Cold Atoms Research Articles

Searching for new fundamental interactions via isotopic shifts in molecular lattice clocks

Precision measurements with ultracold atoms and molecules are primed to probe beyond-the-standard model physics. Isotopologues of homonuclear molecules are a natural testbed for new Yukawa-type mass-dependent forces at nanometer scales, complementing existing mesoscopic-body and neutron scattering experiments. Here, we propose using isotopic shift measurements in molecular lattice clocks to constrain these new interactions from new massive scalar particles in the keV/c2 range: The new interaction would impart an extra isotopic shift to molecular levels on top of one predicted by the standard model. For the strontium dimer, a Hz-level agreement between experiment and theory could constrain the coupling of the new particles to hadrons by up to an order of magnitude over the most stringent existing experiments. Published by the American Physical Society 2024

Observation of highly correlated ultrabright biphotons through increased atomic ensemble density in spontaneous four-wave mixing

The pairing ratio, a crucial metric assessing a biphoton source's ability to generate correlated photon pairs, remains underexplored despite theoretical predictions. This study presents experimental findings on the pairing ratio, utilizing a double-Λ spontaneous four-wave mixing biphoton source in cold atoms. At an optical depth (OD) of 20, we achieved an ultrahigh biphoton generation rate of up to 1.3×107 per second, with a successful pairing ratio of 61%. Increasing the OD to 120 significantly improved the pairing ratio to 89%, while maintaining a consistent biphoton generation rate. This achievement, marked by high generation rates and robust biphoton pairing, holds great promise for advancing efficiency in quantum communication and information processing. Additionally, in a scenario with a lower biphoton generation rate of 5.0×104 per second, we attained an impressive signal-to-background ratio of 241 for the biphoton wavepacket, surpassing the Cauchy-Schwarz criterion by approximately 1.5×104 times. Published by the American Physical Society 2024

Condensate and superfluid fraction of homogeneous Bose gases in a self-consistent Popov approximation

We study the condensate and superfluid fraction of a homogeneous gas of weakly interacting bosons in three spatial dimensions by adopting a self-consistent Popov approximation, comparing this approach with other theoretical schemes. Differently from the superfluid fraction, we find that at finite temperature the condensate fraction is a non-monotonic function of the interaction strength, presenting a global maximum at a characteristic value of the gas parameter, which grows as the temperature increases. This non-monotonic behavior has not yet been observed, but could be tested with the available experimental setups of ultracold bosonic atoms confined in a box potential. We clearly identify the region of parameter space that is of experimental interest to look for this behavior and provide explicit expressions for the relevant observables. Finite size effects are also discussed within a semiclassical approximation.

Fractional quantum Hall effect of a rapidly rotating gas of ultracold bosonic atoms

Fractional quantum Hall effect of a rapidly rotating gas of ultracold bosonic atoms

Atomic transport dynamics in crossed optical dipole trap

Abstract We study the dynamical evolution of cold atoms in crossed optical dipole trap theoretically and experimentally. The atomic transport process is accompanied by two competitive kinds of physical mechanics, atomic loading and atomic loss. The loading process normally is negligible in the evaporative cooling experiment on the ground, while it is significant in preparation of ultra-cold atoms in the space station. Normally, the atomic loading process is much weaker than the atomic loss process, and the atomic number in the central region of the trap decreases monotonically, as reported in previous research. However, when the atomic loading process is comparable to the atomic loss process, the atomic number in the central region of the trap will initially increase to a maximum value and then slowly decrease, and we have observed the phenomenon first. The increase of atomic number in the central region of the trap shows the presence of the loading process, and this will be significant especially under microgravity conditions. We build a theoretical model to analyze the competitive relationship, which coincides with the experimental results well. Furthermore, we have also given the predicted evolutionary behaviors under different conditions. This research provides a solid foundation for further understanding of the atomic transport process in traps. The analysis of loading process is of significant importance for preparation of ultra-cold atoms in a crossed optical dipole trap under microgravity conditions.

Improved gravity field estimation by incorporating the Tiangong Space Station and next-generation CAI satellite gravity mission

The next-generation gravity satellite mission equipped with the Cold Atom Interferometry (CAI) gradiometer has great potential for the Earth's gravity field estimation. Deploying a CAI gradiometer on the Chinese Tiangong Space Station launched for long-term Earth science research not only reduces the cost compared to a dual-satellite constellation but also enhances interdisciplinary collaboration in the Earth's gravity field detection. In this study, we conducted gravity gradient-based simulations to assess the contribution of deploying a CAI gradiometer on the Tiangong Space Station to collaboratively observe the Earth's gravity field with a polar-orbit gravity satellite. The simulation results demonstrate that whether utilizing Vyy component, three diagonal components or full components, the derived gravity field models show significant improvements within 100 degree and above 200 degree after incorporating Tiangong Space Station. In particular, the gravity field solution recovered from three diagonal components achieves the best accuracy. In the case of using diagonal components, the collaboration observation scheme effectively reduced the cumulative geoid height error by approximately 5.3cm (300 d/o). In the spatial domain, the incorporation of the Tiangong Space Station primarily impacts the estimated gravity field within the orbital coverage area of the space station, and this effect is particularly pronounced when just employing Vyy component. However, due to the limitation of angular velocity observation inaccuracy associated with the CAI gradiometer in nadir mode, there is no substantial accuracy improvement observed above 200 degree when adding gradient components.

Characteristic features of strong correlation: lessons from a 3-fermion one-dimensional harmonic trap

Abstract Many quantum phenomena responsible for key applications in material science and quantum chemistry arise in the strongly correlated regime. This is at the same time, a costly regime for computer simulations. In the limit of strong correlation analytic solutions exist, but as we move away from this limit numerical simulation are needed, and accurate quantum solutions do not scale well with the number of interacting particles. In this work we propose to use few-particle harmonic traps in combination with twisted light as a quantum emulator to investigate the transition into a strongly-correlated regime. Using both analytic derivations and numerical simulations we generalize previous findings on 2 Coulomb interacting fermions trapped in a one-dimensional harmonic trap to the case of 3 fermions. The 4 signatures of strong correlation we have identified in the one-dimensional harmonic trap are: (i) the ground state density is highly localized around N equilibrium positions, where N is the number of particles, (ii) the symmetric and antisymmetric ground state wavefunctions become degenerate, (iii) the von Neumann entropy grows, (iv) the energy spectrum is fully characterized by N normal modes or less. Our findings describe the low-energy behavior of electrons in quantum wires and ions in Paul traps. Similar features have also been reported for cold atoms in optical lattices.

Fast entangling quantum gates with almost-resonant modulated driving

Recently, the method of using a special category of synthetic analytical pulses under off-resonant conditions has improved the experimental performance of two- and multi-qubit gates in the cold atom qubit platform. In order to explore more possibilities and wider ranges of options, we design and analyze the method of almost-resonant modulated driving (ARMD) in constructing fast-speed and high-fidelity quantum logic gates. Whilst the modulation forms the key concept of high-fidelity Rydberg blockade gates so far, the on-off resonance condition can lead to nontrivial nuances in the styles of dynamics, and the ARMD gate protocols have its different characteristic mechanisms in quantum physics compared with the off-resonant gate processes. In particular, ARMD gates usually have abrupt phase changes and at certain points during the time evolution. From a more fundamental point of view, both the resonant and off-resonant methods all together belong to the unitary operation family of fast modulated driving with respect to precisely characterized inter-qubit interactions, which usually allows the quantum logic gate to conclude within one continuous pulse. We anticipate the ARMD gates to work well with the Rydberg dipole-dipole interaction and will serve as a helpful tool in the future studies of the cold atom qubit platform.

The synthetic gauge field and exotic vortex phase with spin-orbital-angular-momentum coupling

Ultracold atoms endowed with tunable spin-orbital-angular-momentum coupling (SOAMC) represent a promising avenue for delving into exotic quantum phenomena. Building on recent experimental advancements, we propose the generation of synthetic gauge fields, and by including exotic vortex phases within spinor Bose–Einstein condensates, employing a combination of a running wave and Laguerre–Gaussian laser fields. We investigate the ground-state characteristics of the SOAMC condensate, revealing the emergence of exotic vortex states with controllable orbital angular momenta. It is shown that the interplay of the SOAMC and conventional spin-linear-momentum coupling induced by the running wave beam leads to the formation of a vortex state exhibiting a phase stripe hosting single multiply quantized singularity. The phase of the ground state will undergo the phase transition corresponding to the breaking of rotational symmetry while preserving the mirror symmetry. Importantly, the observed density distribution of the ground-state wavefunction, exhibiting broken rotational symmetry, can be well characterized by the synthetic magnetic field generated through light interaction with the dressed spin state. Our findings pave the way for further exploration into the rotational properties of stable exotic vortices with higher orbital angular momenta against splitting in the presence of synthetic gauge fields in ultracold quantum gases.

Realizing Altermagnetism in Fermi-Hubbard Models with Ultracold Atoms.

Altermagnetism represents a type of collinear magnetism, that is in some aspects distinct from ferromagnetism and from conventional antiferromagnetism. In contrast to the latter, sublattices of opposite spin are related by spatial rotations and not only by translations and inversions. As a result, altermagnets have spin-split bands leading to unique experimental signatures. Here, we show theoretically how a d-wave altermagnetic phase can be realized with ultracold fermionic atoms in optical lattices. We propose an altermagnetic Hubbard model with anisotropic next-nearest neighbor hopping and obtain the Hartree-Fock phase diagram. The altermagnetic phase separates in a metallic and an insulating phase and is robust over a large parameter regime. We show that one of the defining characteristics of altermagnetism, the anisotropic spin transport, can be probed with trap-expansion experiments.

Advances in Atom Interferometry and their Impacts on the Performance of Quantum Accelerometers On-board Future Satellite Gravity Missions

Recent advances in cold atom interferometry have cleared the path for space applications of quantum inertial sensors, whose level of stability is expected to increase dramatically with the longer interrogation times accessible in space. In this study, an in-orbit model is developed for a Mach–Zehnder-type cold-atom accelerometer. Performance tests are realized under different assumptions about the positioning and rotation compensation method, and the impact of various sources of errors on instrument stability is evaluated. Current and future advances for space-based atom interferometry are discussed, and their impact on the performance of quantum sensors on-board satellite gravity missions is investigated in three different scenarios: state-of-the-art scenario (expected to be ready to launch in 5 years), near-future (expected to be launched in the next 10 to 15 years) and far-future scenarios (expected for the next 20 to 25 years). Our results indicate that the highest sensitivity is achievable by positioning the electrostatic accelerometer at the center of mass of the satellite and the quantum accelerometer aside, on the cross-track axis of the satellite. We show that one can achieve a sensitivity level close to 5 × 10−10 m/s2/Hz with the current state-of-the-art technology. We also estimate that in the near and far-future, atom interferometry in space is expected to achieve sensitivity levels of 1 × 10−11 m/s2/Hz and 1 × 10−12 m/s2/Hz, respectively. A roadmap for improvements in atom interferometry is provided that would maximize the performance of future quantum accelerometers, considering their technical capabilities. Finally, the possibility and challenges of having ultra-sensitive atom interferometry in space for future space missions are discussed.

Bi-planar magnetic stabilisation coils for an inertial sensor based on atom interferometry

Inertial sensors that measure the acceleration of ultracold atoms promise unrivalled accuracy compared to classical equivalents. However, atomic systems are sensitive to various perturbations, including magnetic fields, which can introduce measurement inaccuracies. To address this challenge, we have designed, manufactured, and validated a magnetic field stabilisation system for a quantum sensor based on atom interferometry. We solve for the magnetic field generated by surface currents in-between a pair of bi-rectangular coils and approximate the surface current using discrete wires. The wires are wound by-hand onto machined panels which are retrofitted onto the existing mounting structure of the sensor without interfering with any experimental components. Along the central 60mm of the y-axis, which aligns with the trajectory of the atoms during interferometry, the coils are measured to generate an independent uniform axial magnetic field with a strength of Bz=22.81±0.01μT/A [mean±2σstd. error] and an independent linear axial field gradient of strength dBz/dy=10.6±0.1μT/Am. The uniform Bz field is measured to deviate by a maximum value of 1.3% in the same region, which is a factor of three times more uniform than the previously-used on-sensor rectangular Bz compensation set.

Exceptional Nexus in Bose-Einstein Condensates with Collective Dissipation.

In multistate non-Hermitian systems, higher-order exceptional points and exotic phenomena with no analogues in two-level systems arise. A paradigm is the exceptional nexus (EX), a third-order EP as the cusp singularity of exceptional arcs (EAs), that has a hybrid topological nature. Using atomic Bose-Einstein condensates to implement a dissipative three-state system, we experimentally realize an EX within a two-parameter space, despite the absence of symmetry. The engineered dissipation exhibits density dependence due to the collective atomic response to resonant light. Based on extensive analysis of the system's decay dynamics, we demonstrate the formation of an EX from the coalescence of two EAs with distinct geometries. These structures arise from the different roles played by dissipation in the strong coupling limit and quantum Zeno regime. Our Letter paves the way for exploring higher-order exceptional physics in the many-body setting of ultracold atoms.

Ultra-High Vacuum Cells Realized by Miniature Ion Pump Using High-Efficiency Plasma Source.

In recent years, there has been significant interest in quantum technology, characterized by the emergence of quantum computers boasting immense processing power, ultra-sensitive quantum sensors, and ultra-precise atomic clocks. Miniaturization of quantum devices using cold atoms necessitates the employment of an ultra-high vacuum miniature cell with a pressure of approximately 10-6 Pa or even lower. In this study, we developed an ultra-high vacuum cell realized by a miniature ion pump using a high-efficiency plasma source. Initially, an unsealed miniature ion pump was introduced into a vacuum chamber, after which the ion pump's discharge current, depending on vacuum pressures, was evaluated. Subsequently, a miniature vacuum cell was fabricated by hermetically sealing the miniature vacuum pump. The cell was successfully evacuated by a miniature ion pump down to an ultra-high vacuum region, which was derived by the measured discharge current. Our findings demonstrate the feasibility of achieving an ultra-high vacuum cell necessary for the operation of miniature quantum devices.

Effects of forward disorder on quasi-one-dimensional superconductors

We study the competition between disorder and singlet superconductivity in a quasi-one-dimensional (1D) system. We investigate the applicability of the Anderson theorem, namely that time-reversal conserving (nonmagnetic) disorder does not impact the critical temperature, by opposition to time-reversal breaking disorder (magnetic). To do so, we examine a quasi-1D system of spin 1/2 fermions with attractive interactions and forward scattering disorder using field theory (bosonization). By computing the superconducting critical temperature (Tc), we find that, for nonmagnetic disorder, the Anderson theorem also holds in the quasi-1D geometry. In contrast, magnetic disorder has an impact on the critical temperature, which we investigate by deriving renormalization group equations describing the competition between disorder and interactions. Computing the critical temperature as a function of disorder strength, we observe different regimes depending on the strength of interactions. We discuss possible platforms where this can be observed in cold atoms and condensed matter. Published by the American Physical Society 2024

Gravitational wave analogs in spin nematics and cold atoms

Large-scale gravitational phenomena are famously difficult to observe, making parallels in condensed matter physics a valuable resource. Here we show how spin nematic phases, found in magnets and cold atoms, can provide an analog to linearized gravity. In particular, we show that the Goldstone modes of these systems are massless spin-2 bosons, in one-to-one correspondence with quantized gravitational waves in flat spacetime. We identify a spin-1 model supporting these excitations and, using simulation, outline a procedure for their observation in a 23Na spinor condensate. Published by the American Physical Society 2024

Spatial calibration of high-density absorption imaging

The accurate determination of atom numbers is an ubiquitous problem in the field of ultracold atoms. For modest atom numbers, absolute calibration techniques are available, however, for large numbers and high densities, the available techniques neglect many-body scattering processes. Here, a spatial calibration technique for time-of-flight absorption images of ultracold atomic clouds is presented. The calibration is obtained from radially averaged absorption images and we provide a practical guide to the calibration process. It is shown that the calibration coefficient scales linearly with optical density and depends on the absorbed photon number for the experimental conditions explored here. This allows for the direct inclusion of a spatially dependent calibration in the image analysis. For typical ultracold atom clouds the spatial calibration technique leads to corrections in the detected atom number up to ≈12% and temperature up to ≈14% in comparison to previous calibration techniques. The technique presented here addresses a major difficulty in absorption imaging of ultracold atomic clouds and prompts further theoretical work to understand the scattering processes in ultracold dense clouds of atoms for accurate atom number calibration.

Few-body bound topological and flat-band states in a Creutz ladder

We investigate the properties of a few interacting bosons in a Creutz ladder, which has become a standard model for topological systems, and which can be realized in experiments with cold atoms in optical lattices. At the single-particle level, this system may exhibit a completely flat energy landscape with nontrivial topological properties. In this scenario, we identify topological two-body edge states resulting from the bonding of single-particle edge and flat-band states. We also explore the formation of two- and three-body bound states in the strongly interacting limit, and we show how these quasiparticles can be engineered to replicate the flat-band and topological features of the original single-particle model. Furthermore, we show that in this geometry perfect Aharonov-Bohm caging of two-body bound states may occur for arbitrary interaction strengths, and we provide numerical evidence that the main features of this effect are preserved in an interacting many-body scenario resulting in many-body Aharonov-Bohm caging. Published by the American Physical Society 2024

Real-space detection and manipulation of topological edge modes with ultracold atoms

Real-space detection and manipulation of topological edge modes with ultracold atoms

Photon superfluidity through dissipation

Superfluidity—frictionless flow—has been observed in various physical systems such as liquid helium, cold atoms, and exciton polaritons. Superfluidity is usually realized by cooling and suppressing all dissipation. Here we challenge this paradigm by demonstrating signatures of superfluidity, enabled by dissipation, in the flow of light within a room-temperature oil-filled cavity. Dissipation in the oil mediates effective photon-photon interactions which are noninstantaneous and nonlocal. Such interactions were expected to severely limit the emergence of superfluidity in conservative photonic systems. Surprisingly, when launching a photon fluid with sufficiently high density and low velocity against an obstacle in our driven-dissipative cavity, we observe a record suppression of backscattering. Our experiments also reveal the reorganization dynamics of photons into a nonscattering steady state and a qualitatively changing behavior of the optical phase as light propagates around the obstacle. The phase is locked between the laser and the obstacle but evolves with the intensity in the wake of the obstacle where the density of the photon fluid and its mean-field interaction energy decrease. Using a generalized Gross–Pitaevskii equation for photons coupled to a thermal field, we model our experiments and elucidate how the noninstantaneous and nonlocal character of interactions influences the suppression of scattering associated with superfluidity. Beyond providing the first signatures of cavity photon superfluidity, and of any superfluid both at room temperature and in steady state, our results pave the way for probing photon hydrodynamics in arbitrary potential landscapes using structured mirrors. Published by the American Physical Society 2024