  We develop a scheme to prepare a macroscopic maximally entangled state (MMES)
between two atomic ensembles using adaptive quantum nondemolition (QND)
measurements. The quantum state of the system is evolved using a sequence of
QND measurements followed by adaptive unitaries, such that the desired
measurement outcome is obtained with asymptotically unit probability. This
procedure is repeated in z and x spin basis alternately such that the state
converges deterministically towards the maximally entangled state. Up to a
local spin-basis rotation, the maximally entangled state has zero total spin
angular momentum, i.e. it is a singlet state. Our protocol does not perform
postselection and works beyond the Holstein-Primakoff regime for the atomic
spin degrees of freedom, producing genuine macroscopic entanglement.
