patched Trotterisation of order >= 4

Previously, the order >= 4 scenario of Trotterisation did not correctly invoke recursion, but instead called first order Trotterisation five times without symmetrisation. This meant passing order=4 erroneously excluded symmetrisation (halving the Trotter depth), and passing order>=6 merely performed unsymmetrised fourth-order Trotterisation.

Thankfully the exacted operation was still a valid Trotter approximation of the intended unitary, albeit of lower order and ergo accuracy than expected. This was not caught by the unit tests since they do not exist, as warned in the function documentation. Eep!

Author: TysonRayJones

quest/src/api/trotterisation.cpp

@@ -68,11 +68,11 @@ void internal_applyHigherOrderTrotterRepetition(
         qcomp b = (1-4*p) * angle;
 
         int lower = order - 2;
-        internal_applyFirstOrderTrotterRepetition(qureg, ketCtrls, braCtrls, states, sum, a, lower);
-        internal_applyFirstOrderTrotterRepetition(qureg, ketCtrls, braCtrls, states, sum, a, lower);
-        internal_applyFirstOrderTrotterRepetition(qureg, ketCtrls, braCtrls, states, sum, b, lower);
-        internal_applyFirstOrderTrotterRepetition(qureg, ketCtrls, braCtrls, states, sum, a, lower);
-        internal_applyFirstOrderTrotterRepetition(qureg, ketCtrls, braCtrls, states, sum, a, lower);
+        internal_applyHigherOrderTrotterRepetition(qureg, ketCtrls, braCtrls, states, sum, a, lower); // angle -> a
+        internal_applyHigherOrderTrotterRepetition(qureg, ketCtrls, braCtrls, states, sum, a, lower);
+        internal_applyHigherOrderTrotterRepetition(qureg, ketCtrls, braCtrls, states, sum, b, lower); // angle -> b
+        internal_applyHigherOrderTrotterRepetition(qureg, ketCtrls, braCtrls, states, sum, a, lower);
+        internal_applyHigherOrderTrotterRepetition(qureg, ketCtrls, braCtrls, states, sum, a, lower);
     }
 }