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## Analytical stable Gaussian soliton supported by a parity-time-symmetric potential with power-law nonlinearity

Bikashkali Midya

We address the existence and stability of spatial localized modes supported by a parity-time-symmetric complex potential in the presence of power-law nonlinearity. The analytical expressions of the localized modes, which are Gaussian in nature, are obtained in both (1+1) and (2+1) dimensions. A linear stability analysis corroborated by the direct numerical simulations reveals that these analytical localized modes can propagate stably for a wide range of the potential parameters and for various order nonlinearities. Some dynamical characteristics of these solutions, such as the power and the transverse power-flow density, are also examined.

http://arxiv.org/abs/1404.7322
Quantum Physics (quant-ph); Pattern Formation and Solitons (nlin.PS); Exactly Solvable and Integrable Systems (nlin.SI)

## Integrable Generalized KdV, MKdV, and Nonlocal PT-Symmetric NLS Equations with Spatiotemporally Varying Coefficients

Matthew Russo, S. Roy Choudhury

We present a technique based on extended Lax Pairs to derive variable-coefficient generalizations of various Lax-integrable NLPDE hierarchies. As illustrative examples, we consider generalizations of KdV equations, three variants of generalized MKdV equations, and a recently-considered nonlocal PT-symmetric NLS equation. It is demonstrated that the technique yields Lax- or S-integrable NLPDEs with both time- AND space-dependent coefficients which are thus more general than almost all cases considered earlier via other methods such as the Painleve Test, Bell Polynomials, and various similarity methods. Employing the Painleve singular manifold method, some solutions are also presented for the generalized variable-coefficient integrable KdV and MKdV equations derived here. Current and future work is centered on generalizing other integrable hierarchies of NLPDEs similarly, and deriving various integrability properties such as solutions, Backlund Transformations, and hierarchies of conservation laws for these new integrable systems with variable coefficients.

http://arxiv.org/abs/1404.4602

Mathematical Physics (math-ph)

## Exceptional points and Bloch oscillations in non-Hermitian lattices with unidirectional hopping

Stefano Longhi

The spectral and transport properties of a non-Hermitian tight-binding lattice with unidirectional hopping are theoretically investigated in three different geometrical settings. It is shown that, while for the infinitely-extended (open) and for the ring lattice geometries the spectrum is complex, lattice truncation makes the spectrum real. However, an exceptional point of order equal to the number of lattice sites emerges. When a homogeneous dc force is applied to the lattice, in all cases an equally-spaced real Wannier-Stark ladder spectrum is obtained, corresponding to periodic oscillatory dynamics in real space. Possible physical realizations of non-Hermitian lattices with unidirectional hopping are briefly discussed.

http://arxiv.org/abs/1404.3662
Quantum Physics (quant-ph)

## Reversing the Pump-Dependence of a Laser at an Exceptional Point

M. Brandstetter, M. Liertzer, C. Deutsch, P. Klang, J. Schöberl, H. E. Türeci, G. Strasser, K. Unterrainer, S. Rotter

When two resonant modes in a system with gain or loss coalesce in both their resonance position and their width, a so-called “Exceptional Point” occurs which acts as a source of non-trivial physics in a diverse range of systems. Lasers provide a natural setting to study such “non-Hermitian degeneracies”, since they feature resonant modes and a gain material as their basic constituents. Here we show that Exceptional Points can be conveniently induced in a photonic molecule laser by a suitable variation of the applied pump. Using a pair of coupled micro-disk quantum cascade lasers, we demonstrate that in the vicinity of these Exceptional Points the laser shows a characteristic reversal of its pump-dependence, including a strongly decreasing intensity of the emitted laser light for increasing pump power. This result establishes photonic molecule lasers as promising tools for exploring many further fascinating aspects of Exceptional Points, like a strong line-width enhancement and the coherent perfect absorption of light in their vicinity as well as non-trivial mode-switching and the accumulation of a geometric phase when encircling an Exceptional Point parametrically.

http://arxiv.org/abs/1404.1837
Optics (physics.optics); Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Chaotic Dynamics (nlin.CD)

## Coherent perfect absorption with and without lasing in complex potentials

Zafar Ahmed

We study the coherent scattering from complex potentials to find that the coherent perfect absorption (CPA) without lasing is not possible in the PT-symmetric domain as the s-matrix is such that $$|\det S(k)|=1$$. We confirm that in the domain of broken PT-symmetry$$|\det S(k)|$$ can become indeterminate 0/0 at the spectral singularity (SS), k=k∗, of the potential signifying CPA with lasing at threshold gain. We also find that in the domain of unbroken symmetry (when the potential has real discrete spectrum) neither SS nor CPA can occur. In this, regard, we find that exactly solvable Scarf II potential is the unique model that can exhibit these novel phenomena and their subtleties analytically and explicitly. However, we show that the other numerically solved models also behave similarly.

http://arxiv.org/abs/1404.1679

Quantum Physics (quant-ph)

## Spectral Singularities and CPA-Laser Action in a Weakly Nonlinear PT-Symmetric Bilayer Slab

We study optical spectral singularities of a weakly nonlinear PT-symmetric bilinear planar slab of optically active material. In particular, we derive the lasing threshold condition and calculate the laser output intensity. These reveal the following unexpected features of the system: 1. For the case that the real part of the refractive index η of the layers are equal to unity, the presence of the lossy layer decreases the threshold gain; 2. For the more commonly encountered situations when η−1 is much larger than the magnitude of the imaginary part of the refractive index, the threshold gain coefficient is a function of η that has a local minimum. The latter is in sharp contrast to the threshold gain coefficient of a homogeneous slab of gain material which is a decreasing function of η. We use these results to comment on the effect of nonlinearity on the prospects of using this system as a CPA-laser.

http://arxiv.org/abs/1404.1737
Quantum Physics (quant-ph); Mathematical Physics (math-ph); Optics (physics.optics)

## Exceptional points and lasing self-termination in photonic molecules

Ramy El-Ganainy, Mercedeh Khajavikhan, Li Ge

We investigate the rich physics of photonic molecule lasers using a non-Hermitian dimer model. We show that several interesting features, predicted recently using a rigorous steady state ab-initio laser theory (SALT), can be captured by this toy model. In particular, we demonstrate the central role played by exceptional points in both pump-selective lasing and laser self-terminations phenomena. Due to its transparent mathematical structure, our model provides a lucid understanding for how different physical parameters (optical loss, modal coupling between microcavities and pump profiles) affect the lasing action. Interestingly, our analysis also confirms that, for frequency mismatched cavities, operation in the proximity of exceptional points (without actually crossing the square root singularities) can still lead to laser self-termination. We confirm this latter prediction for two coupled slab cavities using scattering matrix analysis and SALT technique. In addition, we employ our model to investigate the pump-controlled lasing action and we show that emission patterns are governed by the locations of exceptional points in the gain parameter space. Finally we extend these results to multi-cavity photonic molecules, where we found the existence of higher-order EPs and pump-induced localization.

http://arxiv.org/abs/1404.1242

Optics (physics.optics)

## Mathematical and physical aspects of complex symmetric operators

Stephan Ramon Garcia, Emil Prodan, Mihai Putinar

Recent advances in the theory of complex symmetric operators are presented and related to current studies in non-hermitian quantum mechanics. The main themes of the survey are: the structure of complex symmetric operators, C-selfadjoint extensions of C-symmetric unbounded operators, resolvent estimates, reality of spectrum, bases of C-orthonormal vectors, and conjugate-linear symmetric operators. The main results are complemented by a variety of natural examples arising in field theory, quantum physics, and complex variables.

http://arxiv.org/abs/1404.1304
Functional Analysis (math.FA); Other Condensed Matter (cond-mat.other); Mathematical Physics (math-ph); Operator Algebras (math.OA); Spectral Theory (math.SP)

## Comment on letter “Local PT-symmetry violates the no-signaling principle” by Yi-Chan Lee et al, Phys. Rev. Lett. 112, 130404 (2014)

Miloslav Znojil

It is shown that the toy-model-based considerations of loc. cit. (see also arXiv:1312.3395) are based on an incorrect, manifestly unphysical choice of the Hilbert space of admissible quantum states. A two-parametric family of all of the eligible correct and potentially physical Hilbert spaces of the model is then constructed. The implications of this construction are discussed. In particular, it is emphasized that contrary to the conclusions of loc. cit. there is no reason to believe that the current form of the PT-symmetric quantum theory should be false as a fundamental theory.

http://arxiv.org/abs/1404.1555
Quantum Physics (quant-ph); High Energy Physics – Theory (hep-th); Mathematical Physics (math-ph)

## Non-Hermitian PT-symmetric relativistic Quantum mechanics with a maximal mass in an external magnetic field

V.N.Rodionov

Starting with the modified Dirac equations for free massive particles with the γ5-extension of the physical mass $$m\to m_1+\gamma_5m_2$$, we consider equations of relativistic quantum mechanics in the presence of an external electromagnetic field. The new approach is developing on the basis of existing methods for study the unbroken PT symmetry of Non-Hermitian Hamiltonians. The paper shows that this modified model contains the definition of the mass parameter, which may use as the determination of the magnitude scaling of energy M. Obviously that the transition to the standard approach is valid when small in comparison with M energies and momenta. Formally, this limit is performed when $$M\to\infty$$, which simultaneously should correspond to the transition to a Hermitian limit $$m2\to0$$. Inequality $$m\leq M$$ may be considered and as the restriction of the mass spectrum of fermions considered in the model. Within of this approach, the effects of possible observability mass parameters: $$m_1, m_2, M$$ are investigated taking into account the interaction of the magnetic field with charged fermions together with the accounting of their anomalous magnetic moments.

http://arxiv.org/abs/1404.0503
High Energy Physics – Theory (hep-th); High Energy Physics – Phenomenology (hep-ph); Mathematical Physics (math-ph); Quantum Physics (quant-ph)