Ultracold Neutral Atoms Research Articles

Realization of 87Rb Bose–Einstein Condensates in Higher Bands of a Hexagonal Boron-Nitride Optical Lattice

Ultracold neutral atoms in higher bands of an optical lattice provide a natural avenue to emulate orbital physics in solid state materials. Here, we report the realization of 87Rb Bose–Einstein condensates in the fourth and seventh Bloch bands of a hexagonal boron-nitride optical lattice, exhibiting remarkably long coherence time through active cooling. Using band mapping spectroscopy, we observe that atoms condensed at the energy minimum of Γ point (K 1 and K 2 points) in the fourth (seventh) band as sharp Bragg peaks. The lifetime for the condensate in the fourth (seventh) band is about 57.6 (4.8) ms, and the phase coherence of atoms in the fourth band persists for a long time larger than 110 ms. Our work thus offers great promise for studying unconventional bosonic superfluidity of neutral atoms in higher bands of optical lattices.

Electric Field Analysis in a Cold-Ion Source Using Stark Spectroscopy of Rydberg Atoms

Cold-atom-based ion sources (CABIS) are a timely example of how laser cooling and trapping can be used in practical applications, such as nanofabrication and microscopy. Unfortunately, Coulomb repulsion between ions can significantly deteriorate CABIS performance. The authors demonstrate that embedded Rydberg atoms can be used for noninvasive, near-real-time measurement of the electric fields between ions in CABIS, and thereby to control Coulomb-induced degradation via feedback. The spectra of different Rydberg states reveal which states are suitable for low- and high-field monitoring, and this work also sheds light on many-body interactions between ultracold ions and neutral atoms.

Rapid generation and number-resolved detection of spinor rubidium Bose-Einstein condensates

High data acquisition rates and low-noise detection of ultracold neutral atoms present important challenges for the state tomography and interferometric application of entangled quantum states in Bose-Einstein condensates. In this article, we present a high-flux source of $^{87}$Rb Bose-Einstein condensates combined with a number-resolving detection. We create Bose-Einstein condensates of $2\times10^5$ atoms with no discernible thermal fraction within $3.3$ s using a hybrid evaporation approach in a magnetic/optical trap. For the high-fidelity tomography of many-body quantum states in the spin degree of freedom [arXiv:2207.01270], it is desirable to select a single mode for a number-resolving detection. We demonstrate the low-noise selection of subsamples of up to $16$ atoms and their subsequent detection with a counting noise below $0.2$ atoms. The presented techniques offer an exciting path towards the creation and analysis of mesoscopic quantum states with unprecedented fidelities, and their exploitation for fundamental and metrological applications.

Simulation of exact quantum Ising models with a Mott insulator of paired atoms

Quantum simulation of the XXZ model with a two-component Bose or Fermi Hubbard model based on a Mott insulator background has been widely used in the investigations of quantum magnetism with ultracold neutral atoms. In most cases, the diagonal spin-spin interaction is always accompanied by a large spin-exchange interaction which hinders the formation of long-range magnetic order at low temperature. Here we show that the spin-exchange interaction can be strongly reduced in a Mott insulator of paired atoms, while the diagonal spin-spin interaction remains unaffected. Thus, the effective magnetic model is quite close to an exact classical Ising model in the textbook. And we analysed an experimentally achievable three-component Fermi-Hubbard model of $\mathrm{{}^{6}Li}$ with two hyperfine levels of atoms paired in the lattice. We find the long-range antiferromagnetic order of such a three-component Fermi-Hubbard model can be much stronger than that of a typical two-component Fermi-Hubbard model at low temperature. And we discussed the possiblity of simulating an exact two-dimensional ferromagnetic Ising model in a Mott insulator of paired bosonic atoms. Our results may be useful for experimental investigation of the long-range Ising-like magnetism with ultracold neutral atoms under thermal equilibrium.

A perspective on integrated atomo-photonic waveguide circuits

Integrated photonic circuits based on suspended photonic rib waveguides, which can be used for coherent trapping, guiding, and splitting of ultra-cold neutral atoms in two-color evanescent light fields near their surfaces, are described. Configurations of quantum inertial sensors based on such integrated atomo-photonic waveguides, which are simultaneously guiding photons and atoms along the same paths, are presented. The difference between free-space and guided atom interferometers in the presence of external forces is explained. The theoretical and technological challenges, to be overcome on the way to the realization of such a platform for quantum technologies, are discussed.

Observation of electric-dipole transitions in the laser-cooling candidate Th− and its application for cooling antiprotons

Despite the fact that the laser-cooling method is a well-established technique to obtain ultracold neutral atoms and atomic cations, it has rarely if ever been applied to atomic anions due to the lack of suitable electric-dipole transitions. Efforts of more than a decade have until recently only resulted in ${\mathrm{La}}^{\ensuremath{-}}$ as a promising anion candidate for laser cooling, but our previous work [Tang et al., Phys. Rev. Lett. 123, 203002 (2019)] showed that ${\mathrm{Th}}^{\ensuremath{-}}$ is also a potential candidate. Here we report on a combination of experimental and theoretical studies to determine the frequencies and rates, as well as branching ratios, for the relevant transitions in ${\mathrm{Th}}^{\ensuremath{-}}$. The resonant frequency of the laser-cooling transition is determined to be $\ensuremath{\nu}=123.455(30)$ THz $[\ensuremath{\lambda}=2428.4(6)\phantom{\rule{0.16em}{0ex}}\mathrm{nm}]$. The transition rate is calculated as $A=1.17\ifmmode\times\else\texttimes\fi{}{10}^{4}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$. Since the branching fraction to dark states is negligible, $1.47\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}$, this represents an ideal closed cycle in ${\mathrm{Th}}^{\ensuremath{-}}$ for laser cooling. Furthermore, the zero nuclear spin of $^{232}\mathrm{Th}$ makes the cooling process possible in a Penning trap, which can be used to confine both antiprotons and ${\mathrm{Th}}^{\ensuremath{-}}$ ions. The presented ion dynamics simulations show that the laser-cooled ${\mathrm{Th}}^{\ensuremath{-}}$ anions can effectively cool antiprotons to a temperature around 10 mK.

Stripe and junction-vortex phases in linearly coupled Bose-Einstein condensates

Soon after its theoretical prediction, striped-density states in the presence of synthetic spin-orbit coupling were realized in Bose-Einstein condensates of ultracold neutral atoms [J.-R. Li et al., Nature 543, 91 (2017)]. The achievement opens avenues to explore the interplay of superfluidity and crystalline order in the search for supersolid features and materials. The system considered is essentially made of two linearly coupled Bose-Einstein condensates, that is, a pseudo-spin-$1/2$ system, subject to a spin-dependent gauge field ${\ensuremath{\sigma}}_{z}\ensuremath{\hbar}{k}_{\ensuremath{\ell}}$. Under these conditions, the stripe phase is achieved when the linear coupling $\ensuremath{\hbar}\mathrm{\ensuremath{\Omega}}/2$ is small against the gauge energy $m\mathrm{\ensuremath{\Omega}}/\ensuremath{\hbar}{k}_{\ensuremath{\ell}}^{2}<1$. The resulting density stripes have been interpreted as a standing-wave interference pattern with approximate wave number $2{k}_{\ensuremath{\ell}}$. Here, we show that the emergence of the stripe phase is induced by an array of Josephson vortices living in the junction defined by the linear coupling. As happens in superconducting junctions subject to external magnetic fields, a vortex array is the natural response of the superfluid system to the presence of a gauge field. Also similar to superconductors, the Josephson currents and their associated vortices can be present as a metastable state in the absence of gauge field. We provide closed-form solutions to the 1D mean-field equations that account for such vortex arrays. The underlying Josephson currents coincide with the analytical solutions to the sine-Gordon equation for the relative phase of superconducting junctions [C. Owen and D. Scalapino, Phys. Rev. 164, 538 (1967)].

Fast universal two-qubit gate for neutral fermionic atoms in optical tweezers

An array of ultracold neutral atoms held in optical micro-traps is a promising platform for quantum computation. One of the major bottlenecks of this platform is the weak coupling strength between adjacent atoms, which limits the speed of two-qubit gates. Here, we present a method to perform a fast universal square-root-SWAP gate with fermionic atoms. The basic idea of the gate is to release the atoms into a harmonic potential positioned in between the two atoms. By properly tailoring the interaction parameter, the collision process between the atoms generates entanglement and yields the desired gate. We prove analytically that in the limit of broad atomic wave-packets, the fidelity of the gate approaches unity. We demonstrate numerically that with typical experimental parameters, our gate can operate on a microsecond timescale and achieves a fidelity higher than 0.998. Moreover, the gate duration is independent of the initial distance between the atoms. A gate with such features is an important milestone towards all-to-all connectivity and fault tolerance in quantum computation with neutral atoms.

Fractional quantum Hall physics and higher-order momentum correlations in a few spinful fermionic contact-interacting ultracold atoms in rotating traps

The fractional quantum Hall effect (FQHE) is theoretically investigated, with numerical and algebraic approaches, in assemblies of a few spinful ultracold neutral fermionic atoms, interacting via repulsive contact potentials and confined in a single rapidly rotating two-dimensional harmonic trap. Going beyond the commonly used second-order correlations in the real configuration space, the methodology in this paper will assist the analysis of experimental observations by providing benchmark results for $N$-body spin-unresolved, as well as spin-resolved, momentum correlations measurable in time-of-flight experiments with individual particle detection. Our analysis shows that the few-body lowest-Landau-level (LLL) states with good magic angular momenta exhibit inherent ordered quantum structures in the $N$-body correlations, similar to those associated with rotating Wigner molecules (WMs), familiar from the field of semiconductor quantum dots under high magnetic fields. The application of a small perturbing stirring potential induces, at the ensuing avoided crossings, formation of symmetry broken states exhibiting ordered polygonal-ring structures, explicitly manifest in the single-particle density profile of the trapped particles. Away from the crossings, an LLL state obtained from exact diagonalization of the microscopic Hamiltonian, found to be well-described by a (1,1,1) Halperin two-component variational wavefunction, represents also a spinful rotating WM. Analysis of the calculated LLL wavefunction enables a two-dimensional generalization of the Girardeau one-dimensional 'fermionization' scheme, originally invoked for mapping of bosonic-type wave functions to those of spinless fermions.

Transport dynamics in a high-brightness magneto-optical-trap Li ion source

Laser-cooled gases offer an alternative to tip-based methods for generating high-brightness ion beams for focused ion beam applications. These sources produce ions by photoionization of ultracold neutral atoms, where the narrow velocity distribution associated with microkelvin-level temperatures results in a very low emittance, high-brightness ion beam. In a magneto-optical trap-based ion source, the brightness is ultimately limited by the transport of cold neutral atoms, which restricts the current that can be extracted from the ion-generating volume. We explore the dynamics of this transport in a 7Li magneto-optical trap ion source by performing time-dependent measurements of the depletion and refilling of the ionization volume in a pulsed source. An analytic microscopic model for the transport is developed, and this model shows excellent agreement with the measured results.

Optical control of atom-ion collisions using a Rydberg state

We present a method to control collisions between ultracold neutral atoms in the electronic ground state and trapped ions. During the collision, the neutral atom is resonantly excited by a laser to a low-field-seeking Rydberg state, which is repelled by the ion. As the atom is reflected from the ion, it is de-excited back into its electronic ground level. The efficiency of shielding is analyzed as a function of laser frequency and power, initial atom-ion collision energy, and collision angle. The suitability of several Rydberg levels of Na and Rb for shielding is discussed. Useful applications of shielding include the suppression of unwanted chemical reactions between atoms and ions, a prerequisite for controlled atom-ion interactions.

The Integration of Photonic Crystal Waveguides with Atom Arrays in Optical Tweezers

AbstractIntegrating nanophotonics and cold atoms has drawn increasing interest in recent years due to diverse applications in quantum information science and the exploration of quantum many‐body physics. For example, dispersion‐engineered photonic crystal waveguides (PCWs) permit not only stable trapping and probing of ultracold neutral atoms via interactions with guided‐mode light, but also the possibility to explore the physics of strong, photon‐mediated interactions between atoms, as well as atom‐mediated interactions between photons. While diverse theoretical opportunities involving atoms and photons in 1D and 2D nanophotonic lattices have been analyzed, a grand challenge remains the experimental integration of PCWs with ultracold atoms. Here, an advanced apparatus that overcomes several significant barriers to current experimental progress is described, with the goal of achieving strong quantum interactions of light and matter by way of single‐atom tweezer arrays strongly coupled to photons in 1D and 2D PCWs. Principal technical advances relate to efficient free‐space coupling of light to and from guided modes of PCWs, silicate bonding of silicon chips within small glass vacuum cells, and deterministic, mechanical delivery of single‐atom tweezer arrays to the near fields of photonic crystal waveguides.

Magnetically trapped atoms in the vicinity of an optical nanofibre

Magnetically trapped ^{87}Rb atoms have been observed near a single-mode optical nanofibre. Approximately 1times 10^6 atoms were optically pumped to the {|{F=2,m_mathrm{F}=2}rangle } state and held in the trap with a trap lifetime of up to 2 s. The temperature of the atomic sample within the magnetic trap was measured to be below 230,upmu hbox {K}. The compact vacuum system and high-temperature fibre feedthroughs are presented, and the feasibility of creating a quantum degenerate gas of ultracold neutral atoms near an optical nanofibre is discussed.

Optical Traps for Sympathetic Cooling of Ions with Ultracold Neutral Atoms

We report the trapping of ultracold neutral Rb atoms and Ba^{+} ions in a common optical potential in absence of any radio frequency (rf) fields. We prepare Ba^{+} at 370 μK and demonstrate efficient sympathetic cooling by 100 μK after one collision. Our approach is currently limited by the Rb density and related three-body losses, but it overcomes the fundamental limitation in rf traps set by rf-driven, micromotion-induced heating. It is applicable to a wide range of ion-atom species, and may enable novel ultracold chemistry experiments and complex many-body dynamics.

Quasiperiodic quantum heat engines with a mobility edge

Steady-state thermoelectric machines convert heat into work by driving a thermally-generated charge current against a voltage gradient. In this work, we propose a new class of steady-state heat engines operating in the quantum regime, where a quasi-periodic tight-binding model that features a mobility edge forms the working medium. In particular, we focus on a generalization of the paradigmatic Aubrey-Andr\'e-Harper (AAH) model, known to display a single-particle mobility edge that separates the energy spectrum into regions of completely delocalized and localized eigenstates. Remarkably, these two regions can be exploited in the context of steady-state heat engines as they correspond to ballistic and insulating transport regimes. This model also presents the advantage that the position of the mobility edge can be controlled via a single parameter in the Hamiltonian. We exploit this highly tunable energy filter, along with the peculiar spectral structure of quasiperiodic systems, to demonstrate large thermoelectric effects, exceeding existing predictions by several orders of magnitude. This opens the route to a new class of highly efficient and versatile quasi-periodic steady-state heat engines, with a possible implementation using ultracold neutral atoms in bichromatic optical lattices.

Effect of ion-trap parameters on energy distributions of ultra-cold atom–ion mixtures

Experiments in which ultra-cold neutral atoms and charged ions are overlapped, constitute a new field in atomic and molecular physics, with applications ranging from studying out-of-equilibrium dynamics to simulating quantum many-body systems. The holy grail of ion-neutral systems is reaching the quantum low-energy scattering regime, known as the s-wave scattering. However, in most atom–ion systems, there is a fundamental limit that prohibits reaching this regime. This limit arises from the time-dependent trapping potential of the ion, the Paul trap, which sets a lower collision energy limit which is higher than the s-wave energy. In this work, we studied both theoretically and experimentally, the way the Paul trap parameters affect the energy distribution of an ion that is immersed in a bath of ultra-cold atoms. Heating rates and energy distributions of the ion are calculated for various trap parameters by a molecular dynamics (MD) simulation that takes into account the attractive atom–ion potential. The deviation of the energy distribution from a thermal one is discussed. Using the MD simulation, the heating dynamics for different atom–ion combinations is also investigated. In addition, we performed measurements of the heating rates of a ground-state cooled 88Sr+ ion that is immersed in an ultra-cold cloud of 87Rb atoms, over a wide range of trap parameters, and compare our results to the MD simulation. Both the simulation and the experiment reveal no significant change in the heating for different parameters of the trap. However, in the experiment a slightly higher global heating is observed, relative to the simulation.

Candidate for Laser Cooling of a Negative Ion: High-Resolution Photoelectron Imaging of Th^{-}.

Laser cooling is a well-established technique for the creation of ensembles of ultracold neutral atoms or positive ions. This ability has opened many exciting new research fields over the past 40years. However, no negatively charged ions have been directly laser cooled because a cycling transition is very rare in atomic anions. Efforts of more than a decade currently have La^{-} as the most promising candidate. We report on experimental and theoretical studies supporting Th^{-} as a new promising candidate for laser cooling. The measured and calculated electron affinities of Th are, respectively, 4901.35(48) cm^{-1} and 4832 cm^{-1}, or 0.607 690(60) and 0.599eV, almost a factor of 2 larger than the previous theoretical value of 0.368eV. The ground state of Th^{-} is determined to be 6d^{3}7s^{2} ^{4}F_{3/2}^{e} rather than 6d^{2}7s^{2}7p ^{4}G_{5/2}^{o}. The consequence of this is that there are several strong electric dipole transitions between the bound levels arising from configurations 6d^{3}7s^{2} and 6d^{2}7s^{2}7p in Th^{-}. The potential laser-cooling transition is ^{2}S_{1/2}^{o}↔^{4}F_{3/2}^{e} with a wavelength of 2.6 μm. The zero nuclear spin and hence lack of hyperfine structure in Th^{-} reduces the potential complications in laser cooling as encountered in La^{-}, making Th^{-} a new and exciting candidate for laser cooling.

Width and shift of Fano-Feshbach resonances for van der Waals interactions

We revisit the basic properties of Fano-Feshbach resonances in two-body systems with van der Waals tail interactions, such as ultracold neutral atoms. Using a two-channel model and two different methods, we investigate the relationship between the width and shift of the resonances and their dependence on the low-energy parameters of the system. Unlike what was previously believed [Rev. Mod. Phys. 82, 1225 (2010)] for magnetic resonances, we find that the ratio between the width and the shift of a resonance does not depend only on the background scattering length, but also on a closed-channel scattering length. We obtain different limits corresponding to different cases of optical and magnetic resonances. Although the generalisation of the theory to the multi-channel case remains to be done, we found that our two-channel predictions are verified for a specific resonance of lithium-6.

Minimizing rf-induced excess micromotion of a trapped ion with the help of ultracold atoms

We report on the compensation of excess micromotion due to parasitic rf-electric fields in a Paul trap. The parasitic rf-electric fields stem from the Paul trap drive but cause excess micromotion, e.g. due to imperfections in the setup of the Paul trap. We compensate these fields by applying rf-voltages of the same frequency but adequate phases and amplitudes to Paul trap electrodes. The magnitude of micromotion is probed by studying elastic collision rates of the trapped ion with a gas of ultracold neutral atoms. Furthermore, we demonstrate that also reactive collisions can be used to quantify micromotion. We achieve compensation efficiencies of about 1$\:\text{Vm}^{-1}$, which is comparable to other conventional methods.

Imaging topology of Hofstadter ribbons

Physical systems with non-trivial topological order find direct applications in metrology (Klitzing et al 1980 Phys. Rev. Lett. 45 494–7) and promise future applications in quantum computing (Freedman 2001 Found. Comput. Math. 1 183–204; Kitaev 2003 Ann. Phys. 303 2–30). The quantum Hall effect derives from transverse conductance, quantized to unprecedented precision in accordance with the system’s topology (Laughlin 1981 Phys. Rev. B 23 5632–33). At magnetic fields beyond the reach of current condensed matter experiment, around T, this conductance remains precisely quantized with values based on the topological order (Thouless et al 1982 Phys. Rev. Lett. 49 405–8). Hitherto, quantized conductance has only been measured in extended 2D systems. Here, we experimentally studied narrow 2D ribbons, just 3 or 5 sites wide along one direction, using ultracold neutral atoms where such large magnetic fields can be engineered (Jaksch and Zoller 2003 New J. Phys. 5 56; Miyake et al 2013 Phys. Rev. Lett. 111 185302; Aidelsburger et al 2013 Phys. Rev. Lett. 111 185301; Celi et al 2014 Phys. Rev. Lett. 112 043001; Stuhl et al 2015 Science 349 1514; Mancini et al 2015 Science 349 1510; An et al 2017 Sci. Adv. 3). We microscopically imaged the transverse spatial motion underlying the quantized Hall effect. Our measurements identify the topological Chern numbers with typical uncertainty of , and show that although band topology is only properly defined in infinite systems, its signatures are striking even in nearly vanishingly thin systems.