Atomic Condensate Research Articles

Nonuniform Bose–Einstein condensate: II. Doubly coherent states

Abstract We find stationary excited states of a one-dimensional system of N spinless point bosons with repulsive interaction and zero boundary conditions by numerically solving the time-independent Gross–Pitaevskii equation. The solutions are compared with the exact ones found in the Bethe-ansatz approach. We show that the jth stationary excited state of a nonuniform condensate of atoms corresponds to a Bethe-ansatz solution with the quantum numbers n 1 = n 2 = … = n N = j + 1 . On the other hand, such values of n 1 , … , n N correspond to a condensate of N elementary excitations (the Bogoliubov quasiparticles) with the quasimomentum ℏ π j / L , where L is the system size. Thus, each stationary excited state of the condensate is ‘doubly coherent’, since it corresponds simultaneously to a condensate of N atoms and a condensate of N elementary excitations. We find the energy E and the particle density profile ρ ( x ) for such states. The possibility of experimental production of these states is also discussed.

Hybrid Boson Sampling.

We propose boson sampling from a system of coupled photons and Bose-Einstein condensed atoms placed inside a multi-mode cavity as a simulation process testing the quantum advantage of quantum systems over classical computers. Consider a two-level atomic transition far-detuned from photon frequency. An atom-photon scattering and interatomic collisions provide interactions that create quasiparticles and excite atoms and photons into squeezed entangled states, orthogonal to the atomic condensate and classical field driving the two-level transition, respectively. We find a joint probability distribution of atom and photon numbers within a quasi-equilibrium model via a hafnian of an extended covariance matrix. It shows a sampling statistics that is ♯P-hard for computing, even if only photon numbers are sampled. Merging cavity-QED and quantum-gas technologies into a hybrid boson sampling setup has the potential to overcome the limitations of separate, photon or atom, sampling schemes and reveal quantum advantage.

Mirror symmetry breaking of superradiance in a dipolar Bose-Einstein condensate

Dicke superradiance occurs when two or more emitters cooperatively interact via the electromagnetic field. This collective light-scattering process has been extensively studied across various platforms, from atoms to quantum dots and organic molecules. Despite extensive research, the precise role of direct interactions between emitters in superradiance remains elusive, particularly in many-body systems where the complexity of interactions poses significant challenges. In this study, we investigate the effect of dipole-dipole interaction between 18 000 atoms in dipolar Bose-Einstein condensates (BECs) on the superradiance process. In dipolar BECs, we simplify the complex effect of anisotropic magnetic dipole-dipole interaction with Bogoliubov transformation. We observe that anisotropic Bogoliubov excitation breaks the mirror symmetry in decay modes of superradiance. Published by the American Physical Society 2024

Hybrid rotational cavity optomechanics using an atomic superfluid in a ring

We introduce a hybrid optomechanical system containing an annularly trapped Bose-Einstein condensate (BEC) inside an optical cavity driven by Lauguerre-Gaussian (LG) modes. Spiral phase elements serve as the end mirrors of the cavity such that the rear mirror oscillates torsionally about the cavity axis through a clamped support. As described earlier in a related system [P. Kumar , ], the condensate atoms interact with the optical cavity modes carrying orbital angular momentum which create two atomic side modes. We observe three peaks in the output noise spectrum corresponding to the atomic side modes and rotating mirror frequencies, respectively. We find that the trapped BEC's rotation reduces quantum fluctuations at the mirror's resonance frequency. We also find that the atomic side modes-cavity coupling and the optorotational coupling can produce bipartite and tripartite entanglements between various constituents of our hybrid system. We reduce the frequency difference between the side modes and the mirror by tuning the drive field's topological charge and the condensate atoms' rotation. When the atomic side modes become degenerate with the mirror, the stationary entanglement between the cavity and the mirror mode diminishes due to the suppression of cooling. Our proposal, which combines atomic superfluid circulation with mechanical rotation, provides a versatile platform for reducing quantum fluctuations and producing macroscopic entanglement with experimentally realizable parameters. Published by the American Physical Society 2024

Quantum Accelerometry Based on a Geometric Phase

A conceptual model of a promising quantum accelerometer based on a two-mode atomic Bose–Einstein condensate has been proposed. Acceleration generates a specific difference in geometric phases between the condensate modes, which shifts the interference pattern of matter waves. The modes have ring configurations, in the plane of which the measured acceleration vector lies. The homogeneity of the potentials of the ring configurations is interrupted by additional localized potentials generated by defects. Under the variation of the parameters of appropriately located defects with a certain structure, the wavefunctions of the condensate modes acquire geometric phases that differ in the presence of acceleration. Calculations performed for ring configurations of the condensate of 87Rb atoms with a radius of 0.25 mm has showed that the proposed scheme can detect a microgravity of ~10–6–10–7g.

STRUCTURAL CHANGES DURING THERMAL STRENGTHENING OF THE RAILWAY WHEEL

Purpose. Justification of mechanism of the structure transformations in the carbon steel of the railway wheel during disk thermal strengthening. Research methods. The material for the study was carbon steel of a railway wheel with a content of 0.57 % C, 0.65 % Si, 0.45 % Mn, 0.0029 % S, 0.014 % P and 0.11 % Cr. The railway wheel was heated to temperatures higher than Ac3, kept at this temperature to complete austenite homogenization process, and disk was rapidly cooled to the specified temperature. Degree defectiveness structure of the metal after accelerated cooling was assessed using technique of X-ray structural analysis. Strength stress, yield stress and relative elongation of the carbon steel were determined at stretching at rate of deformation 10-3 s-1. Results. At accelerated cooling of the carbon steel, the sources of strengthening are the processes of blocking mobile dislocations due to the condensation of carbon atoms on them and dispersion strengthening from the formed particles of the carbide phase. At temperatures of termination of forced cooling of carbon steel above 300…350 °C, the reduction rate of strength properties is determined by the excess of total effect of softening from decomposition of the solid solution, acceleration of spheroidization and coalescence of cementite particles over the blocking of dislocations by carbon atoms and dispersion hardening. Scientific novelty. The level of strength and plasticity characteristics of carbon steel of the railway wheel disc, depending on the temperature end forced cooling, is determined by the ratio of the influence of degree super saturation of the solid solution and the dispersion strengthening by carbide phase. Practical value. For temperatures termination of accelerated cooling of 200…300 °C, degree of super saturation of the solid solution is the main factor that determines the level of strength and plasticity characteristics. When manufacturing an all-rolled railway wheel, the strength limit of the disc metal can be increased by accelerated cooling to the middle range of temperature.

Engineering and revealing Dirac strings in spinor condensates

Artificial monopoles have been engineered in various systems, yet there has been no systematic study of the singular vector potentials associated with the monopole field. We show that the Dirac string, the line singularity of the vector potential, can be engineered, manipulated, and made manifest in a spinor atomic condensate. We elucidate the connection among spin, orbital degrees of freedom, and the artificial gauge, and show that there exists a mapping between the vortex filament and the Dirac string. We also devise a proposal where preparing initial spin states with relevant symmetries can result in different vortex patterns, revealing an underlying correspondence between the internal spin states and the spherical vortex structures. Such a mapping also leads to a new way of constructing spherical Landau levels, and monopole harmonics. Our observation provides insights into the behavior of quantum matter possessing internal symmetries in curved spaces. Published by the American Physical Society 2024

Observation of dense collisional soliton complexes in a two-component Bose-Einstein condensate

Solitons are nonlinear solitary waves which maintain their shape over time and through collisions, occurring in a variety of nonlinear media from plasmas to optics. We present an experimental and theoretical study of hydrodynamic phenomena in a two-component atomic Bose-Einstein condensate where a soliton array emerges from the imprinting of a periodic spin pattern by a microwave pulse-based winding technique. We observe the ensuing dynamics which include shape deformations, the emergence of dark-antidark solitons, apparent spatial frequency tripling, and decay and revival of contrast related to soliton collisions. For the densest arrays, we obtain soliton complexes where solitons undergo continued collisions for long evolution times providing an avenue towards the investigation of soliton gases in atomic condensates.

Bose-Einstein condensation of non-ground-state caesium atoms

Bose-Einstein condensates of ultracold atoms serve as low-entropy sources for a multitude of quantum-science applications, ranging from quantum simulation and quantum many-body physics to proof-of-principle experiments in quantum metrology and quantum computing. For stability reasons, in the majority of cases the energetically lowest-lying atomic spin state is used. Here, we report the Bose-Einstein condensation of caesium atoms in the Zeeman-excited mf = 2 state, realizing a non-ground-state Bose-Einstein condensate with tunable interactions and tunable loss. We identify two regions of magnetic field in which the two-body relaxation rate is low enough that condensation is possible. We characterize the phase transition and quantify the loss processes, finding unusually high three-body losses in one of the two regions. Our results open up new possibilities for the mixing of quantum-degenerate gases, for polaron and impurity physics, and in particular for the study of impurity transport in strongly correlated one-dimensional quantum wires.

Dissipation-Driven Superradiant Phase Transition of a Two-Dimensional Bose–Einstein Condensate in a Double Cavity

Abstract We study superradiant phase transitions in a hybrid system of a two-dimensional Bose-Einstein condensate of atoms and two cavities arranged with a tilt angle. By adjusting the loss rate of cavities, we map out the phase diagram of steady states within a mean field framework. We find that when the loss rates of the two cavities are different, superradiant transitions may not occur at the same time in the two cavities. A first-order phase transition is observed between the states with only one cavity in superradiance and both in superradiance. In the case of both cavities are superradiant, a net photon current is observed flowing from the cavity with small decay rate to the one with large decay rate. The photon current shows a non-monotonic dependence on the loss rate difference, owing to the competition of photon number difference and cavity field phase difference. Our findings can be realized and detected in experiments.

Mixed localized matter wave solitons in Bose–Einstein condensates with time-varying interatomic interaction and a time-varying complex harmonic trapping potential

We study the generation of mixed localized matter wave solitons in the quasi-one-dimensional Gross–Pitaevskii equation with a complex potential (model equation) that describes the dynamics of Bose–Einstein condensates trapped in an harmonic potential when the loss/gain of condensate atoms is taken into account. Performing a modified lens-type transformation, the integrability condition is derived and the model equation is converted to a Kundu-like nonlinear Schrödinger equation. Through the linear stability analysis, the phenomenon of the modulational instability is analyzed and the criterion of the modulational instability is established. Based on the generalized perturbation (n,N−n)−fold Darboux transformation, new mixed localized wave solutions are presented and used for analyzing the generation of mixed matter-wave solitons in Bose–Einstein condensates with two-body interatomic interactions. We show that mixed localized waves generated with these exact solutions can change from a strong interaction to a weak interaction by choosing the parameters such as the similarity parameters, the spectrum parameter, or the seed solution parameter. Our results show that the spectrum parameter is useful for generating interesting structures which are beneficial to understanding the complex physical phenomena in Bose–Einstein condensates with atomic gain/loss.

Properties of laser-targeted Rydberg Matter with changes in pressure and magnetic field

Rydberg Matter (RM) is a condensate of Rydberg atoms, forming planar hexagonal clusters on a surface. It can exhibit several interesting properties when targeted by a laser pulse, such as expelling particles that can be measured using Time-of-Flight (TOF). Four peaks, shown by voltage spikes caused by a distinct increase in the number of measured particles, were identified in the current experiments. Three of them were in the ns range and one was in the µs range, over 100 cm from emitter to detector. The third ns peak showed a logarithmic correlation between TOF and pressure, reducing TOF from 31-34 ns to 21-24 ns with an increase in pressure from 1 × 10−4 mbar to 5 × 10−2 mbar. This is suggested to arise from a reaction in RM that have reduced reaction time with higher pressure, resulting in photon emission. Additionally, a linear correlation between time and magnitude was found for the µs peak, and the magnitude of this peak could be increased 30 times by setting up a magnetic field compared to having no magnetic field. Still, the concept of RM is not fully developed or understood, and while these findings give more knowledge to the area, they also produce more questions.

Inspiration from machine learning on the example of optimization of the Bose-Einstein condensate of thulium atoms in a 1064-nm trap

Inspiration from machine learning on the example of optimization of the Bose-Einstein condensate of thulium atoms in a 1064-nm trap

Parity-enhanced quantum optimal measurements

In quantum metrology, measurement and estimation schemes are vital for achieving higher precision, along with initial state preparation. This article presents the compound measurement of parity and particle number, which is optimal for a broad range of states named equator states (ESs). ES encompasses most pure input states used in current studies and, more significantly, a wide range of mixed states. Moreover, the ES can be prepared directly using non-demolition parity measurement. We thus propose an improved quantum phase estimation protocol applicable to arbitrary input states, ensuring precision consistently surpassing that of the standard protocol. The proposed scheme is also demonstrated using a nonlinear interferometer, with the realization of the non-demolition parity measurement in atomic condensates.

Topological phase in one-dimensional momentum space lattice of ultracold atoms without chiral symmetry

Symmetry plays a crucial role in understanding topological phases in materials. In one-dimensional systems, such as the Su-Schrieffer-Heeger (SSH) model, chiral symmetry is thought to ensure the quantization of the Zak phase and the nontrivial topological phase. However, our work demonstrates that the one-dimensional lattice system with broken chiral symmetry can still possess quantized Zak phase and nontrivial topological phase. Specifically, we use a Bose-Einstein condensate of <sup>87</sup>Rb atoms in a momentum space lattice of ultracold atoms to effectively simulate a one-dimensional Zigzag model of 26 sites, which intrinsically breaks the chiral symmetry by additional next-nearest-neighbor coupling. To ensure the existence of the nontrivial topological phase, where the Zak phase can be measured from the time-averaged displacement during the system’s evolution, we need to preserve the inversion symmetry by modulating laser power so that all next-nearest-neighbor coupling strengths are equal. Furthermore, by changing the ratio of nearest-neighbor coupling strengths, we observe a topological phase transition from a nontrivial topological phase to a trivial topological phase at the point where the ratio equals 1. Our work demonstrates that the ultracold atom system provides a controllable platform for studying the symmetrical phase and topological phase, with the potential to explore nonlinear topological phenomena by increasing the interactions among atoms. In addition, our system can be used to investigate other interesting topological phenomena with more complex models, such as critical phenomena at the phase transitions and flat band structures in the extended SSH model with long-range coupling. By controlling the coupling strengths, we can also explore the influence of different symmetries on the topological properties of extended SSH models in the future. Moreover, our platform makes it possible to studythe models with more energy bands, such as the Aharonov-Bohm caging model with a three-level structure, which shows peculiar flat-band properties. This work provides opportunities for various studies in the fields of symmetry, topology, and the interaction of controllable quantum systems.

Preparation of Bose-Einstein condensate of dysprosium atoms based on demagnetization cooling

In magnetic atomic gases, the dipolar relaxation process couples the system spin and kinetic degrees of freedom. When the average kinetic energy is significantly lower than the Zeeman splitting, the atoms predominantly occupy the lowest Zeeman state. As the Zeeman splitting approaches the average kinetic energy, some atoms transfer to adjacent Zeeman states through dipolar relaxation, converting kinetic energy into Zeeman energy. By utilizing optical pumping, atoms transferred to higher spin states can be repumped to the ground state, thereby achieving a continuous cooling cycle and effectively lowering the system’s temperature. As the energy removed in a single cooling cycle is much larger than the energy of scattered photons, this demagnetization cooling scheme significantly enhances cooling effciency and reduces atomic loss. In this work, we establish state-coupled equations that incorporate dipolar relaxation and optical pumping to analyze the demagnetization cooling process, modeling the evolution of atom number and temperature during the cooling of <inline-formula><tex-math id="M1">\begin{document}$^{164}{\text{Dy}}$\end{document}</tex-math></inline-formula> atoms. We develop a strategy to generate an optimal magnetic field waveform by maximizing the demagnetization rate. Based on this strategy, we investigate the influence of crucial experimental parameters on demagnetization cooling and determine their specific ranges for producing large atom number of BEC, including the optical dipole trap frequency, as well as the intensity and polarization purity of the optical pumping light. The results indicate that demagnetization cooling enables the direct preparation of a large number of dysprosium BEC with sub-second timescales, reducing the cooling time by an order of magnitude compared to conventional methods for dysprosium atoms. Furthermore, it could achieve a cooling effciency of <inline-formula><tex-math id="M2">\begin{document}$\chi \approx 44.92$\end{document}</tex-math></inline-formula>, an order of magnitude higher than that of traditional evaporative cooling.

Opticheskiy analog vrashchayushchegosya binarnogo boze-kondensata

Coupled nonlinear Schrödinger equations for paraxial optics with two circular polarizations of light in a defocusing Kerr medium with anomalous dispersion coincide in form with the Gross–Pitaevskii equations for a binary Bose—Einstein condensate (BEC) of cold atoms in the phase separation regime. In this case, the helical symmetry of an optical waveguide corresponds to rotation of the transverse potential confining the BEC. The “centrifugal force” considerably affects the propagation of a light wave in such a system. Numerical experiments for a waveguide with an elliptical cross section have revealed characteristic structures consisting of quantized vortices and domain walls between two polarizations, which have not been observed earlier in optics.

Hydrogen-dislocation interaction in relation to hydrogen embrittlement and enhanced plasticity of metals

Interaction between hydrogen atoms and dislocations is studied in the iron-, nickel-, and titanium-based alloys using ab initio calculations and experimental studies of the electron structure, as well as mechanical spectroscopy. It is obtained that hydrogen increases the density of electron states, DOS, at the Fermi level, which is responsible for the increased concentration of free electrons decreasing thereby the elasticity moduli, and results in the decreased line tension of dislocations causing their increased mobility. By means of mechanical spectroscopy, the hydrogen diffusion enthalpy, ΔHd, and that of binding between hydrogen atoms and dislocations, ΔHb, have been measured. It is shown that these values control temperature Tc for condensation of hydrogen atoms at dislocations. Consequently, in a definite range of temperatures and strain rates, dislocations move accompanied by hydrogen atmospheres, which is a reason for hydrogen embrittlement. Hydrogen embrittlement in the iron-, nickel- and titaniun-based alloys is analyzed in terms of this concept. Based on the obtained results, the use of hydrogen as a temporary alloying element increasing techological plasticity of β titanium alloys is interpreted.

Three-dimensional solitons in Rydberg-dressed cold atomic gases with spin–orbit coupling

We present numerical results for three-dimensional (3D) solitons with symmetries of the semi-vortex (SV) and mixed-mode (MM) types, which can be created in spinor Bose–Einstein condensates of Rydberg atoms under the action of the spin–orbit coupling (SOC). By means of systematic numerical computations, we demonstrate that the interplay of SOC and long-range spherically symmetric Rydberg interactions stabilize the 3D solitons, improving their resistance to collapse. We find how the stability range depends on the strengths of the SOC and Rydberg interactions and the soft-core atomic radius.

Carbon nanodot with highly localized excitonic emission for efficient luminescent solar concentrator.

Luminescent solar concentrators (LSCs) are attractive for the easy operation and high compatibility with building integrated photovoltaics due to their low cost, large-scale and applicability. However, underutilized sunlight in visible wavelengths often impedes the advance of LSCs. Here, we demonstrate an orange-emitting carbon nanodots-based LSC (O-CDs) with excitation concentrated in the visible wavelengths. The orange-emitting carbon nanodots (O-CDs) with highly localized excitonic emission are prepared via atomic condensation of doped pyrrolic nitrogen, delivering a high photoluminescence quantum yield of 80 % and a suitable Stokes shift with absorption spectrum situated in the visible region. The O-CDs are embedded in polyvinylpyrrolidone to obtain a highly transparent, stable and environmentally friendly O-CDs-based LSC. Thanks to efficient utilization of solar radiation in visible areas and well match between the emission of O-CDs and the response bands of photovoltaic cells, the O-CDs-based LSC reveals an optical conversion efficiency of 5.17 %, superior to that of most carbon nanodots-based LSCs. These results provide an effective strategy to develop carbon-based luminescent concentrated materials for architectural integrated photovoltaic technology.