arXiv subjects

Alexander Poddubny

Publications and source records attributed to Alexander Poddubny.

17 recordsLinked to original sources

arXiv2026

Blueprint for a fault-tolerant compound photon-atom quantum architecture

Fault-tolerant quantum computing requires architectures that simultaneously address scalability, connectivity, and error correction under realistic noise constraints. We present a compound photonic-atomic quantum computing platform that uses cavity QED to realize near-deterministic entangling operations between flying photonic qubits and stationary atomic qubits. Photons provide long-range connectivity and scalability via measurement-based quantum computing (MBQC), while atoms supply reusable, near-deterministic resources for photon generation and entanglement, overcoming the inefficiency of purely photonic platforms. The core primitive is a symmetrized Duan-Kimble photon-atom controlled-phase (CZ) gate, robust to experimental imperfections and high-fidelity. Using single $^{87}$Rb atoms coupled to optical cavities, we give protocols for state preparation, measurement, photon generation, and entangling gates on tens-of-nanosecond timescales, and show how large-scale cluster states with effectively unrestricted connectivity and reduced overhead can be generated through atomic reuse. We analyze fault tolerance on the Raussendorf-Harrington-Goyal (RHG) lattice with a hardware-aware noise model capturing asymmetric loss and correlated photonic-atomic errors. Logical memory simulations yield a photon-loss threshold near $2.6\%$ per physical gate ($\sim$15\% total per trajectory). The full Clifford set -- Hadamard, phase, CNOT -- is implementable transversally or fold-transversally at thresholds matching the identity channel, and we propose two non-Clifford resource-state routes (code teleportation and magic state cultivation) within the foliated cluster-state architecture.

quant-ph↗

arXiv2026

Bound, antibound and resonance two-photon states in chiral waveguide QED

We present a theoretical study of the two-particle spectrum $\omega(K)$ for the chiral waveguide QED setup of an array of two-level atoms directionally interacting with photons propagating along the waveguide. We demonstrate that for each pair center-of-mass momentum $K$ there exist distinct solutions with $\Im\omega\le 0$ in the two-particle spectrum, corresponding to bound, antibound and resonance states, in addition to the continuum of scattering states. Contrary to previous studies, which showed the bound and resonance-state spectra only over a limited range of $K$, the calculated spectrum is consistent across all $K$ values. An interesting finding is that the real part of the spectrum $\Re \omega(K)$ in the chiral model is gapless. The calculated dispersion law $\omega(K)$ provides an effective model for the bound photon pairs also in a finite-size array, manifesting the topological non-Hermitian skin effect.

quant-ph↗

arXiv2025

Isaac Lab: A GPU-Accelerated Simulation Framework for Multi-Modal Robot Learning

We present Isaac Lab, the natural successor to Isaac Gym, which extends the paradigm of GPU-native robotics simulation into the era of large-scale multi-modal learning. Isaac Lab combines high-fidelity GPU parallel physics, photorealistic rendering, and a modular, composable architecture for designing environments and training robot policies. Beyond physics and rendering, the framework integrates actuator models, multi-frequency sensor simulation, data collection pipelines, and domain randomization tools, unifying best practices for reinforcement and imitation learning at scale within a single extensible platform. We highlight its application to a diverse set of challenges, including whole-body control, cross-embodiment mobility, contact-rich and dexterous manipulation, and the integration of human demonstrations for skill acquisition. Finally, we discuss upcoming integration with the differentiable, GPU-accelerated Newton physics engine, which promises new opportunities for scalable, data-efficient, and gradient-based approaches to robot learning. We believe Isaac Lab's combination of advanced simulation capabilities, rich sensing, and data-center scale execution will help unlock the next generation of breakthroughs in robotics research.

cs.RO↗

arXiv2025

Multiband dispersion and warped vortices of strongly-interacting photons

We present a theoretical study of quantum correlations between interacting photons realized through co-propagating Rydberg polaritons. We show that the spatial evolution of the $n$-photon wavefunction is governed by a multiband dispersion featuring one massive mode and multiple massless modes with degenerate Dirac points and $n$-fold rotational symmetry. The resulting band structure is warped, departing from the single-band, parabolic approximation commonly assumed for interacting polaritons. Our analytical results are supported by rigorous numerical modeling that fully accounts for photon propagation inside the finite atomic medium. These findings advance the understanding of multi-photon interactions and support the development of future multi-photon control tools.

quant-ph↗

arXiv2023

Mesoscopic non-Hermitian skin effect

We discuss a generalization of the non-Hermitian skin effect to finite-size photonic structures with neither gain nor loss in the bulk and purely real energy spectrum under periodic boundary conditions (PBC). We show that such systems can still have significant portions of eigenmodes concentrated at the edges and that this edge concentration can be linked to the non-trivial point-gap topology of the size-dependent regularized PBC spectrum, accounting for the radiative losses. As an example, we consider the chiral waveguide quantum electrodynamics platform with an array of atoms coupled to the waveguide. The proposed mesoscopic analogue of the non-Hermitian skin effect could be potentially applied to other seemingly lossless photonic structures, such as chiral resonant all-dielectric metamaterials.

physics.optics↗

arXiv2023

Long persistent anticorrelations in few-qubit arrays

We consider theoretically the mechanisms to realize antibunching between the photons scattered on the array of two-level atoms in a general electromagnetic environment. Our goal is the antibunching that persists for the times much longer than the spontaneous emission lifetime of an individual atom. We identify two mechanisms for such persistent antibunching. The first one is based on subradiant states of the atomic array, and the second one does not require any subradiant states. We provided two specific examples of array parameters with optimized antibunching, based on an array in a free space and an array coupled to a waveguide.

quant-ph↗

arXiv2023

Quantum vortices of strongly interacting photons

Vortices are a hallmark of topologically nontrivial dynamics in nonlinear physics and arise in a huge variety of systems, from space and atmosphere to condensed matter and quantum gases. In optics, vortices manifest as phase twists of the electromagnetic field, commonly formed by the interaction of light and matter. Formation of vortices by effective interaction of light with itself requires strong optical nonlinearity and has therefore been confined, until now, to the classical regime. Here we report on the realization of quantum vortices resulting from a strong photon-photon interaction in a quantum nonlinear optical medium. The interaction causes faster phase accumulation for co-propagating photons. Similarly to a plate pushing water, the local phase accumulation produces a quantum vortex-antivortex pair within the two-photon wavefunction. For three photons, the formation of vortex lines and a central vortex ring attests to a genuine three-photon interaction. The wavefunction topology, governed by two- and three-photon bound states, imposes a conditional phase shift of $\pi$-per-photon, a potential resource for deterministic quantum logic operations.

quant-ph↗

arXiv2021

Localized vibrational modes in waveguide quantum optomechanics with spontaneously broken PT symmetry

We study theoretically two vibrating quantum emitters trapped near a one-dimensional waveguide and interacting with propagating photons. We demonstrate, that in the regime of strong optomechanical interaction the light-induced coupling of emitter vibrations can lead to formation of spatially localized vibration modes, exhibiting parity-time (PT ) symmetry breaking. These localized vibrations can be interpreted as topological defects in the quasiclassical energy spectrum.

quant-ph↗

arXiv2021

Enhanced generation of non-degenerate photon-pairs in nonlinear metasurfaces

We reveal a novel regime of photon-pair generation driven by the interplay of multiple bound states in the continuum resonances in nonlinear metasurfaces. This non-degenerate photon-pair generation is derived from the hyperbolic topology of the transverse phase-matching and can enable orders-of-magnitude enhancement of the photon rate and spectral brightness, as compared to the degenerate regime. We show that the entanglement of the photon-pairs can be tuned by varying the pump polarization, which can underpin future advances and applications of ultra-compact quantum light sources.

physics.optics↗

arXiv2020

Waveguide quantum optomechanics: parity-time phase transitions in ultrastrong coupling regime

We develop a rigorous theoretical framework for interaction-induced phenomena in the waveguide quantum electrodynamics (QED) driven by mechanical oscillations of the qubits. Specifically, we predict that the simplest set-up of two qubits, harmonically trapped over an optical waveguide, enables the ultrastrong coupling regime of the quantum optomechanical interaction. Moreover, the combination of the inherent open nature of the system and the strong optomechanical coupling leads to emerging parity-time (\PT) symmetry, quite unexpected for a purely quantum system without artificially engineered gain and loss. The $\mathcal{PT}$ phase transition drives long-living subradiant states, observable in the state-of-the-art waveguide QED setups.

cond-mat.mes-hall↗

arXiv2019

Edge states of photon pairs in cavity arrays with spatially modulated nonlinearity

We study theoretically an extended Bose-Hubbard model with the spatially modulated interaction strength, describing a one-dimensional array of tunneling-coupled nonlinear cavities. It is demonstrated that the spatial modulation of the nonlinearity induces bound two-photon edge states. The formation of these edge states has been understood analytically in terms of nonlinear self-localization.

physics.optics↗

arXiv2019

Second-harmonic generation in Mie-resonant dielectric nanoparticles made of noncentrosymmetric materials

We develop a multipolar theory of second-harmonic generation (SHG) by dielectric nanoparticles made of noncentrosymmetric materials with bulk quadratic nonlinearity. We specifically analyze two regimes of optical excitation: illumination by a plane wave and single-mode excitation, when the laser pump drives the magnetic dipole mode only. Considering two classes of nonlinear crystalline solids (dielectric perovskite material and III-V semiconductor), we apply a symmetry approach to derive selection rules for the multipolar composition of the nonlinear radiation. The developed description can be used for design of efficient nonlinear optical nanoantennas with reconfigurable radiation characteristics.

physics.optics↗

arXiv2019

Photonic quadrupole topological phases

The topological phases of matter are characterized using the Berry phase, a geometrical phase, associated with the energy-momentum band structure. The quantization of the Berry phase, and the associated wavefunction polarization, manifest themselves as remarkably robust physical observables, such as quantized Hall conductivity and disorder-insensitive photonic transport. Recently, a novel class of topological phases, called higher-order topological phases, were proposed by generalizing the fundamental relationship between the Berry phase and the quantized polarization, from dipole to multipole moments. Here, we demonstrate the first photonic realization of the quantized quadrupole topological phase, using silicon photonics. In this 2nd-order topological phase, the quantization of the bulk quadrupole moment in a two-dimensional system manifests as topologically robust corner states. We unambiguously show the presence of localized corner states and establish their robustness against certain defects. Furthermore, we contrast these topological states against topologically-trivial corner states, in a system without bulk quadrupole moment, and observe no robustness. Our photonic platform could enable the development of robust on-chip classical and quantum optical devices with higher-order topological protection.

physics.optics↗

arXiv2018

Exceptional points for photon pairs bound by nonlinear dissipation in cavity arrays

We study theoretically the dissipative Bose-Hubbard model describing array of tunneling-coupled cavities with non-conservative photon-photon interaction. Our calculation of the complex energy spectrum for the photon pairs reveals exceptional points where the two-photon states bound by nonlinear dissipation are formed. This improves fundamental understanding of the interplay of non-Hermiticity and interactions in the quantum structures and can be potentially used for on-demand nonlinear light generation in photonic lattices.

physics.optics↗

arXiv2018

Exceptional points in topological edge spectrum of PT symmetric domain walls

We demonstrate that the non-Hermitian parity-time (PT) symmetric interfaces formed between amplifying and lossy crystals support dissipationless edge states. These PT edge states exhibit gapless spectra in the complex band structure interconnecting complex-valued bulk bands as long as exceptional points (EPs) of edge states exist. As a result, regimes exist where the edge states can spectrally overlap with the bulk continuum without hybridization, and leakage into the bulk states is suppressed due to the PT symmetry. Two exemplary PT symmetric systems, based on valley and quantum hall topological phases, are investigated, and the connection with the corresponding Hermitian systems is established. We find that the edge states smoothly transit to the valley edge states found in Hermitian systems if the magnitude of gain/loss vanishes. The topological nature of the PT edge states can be established within the non-Hermitian Haldane model, where the topological invariance is found to be unaffected by gain or loss. Nonreciprocal PT edge states are discovered at the interfaces between PT-Haldane phases, indicating the interplay between the gain/loss and the magnetic flux. The proposed systems are experimentally feasible to realize in photonics. This has been verified by our rigorous full-wave simulations of edge states in PT-symmetric silicon-based photonic graphene.

cond-mat.mes-hall↗

arXiv2011

Dramatic impact of pumping mechanism on photon entanglement in microcavity

A theory of entangled photons emission from quantum dot in microcavity under continuous and pulsed incoherent pumping is presented. It is shown that the time-resolved two-photon correlations drastically depend on the pumping mechanism: the continuous pumping quenches the polarization entanglement and strongly suppresses photon correlation times. Analytical theory of the effect is presented.

cond-mat.mes-hall↗

arXiv2011

Spontaneous emission enhancement in metal-dielectric metamaterials

We study the spontaneous emission of a dipole emitter imbedded into a layered metal-dielectric metamaterial. We demonstrate ultra-high values of the Purcell factor in such structures due to a high density of states with hyperbolic isofrequency surfaces. We reveal that the traditional effective-medium approach greatly underestimates the value of the Purcell factor due to the presence of an effective nonlocality, and we present an analytical model which agrees well with numerical calculations.

physics.optics↗