Fix for https://github.com/Qiskit/qiskit-aer/issues/2235

releasenotes/notes/fix-amplitude-e28a8d127ae1d5e7.yaml

@@ -0,0 +1,10 @@
+---
+prelude: >
+    AerState::probability and AerState::amplitude returned the wrong states
+    for the MPS simulator.
+fixes:
+  - |
+    Added a function that provides the correct states, ``reorder_qubits_rev``,
+    which is called now by ``MPS::get_amplitude_vector``.
+    For more details, refer to:
+    `#2235 <https://github.com/Qiskit/qiskit-aer/issues/2235>`

src/simulators/matrix_product_state/matrix_product_state_internal.cpp

@@ -82,6 +82,7 @@ template <class vec_t>
 void reorder_all_qubits(const vec_t &orig_probvector, const reg_t &qubits,
                         vec_t &new_probvector);
 uint_t reorder_qubits(const reg_t &qubits, uint_t index);
+uint_t reorder_qubits_rev(const reg_t &qubits, uint_t index);
 
 //------------------------------------------------------------------------
 // Function name: permute_all_qubits
@@ -208,6 +209,29 @@ uint_t reorder_qubits(const reg_t &qubits, uint_t index) {
   return new_index;
 }
 
+uint_t reorder_qubits_rev(const reg_t &qubits, uint_t index) {
+  uint_t new_index = 0;
+
+  int_t current_pos = 0, current_val = 0, new_pos = 0, shift = 0;
+  uint_t num_qubits = qubits.size();
+  for (uint_t i = 0; i < num_qubits; i++) {
+    current_pos = qubits[i];
+    current_val = 1ULL << current_pos;
+    new_pos = i;
+    shift = new_pos - current_pos;
+    if (index & current_val) {
+      if (shift > 0) {
+        new_index += current_val << shift;
+      } else if (shift < 0) {
+        new_index += current_val >> -shift;
+      } else {
+        new_index += current_val;
+      }
+    }
+  }
+  return new_index;
+}
+
 template <class vec_t>
 void permute_all_qubits(const vec_t &orig_statevector,
                         const reg_t &input_qubits, const reg_t &output_qubits,
@@ -1350,7 +1374,7 @@ Vector<complex_t> MPS::get_amplitude_vector(const reg_t &base_values) {
     // Since the qubits may not be ordered, we determine the actual index
     // by the internal order of the qubits, to obtain the actual_base_value
     uint_t actual_base_value =
-        reorder_qubits(qubit_ordering_.order_, base_values[i]);
+        reorder_qubits_rev(qubit_ordering_.order_, base_values[i]);
     base_value = AER::Utils::int2string(actual_base_value);
     amplitude_vector[i] = get_single_amplitude(base_value);
   }