  In this paper, we have developed a unitary variant of a double exponential
coupled cluster theory, which is capable of mimicking the effects of connected
excitations of arbitrarily high rank, using only rank-one and rank-two
parametrization of the wavefunction ansatz. While its implementation in a
classical computer necessitates the construction of an effective Hamiltonian
which involves infinite number of terms with arbitrarily high many-body rank,
the same can easily be implemented in the hybrid quantum-classical variational
quantum eigensolver framework with a reasonably shallow quantum circuit. The
method relies upon the nontrivial action of a unitary, containing a set of
rank-two scattering operators, on entangled states generated via cluster
operators. We have further introduced a number of variants of the ansatz with
different degrees of expressibility by judiciously approximating the scattering
operators. With a number of applications on strongly correlated molecules, we
have shown that all our schemes can perform uniformly well throughout the
molecular potential energy surface without significant additional
implementation cost and quantum complexity over the unitary coupled cluster
approach with single and double excitations.
