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Restoration of the nonHermitian bulkboundary correspondence via topological amplification
by Matteo Brunelli, Clara C. Wanjura, Andreas Nunnenkamp
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Submission summary
Authors (as registered SciPost users):  Andreas Nunnenkamp 
Submission information  

Preprint Link:  https://arxiv.org/abs/2207.12427v4 (pdf) 
Date accepted:  20230920 
Date submitted:  20230906 09:23 
Submitted by:  Nunnenkamp, Andreas 
Submitted to:  SciPost Physics 
Ontological classification  

Academic field:  Physics 
Specialties: 

Approach:  Theoretical 
Abstract
NonHermitian (NH) lattice Hamiltonians display a unique kind of energy gap and extreme sensitivity to boundary conditions. Due to the NH skin effect, the separation between edge and bulk states is blurred and the (conventional) bulkboundary correspondence is lost. Here, we restore the bulkboundary correspondence for the most paradigmatic class of NH Hamiltonians, namely those with one complex band and without symmetries. We obtain the desired NH Hamiltonian from the (meanfield) unconditional evolution of drivendissipative cavity arrays, in which NH terms  in the form of nonreciprocal hopping amplitudes, gain and loss  are explicitly modeled via coupling to (engineered and nonengineered) reservoirs. This approach removes the arbitrariness in the definition of the topological invariant, as pointgapped spectra differing by a complexenergy shift are not treated as equivalent; the origin of the complex plane provides a common reference (base point) for the evaluation of the topological invariant. This implies that topologically nontrivial Hamiltonians are only a strict subset of those with a point gap and that the NH skin effect does not have a topological origin. We analyze the NH Hamiltonians so obtained via the singular value decomposition, which allows to express the NH bulkboundary correspondence in the following simple form: an integer value $\nu$ of the topological invariant defined in the bulk corresponds to $\vert \nu\vert$ singular vectors exponentially localized at the system edge under open boundary conditions, in which the sign of $\nu$ determines which edge. Nontrivial topology manifests as directional amplification of a coherent input with gain exponential in system size. Our work solves an outstanding problem in the theory of NH topological phases and opens up new avenues in topological photonics.
Author comments upon resubmission
In the revised manuscript we address the remaining questions and suggestions in Anonymous Report 2.
Best regards,
The Authors
List of changes
(1) We added the following sentence in the caption of Fig. 6(b): βNote that the diagonal entries are of order 1, although not discernible from the plot.β
(2) We added the following sentence at the end of Sec. VII, before subsection A (page 9, right column): βTo obtain the OBC spectrum in (c)(e), we write the NH Hamiltonian Eqs. (10) and (11) as the PBC Hamiltonian minus the matrix boundary terms, and express it in the planewave basis π > where the PBC Hamiltonian is diagonal. We then diagonalize the Hamiltonian < ππ»ππ΅πΆπ' > and label the eigenstates with π. The same approach is used for computing the singular value spectrum in Fig. 1 (third column from the left).β
(3) We amended this sentence: βWe start from the description of the underlying open quantum system (in order to model explicitly both engineered and nonengineered dissipative processes) and study the dynamics of the classical amplitudes.β
(4) A few lines before the beginning of Sec.III, we amended the sentence to βnearunit gainβ.
(5) We added a comment and cited the paper Wang et al. Science 371, 1240 (2021) at the top of page 13, right column: βNH topological amplification entails that the ZSMs are directly measurable in a simple transmission experiment and the topological winding number Eq. (19) can be extracted by counting the number and direction of amplified edge modes, without having to measure the momentumresolved complex energy band [86].β
Published as SciPost Phys. 15, 173 (2023)