  Goldstone and Higgs modes have been detected in various condensed matter,
cold atom and particle physics experiments. Here, we demonstrate that the two
modes can also be observed in optical systems with only a few (artificial)
atoms inside a cavity. We establish this connection by studying the $ U(1)/Z_2
$ Dicke model where $ N $ qubits (atoms) coupled to a single photon mode. We
determine the Goldstone and Higgs modes inside the super-radiant phase and
their corresponding spectral weights by performing both $ 1/J=2/N $ expansion
and exact diagonization (ED) study at a finite $ N $. We find nearly perfect
agreements between the results achieved by the two approaches when $ N $ gets
down even to $ N = 2 $. The quantum finite size effects at a few qubits make
the two modes quite robust against an effectively small counter-rotating wave
term. We present a few schemes to reduce the critical coupling strength, so the
two modes can be observed in several experimental systems of (artificial) atoms
inside a cavity by just conventional optical measurements.
