  Quantum many-body scars have attracted much interest as a violation of the
eigenstate thermalization hypothesis (ETH) due to recent experimental
observation in Rydberg atoms and related theoretical studies. In this paper, we
construct a model hosting exact Floquet quantum many-body scars, which violate
the Floquet version of ETH. We consider two uniformly-driven static
Hamiltonians prohibiting neighboring up spins (Rydberg blockade) like the PXP
model, and construct a binary drive composed of them. We show that there exists
a four-dimensional subspace which completely avoids thermalization to infinite
temperature and that any other states, including some special scar states
reported in the static PXP model, are vulnerable to heating and relax to
infinite temperature. We also construct a more generalized periodic drive
composed of time-dependent PXP-type Hamiltonians showing exact Floquet quantum
many-body scars and discuss possible experimental realization of the model in
Rydberg atoms.
