  Although quantum computers have the potential to efficiently solve certain
problems considered difficult by known classical approaches, the design of a
quantum circuit remains computationally difficult. It is known that the optimal
gate design problem is equivalent to the solution of an associated optimal
control problem, the solution to which is also computationally intensive.
Hence, in this article, we introduce the application of a class of numerical
methods (termed the max-plus curse of dimensionality free techniques) that
determine the optimal control thereby synthesizing the desired unitary gate.
The application of this technique to quantum systems has a growth in complexity
that depends on the cardinality of the control set approximation rather than
the much larger growth with respect to spatial dimensions in approaches based
on gridding of the space, used in previous literature. This technique is
demonstrated by obtaining an approximate solution for the gate synthesis on
$SU(4)$- a problem that is computationally intractable by grid based
approaches.
