1,721,683 research outputs found
Non-Bloch PT symmetry breaking in non-Hermitian photonic quantum walks
No full textA hallmark of topological band theory in periodic media is that bulk properties are not affected by boundary conditions. Remarkably, in certain non-Hermitian lattices, the bulk properties are affected largely by boundaries, leading to such major effects as the non-Hermitian skin effect and violation of the bulk-boundary correspondence. Here, we unveil that non-unitary discrete-time quantum walks of photons in systems involving gain and loss show rather generally non-Bloch parity-time symmetry-breaking phase transitions and suggest a bulk probing method to detect such boundary-driven phase transitions
Quantum decay in a topological continuum
No full textThe quantum-mechanical decay of two or more overlapped resonances in a common continuum is largely influenced by Fano interference, leading to important phenomena such as the existence of bound states in the continuum, fractional decay and quiescent dynamics for single particle decay, and signature of particle statistics in the many-body quantum decay. An overlooked yet essential requirement to observe Fano interference is time-reversal symmetry of the bath. Here we consider multilevel quantum decay in a bath sustaining unidirectional (chiral) propagating states, such as in quantum Hall or in Floquet topological insulators, and show that the chiral nature of scattering states fully suppresses Fano interference among overlapping resonances. As a result, there are not bound states in the continuum, quantum decay is complete, and there is not any signature of particle statistics in the decay process. Nonetheless, some interesting features are disclosed in the multilevel decay dynamics in a topological bath, such as the appearance of high-order exceptional points, long quiescent dynamics followed by a fast decay, and the possibility to observe damped non-Hermitian Bloch oscillations
Non-Hermitian topological phase transition in PT -symmetric mode-locked lasers
No full textThe delocalization transition in the Anderson model in the presence of an imaginary gauge field is a landmark example of non-Hermitian topological phase transition. Originally proposed by Hatano and Nelson, its topological nature has been disclosed only very recently. Here it shown that a non-Hermitian phase transition can be observed in the frequency domain of an actively mode-locked laser in a parity-time (PT)-symmetric configuration with incommensurate spacing of cavity axial frequencies
Metal-insulator phase transition in a non-Hermitian Aubry-André-Harper model
No full textNon-Hermitian extensions of the Anderson and Aubry-André-Harper models are attracting considerable interest as platforms to study localization phenomena, metal-insulator, and topological phase transitions in disordered non-Hermitian systems. Most of the available studies, however, resort to numerical results, while few analytical and rigorous results are available owing to the extraordinary complexity of the underlying problem. Here we consider a parity-time symmetric extension of the Aubry-André-Harper model, undergoing a topological metal-insulator phase transition, and provide rigorous analytical results of energy spectrum, symmetry breaking phase transition, and localization length. In particular, by extending to the non-Hermitian realm the Thouless result relating localization length and density of states, we derive an analytical form of the localization length in the insulating phase, showing that-like in the Hermitian Aubry-André-Harper model-the localization length is independent of energy
Photonic flat-band laser
No full textFlat-band photonic lattices, i.e., arrays of waveguides or resonators displaying a flat Bloch band, offer new routes for light trapping and distortion-free imaging. Here it is shown that flat-band lattices can show stable and cooperative laser emission when optical gain is supplied to the system, despite the large degree of degeneracy of flat-band supermodes. By considering a quasi one-dimensional rhombic lattice of coupled semiconductor microrings, selective pumping of the outer sublattices can induce cooperative lasing in a supermode of the flat band
Probing one-dimensional topological phases in waveguide lattices with broken chiral symmetry
Full text linkOne-dimensional lattices with chiral symmetry are known to possess quantized Zak phase and nontrivial topological phases. Here it is shown that the quantized Zak phase and nontrivial edge states, partially protected by inversion symmetry rather than chiral symmetry, can be observed and probed in the bulk exploiting continuous-time photonic quantum walk in zig-zag waveguide arrays. The averaged beam displacement measurements can detect quantized Zak phase and nontrivial topological phases in the extended Su–Schrieffer–Heeger model with broken chiral symmetry
Topological Phase Transition in non-Hermitian Quasicrystals
Full text linkThe discovery of topological phases in non-Hermitian open classical and quantum systems challenges our current understanding of topological order. Non-Hermitian systems exhibit unique features with no counterparts in topological Hermitian models, such as failure of the conventional bulk-boundary correspondence and non-Hermitian skin effect. Advances in the understanding of the topological properties of non-Hermitian lattices with translational invariance have been reported in several recent studies; however little is known about non-Hermitian quasicrystals. Here we disclose topological phases in a quasicrystal with parity-time (PT) symmetry, described by a non-Hermitian extension of the Aubry-André-Harper model. It is shown that the metal-insulating phase transition, observed at the PT symmetry breaking point, is of topological nature and can be expressed in terms of a winding number. A photonic realization of a non-Hermitian quasicrystal is also suggested
Quantum interference and exceptional points
No full textExceptional points (EPs), that is, branch point singularities of non-Hermitian Hamiltonians, are ubiquitous in optics. So far, the signatures of EPs have been mostly studied assuming classical light. In the passive parity-time (PT) optical coupler, a fingerprint of EPs resulting from the coalescence of two resonance modes is a qualitative change of the photon decay law, from damped Rabi-like oscillations to transparency, as the EP is crossed by increasing the loss rate. However, when probed by nonclassical states of light, quantum interference can hide EPs. Here it is shown that, under excitation with polarization-entangled two-photon states, the EP phase transition is smoothed until it disappears as the effective particle statistics are changed from bosonic to fermionic
Probing topological phases in waveguide superlattices
No full textOne-dimensional superlattices with modulated coupling constants show rich topological properties and tunable edge states. Beyond the dimeric case, probing the topological properties of superlattices is a challenge. Here we suggest a rather general method of bulk probing topological invariants in waveguide superlattices based on spatial displacement of discretized beams. A judiciously tailored initial beam excitation of the lattice, corresponding to superposition of Wannier functions, provides a direct measure of the band gap topological numbers. For a quadrimeric superlattice, a simple bulk probing method, which avoids Wannier states, is proposed to discriminate the existence of zero-energy topological edge states from non-topological ones
Topological pumping of edge states via adiabatic passage
No full textTopological pumping of edge states in finite crystals or quasicrystals with nontrivial topological phases provides a powerful means for robust excitation transfer. In most schemes of topological pumping, the edge states become delocalized and immersed into the continuum during the adiabatic cycle, requiring extremely slow evolution to avoid nonadiabatic effects. Here, a scheme of topological pumping based on adiabatic passage of edge and interface states is proposed, which is more robust to nonadiabatic effects and avoids delocalization of the state over the entire adiabatic cycle. The scheme is illustrated by considering adiabatic passage in a system sustaining two topologically protected edge states and one interface state, realized by interfacing two dimerized Su-Schrieffer-Heeger chains with different topological order
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