  Current proposals focusing on neutral atoms for quantum computing are mostly
based on using single atoms as quantum bits (qubits), while using cavity
induced coupling or dipole-dipole interaction for two-qubit operations. An
alternative approach is to use atomic ensembles as qubits. However, when an
atomic ensemble is excited, by a laser beam matched to a two-level transition
(or a Raman transition) for example, it leads to a cascade of many states as
more and more photons are absorbed^1. In order to make use of an ensemble as a
qubit, it is necessary to disrupt this cascade, and restrict the excitation to
the absorption (and emission) of a single photon only. Here, we show how this
can be achieved by using a new type of blockade mechanism, based on the
light-shift imbalance (LSI) in a Raman transition. We describe first a simple
example illustrating the concept of light shift imbalanced induced blockade
(LSIIB) using a multi-level structure in a single atom, and show verifications
of the analytic prediction using numerical simulations. We then extend this
model to show how a blockade can be realized by using LSI in the excitation of
an ensemble. Specifically, we show how the LSIIB process enables one to treat
the ensemble as a two level atom that undergoes fully deterministic Rabi
oscillations between two collective quantum states, while suppressing
excitations of higher order collective states.
