## Isospectral Hermitian counterpart of complex non Hermitian Hamiltonian $$p^{2}-gx^{4}+a/x^{2}$$

Asiri Nanayakkara, Thilagarajah Mathanaranjan

In this paper we show that the non-Hermitian Hamiltonians $$H=p^{2}-gx^{4}+a/x^2$$ and the conventional Hermitian Hamiltonians $$h=p^2+4gx^{4}+bx$$ ($$a,b\in \mathbb{R}$$) are isospectral if $$a=(b^2-4g\hbar^2)/16g$$ and $$a\geq -\hbar^2/4$$. This new class includes the equivalent non-Hermitian – Hermitian Hamiltonian pair, $$p^{2}-gx^{4}$$ and $$p^{2}+4gx^{4}-2\hbar \sqrt{g}x$$, found by Jones and Mateo six years ago as a special case. When $$a=\left(b^{2}-4g\hbar ^{2}\right) /16g$$ and $$a<-\hbar^2/4$$, although $$h$$ and $$H$$ are still isospectral, $$b$$ is complex and $$h$$ is no longer the Hermitian counterpart of $$H$$.

http://arxiv.org/abs/1407.4633
Mathematical Physics (math-ph)

## PT spectroscopy of the Rabi problem

Yogesh N. Joglekar, Rahul Marathe, P. Durganandini, Rajeev K. Pathak

We investigate the effects of a time-periodic, non-hermitian, PT-symmetric perturbation on a system with two (or few) levels, and obtain its phase diagram as a function of the perturbation strength and frequency. We demonstrate that when the perturbation frequency is close to one of the system resonances, even a vanishingly small perturbation leads to PT symmetry breaking. We also find a restored PT-symmetric phase at high frequencies, and at moderate perturbation strengths, we find multiple frequency windows where PT-symmetry is broken and restored. Our results imply that the PT-symmetric Rabi problem shows surprisingly rich phenomena absent in its hermitian or static counterparts.

http://arxiv.org/abs/1407.4535

Optics (physics.optics); Other Condensed Matter (cond-mat.other); Quantum Physics (quant-ph)

## Selective enhancement of topologically induced interface states

C. Poli, M. Bellec, U.Kuhl, F. Mortessagne, H. Schomerus

An attractive mechanism to induce robust spatially confined states utilizes interfaces between regions with topologically distinct gapped band structures. For electromagnetic waves, this mechanism can be realized in two dimensions by breaking symmetries in analogy to the quantum Hall effect or by employing analogies to the quantum spin Hall effect, while in one dimension it can be obtained by geometric lattice modulation. Induced by the presence of the interface, a topologically protected, exponentially confined state appears in the middle of the band gap. The intrinsic robustness of such states raises the question whether their properties can be controlled and modified independently of the other states in the system. Here, we draw on concepts from passive non-hermitian parity-time (PT)-symmetry to demonstrate the selective control and enhancement of a topologically induced state in a one-dimensional microwave set-up. In particular, we show that the state can be isolated from losses that affect all other modes in the system, which enhances its visibility in the temporal evolution of a pulse. The intrinsic robustness of the state to structural disorder persists in the presence of the losses. The combination of concepts from topology and non-hermitian symmetry is a promising addition to the set of design tools for optical structures with novel functionality.

http://arxiv.org/abs/1407.3703
Optics (physics.optics); Mesoscale and Nanoscale Physics (cond-mat.mes-hall)

## Supercritical blowup in coupled parity-time-symmetric nonlinear Schrödinger equations

João-Paulo Dias, Mário Figueira, Vladimir V. Konotop, Dmitry A. Zezyulin

We prove finite time supercritical blowup in a parity-time-symmetric system of the two coupled nonlinear Schrodinger (NLS) equations. One of the equations contains gain and the other one contains dissipation such that strengths of the gain and dissipation are equal. We address two cases: in the first model all nonlinear coefficients (i.e. the ones describing self-action and non-linear coupling) correspond to attractive (focusing) nonlinearities, and in the second case the NLS equation with gain has attractive nonlinearity while the NLS equation with dissipation has repulsive (defocusing) nonlinearity and the nonlinear coupling is repulsive, as well. The proofs are based on the virial technique arguments. Several particular cases are also illustrated numerically.

http://arxiv.org/abs/1407.2438
Analysis of PDEs (math.AP); Optics (physics.optics)

## \$$$PT$$ Symmetry, Conformal Symmetry, and the Metrication of Electromagnetism

Philip D. Mannheim

We present some interesting connections between $$PT$$ symmetry and conformal symmetry. We use them to develop a metricated theory of electromagnetism in which the electromagnetic field is present in the geometric connection. However, unlike Weyl who first advanced this possibility, we do not take the connection to be real but to instead be $$PT$$ symmetric, with it being $$iA_{\mu}$$ rather than $$A_{\mu}$$ itself that then appears in the connection. With this modification the standard minimal coupling of electromagnetism to fermions is obtained. Through the use of torsion we obtain a fully metricated theory of electromagnetism that treats its electric and magnetic sectors completely symmetrically, with a conformal invariant theory of gravity being found to emerge.

http://arxiv.org/abs/1407.1820
High Energy Physics – Theory (hep-th); General Relativity and Quantum Cosmology (gr-qc)

## Multi-stability and condensation of exciton-polaritons below threshold

Jiun-Yi Lien, Yueh-Nan Chen, Natsuko Ishida, Hong-Bin Chen, Chi-Chuan Hwang, Franco Nori

Exciton-polaritons can condense to a macroscopic quantum state through a non-equilibrium process of pumping and decay. In recent experiments, polariton condensates are used to observe, for a short time, nonlinear Josephson phenomena by coupling two condensates. However, it is still not clear how these phenomena are affected by the pumping and decay at long times and how the coupling alters the polariton condensation. Here, we consider a polariton Josephson junction pumped on one side and study its dynamics within a mean-field theory. The Josephson current is found to give rise to multi-stability of the stationary states, which are sensitive to the initial conditions and incoherent noises. These states can be attributed to either the self-trapping effect or the parity-time (PT) symmetry of the system. These results can be used to explain the emission spectra and the $$\pi$$-phase locking observed in recent experiments. We further predict that the multi-stability can reduce to the self-trapped state if the PT symmetry is broken. Moreover, the polaritons can condense even below the threshold, exhibiting hysteresis.

http://arxiv.org/abs/1407.1271
Quantum Physics (quant-ph); Mesoscale and Nanoscale Physics (cond-mat.mes-hall)

## Unidirectionally Invisible Potentials as Local Building Blocks of all Scattering Potentials

We give a complete solution of the problem of constructing a scattering potential v(x) that possesses scattering properties of one’s choice at an arbitrary prescribed wavenumber. Our solution involves expressing v(x) as the sum of at most six unidirectionally invisible finite-range potentials for which we give explicit formulas. Our results can be employed for designing optical potentials. We discuss its application in modeling threshold lasers, coherent perfect absorbers, and bidirectionally and unidirectionally reflectionless absorbers, amplifiers, and phase shifters.

http://arxiv.org/abs/1407.1760
Quantum Physics (quant-ph); Optics (physics.optics)

## Explicit energy expansion for general odd degree polynomial potentials

Asiri Nanayakkara, Thilagarajah Mathanaranjan

In this paper we derive an almost explicit analytic formula for asymptotic eigenenergy expansion of arbitrary odd degree polynomial potentials of the form $$V(x)=(ix)^{2N+1}+\beta _{1}x^{2N}+\beta _{2}x^{2N-1}+\cdot \cdot \cdot \cdot \cdot +\beta _{2N}x$$ where $$\beta _{k}^{\prime }$$s are real or complex for $$1\leq k\leq 2N$$. The formula can be used to find semiclassical analytic expressions for eigenenergies up to any order very efficiently. Each term of the expansion is given explicitly as a multinomial of the parameters $$\beta _{1},\beta _{2}….$$ and $$\beta _{2N}$$ of the potential. Unlike in the even degree polynomial case, the highest order term in the potential is pure imaginary and hence the system is non-Hermitian. Therefore all the integrations have been carried out along a contour enclosing two complex turning points which lies within a wedge in the complex plane. With the help of some examples we demonstrate the accuracy of the method for both real and complex eigenspectra.

http://arxiv.org/abs/1407.0191

Mathematical Physics (math-ph)

Katherine Jones-Smith, Connor Wallace

Hofstadter showed that the energy levels of electrons on a lattice plotted as a function of magnetic field form an beautiful structure now referred to as “Hofstadter’s butterfly”. We study a non-Hermitian continuation of Hofstadter’s model; as the non-Hermiticity parameter $$g$$ increases past a sequence of critical values the eigenvalues successively go complex in a sequence of “double-pitchfork bifurcations” wherein pairs of real eigenvalues degenerate and then become complex conjugate pairs. The associated wavefunctions undergo a spontaneous symmetry breaking transition that we elucidate. Beyond the transition a plot of the real parts of the eigenvalues against magnetic field resembles the Hofstadter butterfly; a plot of the imaginary parts plotted against magnetic fields forms an intricate structure that we call the Hofstadter cocoon. The symmetries of the cocoon are described. Hatano and Nelson have studied a non-Hermitian continuation of the Anderson model of localization that has close parallels to the model studied here. The relationship of our work to that of Hatano and Nelson and to PT transitions studied in PT quantum mechanics is discussed.

http://arxiv.org/abs/1407.0093

Quantum Physics (quant-ph); Mesoscale and Nanoscale Physics (cond-mat.mes-hall)

## $$\mathcal{PT}$$-symmetric Hamiltonian Model and Exactly Solvable Potentials

Özlem Yeşiltaş

Searching for non-Hermitian (parity-time)$$\mathcal{PT}$$-symmetric Hamiltonians with real spectra has been acquiring much interest for fourteen years. In this article, we have introduced a $$\mathcal{PT}$$-symmetric non-Hermitian Hamiltonian model which is given as $$\hat{\mathcal{H}}=\omega (\hat{b}^\dagger\hat{b}+\frac{1}{2})+\alpha (\hat{b}^{2}-(\hat{b}^\dagger)^{2})$$ where $$\omega$$ and $$\alpha$$ are real constants, $$\hat{b}$$ and $$\hat{b^\dagger}$$ are first order differential operators. Moreover, Pseudo-Hermiticity that is a generalization of $$\mathcal{PT}$$ symmetry has been attracting a growing interest \cite{mos}. Because the Hamiltonian $$\mathcal{H}$$ is pseudo-Hermitian, we have obtained the Hermitian equivalent of $$\mathcal{H}$$, which is in Sturm- Liouville form, leads to exactly solvable potential models which are effective screened potential and hyperbolic Rosen-Morse II potential. $$\mathcal{H}$$ is called pseudo-Hermitian, if there exists a Hermitian and invertible operator $$\eta$$ satisfying $$\mathcal{H^\dagger}=\eta \mathcal{H} \eta^{-1}$$. For the Hermitian Hamiltonian $$h$$, one can write $$h=\rho \mathcal{H} \rho^{-1}$$ where $$\rho=\sqrt{\eta}$$ is unitary. Using this $$\rho$$ we have obtained a physical Hamiltonian $$h$$ for each case. Then, the Schr\”{o}dinger equation is solved exactly using Shape Invariance method of Supersymmetric Quantum Mechanics. Mapping function $$\rho$$ is obtained for each potential case.

http://arxiv.org/abs/1406.3298

Quantum Physics (quant-ph)