Experimental observation of partial parity-time symmetry and its phase transition with a laser-driven cesium atomic gas

Realizing and manipulating parity-time (PT) symmetry in multidimensional systems are highly desirable for the exploration of nontrivial physics and the discovery of exotic phenomena in non-Hermitian systems. Finding non-Hermitian systems that still have all-real spectra even if their Hamiltonians possess only partial PT symmetry has also attracted tremendous attention in recent years. Here, we report the experimental observation of partial PT symmetry in a cesium atomic gas coupled with laser fields, where a two-dimensional partially PT-symmetric optical potential for a probe laser beam is created. A transition of the partial PT symmetry from an unbroken phase to a broken one is observed through changing the beam-waist ratio of the control and probe laser beams, and the domains of unbroken, broken, and nonpartial PT phases are also discriminated unambiguously. Moreover, we develop a technique to precisely determine the exceptional point location of the partial PT symmetry breaking by measuring the asymmetry degree of the probe-beam intensity distribution. The findings reported here pave the way for controlling multidimensional laser beams in non-Hermitian systems via laser-induced atomic coherence, and have potential applications for designing light amplifiers and attenuators in different parts of laser beams.