Corrected qubits ordering in MPS::sample_measure_using_probabilities.

releasenotes/notes/sample_measure-bug-4ce3a1a6553e4900.yaml

@@ -0,0 +1,10 @@
+---
+
+fixes:
+  - |
+    When invoking MPS::sample_measure, we need to first sort the qubits to the 
+    default ordering because this is the assumption in qasm_controller.This is 
+    done by invoking the method move_all_qubits_to_sorted_ordering. It was 
+    correct in sample_measure_using_apply_measure, but missing in 
+    sample_measure_using_probabilities. 
+

src/simulators/matrix_product_state/matrix_product_state.hpp

@@ -141,7 +141,7 @@ class State : public Base::State<matrixproductstate_t> {
   std::vector<reg_t> 
   sample_measure_using_probabilities(const reg_t &qubits,
 				     uint_t shots,
-				     RngEngine &rng) const;
+				     RngEngine &rng);
 
   // Computes sample_measure by copying the MPS to a temporary structure, and
   // applying a measurement on the temporary MPS. This is done for every shot,
@@ -765,9 +765,9 @@ std::vector<reg_t> State::sample_measure(const reg_t &qubits,
   // The parameters used below are based on experimentation.
   // The user can override this by setting the parameter "mps_sample_measure_algorithm"
   uint_t num_qubits = qubits.size();
-
-  if (MPS::get_sample_measure_alg() == Sample_measure_alg::PROB || num_qubits < 10)
+  if (MPS::get_sample_measure_alg() == Sample_measure_alg::PROB){
     return sample_measure_using_probabilities(qubits, shots, rng);
+  }
   if (MPS::get_sample_measure_alg() == Sample_measure_alg::APPLY_MEASURE ||
       num_qubits >26 )
      return sample_measure_using_apply_measure(qubits, shots, rng);
@@ -778,6 +778,8 @@ std::vector<reg_t> State::sample_measure(const reg_t &qubits,
   // Sample_measure_alg::HEURISTIC
   uint_t max_bond_dim = qreg_.get_max_bond_dimensions();
 
+  if (num_qubits <10)
+    return sample_measure_using_probabilities(qubits, shots, rng);
   if (max_bond_dim <= 2) {
     if (shots_dbl < 12.0 * pow(1.85, (num_qubits_dbl-10.0)))
        return sample_measure_using_apply_measure(qubits, shots, rng);
@@ -805,7 +807,7 @@ std::vector<reg_t> State::sample_measure(const reg_t &qubits,
 std::vector<reg_t> State::
 sample_measure_using_probabilities(const reg_t &qubits,
 				   uint_t shots,
-				   RngEngine &rng) const {
+				   RngEngine &rng) {
 
   // Generate flat register for storing
   rvector_t rnds;

src/simulators/matrix_product_state/matrix_product_state_internal.cpp

@@ -1254,7 +1254,15 @@ double MPS::norm(const reg_t &qubits, const cmatrix_t &mat) const {
 
 //-----------------------------------------------------------------------------
 reg_t MPS::sample_measure_using_probabilities(const rvector_t &rnds, 
-					      const reg_t &qubits) const {
+					      const reg_t &qubits) {
+  // since input is always sorted in qasm_controller, therefore, we must return the qubits 
+  // to their original location (sorted)
+  move_all_qubits_to_sorted_ordering();
+  return sample_measure_using_probabilities_internal(rnds, qubits);
+}
+
+reg_t MPS::sample_measure_using_probabilities_internal(const rvector_t &rnds, 
+						       const reg_t &qubits) const {
   const uint_t SHOTS = rnds.size();
   reg_t samples;
   samples.assign(SHOTS, 0);

src/simulators/matrix_product_state/matrix_product_state_internal.hpp

@@ -244,7 +244,7 @@ class MPS{
   double norm(const reg_t &qubits, const cmatrix_t &mat) const; 
 
   reg_t sample_measure_using_probabilities(const rvector_t &rnds, 
-					   const reg_t &qubits) const;
+					   const reg_t &qubits);
 
   reg_t apply_measure(const reg_t &qubits,
 		      RngEngine &rng);
@@ -342,6 +342,9 @@ class MPS{
    uint_t apply_measure(uint_t qubit, 
 			  RngEngine &rng);
 
+  reg_t sample_measure_using_probabilities_internal(const rvector_t &rnds, 
+						    const reg_t &qubits) const;
+
   void initialize_from_matrix(uint_t num_qubits, cmatrix_t mat);
   //----------------------------------------------------------------
   // Function name: centralize_qubits