  We propose a new physical approach for encoding and processing of quantum
information in ensembles of multi-level quantum systems, where the different
bits are not carried by individual particles but associated with the collective
population of different internal levels. One- and two-bit gates are implemented
by collective internal state transitions taking place in the presence of an
excitation blockade mechanism which restricts the population of each internal
state to the values zero and unity. 10-20 bit quantum computers can be built
via this scheme in single trapped clouds of ground state atoms subject to the
Rydberg excitation blockade mechanism, and the linear dependence between
register size and the number of internal quantum states in atoms offers
realistic means to reach larger registers.
