  The fidelity of quantum programs in the NISQ era is limited by high levels of
device noise. To increase the fidelity of quantum programs running on NISQ
devices, a variety of optimizations have been proposed. These include mapping
passes, routing passes, scheduling methods and standalone optimisations which
are usually incorporated into a transpiler as passes. Popular transpilers such
as those proposed by Qiskit, Cirq and Cambridge Quantum Computing make use of
these extensively. However, choosing the right set of transpiler passes and the
right configuration for each pass is a challenging problem. Transpilers often
make critical decisions using heuristics since the ideal choices are impossible
to identify without knowing the target application outcome. Further, the
transpiler also makes simplifying assumptions about device noise that often do
not hold in the real world. As a result, we often see effects where the
fidelity of a target application decreases despite using state-of-the-art
optimisations. To overcome this challenge, we propose OPTRAN, a framework for
Choosing an Optimal Pass Set for Quantum Transpilation. OPTRAN uses classically
simulable quantum circuits composed entirely of Clifford gates, that resemble
the target application, to estimate how different passes interact with each
other in the context of the target application. OPTRAN then uses this
information to choose the optimal combination of passes that maximizes the
target application's fidelity when run on the actual device. Our experiments on
IBM machines show that OPTRAN improves fidelity by 87.66% of the maximum
possible limit over the baseline used by IBM Qiskit. We also propose low-cost
variants of OPTRAN, called OPTRAN-E-3 and OPTRAN-E-1 that improve fidelity by
78.33% and 76.66% of the maximum permissible limit over the baseline at a
58.33% and 69.44% reduction in cost compared to OPTRAN respectively.
