  Dicke state, a coherent state of multiple particles, is fundamentally
responsible for various intriguing collective behaviors of many-body systems.
Experimental access to Dicke states with a scalable and deterministic number of
particles is essential to study how many-body properties depend on the particle
number. We report the observation of Dicke states consisting of two, three,
four, five, six, and seven atoms in an atomic vapor. Quantum coherences between
the ground state and multi-atom states are created and detected by using
optical two-dimensional coherent spectroscopy. The signal originated from
multi-atom states is manifested as correlation between the multi-quantum
coherence and the emission signal, providing direct and unambiguous detection
of Dicke states. The manipulation of deterministic atomic Dicke states has
possible implications in quantum information processing and fundamental
many-body physics.
