Cold Atoms Research Articles

Measuring the nuclear magnetic quadrupole moment of optically trapped ytterbium atoms in the metastable state

We propose a scheme to measure a nuclear magnetic quadrupole moment (MQM), a CP -violating electromagnetic moment that appears in the nuclear sector, using the long-lived 3P2 metastable state in neutral 173Yb atoms. Laser-cooling and trapping techniques enable us to prepare ultracold 173Yb atoms in the 3P2 state trapped in an optical lattice or an optical tweezer array, providing an ideal experimental platform with long spin coherence time. In addition, our relativistic configuration interaction calculation for the 3P2 electronic wavefunction reveals a large magnetic field gradient generated by the atomic electrons in this state, which amplifies the measurable effect of an MQM. Our scheme could lead to an improvement of more than one order of magnitude in MQM sensitivity compared to the best previous measurement (Murthy et al 1989 Phys. Rev. Lett. 63, 965).

Diffusion transitions induced by shear-thinning viscosity: application to laser-cooled atomic gases

We study the diffusive dynamics of a system in a nonlinear velocity-dependent frictional environment within a continuous time random walk model. In this model, the motion is governed by a shear-thinning frictional force, −γ0v/[1+(v2/vc2)]μ ( 0<μ⩽1 ), where γ 0 represents the coefficient of static friction and µ is the scaling index. Through analytical and numerical results, we construct a diffusion phase diagram that encompasses different regimes upon variations in parameters γ 0 and µ: normal diffusion; superdiffusion; and hyperdiffusion. These transitions occur because the induced weaker friction enhances the diffusion. With a decrease in the scaling index, we find that the γ 0-dependent exponent of diffusion converges towards the experimental findings for ultracold 87Rb atoms because the strong effective friction arises. The discrepancies between the fractional Lévy kinetics and the experimental findings may be potentially reconciled. We believe that these findings are helpful for analyzing experimental observations of cold atoms diffusing in optical lattices.

Ultracold atoms carrying orbital angular momentum: Engineering topological phases in lattices

Ultracold atoms carrying Orbital Angular Momentum (OAM) loaded in lattices constitute a promising platform for engineering topological systems either at the single-particle limit or in the presence of interactions. In this review, we report recent progress on this topic with the focus on bosons with OAM l = 1 in lattices of coplanar ring potentials, which provide an ideal scenario to realise topological non-trivial phases of matter.

Systematic design of a robust half-W1 photonic crystal waveguide for interfacing slow light and trapped cold atoms

Novel platforms interfacing trapped cold atoms and guided light in nanoscale waveguides are a promising route to achieve a regime of strong coupling between light and atoms in single pass, with applications to quantum non-linear optics and quantum simulation. A strong challenge for the experimental development of this emerging waveguide-QED field of research is to combine facilitated optical access for atom transport, atom trapping via guided modes and robustness to inherent nanofabrication imperfections. In this endeavor, here we propose to interface Rubidium atoms with a photonic-crystal waveguide based on a large-index GaInP slab. With a specifically tailored half-W1 design, we show that a large chiral coupling to the waveguide can be obtained and guided modes can be used to form two-color dipole traps for atoms down to 115 nm from the edge of the structure. This optimized device should greatly improve the level of experimental control and facilitate the atom integration.

Towards the generation of mechanical Kerr-cats: awakening the perturbative quantum Moyal corrections to classical motion

The quantum to classical transition is determined by the interplay of a trio of parameters: dissipation, nonlinearity, and macroscopicity. Why is nonlinearity needed to see quantum effects? And, is not an ordinary pendulum quite nonlinear already? In this manuscript, we discuss the parameter regime where the dynamics of a massive oscillator should be quantum mechanical in the presence of dissipation. We review the outstanding challenge of the dynamical generation of highly quantum mechanical cat states of a massive ‘pendulum’, known as Kerr-cats. We argue that state-of-the-art cold atom experiments may be in a position to reach such a nonlinear regime, which today singles out superconducting quantum circuits. A way to stabilize Schrödinger cat superpositions of a mechanical atomic oscillator via parametric squeezing and further protected by an unusual form of quantum interference is discussed. The encoding of a neutral atom Kerr-cat qubit is proposed.

Bounding entanglement dimensionality from the covariance matrix

High-dimensional entanglement has been identified as an important resource in quantum information processing, and also as a main obstacle for simulating quantum systems. Its certification is often difficult, and most widely used methods for experiments are based on fidelity measurements with respect to highly entangled states. Here, instead, we consider covariances of collective observables, as in the well-known Covariance Matrix Criterion (CMC) \cite{guhnecova} and present a generalization of the CMC for determining the Schmidt number of a bipartite system. This is potentially particularly advantageous in many-body systems, such as cold atoms, where the set of practical measurements is very limited and only variances of collective operators can typically be estimated. To show the practical relevance of our results, we derive simpler Schmidt-number criteria that require similar information as the fidelity-based witnesses, yet can detect a wider set of states. We also consider paradigmatic criteria based on spin covariances, which would be very helpful for experimental detection of high-dimensional entanglement in cold atom systems. We conclude by discussing the applicability of our results to a multiparticle ensemble and some open questions for future work.

Dissipative preparation of many-body spin steady states using trapped ultracold atoms

This article presents a dissipative method of creating a spin steady state, or a state whose spin expectation values approaches a fixed value over time, using a trapped gas of ultracold atoms coupled to a background BEC. The ultracold atoms are trapped in a double potential well embedded in a wide harmonic trap, which has a higher energy level than the double wells. The trapped atoms are then excited out of the double well trap into the harmonic trap using Raman lasers. Due to the coupling of the system to the background BEC, the atoms are then able to return to the double potential well by emitting an excitation into the background BEC, which serves as a reservoir of these excitations. By repeatedly coupling and uncoupling the trapped ultracold atoms and the background BEC over fixed intervals of time, the expectation value of the total spin of these atoms will, over time, reach a steady-state value.

Universal transport in periodically driven systems without long-lived quasiparticles

An intriguing regime of universal charge transport at high entropy density has been proposed for periodically driven interacting one-dimensional systems with Bloch bands separated by a large single-particle band gap. For weak interactions, a simple picture based on well-defined Floquet quasiparticles suggests that the system should host a quasisteady state current that depends only on the populations of the system's Floquet-Bloch bands and their associated quasienergy winding numbers. Here we show that such topological transport persists into the strongly interacting regime where the single-particle lifetime becomes shorter than the drive period. Analytically, we show that the value of the current is insensitive to interaction-induced band renormalizations and lifetime broadening when certain conditions are met by the system's nonequilibrium distribution function. We show that these conditions correspond to a quasisteady state. We support these predictions through numerical simulation of a system of strongly interacting fermions in a periodically modulated chain of Sachdev-Ye-Kitaev dots. Our paper establishes universal transport at high entropy density as a robust far from equilibrium topological phenomenon, which can be readily realized with cold atoms in optical lattices. Published by the American Physical Society 2024

Momentum-space signatures of the Anderson transition in a symplectic, two-dimensional, disordered ultracold gas

We study Anderson localization in two-dimensional, disordered, spin-orbit systems belonging to the symplectic symmetry class using momentum-space signatures such as the coherent backscattering antipeak and the coherent forward-scattering peak. Significantly, these momentum-space features are readily accessible in ultracold atom experiments through absorption imaging after time-of-flight expansion. Here, the critical exponent and mobility edge of the metal-insulator transition are successfully obtained through a finite-time analysis of the coherent backscattering width. An anomalous residual diffusion, unique to two dimensions, is identified at the transition point where the system changes from a metal to an insulator. A spin localization phenomenon is also observed in the deep localized regime. Published by the American Physical Society 2024

Quantum entanglement: Principles and research progress in quantum information processing

Quantum entanglement is a peculiar phenomenon in quantum information science, characterized by nonclassical correlations between quantum states of subsystems in a quantum system. Since the proposal of the Einstein-Podolsky-Rosen (EPR) paradox by Einstein, Podolsky, and Rosen, quantum entanglement has sparked intense debates on local realism. Bells inequality experiment established the nonlocality of quantum mechanics. Currently, high-dimensional quantum entanglement of both deterministic and random states can be realized in systems such as photons and cold atoms. Technologies such as quantum teleportation, quantum teleportation, quantum computing, and others rely on quantum entanglement to achieve effects beyond classical limitations. Current research focuses on the implementation of macroscopic quantum entanglement and its significance in fundamental problems of quantum mechanics. Quantum entanglement opens up a new paradigm for information processing with broad application prospects. It is necessary to conduct in-depth research on the nature of quantum entanglement and its advantages in information processing. This paper reviews the theoretical foundations of quantum entanglement, methods of generation and detection, and research progress in its applications in the field of quantum information. It discusses the important applications of quantum entanglement in quantum communication, computing, and sensing and provides an outlook on the future development prospects of quantum entanglement technologies.

Observing dynamical phases of BCS superconductors in a cavity QED simulator.

In conventional Bardeen-Cooper-Schrieffer superconductors1, electrons with opposite momenta bind into Cooper pairs due to an attractive interaction mediated by phonons in the material. Although superconductivity naturally emerges at thermal equilibrium, it can also emerge out of equilibrium when the system parameters are abruptly changed2-8. The resulting out-of-equilibrium phases are predicted to occur in real materials and ultracold fermionic atoms, but not all have yetbeen directly observed. Here we realize an alternative way to generate the proposed dynamical phases using cavity quantum electrodynamics (QED). Our system encodes the presence or absence of a Cooper pair in a long-lived electronic transition in 88Sr atoms coupled to an optical cavity and represents interactions between electrons as photon-mediated interactions through the cavity9,10. To fully explore the phase diagram, we manipulate the ratio between the single-particle dispersion and the interactions after a quench and perform real-time tracking of the subsequent dynamics of the superconducting order parameter using nondestructive measurements. We observe regimes in which the order parameter decays to zero (phase I)3,4, assumes a non-equilibrium steady-state value (phase II)2,3 or exhibits persistent oscillations (phase III)2,3. This opens up exciting prospects for quantum simulation, including the potential to engineer unconventional superconductors and to probe beyond mean-field effects like the spectral form factor11,12, and for increasing the coherence time for quantum sensing.

Superradiance of ultracold cesium Rydberg |65D5/2〉 → |66P3/2〉

We investigate Rydberg |65D5/2〉 → |66P3/2〉 superradiance in dense ultracold cesium atoms, where the ground atoms are excited to |65D5/2〉 Rydberg states via two-photon excitation in a standard magneto-optical trap. The superradiant spectrum of |65D5/2〉 → |66P3/2〉 is obtained using the state-selective field ionization technique. We observe its dynamic evolution process by varying the delay time of ionization field t d. The results show that the evolution process of |65D5/2〉 → |66P3/2〉 is much shorter than its radiation lifetime at room temperature, which verifies the superradiance effect. The dependence of the superradiance process on Rydberg atoms number N e and principal quantum number n is investigated. The results show that the superradiance becomes faster with increasing N e, while it is suppressed for stronger van der Waals (vdW) interactions. Superradiance has potential applications in quantum technologies, and the Rydberg atom is an ideal medium for superradiance. Our system is effective for studying the strong two-body interaction between Rydberg atoms.

Injection spectroscopy of momentum state lattices

The energy spectrum of quantum systems contain a wealth of information about their underlying properties. Spectroscopic techniques, especially those with access to spatially resolved measurements, can be challenging to implement in real-space systems of cold atoms in optical lattices. Here we explore a technique for probing energy spectra in synthetic lattices that is analogous to scanning tunneling microscopy. Using one-dimensional synthetic lattices of coupled atomic momentum states, we explore this spectroscopic technique and observe qualitative agreement between the measured and simulated energy spectra for small two- and three-site lattices as well as a uniform many-site lattice. Finally, through simulations, we show that this technique should allow for the exploration of the topological bands and the fractal energy spectrum of the Hofstadter model as realized in synthetic lattices.

Topologically Invisible Defects in Chiral Mirror Lattices

AbstractOne of the hallmark of topological insulators is having conductivity properties that are unaffected by the possible presence of defects. In this work, by going beyond backscattering immunity and topological invisibility across defects or disorder is obtained. Using a combination of chiral and mirror symmetry, the transmission coefficient is guaranteed to be unity. Importantly, but no phase shift is induced making the defect completely invisible. Many lattices possess the chiral‐mirror symmetry, and the principle is chosen to be demonstrated on an hexagonal lattice model with Kekulé distortion displaying topological edge waves, and analytically and numerically is shown that the transmission across symmetry preserving defects is unity. Then this lattice in an acoustic system is realized, and the invisibility is confirmed with numerical experiments. It is foreseen that the versatility of the model will trigger new experiments to observe topological invisibility in various wave systems, such as photonics, cold atoms or elastic waves.

Investigation of Floquet engineered non-Abelian geometric phase for holonomic quantum computing

Holonomic quantum computing functions by transporting an adiabatically degenerate manifold of computational states around a closed loop in a control-parameter space; this cyclic evolution results in a non-Abelian geometric phase which may couple states within the manifold. Realizing the required degeneracy is challenging and typically requires auxiliary levels or intermediate-level couplings. One potential way to circumvent this is through Floquet engineering, where the periodic driving of a nondegenerate Hamiltonian leads to degenerate Floquet bands, and subsequently non-Abelian gauge structures may emerge. Here we present an experiment in ultracold Rb87 atoms where atomic spin states are dressed by modulated RF fields to induce periodic driving of a family of Hamiltonians linked through a fully tuneable parameter space. The adiabatic motion through this parameter space leads to the holonomic evolution of the degenerate spin states in SU(2), characterized by a non-Abelian connection. We study the holonomic transformations of spin eigenstates in the presence of a background magnetic field, characterizing the fidelity of these single-qubit gate operations. Results indicate that while the Floquet engineering technique removes the need for explicit degeneracies, it inherits many of the same limitations present in degenerate systems. Published by the American Physical Society 2024

Emergent topological ordered phase for the Ising-XY model revealed by cluster-updating Monte Carlo method

The two-component cold atom systems with anisotropic hopping amplitudes can be phenomenologically described by a two-dimensional Ising-XY coupled model with spatial anisotropy. At low temperatures, theoretical predictions [Phys. Rev. A 72 053604 (2005)] and [arXiv: 0706.1609] indicate the existence of a topological ordered phase characterized by Ising and XY disorder but with 2XY ordering. However, due to ergodic difficulties faced by Monte Carlo methods at low temperatures, this topological phase has not been numerically explored. We propose a linear cluster updating Monte Carlo method, which flips spins without rejection in the anisotropy limit but does not change the energy. Using this scheme and conventional Monte Carlo methods, we succeed in revealing the nature of topological phases with half-vortices and domain walls. In the constructed global phase diagram, Ising and XY-type transitions are very close to each other and differ significantly from the schematic phase diagram reported earlier. We also propose and explore a wide range of quantities, including magnetism, superfluidity, specific heat, susceptibility, and even percolation susceptibility, and obtain consistent and reliable results. Furthermore, we observed first-order transitions characterized by common intersection points in magnetizations for different system sizes, as opposed to the conventional phase transition where Binder cumulants of various sizes share common intersections. The critical exponents of different types of phase transitions are reasonably fitted. The results are useful to help cold atom experiments explore the half-vortex topological phase.

Singlet-triplets and excited triplets in trapped medium-body systems with particle number 3 ≤ N ≤ 13 and with spin-4 cold atoms

The spin-textures of trapped medium-body cold systems of spin-4 atoms with the interaction dominated by the attractive λ = 4 channel (the two interacting spins are coupled to 4) have been studied via an exact diagonalization of the spin-dependent Hamiltonian under the single spatial mode approximation. Particle number N (3 ≤ N ≤ 13) is assumed. The eigenstates have been analyzed to find out the details of spin-structures and the inherent excitation modes, and the relation between all of them. The emphasis is to find out how the spectra depend on N. We found that the singlet-triplets (a 3-body subsystem where the three spins are coupled to zero) and various excited triplets are basic constituents and all the eigenstates are composed of them. For low-lying states, only a few particles are involved in the excitation modes, while the other particles would 3-by-3 keep themselves in singlet-triplets.

Perfect Excitation of Topological States by Supersymmetric Waveguides.

Topological photonic states provide intriguing strategies for robust light manipulations, however, it remains challenging to perfectly excite these topological eigenstates due to their complicated mode profiles. In this work, we propose to realize the exact eigenmode of the topological edge states by supersymmetric (SUSY) structures. By adiabatically transforming the SUSY partner to its main topological structure, the edge modes can be perfectly excited with simple single-site input. We experimentally verify our strategy in integrated silicon waveguides in telecommunication wavelength, showing a broad working bandwidth. Moreover, a shortcut-to-adiabaticity strategy is further applied to speed up the adiabatic pump process by inverse-design approaches, thus enabling fast mode evolutions and leading to reduced device size. Our method is universal and beneficial to the topology-based or complex eigenmodes systems, ranging from photonics and microwaves to cold atoms and acoustics.

What ultracold atoms tell us about the real-time dynamics of QCD in extreme conditions

I review developments of how compact table-top setups with ultracold atoms can help us to understand the more complex real-time dynamics of QCD probed in heavy-ion collision experiments.

Enhanced production of &lt;sup&gt;199&lt;/sup&gt;Hg cold atoms based on two-dimensional magneto-optical trap

Efficient preparation of cold atoms plays an important role in realizing precision measurement including optical lattice clocks (OLCs). Fast preparation of cold atoms reduces Dick noise by shortening dead time in a clock interrogation cycle, which improves the stability of OLCs. Here, we increase the loading rate of the three-dimensional magneto-optical trap (3D-MOT) in the ultra-high vacuum environment by utilizing the two-dimensional magneto-optical trap (2D-MOT) with a push beam, reduce the temperature of cold atoms with the compression-MOT technique which is implemented by reducing the detuning of 3D-MOT rapidly at the end of atom preparation, and realize the enhanced production of cold atoms for &lt;sup&gt;199&lt;/sup&gt;Hg OLCs. To achieve 3D-MOT and 2D-MOT of mercury atoms, a deep ultraviolet laser (DUVL) system composed of three DUVLs is developed with one working in lower power for frequency locking and the other two in high power for laser cooling. Such a configuration improves the long-term frequency stability and shows greater robustness than our previous system consisting of two DUVLs. To maximize the 3D-MOT loading rate, we orderly optimize the detuning and the magnetic field gradient of 3D-MOT and those of 2D-MOT as well as the detuning and the power of the push beam. After all parameters are optimized, we measure the maximum loading rate of 3D-MOT to be 3.1×10&lt;sup&gt;5&lt;/sup&gt; s&lt;sup&gt;–1&lt;/sup&gt; and prepare cold atoms of 1.8×10&lt;sup&gt;6&lt;/sup&gt; in 9 s. The loading rate is greatly enhanced by a factor of 51 by using 2D-MOT and the push beam. In order to improve the efficiency of transferring cold atoms from 3D-MOT to optical lattice, we use compression-MOT technique to reduce the temperature of cold atoms and produce cold &lt;sup&gt;199&lt;/sup&gt;Hg atoms which are about 45 μK, lower than the expected temperature of Doppler cooling theory. By achieving the high gain of the 3D-MOT loading rate under the ultra-high vacuum and reducing the temperature of cold atoms, this enhanced preparation of cold atoms based on 2D-MOT effectively shortens the preparation time of cold atoms and improves the transfer efficiency of optical lattice, which provides a significant scheme for efficiently preparing cold mercury atoms in other experiments.