FasterAmplitudeEstimation(0.1, 4), {"estimation": 0.000551}],
+            [4, 1000, AmplitudeEstimation(2), {"estimation": 0.5, "mle": 0.2636}],
+            [3, 10, MaximumLikelihoodAmplitudeEstimation(2), {"estimation": 0.2904}],
+            [3, 1000, IterativeAmplitudeEstimation(0.01, 0.01), {"estimation": 0.2706}],
+            [3, 1000, FasterAmplitudeEstimation(0.1, 4), {"estimation": 0.2764}],
         ]
     )
     @unpack