  We experimentally study quantum Zeno effects in a parity-time (PT) symmetric
cold atom gas periodically coupled to a reservoir. Based on the
state-of-the-art control of inter-site couplings of atoms in a momentum
lattice, we implement a synthetic two-level system with passive PT symmetry
over two lattice sites, where an effective dissipation is introduced through
repeated couplings to the rest of the lattice. Quantum Zeno (anti-Zeno) effects
manifest in our experiment as the overall dissipation of the two-level system
becoming suppressed (enhanced) with increasing coupling intensity or frequency.
We demonstrate that quantum Zeno regimes exist in the broken PT symmetry phase,
and are bounded by exceptional points separating the PT symmetric and PT broken
phases, as well as by a discrete set of critical coupling frequencies. Our
experiment establishes the connection between PT-symmetry-breaking transitions
and quantum Zeno effects, and is extendable to higher dimensions or to
interacting regimes, thanks to the flexible control with atoms in a momentum
lattice.
