  There is increasing interest in quantum algorithms that are based on the
imaginary-time evolution (ITE), a successful classical numerical approach to
obtain ground states. However, most of the proposals so far require heavy
post-processing computational steps on a classical computer, such as solving
linear equations. Here we provide an alternative approach to implement ITE. A
key feature in our approach is the use of an orthogonal basis set: the
propagated state is efficiently expressed in terms of orthogonal basis states
at every step of the evolution. We argue that the number of basis states needed
at those steps to achieve an accurate solution can be kept of the order of $n$,
the number of qubits, by controlling the precision (number of significant
digits) and the imaginary-time increment. The number of quantum gates per
imaginary-time step is estimated to be polynomial in $n$. Additionally, while
in many QAs the locality of the Hamiltonian is a key assumption, in our
algorithm this restriction is not required. This characteristic of our
algorithm renders it useful for studying highly nonlocal systems, such as the
occupation-representation nuclear shell model. We illustrate our algorithm
through numerical implementation on an IBM quantum simulator.
