  We introduce a new class of quantum many-particle entangled states, called
the Dicke squeezed (or DS) states, which can be used to improve the precision
in quantum metrology beyond the standard quantum limit. We show that the
enhancement in measurement precision is characterized by a single
experimentally detectable parameter, called the Dicke squeezing $\xi_{D}$,
which also bounds the entanglement depth for this class of states. The
measurement precision approaches the ultimate Heisenberg limit as $\xi_{D}$
attains the minimum in an ideal Dicke state. Compared with other entangled
states, we show that the Dicke squeezed states are more robust to decoherence
and give better measurement precision under typical experimental noise.
