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Whispering gallery modes in a microwire are characterized by a nearly equidistant energy spectrum. In the strong exciton-photon coupling regime, this system represents a bosonic cascade: a ladder of discrete energy levels that sustains stimulated transitions between neighboring steps. Here, by using a femtosecond angle-resolved spectroscopic imaging technique, the ultrafast dynamics of polaritons in a bosonic cascade based on a one-dimensional ZnO whispering gallery microcavity are explicitly visualized. Clear ladder-form build-up processes from higher to lower energy branches of the polariton condensates are observed, which are well reproduced by modeling using rate equations. Remarkably, a pronounced superbunching feature, which could serve as solid evidence for bosonic cascades, is demonstrated by the measured second-order time correlation factor. In addition, the nonlinear polariton parametric scattering dynamics on a time scale of hundreds of femtoseconds are revealed. Our understandings pave the way toward ultrafast coherent control of polaritons at room temperature.

We predict the spontaneous symmetry breaking in a spinor Bose–Einstein condensate of exciton-polaritons (polaritons) caused by the coupling of its spin and orbital degrees of freedom. We study a polariton condensate trapped in a ring-shaped effective potential with a broken rotational symmetry. We propose a realistic scheme of generating controllable spinor azimuthal persistent currents of polaritons in the trap under the continuous wave optical pump. We propose a new type of half-quantum circulating states in a spinor system characterized by azimuthal currents in both circular polarizations and a vortex in only one of the polarizations. The spontaneous symmetry breaking in the spinor polariton condensate that consists in the switching from co-winding to opposite-winding currents in opposite spin states is revealed. It is characterized by the change of the average orbital angular momentum of the condensate from zero to non-zero values. The radial displacement of the pump spot and the polarization of the pump act as the control parameters. The considered system exhibits a fundamental similarity to a superconducting flux qubit, which makes it highly promising for applications in quantum computing.

Oscillations of the trajectory of exciton–polaritons propagating in the plane of an optical microcavity with integral quantum wells are studied. This effect is characteristic of systems with the splitting of spin states, and arises as a result of the mutual influence of the spin (polarization) degree of freedom and the translational motion of polaritons. The trajectory’s period of oscillation is determined by the splitting of the photon and exciton components of the polaritons.

We create exciton-polariton condensates in half-moon shapes by the non-resonant optical excitation of GaAs-based cylindrical pillar microcavities. In this geometry, the π-jump of the phase of the c...

We study the manifestation of the Nernst effect in the Corbino disk subjected to the normal external magnetic field and to the radial temperature gradient. The Corbino geometry offers a precious opportunity for the direct measurement of the magnetization currents that are masked by kinetic contributions to the Nernst current in the conventional geometry. The magnetization currents, also referred to as the edge currents, are independent on the conductivity of the sample which is why they can be conveniently described within the thermodynamic approach. They can be related to the Landau thermodynamic potential for an infinite system. We demonstrate that the observable manifestation of this, purely thermodynamic, Nernst effect consists in the strong oscillations of the magnetic field measured in the center of the disk as a function of the external field. The oscillations depend on the temperature difference at the edges of the disk. Dirac fermions and 2D electrons with a parabolic spectrum are characterized by oscillations of different phase and frequency. We predict qualitatively different power dependencies of the magnitude of the Nernst signal on the chemical potential for normal and Dirac carriers.

We propose an approach for the excitation of polariton Tamm states with a controllable nontrivial topology. The Tamm polaritons emerge at the interface of two binary one-dimensional photonic crystals belonging to a C3v point group, with an exciton resonance within their matching band gaps. The external magnetic field applied in the Faraday geometry endows the dispersion of the Tamm polaritons with the nonreciprocity: it lifts the degeneracy between opposite propagation directions in the interface plane. The phenomenology of Tamm polariton currents closely resembles one of a Z2 topological insulator. The proposed structure acts as an optical spin splitter controlled by the magnetic field magnitude.

Concentric ring exciton polariton condensates emerging under non-resonant laser pump in an annular trapping potential support persistent circular currents of polaritons. The trapping potential is formed by a cylindrical micropillar etched in a semiconductor microcavity with embedded quantum wells and a repulsive cloud of optically excited excitons under the pump spot. The symmetry of the potential is subject to external control via manipulation by its pump-induced component. In the manuscript, we demonstrate excitation of concentric ring polariton current states with predetermined vorticity which we trace using interferometry measurements with a spherical reference wave. We also observe the polariton condensate dynamically changing its vorticity during observation, which results in pairs of fork-like dislocations on the time-averaged interferogram coexisting with azimuthally homogeneous photoluminescence distribution in the micropillar.

Optically inactive or dark excitons play an important role in exciton and polariton devices. On one hand, they supply excitons to the light cone and feed the photoluminescence signal. On the other hand, they repel radiatively active excitons due to the exchange interaction and contribute to the formation of lateral potentials for exciton and polariton condensates. On top of this, they play an important role in scattering and energy relaxation dynamics of quasiparticles in semiconductors. So far, because of optical inaccessibility, studies were focused typically on one experimental technique, giving information about one quantity of dark excitons. Here we present a comprehensive study of the dark exciton reservoir in a high-quality 14-nm GaAs/AlGaAs quantum well using several experimental techniques. We develop a new method of nonradiative broadening spectroscopy of exciton resonances and combine it with nondegenerate pump-probe spectroscopy. The exciton and carrier dynamics in the reservoir is monitored via dynamic broadening of exciton resonances induced by exciton-exciton and exciton-carrier scattering. The dynamics is found to be strongly dependent on the optical excitation conditions. Based on the experimental results, we develop a model of dynamics in a reservoir of excitons and free carriers. The model allows us to describe the experimentally measured photoluminescence kinetics with no fitting parameters. We also demonstrate the optical control of the dark exciton density by means of an additional excitation that creates imbalance of free carriers depleting the reservoir. These results shed light onto the dynamics of the excitonic ``dark matter'' and pave the way to the high-precision engineering of optically induced potentials in exciton-polariton and integrated photonic devices. We expect that the observed results can be transferred also to other semiconductors so that the current quantum well serves as a high-quality model system.

Semiconducting monolayer crystals have emerged as a new platform for studies of tightly bound excitons and many-body excitations in ultimately thin materials. Their giant dipole coupling to optical fields makes them very appealing for (nano-) photonic devices, and for fundamental investigations in the framework of cavity quantum electrodynamics [1] .

Magnetic field induced edge currents propagate in opposite directions at opposite edges of a Hall bar. We consider a system that possesses just one edge: a M\"obius stripe. We show that if the carrier mean free path is less than the width of the stripe, magnetic field induced edge currents vanish. Instead, a stationary charge density wave is formed. Its profile is governed by the interplay between the magnetic field dependence of the chemical potential and the screened Coulomb repulsion of charge carriers. This effect offers an experimental tool for studies of the electron density of states in two-dimensional crystal films.

Mechanisms of the suppression of the electron-hole exchange interaction in nonradiative excitons with a large in-plane wave vector in high-quality heterostructures with quantum wells are analyzed theoretically. It is shown that the dominant suppression mechanism is exciton-exciton scattering accompanied by the mutual spin flips of like carriers (either two electrons or two holes), comprising the excitons. As a result, the electron spin polarization in nonradiative excitons may be retained for a long time. The analysis of experimental data shows that this relaxation time can exceed one nanosecond. This long-term and optically controllable spin memory in an exciton reservoir may be of interest for future information technologies.

Open-dissipative systems obeying parity-time ($\mathcal{PT}$) symmetry are capable of demonstrating oscillatory dynamics akin to the conservative systems. In contrast to limit cycle solutions characteristic of nonlinear systems, the $\mathcal{PT}$-symmetric oscillations form a continuum of non-isolated orbits. However, precise sculpturing of the real potential and the gain-loss spatial profiles required for establishing of the $\mathcal{PT}$-symmetry is practically challenging. The optical devices, such as lasers, exhibit relaxation dynamics and do not operate as the $\mathcal{PT}$-symmetric systems. Here we demonstrate how these constraints can be overcome. We predict that a pair of optically trapped polariton condensates (a polariton dimer) can be excited and operated in the oscillating regime typical of the isolated systems. This regime can be realized in the presence of both dissipative and conservative coupling between the condensates and can be maintained at an arbitrary external pump intensity. Every orbit is characterised by a frequency comb appearing in the spectrum of a dimer in the presence of the conservative nonlinearity. Our results pave the way for the creation of the optical computing devices operating under the constant-wave external pumping., 12 pages, 8 figures

We propose a technique for the generation of polygonal optical patterns in real space using a combined effect of the spin–orbit interaction and confinement of light in the plane of a dielectric optical microcavity. The spin–orbit interaction emerging from the splitting in transverse electric (TE) and transverse magnetic (TM) optical modes of the microcavity gives rise to oscillations in space of propagating macroscopic wave packets of polarized photons. Confined in a harmonic potential, the latter follow closed trajectories of a polygonal form. We demonstrate the possibility of excitation by a continuous wave resonant optical pumping of polygonal optical patterns with a controllable (both even and odd) number of vertices.

We report on the fabrication of fractal dendrites using laser-induced melting of aluminum alloys. We target boron carbide (B4C), which is one of the most effective radiation-absorbing materials characterized by a low coefficient of thermal expansion. Due to the high fragility of B4C crystals, we were able to introduce its nanoparticles into a stabilization aluminum matrix of AA385.0. The high-intensity laser field action led to the formation of composite dendrite structures under the effect of local surface melting. Modelling the dendrite cluster growth confirms its fractal nature and sheds light on the pattern behavior of the resulting quasicrystal structure.

The formation of macroscale carbon structures characterised by an sp-sp2-hybridization is realised by self-assembly in colloidal solutions under an effect of laser irradiation and electromagnetic fields. The sponge-like morphology, sculptured with gold nanoparticles (NPs) was revealed by Scanning Electron Microscopy (SEM) imaging. Full structural and defect characterization of the self-assembled sponges was provided using the micro-Raman spectroscopic technique. The synthesized clusters manifest themselves in the presence of a strong spectral band in the visible range of the photoluminescence spectra that is quite unusual for ordered sp2-carbon systems.

Superposition states of circular currents of exciton-polaritons mimic the superconducting flux qubits. The phase of a polariton fluid must change by an integer number of $2\pi$, when going around the ring. If one introduces a ${\pi}$-phase delay line in the ring, the fluid is obliged to propagate a clockwise or anticlockwise circular current to reduce the total phase gained over one round-trip to zero or to build it up to $2\pi$. We show that such a $\pi$-delay line can be provided by a dark soliton pinned to a potential well created by a C-shape non-resonant pump-spot. The resulting split-ring polariton condensates exhibit pronounced coherent oscillations passing periodically through clockwise and anticlockwise current states. These oscillations may persist far beyond the coherence time of polariton condensates. The qubits based on split-ring polariton condensates are expected to possess very high figures of merit that makes them a valuable alternative to superconducting qubits. The use of the dipole-polarized polaritons allows to control coherently the state of the qubit with the external electric field. This is shown to be one of the tools for realization of single-qubit logic operations. We propose the design of an $i$SWAP gate based on a pair of coupled polariton qubits. To demonstrate the capacity of the polariton platform for quantum computations, we propose a protocol for the realization of the Deutsch's algorithm with polariton qubit networks., Comment: 18 pages, 15 figures

It is well known that electrically neutral excitons can still be affected by crossed electric and magnetic fields that make them move in a direction perpendicular to both fields. We show that a similar effect appears in the absence of external electric fields, in the case of scattering of an exciton flow by charged impurities in the presence of the external magnetic field. As a result, the exciton flow changes the direction of its propagation that may be described in terms of the Hall conductivity for excitons. We develop a theory of this effect, which we refer to as the anomalous exciton Hall effect, to distinguish it from the exciton Hall effect that arises due to the valley selective exciton transport in transition metal dichalcogenides. According to our estimations, the effect is relatively weak for optically active or bright excitons in conventional GaAs quantum wells, but it becomes significant for optically inactive or dark excitons, because of the difference of the lifetimes. This makes the proposed effect a convenient tool for spatial separation of dark and bright excitons., 6 pages, 2 figures, published in PRL

We predict an optical effect associated with systems which exhibit topologically protected states separated by a finite distance. We develop a tight-binding model to calculate the optical selection rules in linear chains of atoms of different lengths, and show the crucial importance of edge states. For long enough molecules the interband transitions involving these edge states are in the highly sought-after THz frequency range. Although we have specifically considered finite carbon chains terminated by gold nanoparticles, the main results of our work can be generalized to various systems which exhibit topologically protected states separated by a finite distance., 9 Pages, 4 Figures

We consider two concentric rings formed by bosonic condensates of exciton-polaritons. A circular superfluid flow of polaritons in one of the rings can be manipulated by acting upon the second annular polariton condensate. The complex coupling between the rings with different topological charges triggers nucleation of stable Josephson vortices (JVs) which are revealed as topological defects of the angular dependence of the relative phase between rings. Being dependent on the coupling strength, the structure of the JV governs the difference of the mean angular momenta of the inner and the outer rings. At the vanishing coupling the condensates rotate independently demonstrating no correlations of their winding numbers. At the moderate coupling, the interaction between two condensates tends to equalize their mean angular momenta despite of the mismatch of the winding numbers demonstrating the phenomenology of a drag effect. Above the critical coupling strength the synchronous rotation is established via the phase slip events.

The ideal reversible thermodynamic cycle visualization of the Nernst effect in Laughlin geometry, excluding the kinetic contribution is proposed. The Ettingshausen effect is also treated in the fashion using the reverse cycle. The corresponding values of the off-diagonal thermoelectric coefficients are expressed through the ratio of the entropy budget per magnetic flux. Our approach enlightens the profound thermodynamic origin of the relation between the Nernst effect and magnetization currents., 7 pages, 2 figures; final version published in PRR

Linear in the wave-vector terms of an electron Hamiltonian play an important role in topological insulators and spintronic devices. Here we demonstrate how an external electric field controls the magnitude of a linear-in-K term in the exciton Hamiltonian in wide GaAs quantum wells. The dependence of this term on the applied field in a high quality sample was studied by means of the differential reflection spectroscopy. An excellent agreement between the experimental data and the results of calculations using semi-classical non-local dielectric response model confirms the validity of the method and paves the way for the realisation of excitonic Datta-and-Das transistors. In full analogy with the spin-orbit transistor proposed by Datta and Das [Appl. Phys. Lett. {\bf 56}, 665 (1990)], the switch between positive and negative interference of exciton polaritons propagating forward and backward in a GaAs film is achieved by application of an electric field with non-zero component in the plane of the quantum well layer., Comment: 30 pages, 6 figures

The emergence of two-dimensional crystals has revolutionized modern solid-state physics. From a fundamental point of view, the enhancement of charge carrier correlations has sparked enormous research activities in the transport- and quantum optics communities. One of the most intriguing effects, in this regard, is the bosonic condensation and spontaneous coherence of many-particle complexes. Here, we find compelling evidence of bosonic condensation of exciton-polaritons emerging from an atomically thin crystal of MoSe2 embedded in a dielectric microcavity under optical pumping. The formation of the condensate manifests itself in a sudden increase of luminescence intensity in a threshold-like manner, and a significant spin-polarizability in an externally applied magnetic field. Spatial coherence is mapped out via highly resolved real-space interferometry, revealing a spatially extended condensate. Our device represents a decisive step towards the implementation of coherent light-sources based on atomically thin crystals, as well as non-linear, valleytronic coherent devices., Main text: 11 pages, 5 figures, Supplementary Materials: 4 pages, 5 figures

Strong spin-orbit coupling and inversion symmetry breaking in transition metal dichalcogenide monolayers yield the intriguing effects of valley-dependent optical selection rules. As such, it is possible to substantially polarize valley excitons with chiral light and furthermore create coherent superpositions of K and K- polarized states. Yet, at ambient conditions dephasing usually becomes too dominant, and valley coherence typically is not observable. Here, we demonstrate that valley coherence is, however, clearly observable for a single monolayer of WSe2, if it is strongly coupled to the optical mode of a high quality factor microcavity. The azimuthal vector, representing the phase of the valley coherent superposition, can be directly manipulated by applying magnetic fields, and furthermore, it sensibly reacts to the polarization anisotropy of the cavity which represents an artificial magnetic field. Our results are in qualitative and quantitative agreement with our model based on pseudospin rate equations, accounting for both effects of real and pseudo-magnetic fields., 12 Pages, 5 Figures

Bose-Einstein condensates of exciton-polaritons are known for their fascinating coherent and polarization properties. The spin state of the condensate is reflected in polarization of the exciton-polariton emission, with temporal fluctuations of this polarization being, in general, capable of reflecting quantum statistics of polaritons in the condensate. To study the polarization properties of optically trapped polariton condensates, we take advantage of the spin noise spectroscopy technique. The ratio between the noise of ellipticity of the condensate emission and its polarization plane rotation noise is found to be dependent, in a nontrivial way, on the intensity of CW nonresonant laser pumping. We show that the interplay between the ellipticity and the rotation noise can be explained in terms of the competition between the self-induced Larmor precession of the condensate pseudospin and the static polarization anisotropy of the microcavity., 5 pages, 3 figures

Remarkable advancements in the ability to create, manipulate, and measure quantum systems are paving the way to build next generations of devices based on quantum physics. Quantum technologies in Russia are on the list of strategically important cross-cutting directions in the framework of the National Technology Initiative programs and the Digital Economy National Program. The broad focus includes quantum computing and simulation, quantum communications, quantum metrology and sensing. This paper reviews existing research on quantum science and technologies in Russia and summarizes the main goals for the next few years that form the basis of an upcoming major national initiative.

We stabilize monoatomic carbon chains in water by attaching them to gold nanoparticles (NPs) by means of the laser ablation process. Resulting nanoobjects represent pairs of NPs connected by multiple straight carbon chains of several nanometer lengths. If NPs at the opposite ends of a chain differ in size, the structure acquires a dipole moment due to the difference in work functions of the two NPs. We take advantage of the dipole polarisation of carbon chains for ordering them by the external electric field. We deposit them on a glass substrate by the sputtering method in the presence of static electric fields of magnitudes up to 10^5 V/m. The formation of one-dimensional carbyne quasi-crystals deposited on a substrate is evidenced by high-resolution TEM and X-ray diffraction measurements. The original kinetic model describing the dynamics of ballistically flowing nano-dipoles reproduces the experimental diagram of orientation of the deposited chains., 15 pages, 7 figures

We model the generation of vortex modes in exciton-polariton condensates in semiconductor micropillars, arranged into a hexagonal ring molecule, in the presence of TE–TM splitting. This splitting lifts the degeneracy of azimuthally modulated vortex modes with opposite topological charges supported by this structure, so that a number of non-degenerate vortex states characterized by different combinations of topological charges in two polarization components appears. We present a full bifurcation picture for such vortex modes and show that because they have different energies they can be selectively excited by coherent pump beams with specific frequencies and spatial configurations. At high pumping intensity, polariton-polariton interactions give rise to the coupling of different vortex resonances and a bistable regime is achieved.

We extend the thermodynamic approach for the description of the thermal Hall effect in the vicinity of a superconducting phase transition, in the fluctuation dominated regime. We show that the Hall heat conductivity is proportional to the product of temperature derivatives of the chemical potential and of the magnetization of the system. We argue that the latter derivative shows the strong singularity in the vicinity of the phase transition, while the former does not contain the characteristic for fermionic systems smallness (T /EF ), what additionally increases the effect. We derive the analytical formula predicting the temperature dependence of the thermal Hall conductivity in the vicinity of the critical temperature for different magnetic fields. Moreover, we study the phenomenon in the regime of quantum fluctuations, in the vicinity of the second critical field and at very low temperatures. We demonstrate how it fades away in a full agreement with the third law of thermodynamics. The developed approach qualitatively explains the recently observed giant thermal Hall effect in cuprates [1]., Comment: 5 pages, 2 figures

In this paper, we experimentally demonstrate an oscillating energy shift of quantum-confined exciton levels in a semiconductor quantum well after excitation into a superposition of two quantum confined exciton states of different parity. Oscillations are observed at frequencies corresponding to the quantum beats between these states. We show that the observed effect is a manifestation of the exciton density oscillations in the real space similar to the dynamics of a Hertzian dipole. The effect is caused by the exciton-exciton exchange interaction and appears only if the excitons are in a superposition quantum state. Thus, we have found clear evidence for the incoherent exchange interaction in the coherent process of quantum beats. This effect may be harnessed for quantum technologies requiring the quantum coherence of states., Comment: 6 pages, 4 figures

Recently, we have experimentally observed signatures of sharp exciton peaks in the photoluminescence spectra of bundles of monoatomic carbon chains stabilized by gold nanoparticles and deposited on a glass substrate. Here, we estimate the characteristic energies of excitonic transitions in this complex quasi-one-dimensional nano-system with use of the variational method. We show that the characteristic energy scale for the experimentally observed excitonic fine structure is governed by the interplay between the hopping energy in a Van der Waals quasicrystal formed by parallel carbon chains, the neutral-charged exciton splitting and the positive-negative trion splitting. These three characteristic energies are an order of magnitude lower than the direct exciton binding energy., Comment: 6 pages, 3 figures

We exploit the recent proposals for the light-induced superconductivity mediated by a Bose-Einstein condensate of exciton-polaritons to design a superconducting fiber that would enable long-distance transport of a supercurrent at elevated temperatures. The proposed fiber consists of a conventional core made of a silica glass with the first cladding layer formed by a material sustaining dipole-polarised excitons with a binding energy exceeding 25 meV. To be specific, we consider a perovskite cladding layer of 20 nm width. The second cladding layer is made of a conventional superconductor such as aluminium. The fiber is covered by a conventional coating buffer and by a plastic outer jacket. We argue that the critical temperature for a superconducting phase transition in the second cladding layer may be strongly enhanced due to the coupling of the superconductor to a bosonic condensate of exciton-polaritons optically induced by the evanescent part of the guiding mode confined in the core. The guided light mode would penetrate to the first cladding layer and provide the strong exciton-photon coupling regime. We run simulations that confirm the validity of the proposed concept. The fabrication of superconducting fibers where a high-temperature superconductivity could be controlled by light would enable passing superconducting currents over extremely long distances.

We calculate the dispersion of exciton-polariton modes in one-dimensional arrays of microcavities. We consider a two-sublattice array and a one-sublattice array of unevenly spaced spherical microcavities some of which contain embedded quantum dots. In both cases the dispersion of polariton eigen modes is shown to be efficiently controlled by a weak applied uniform strain. The structures we consider sustain slow-light modes that are of a specific importance for quantum optics applications. The high sensitivity of the optical response of microcavity arrays to the applied strain may be used in classical and quantum optical switches and the polariton based integrated photonics.

We discover an unexpected enhancement of the absorption of near-infrared light by aromatic amine overtones on photonic microfibers sculptured with gold nanoparticles. The adsorbed nanoparticles make the near-infrared spectroscopy of aromatic amines on microfibers feasible despite the small absorption cross-section of the molecular vibration overtones. We demonstrate that in the presence of gold nanomediators, the absorption of light by weak overtone transitions in N-methylaniline as a model analyte is dramatically enhanced. We attribute this effect to the increase of the mean trajectory of light in a microfiber due to its resonant scattering on metallic nanoparticles. The spectrally integrated transmittance scales with the concentration of nanoparticles to the power 1/6—the phenomenon of diffusive propagation of light. Practical applications of the discovered effect will include the detection of aromatic amines for efficient treatment of metabolic disorders resulting from the amino acids deficiency, resea...

In this paper, the boundary value problem for the heat equation in the region which degenerates at the initial time is considered. Such problems arise in mathematical models of the processes occurring by opening of electric contacts, in particular, at the description of the heat transfer in a liquid metal bridge and electric arcing. The boundary value problem is reduced to a Volterra integral equation of the second kind which has a singular feature. The class of solutions for the integral equation is defined and the constructive method of its solution is developed.