We study the spectral stability of a two-dimensional discrete Schrödinger equation on a square lattice, in the simultaneous presence of a fractional Laplacian and $\mathcal{PT}$ symmetry. For that purpose, we compute the plane-wave spectrum in closed form, as a function of the gain/loss parameter and the fractional exponent. Examination of the spectrum reveals that an increase of the gain/loss parameter favors the early appearance of complex eigenvalues, that is, the onset of a broken $\mathcal{P}T$ symmetry. On the other hand, as the fractional exponent decreases from unity, at a critical value a gap opens up separating the upper and lower bands, and the spectrum becomes real. A further decrease of the exponent increases the width of the gap and the system remains in the $\mathcal{PT}$-symmetric phase down to a vanishing value of the fractional exponent. An examination of the density of states and the participation ratio reinforces these observations and leads one to conclude that, unlike the standard, nonfractional case where the binary lattice is always in the broken $\mathcal{PT}$ phase, for the fractional case it is possible to have a symmetric $\mathcal{P}\mathcal{T}$ phase in the presence of a finite gain/loss parameter and a small enough fractional exponent.

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• PT-symmetric quantum mechanics
• Waveguide arrays
• PT-symmetry