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## PT-Symmetry in Non-Hermitian Su-Schrieffer-Heeger model with complex boundary potentials

Baogang Zhu, Rong Lu, Shu Chen

We study the parity- and time-reversal PT symmetric non-Hermitian Su-Schrieffer-Heeger (SSH) model with two conjugated imaginary potentials $$\pm i\gamma$$ at two end sites. The SSH model is known as one of the simplest two-band topological models which has topologically trivial and nontrivial phases. We find that the non-Hermitian terms can lead to different effects on the properties of the eigenvalues spectrum in topologically trivial and nontrivial phases. In the topologically trivial phase, the system undergos an abrupt transition from unbroken PT-symmetry region to spontaneously broken $$\mathcal{PT}$$-symmetry region at a certain $$\gamma_{c}$$, and a second transition occurs at another transition point $$\gamma_{c^{‘}}$$ when further increasing the strength of the imaginary potential $$\gamma$$. But in the topologically nontrivial phase, the zero-mode edge states become unstable for arbitrary nonzero $$\gamma$$ and the $$\mathcal{PT}$$-symmetry of the system is spontaneously broken, which is characterized by the emergence of a pair of conjugated imaginary modes.

http://arxiv.org/abs/1405.5591
Other Condensed Matter (cond-mat.other); Quantum Physics (quant-ph)

## Dynamics of mode entanglement in a system of cavities coupled with a chiral mirror

Ali Ü. C. Hardal

We investigate the Hermitian and the non-Hermitian dynamics of the mode entanglement in two identical optical cavities coupled by a chiral mirror. By employing the non-Hermitian quantum evolution, we calculate the logarithmic negativity measure of entanglement for initially Fock, coherent and squeezed states, separately. We verify the non-conservation of mean spin for the initially coherent and squeezed states when the coupling is non-reciprocal and report the associated spin noise for each case. We examine the effects of non-conserved symmetries on the mode correlations and determine the degree of non-reciprocal coupling to establish robust quantum entanglement.

http://arxiv.org/abs/1405.5079
Quantum Physics (quant-ph)

## Parity-Time Synthetic Laser

Liang Feng, Zi Jing Wong, Renmin Ma, Yuan Wang, Xiang Zhang

Parity-time (PT) symmetry is a fundamental notion in quantum field theories1,2. It has opened a new paradigm for non-Hermitian Hamiltonians ranging from quantum mechanics, electronics, to optics. In the realm of optics, optical loss is responsible for power dissipation, therefore typically degrading device performance such as attenuation of a laser beam. By carefully exploiting optical loss in the complex dielectric permittivity, however, recent exploration of PT symmetry revolutionizes our understandings in fundamental physics and intriguing optical phenomena such as exceptional points and phase transition that are critical for high-speed optical modulators3-9. The interplay between optical gain and loss in photonic PT synthetic matters offers a new criterion of positively utilizing loss to efficiently manipulate gain and its associated optical properties10-19. Instead of simply compensating optical loss in conventional lasers, for example, it is theoretically proposed that judiciously designed delicate modulation of optical loss and gain can lead to PT synthetic lasing20,21 that fundamentally broadens laser physics. Here, we report the first experimental demonstration of PT synthetic lasers. By carefully exploiting the interplay between gain and loss, we achieve degenerate eigen modes at the same frequency but with complex conjugate gain and loss coefficients. In contrast to conventional ring cavity lasers with multiple modes, the PT synthetic micro-ring laser exhibits an intrinsic single mode lasing: the non-threshold PT broken phase inherently associated in such a photonic system squeezes broadband optical gain into a single lasing mode regardless of the gain spectral bandwidth. This chip-scale semiconductor platform provides a unique route towards fundamental explorations of PT physics and next generation of optoelectronic devices for optical communications and computing.

http://arxiv.org/abs/1405.2863
Optics (physics.optics)