Atomic Condensate Research Articles

Deciphering Photochemical Reaction Pathways of Aqueous Tetrachloroauric Acid by X-ray Transient Absorption Spectroscopy.

Photolysis reaction pathways of [Au(III)Cl4]- in aqueous solution have been investigated by time-resolved X-ray absorption spectroscopy. Ultraviolet excitation directly breaks the Au-Cl bond in [Au(III)Cl4]- to form [Au(II)Cl3]- that becomes highly reactive within 79 ps. Disproportionation of [Au(II)Cl3]- generates [Au(I)Cl2]-, which is stable for ≤10 μs. In contrast, intense near-infrared lasers photolyze water to generate hydrated electrons, which then reduce [Au(III)Cl4]- to [Au(II)Cl3]- at 5 ns. Hydrated electrons further induce a chain reaction from [Au(II)Cl3]- to [Au(0)Cl]- by successively removing one Cl-. The zero-valency Au anions quickly polymerize and condense to form Au nanoparticles, which become the dominating product after 400 s. Our results reveal that the condensation of zero-valency Au starts with dimerization of gold clusters coordinated with chloride ions rather than direct condensation of pristine Au atoms.

Deep cooling scheme of quantum degenerate gas and ground experimental verification for chinese space station

The Cold Atom Physics Rack (CAPR) of Chinese space station will be launched at the end of 2022. The important goal of CAPR is to achieve BEC at 100 pk. In order to obtain ultracold atoms in microgravity of space station, we propose a two-stage cooling scheme using all-optical trap with different waist beams. The cold atom cloud obtained by this scheme is composed of condensate and thermal atoms around condensate. The design of our two-stage cooling scheme will effectively reduce the temperature of the thermal atom cloud and the effective temperature generated by the interaction energy of the condensate. The atomic temperature of 5 nk is obtained from the ground test experiment, and the corresponding temperature under the microgravity condition of the space station is theoretically predicted to be less than 100 pk. Taking the advantages of ultracold temperature and long-time detection, many scientific experiments will be arranged. In this paper, the ground test experiments based on ground principle prototype and pre-prototype for CAPR are also introduced.

Carbon Nanodots with Nearly Unity Fluorescent Efficiency Realized via Localized Excitons.

Carbon nanodots (CDs) have emerged as an alternative option for traditional nanocrystals due to their excellent optical properties and low toxicity. Nevertheless, high emission efficiency is a long‐lasting pursuit for CDs. Herein, CDs with near‐unity emission efficiency are prepared via atomic condensation of doped pyrrolic nitrogen, which can highly localize the excited states thus lead to the formation of bound excitons and the symmetry break of the π–electron conjugation. The short radiative lifetimes (<8 ns) and diffusion lengths (<50 nm) of the CDs imply that excitons can be efficiently localized by radiative recombination centers for a defect‐insensitive emission of CDs. By incorporating the CDs into polystyrene, flexible light‐converting films with a high solid‐state quantum efficiency of 84% and good resistance to water, heating, and UV light are obtained. With the CD–polymer films as light conversion layers, CD‐based white light‐emitting diodes (WLEDs) with a luminous efficiency of 140 lm W−1 and a flat‐panel illumination system with lighting sizes of more than 100 cm2 are achieved, matching state‐of‐the‐art nanocrystal‐based LEDs. These results pave the way toward carbon‐based luminescent materials for solid‐state lighting technology.

High precision solutions to quantized vortices within Gross–Pitaevskii equation

The dynamics of vortices in Bose–Einstein condensates of dilute cold atoms can be well formulated by Gross–Pitaevskii equation. To better understand the properties of vortices, a systematic method to solve the nonlinear differential equation for the vortex to very high precision is proposed. Through two-point Padé approximants, these solutions are presented in terms of simple rational functions, which can be used in the simulation of vortex dynamics. The precision of the solutions is sensitive to the connecting parameter and the truncation orders. It can be improved significantly with a reasonable extension in the order of rational functions. The errors of the solutions and the limitation of two-point Padé approximants are discussed. This investigation may shed light on the exact solution to the nonlinear vortex equation.

Collapse inhibition in three-dimensional Bose–Einstein condensates: Internal repulsion and particle loss

We consider the dynamics of three-dimensional Bose–Einstein condensates of cold atoms in the presence of internal dynamical counter-pressure that may arise due to three-body atomic collisions. We demonstrate that, in the corresponding phenomenological approach, in addition to the condensate density losses, these three-body interactions may lead to an internal pressure resulting in a repulsive potential in the condensate. This repulsion, being nonlocal and expressed with a specific form of the integral dependence on the condensate density, similar to the manifestation of the Coulomb forces in charged systems, can determine the entire condensate evolution. We find that these effects can inhibit the collapse of a self-attracting condensate and even cause its explosion, and identify the corresponding conditions numerically.

Analogous Hawking radiation and quantum entanglement in two-component Bose-Einstein condensates: The gapped excitations

The condensates of cold atoms at zero temperature in the tunable binary Bose-Einstein condensate system are studied with the Rabi transition between atomic hyperfine states where the system can be represented by a coupled two-field model of gapless excitations and gapped excitations. We set up the configuration of the supersonic and subsonic regimes with the acoustic horizon between them in the elongated two-component Bose-Einstein condensates, trying to mimic Hawking radiations, in particular due to the gapped excitations. The simplified step-like sound speed change is adopted for the subsonic-supersonic transition so that the model can be analytically treatable. The effective energy gap term in the dispersion relation of the gapped excitations introduces the threshold frequency $\omega_\text{min}$ in the subsonic regime, below which the propagating modes do not exist. Thus, the particle spectrum of the Hawking modes significantly deviates from that of the gapless cases near the threshold frequency due to the modified grey-body factor, which vanishes as the mode frequency is below $\omega_\text{min}$. The influence from the gapped excitations to the quantum entanglement of the Hawking mode and its partner of the gapless excitations is also studied according to the Peres-Horodecki-Simon (PHS) criterion. It is found that the presence of the gapped excitations will deteriorate the quantumness of the pair modes of the gapless excitations when the frequency of the pair modes in particular is around $\omega \sim \omega_\text{min}$. On top of that, when the coupling constant between the gapless and gapped excitations becomes large enough, the huge particle density of the gapped excitations in the small $\omega$ regime will significantly disentangle the pair modes of the gapless excitations. The detailed time-dependent PHS criterion will be discussed.

On the survival of the quantum depletion of a condensate after release from a magnetic trap

We present observations of the high momentum tail in expanding Bose–Einstein condensates of metastable Helium atoms released from a harmonic trap. The far-field density profile exhibits features that support identification of the tails of the momentum distribution as originating in the in-situ quantum depletion prior to release. Thus, we corroborate recent observations of slowly-decaying tails in the far-field beyond the thermal component. This observation is in conflict with the hydrodynamic theory, which predicts that the in-situ depletion does not survive when atoms are released from a trap. Indeed, the depleted tails even appear stronger in the far-field than expected before release, and we discuss the challenges of interpreting this in terms of the Tan contact in the trapped gas. In complement to these observations, full quantum simulations of the experiment show that, under the right conditions, the depletion can persist into the far field after expansion. Moreover, the simulations provide mechanisms for survival and for the the large-momentum tails to appear stronger after expansion due to an acceleration of the depleted atoms by the mean-field potential. However, while in qualitative agreement, the final depletion observed in the experiment is much larger than in the simulation.

Two-Dimensional Solitons in Nonlocal Media: A Brief Review

This is a review addressing soliton-like states in systems with nonlocal nonlinearity. The work on this topic has long history in optics and related areas. Some results produced by the work (such as solitons supported by thermal nonlinearity in optical glasses, and orientational nonlinearity, which affects light propagation in liquid crystals) are well known, and have been properly reviewed in the literature, therefore the respective models are outlined in the present review in a brief form. Some other studies, such as those addressing models with fractional diffraction, which is represented by a linear nonlocal operator, have started more recently, therefore it will be relevant to review them in detail when more results will be accumulated; for this reason, the present article provides a short outline of the latter topic. The main part of the article is a summary of results obtained for two-dimensional solitons in specific nonlocal nonlinear models originating in studies of Bose–Einstein condensates (BECs), which are sufficiently mature but have not yet been reviewed previously (some results for three-dimensional solitons are briefly mentioned too). These are, in particular, anisotropic quasi-2D solitons supported by long-range dipole-dipole interactions in a condensate of magnetic atoms and giant vortex solitons (which are stable for high values of the winding number), as well as 2D vortex solitons of the latter type moving with self-acceleration. The vortex solitons are states of a hybrid type, which include matter-wave and electromagnetic-wave components. They are supported, in a binary BEC composed of two different atomic states, by the resonant interaction of the two-component matter waves with a microwave field that couples the two atomic states. The shape, stability, and dynamics of the solitons in such systems are strongly affected by their symmetry. Some other topics are included in the review in a brief form. This review uses the “Harvard style” of referring to the bibliography.

Dynamics of atoms within atoms

Recent experiments with Bose–Einstein condensates have entered a regime in which thousands of ground-state condensate atoms fill the Rydberg-electron orbit. After the excitation of a single atom into a highly excited Rydberg state, scattering off the Rydberg electron sets ground-state atoms into motion, such that one can study the quantum-many-body dynamics of atoms moving within the Rydberg atom. Here we study this many-body dynamics using Gross–Pitaevskii and truncated Wigner theory. Our simulations focus in particular on the scenario of multiple sequential Rydberg excitations on the same rubidium condensate which has become the standard tool to observe quantum impurity dynamics in Rydberg experiments. We investigate to what extent such experiments can be sensitive to details in the electron–atom interaction potential, such as the rapid radial modulation of the Rydberg molecular potential, or p-wave shape resonance. We demonstrate that both effects are crucial for the initial condensate response within the Rydberg orbit, but become less relevant for the density waves emerging outside the Rydberg excitation region at later times. Finally we explore the local dynamics of condensate heating. We find that it provides only minor corrections to the mean-field dynamics. Combining all these insights, our results suggest Bose–Einstein condensates as a viable platform for the in situ and real time interrogation of ultra-cold chemistry dynamics involving Rydberg states.

Continuous Bose–Einstein condensation

Bose–Einstein condensates (BECs) are macroscopic coherent matter waves that have revolutionized quantum science and atomic physics. They are important to quantum simulation1 and sensing2,3, for example, underlying atom interferometers in space4 and ambitious tests of Einstein’s equivalence principle5,6. A long-standing constraint for quantum gas devices has been the need to execute cooling stages time-sequentially, restricting these devices to pulsed operation. Here we demonstrate continuous Bose–Einstein condensation by creating a continuous-wave (CW) condensate of strontium atoms that lasts indefinitely. The coherent matter wave is sustained by amplification through Bose-stimulated gain of atoms from a thermal bath. By steadily replenishing this bath while achieving 1,000 times higher phase-space densities than previous works7,8, we maintain the conditions for condensation. Our experiment is the matter wave analogue of a CW optical laser with fully reflective cavity mirrors. This proof-of-principle demonstration provides a new, hitherto missing piece of atom optics, enabling the construction of continuous coherent-matter-wave devices.

Resolving the puzzle of sound propagation in a dilute Bose–Einstein condensate

A unified model of a dilute Bose–Einstein condensate is proposed, combining the logarithmic and Gross–Pitaevskii (GP) nonlinear terms in a wave equation, where the GP term describes two-body interactions, as suggested by the standard perturbation theory; while the logarithmic term is essentially nonperturbative, and takes into account quantum vacuum effects. The model is shown to have excellent agreement with sound propagation data in the condensate of cold sodium atoms known since the now classic works by Andrews and collaborators. The data also allowed us to place constraints on two of the unified model’s parameters, which describe the strengths of the logarithmic and GP terms. Additionally, we suggest an experiment constraining the value of the third parameter (the characteristic density scale of the logarithmic part of the model), using the conjectured attraction–repulsion transition of many-body interaction inside the condensate.

Bending the rules of low-temperature thermometry with periodic driving

There exist severe limitations on the accuracy of low-temperature thermometry, which poses a major challenge for future quantum-technological applications. Low-temperature sensitivity might be manipulated by tailoring the interactions between probe and sample. Unfortunately, the tunability of these interactions is usually very restricted. Here, we focus on a more practical solution to boost thermometric precision – driving the probe. Specifically, we solve for the limit cycle of a periodically modulated linear probe in an equilibrium sample. We treat the probe-sample interactions exactly and hence, our results are valid for arbitrarily low temperatures T and any spectral density. We find that weak near-resonant modulation strongly enhances the signal-to-noise ratio of low-temperature measurements, while causing minimal back action on the sample. Furthermore, we show that near-resonant driving changes the power law that governs thermal sensitivity over a broad range of temperatures, thus `bending' the fundamental precision limits and enabling more sensitive low-temperature thermometry. We then focus on a concrete example – impurity thermometry in an atomic condensate. We demonstrate that periodic driving allows for a sensitivity improvement of several orders of magnitude in sub-nanokelvin temperature estimates drawn from the density profile of the impurity atoms. We thus provide a feasible upgrade that can be easily integrated into low-T thermometry experiments.

A radio-frequency Bose–Einstein condensate magnetometer

We report on a radio frequency magnetometer employing a Bose–Einstein condensate of 87Rb atoms held in a dipole trap. An AC sensitivity of 296 pT/Hz is achieved at a probing volume of 3.2×10−8 cm3, leading to a volume-normalized sensitivity of 53 fT/Hz cm−3. At larger probing volumes with the atoms released from the magnetic trap used in the initial phase of the evaporation sequence, the AC sensitivity is improved to 1.4 pT/Hz, allowing a two-mode approach for applications requiring improved sensitivity. Immediate application in high-resolution electromagnetic induction imaging is expected when compared to performance of other induction-based imaging platforms.

Superfluid to Mott-insulator transition in a one-dimensional optical lattice

Bose–Einstein condensates (BEC) of sodium atoms are transferred into one-dimensional (1D) optical lattice potentials, formed by two laser beams with a wavelength of 1064 nm, in a shallow optical trap. The phase coherence of the condensate in the lattice potential is studied by changing the lattice depth. A qualitative change in behavior of the BEC is observed at a lattice depth of ∼ 13.7 E r, where the quantum gas undergoes a transition from a superfluid state to a state that lacks well-to-well phase coherence.

Continuous-time crystal from a spontaneous many-body Floquet state

Floquet driven systems represent an extremely interesting arena to study out-of-equilibrium phenomena. For instance, they provide realizations of discrete time crystals, where the discrete time translation symmetry of the periodic Hamiltonian is spontaneously broken by a subharmonic response of the system. However, the continuous presence of an external periodic driving is required within the current Floquet paradigm. We propose here the concept of spontaneous many-body Floquet state. This is a state that, in the absence of external periodic driving, self-oscillates like in the presence of a periodic Hamiltonian, this behavior being spontaneously induced by many-body interactions. In addition, its quantum fluctuations are described by regular Floquet theory. Furthermore, it is also a time crystal, presenting long-range time-periodic order. However, this crystalline behavior is very different to that of conventional Floquet discrete time crystals: here, there is no external periodic driving, energy is conserved, and the nature of the spontaneous symmetry breaking is continuous instead of discrete. We demonstrate that spontaneous many-body Floquet states can emerge in a variety of canonical many-body problems, ranging from interacting fermions to Bose-Hubbard models. We specifically show that a spontaneous many-body Floquet state is a universal intrinsic state of a one-dimensional flowing atom condensate, both subsonic and supersonic, resulting from a dynamical phase transition and robust against external perturbations and quantum fluctuations, proposing also realistic experimental scenarios for its observation. A spontaneous many-body Floquet state not only represents a realization of a continuous time crystal, but also a novel paradigm in Floquet physics.

Dynamical preparation of an atomic condensate in a Hofstadter band

The creation of a Hamiltonian in the quantum regime which has non-trivial topological features is a central goal of the cold-atom community, enabling widespread exploration of novel phases of quantum matter. A general scheme to synthesize such Hamiltonians is based on dynamical modulation of optical lattices which thereby generate vector potentials. At the same time the modulation can lead to heating and serious difficulties with equilibration. Here we show that these challenges can be overcome by demonstrating how a Hofstadter Bose-Einstein condensate (BEC) can be dynamically realized, using experimental protocols. From Gross-Pitaevskii simulations our study reveals a complex, multistage evolution; this includes a chaotic intermediate "heating" stage followed by a spontaneous reentrance to the BEC. The observed behavior is reminiscent of evolution in cosmological models.

Dimensional crossover of Bose–Einstein condensation of atomic gases in anisotropic harmonic traps

We investigate the dimensional crossover of Bose–Einstein condensation of atomic gases trapped in anisotropic harmonic potentials. The crossover from three dimension to lower dimensions is studied by calculating some thermodynamic quantities, such as critical temperature, heat capacity, entropy and atom number fluctuations in Bose–Einstein condensate. The smooth transition behavior indicates that a decimal dimension may be employed to describe the corresponding intervenient states. We compare the theoretical curves with recent experimental results on the fluctuations of the condensate atom numbers, revealing how the geometry and size of the condensates affect the thermodynamic characteristics.

Production of Size-Controlled Gold Nanoclusters for Vapor-Liquid-Solid Method.

This study demonstrated the deposition of size-controlled gold (Au) nanoclusters via direct-current magnetron sputtering and inert gas condensation techniques. The impact of different source parameters, namely, sputtering discharge power, inert gas flow rate, and aggregation length on Au nanoclusters’ size and yield was investigated. Au nanoclusters’ size and size uniformity were confirmed via transmission electron microscopy. In general, Au nanoclusters’ average diameter increased by increasing all source parameters, producing monodispersed nanoclusters of an average size range of 1.7 ± 0.1 nm to 9.1 ± 0.1 nm. Among all source parameters, inert gas flow rate exhibited a strong impact on nanoclusters’ average size, while sputtering discharge power showed great influence on Au nanoclusters’ yield. Results suggest that Au nanoclusters nucleate via a three-body collision mechanism and grow through a two-body collision mechanism, wherein the nanocluster embryos grow in size due to atomic condensation. Ultimately, the usefulness of the produced Au nanoclusters as catalysts for a vapor–liquid–solid technique was put to test to synthesize the phase change material germanium telluride nanowires.

Spectral functions and localization-landscape theory in speckle potentials

Spectral function is a key tool for understanding the behavior of Bose-Einstein condensates of cold atoms in random potentials generated by a laser speckle. In this paper we introduce a method for computing the spectral functions in disordered potentials. Using a combination of the Wigner-Weyl approach with the localization-landscape theory, we build an approximation for the Wigner distributions of the eigenstates in the phase space and show its accuracy in all regimes, from the deep quantum regime to the intermediate and semiclassical. Based on this approximation, we devise a method to compute the spectral functions using only the landscape-based effective potential. The paper demonstrates the efficiency of the proposed approach for disordered potentials with various statistical properties without requiring any adjustable parameters.

A pathway to peptides in space through the condensation of atomic carbon

Organic molecules are widely present in the dense interstellar medium, and many have been synthesized in the laboratory on Earth under the conditions typical for an interstellar environment. Until now, however, only relatively small molecules of biological interest have been demonstrated to form experimentally under typical space conditions. Here we prove experimentally that the condensation of carbon atoms on the surface of cold solid particles (cosmic dust) leads to the formation of isomeric polyglycine monomers (aminoketene molecules). Following encounters between aminoketene molecules, they polymerize to produce peptides of different lengths. The chemistry involves three of the most abundant species (CO, C and NH3) present in star-forming molecular clouds, and proceeds via a novel pathway that skips the stage of amino acid formation in protein synthesis. The process is efficient, even at low temperatures, without irradiation or the presence of water. The delivery of biopolymers formed by this chemistry to rocky planets in the habitable zone might be an important element in the origins of life.