  Designing efficient and robust quantum control strategies is vital for
developing quantum technologies. One recent strategy is the Quantum Alternating
Operator Ansatz (QAOA) sequence that alternatively propagates under two
noncommuting Hamiltonians, whose control parameters can be optimized to
generate a gate or prepare a state. Here, we describe the design of the QAOA
sequence and their variants to prepare long-lived singlet states (LLS) from the
thermal state in NMR. With extraordinarily long lifetimes exceeding the
spin-lattice relaxation time constant $T_1$, LLS have been of great interest
for various applications, from spectroscopy to medical imaging. Accordingly,
designing sequences for efficiently preparing LLS in a general spin system is
crucial. Using numerical analysis, we study the efficiency and robustness of
the QAOA sequences over a wide range of errors in the control parameters. Using
a two-qubit NMR register, we conduct an experimental study to benchmark QAOA
sequences against other prominent methods of LLS preparation and observe the
significantly superior performance of the QAOA sequences.
