This should be a better fix for Qiskit#2292

src/simulators/matrix_product_state/matrix_product_state_internal.cpp

@@ -812,6 +812,8 @@ void MPS::apply_multi_qubit_gate(const reg_t &qubits, const cmatrix_t &mat,
   // change qubit order in the matrix - instead of doing swaps on the qubits
   uint_t nqubits = qubits.size();
   uint_t sidelen = 1 << nqubits;
+
+  /*
   cmatrix_t new_mat(sidelen, sidelen);
   for (uint_t col = 0; col < sidelen; ++col) {
     for (uint_t row = 0; row < sidelen; ++row) {
@@ -821,6 +823,27 @@ void MPS::apply_multi_qubit_gate(const reg_t &qubits, const cmatrix_t &mat,
         new_mat(new_vec[row], new_vec[col]) = is_diagonal ? 0. : mat(row, col);
     }
   }
+  */
+
+  // alternative solution, maybe it's better:
+  cmatrix_t new_mat(is_diagonal ? 1 : sidelen, sidelen);
+
+  if (is_diagonal) {
+    for (uint_t col = 0; col < sidelen; ++col) {
+      new_mat(0, new_vec[col]) =
+          mat.GetRows() == 1 ? mat(0, col) : mat(col, col); // this is just in case something passes a matrix
+                                                            // instead of a vector even if is_diagonal is true
+    }
+  } else {
+    for (uint_t col = 0; col < sidelen; ++col) {
+      for (uint_t row = 0; row < sidelen; ++row) {
+        if (row == col)
+          new_mat(new_vec[row], new_vec[row]) = mat(row, row);
+        else
+          new_mat(new_vec[row], new_vec[col]) = mat(row, col);
+      }
+    }
+  }
 
   if (is_ordered(qubits))
     apply_matrix_to_target_qubits(qubits, new_mat, is_diagonal);