Trapped Cold Atoms Research Articles

All-dielectric metaoptics for the compact generation of double-ring perfect vector beams

Abstract Perfect vortices, whose ring profile is independent of the topological charge, play a key role in telecommunications and particle micro-manipulation. In this work, we report the compact generation of a new kind of double-ring perfect vortices, called double-ring perfect vector beams, by exploiting dual-functional silicon metaoptics. In particular, we develop and test a new paradigm to generate those beams with the possibility of selecting different topological charges between the two rings. The generated beams are characterized through a filtering method, proving that the two rings have a vectorial nature with the same magnitude and either the same or different topological charges. Their unique properties suggest promising applications for optical tweezing and manipulation of low refractive-index particles, trapping of cold atoms, and high-capacity communications.

Optical dipole potential energy of a two-level atom interacting with a tightly focused truncated optical Bessel beam

Truncated optical Bessel (TOB) beams are an orthonormal set of optical vortex beams that possess orbital angular momentum (OAM). It is characterized by a significant longitudinal electric field component at the sub-wavelength scales. The photons of a circularly polarized TOB beam also carry spin angular momentum (SAM) and can interact with a two-level atom, in the far-off resonance regime, giving rise to an optical dipole potential energy that involves a spin–orbit term that stems from the inclusion of the longitudinal field component. When a two-level atom is simultaneously irradiated by two TOB beams, the optical dipole potential energy, in some special cases, is entirely dependent on the OAM–SAM coupling. We explore the cold-atom trapping options enabled by the use of TOB beams, either single ones or combinations thereof. We put emphasis on case in which the OAM–SAM coupling is the dominant contribution. The similarities and differences between the results obtained for the TOB beams and those for the Laguerre–Gaussian beams are also discussed.

Time-orbiting-potential chip trap for cold atoms

We present a design for an atom chip trap that uses the time-orbiting potential technique. The design offers several advantages compared to other chip-trap methods. It uses a simple crossed-wire pattern on the chip, along with a rotating bias field. The trap is naturally close to spherically symmetric, and it can be modified to be exactly symmetric in quadratic order of the coordinates. Loading from a magneto-optical trap is facilitated because the trap can be positioned an arbitrary distance from the chip. The fields can be modified to provide a gradient for support against gravity, and the three-dimensional trap can be adiabatically transformed into a two-dimensional guide.

Controllable production of degenerate Fermi gases of Li6 atoms in the crossover from two dimensions to three dimensions

Many-body physics in dimensional crossover systems has attracted much attention, but is yet to explore intensively. Cold atoms provide an excellent experimental platform to address the questions in this area, but technical difficulties still exist. When an optical lattice is applied to tune the dimensionality of trapped cold atoms, it is usually difficult to precisely tune the occupation ratio of the atom in the different lattice bands. In this paper we report the method to tune the occupation ratio of the energy band when transferring a three-dimensional Fermi gas into a one-dimensional optical lattice, where we could tune the occupation ratio in the lowest band from unity to $50%$ quantitatively by jointly varying the trapping potentials of the optical dipole trap and the one-dimensional lattice. This method provides a route to study the dependence of the many-body interaction on the dimensionality by continuously changing the dimensionality of a Fermi gas in the crossover from two dimensions to three dimensions.

Born–Oppenheimer and the geometry of ray space: an application to cold atoms

Abstract It is known that, within the Born–Oppenheimer approximation, the slow modes of the nuclear motion are altered by three effects that emerge from integrating out the fast modes of the electronic motion. The first is an effective scalar potential V d y n coming from the eigenvalue of the electronic state, the second is an effective magnetic field coming from the Berry phase vector potential A. The third term is an additional potential V g e o m originating in the geometry of ray space and the Fubini–Study metric. In this article, we illustrate these effects and their geometric origin in the context of a simple toy model of a slow neutron interacting with a strong, spatially varying magnetic field. Regarding the neutron spin as a fast degree of freedom, we work out the slow dynamics of the motion of the neutron. Our treatment is geometrical and brings out the effects originating in the Kähler geometry of ray space and the Fubini–Study metric. We then give examples of magnetic field configurations which isolate these three separate effects. Finally we apply these ideas to the trapping of cold atoms. Our main result is that the geometric electric potential V g e o m dominates for smaller traps and can be used to confine cold atoms in static traps. This observation could result in better and smaller atomic clocks. This paper is dedicated to Michael Berry on his 80th birthday.

A single laser-operated magneto-optical trap for Rb atomic fountain

A single diode laser and an electro-optic modulator (EOM)-operated magneto-optical trap (MOT) has been developed for a compact atomic fountain. For generating the required cooling and re-pumping laser beams for the MOT, an EOM operating at 6.58 GHz has been used to modulate the input laser beam. In the trapped cold atom cloud, the population in the two ground hyperfine states ( $$F=1$$ and $$F=2$$ ) of $$^{{87}}\hbox {Rb}$$ atom has been varied by changing the power applied to EOM. Using this MOT, the cold atoms have been launched vertically upwards and the launch velocity of atoms has been measured. Besides being compact, such MOTs can be useful in atom interferometry set-ups where different atom clouds with different initial hyperfine states need to be prepared.

Dielectric microcylinder makes a nanocylindrical trap for atoms and ions

In the diffraction of visible light by a dielectric microcylinder packages of evanescent waves always arise. However, a single-wave incidence corresponds to rather small impact of evanescent waves outside the cylinder. In this paper, we theoretically show that a symmetric pair of plane waves impinging a glass microcylinder corresponds to much higher impact of the evanescent waves. Namely, the interference of the evanescent waves with the propagating ones results in the suppression of the electromagnetic field in an area with very small cross section. This area is located in free space at a substantial distance from the {rear side of the microcylinder and along its axis}. It may serve a linear optical trap for cold atoms and ions.

Study of Optical Modulation based on Binary Masks with Finite Pixels

Binary holography has been applied with fast switching digital masks, e.g., digital micromirror device (DMD), to modulate light in recent years for a wide range of optical and scientific applications, such as trapping of cold atoms and 3D random-access two-photon fluorescence microscopy. However, the binarized modulation errors associated with the effect of discrete sampling, finite pixels, and intrinsic noises of algorithms have yet to be systematically investigated. In this paper, we present the development of an analytical model for a general binary mask with finite pixels. The model has established a deterministic link between the quality of the reconstructed wavefront to important parameters of the binary mask, including pixel size, pixel geometry, fill factor, and aperture. Based on the model, three case studies are presented, including (1) beam shaping, (2) 3D random-access scanning, and (3) multi-focus generation based on binary masks. The results show the model can precisely predict the wavefront distortion, power efficiency, position accuracy, and intensity uniformity for multi-focus generation as a function of binary mask parameters. As digital systems are highly repeatable, the predicted errors, e.g., position errors, can be compensated by adding appropriate phase to the designed holograms. The model and the simulation results may provide useful guidance to select appropriate binary devices and optimization algorithms for specific optical engineering applications, e.g., nanofabrication or nonlinear microscopy.

Compact external cavity diode laser for quantum experiments

We present a compact and practical scheme of building a ∼780 nm external cavity diode laser (ECDL) whose wavelength is mainly determined by an interference filter. The Lorentzian linewidth measured by the heterodyne beating between two identical lasers is 60 kHz, and the geometry size of the laser is only 71.5 mm×65 mm×40 mm. The linear cavity design is less sensitive to misalignment induced by mechanical and thermal disturbances, and in comparison to a common grating-based design, the sensitivity to vibration is substantially reduced. Due to its excellent performance, the laser design has already been applied to cold atom trapping experiments. This interference filter ECDL method can also be extended to other wavelengths and widen the application range of diode laser.

A versatile microwave source for cold atom experiments controlled by a field programmable gate array.

We present a microwave source that is controlled by a commercially available field-programmable gate array (FPGA). Using an FPGA allows for precise control of the time dependent microwave-dressing applied to a sample of trapped cold atoms. We test our microwave source by exciting Rabi oscillations in a Na spinor Bose-Einstein condensate. We include, as supplements, the complete source code, parts' lists, pin connection diagrams, and schematics to make it easy for any group to build and use this device.

Spatial adiabatic passage of massive quantum particles in an optical Lieb lattice

Quantum interference lies at the heart of quantum mechanics. By utilizing destructive interference, it is possible to transfer a physical object between two states without populating an intermediate state which is necessary to connect the initial and final states. A famous application is a technique of stimulated Raman adiabatic passage, where atomic internal states can be transfered with high efficiency regardless of lossy intermediate states. One interesting situation is a case where the initial and final states are spatially well separated. Quantum mechanics allows a particle to move without practical possibility of being found at the intermediate area. Here we demonstrate this spatial adiabatic passage with ultracold atoms in an optical lattice. Key to this is the existence of dark eigenstates forming a flat energy band, with effective transfer between two sublattices being observed. This work sheds light on a study of coherent control of trapped cold atoms.

Cavity QED based on room temperature atoms interacting with a photonic crystal cavity: a feasibility study

The paradigm of cavity QED is a two-level emitter interacting with a high-quality factor single-mode optical resonator. The hybridization of the emitter and photon wave functions mandates large vacuum Rabi frequencies and long coherence times; features that so far have been successfully realized with trapped cold atoms and ions, and localized solid-state quantum emitters such as superconducting circuits, quantum dots, and color centers Reiserer and Rempe (Rev Modern Phys 87:1379, 2015), Faraon et al. (Phys Rev 81:033838, 2010). Thermal atoms, on the other hand, provide us with a dense emitter ensemble and in comparison to the cold systems are more compatible with integration, hence enabling large-scale quantum systems. However, their thermal motion and large transit-time broadening is a major bottleneck that has to be circumvented. A promising remedy could benefit from the highly controllable and tunable electromagnetic fields of a nano-photonic cavity with strong local electric-field enhancements. Utilizing this feature, here we investigate the interaction between fast moving thermal atoms and a nano-beam photonic crystal cavity (PCC) with large quality factor and small mode volume. Through fully quantum mechanical calculations, including Casimir–Polder potential (i.e. the effect of the surface on radiation properties of an atom), we show, when designed properly, the achievable coupling between the flying atom and the cavity photon would be strong enough to lead to quantum interference effects in spite of short interaction times. In addition, the time-resolved detection of different trajectories can be used to identify single and multiple atom counts. This probabilistic approach will find applications in cavity QED studies in dense atomic media and paves the way towards realizing large-scale, room-temperature macroscopic quantum systems aimed at out of the lab quantum devices.

Effective trapping of cold atoms using dipole and radiative forces in an optical trap

We report on highly effective trapping of cold atoms by a new method for a stable single optical trap in the near-optical resonant regime. An optical trap with the near-optical resonance condition consists of not only the dipole but also the radiative forces, while a trap using a far-off resonance dominates only the dipole force. We estimate a near-optical resonant trap for ultracold rubidium atoms in the range between -0.373 and -2.23 THz from the resonance. The time dependence of the trapped atoms indicates some difference of the stable center-of-mass positions in the near-optical resonant trap, and also indicates that the differences are caused by the change of the equilibrium condition of the optical dipole and radiative forces. A stable position depends only on laser detuning due to the change in the radiative force; however, the position is ineffective against the change in the laser intensity, which results in a change in the radiative force.

Bose polaron in spherical trap potentials: Spatial structure and quantum depletion

We investigate how the presence of a localized impurity in a Bose-Einstein condensate of trapped cold atoms that interact with each other weakly and repulsively affects the profile of the condensed and excited components at zero temperature. By solving the Gross-Pitaevskii and Bogoliubov-de Gennes equations, we find that an impurity-boson contact attraction (repulsion) causes both components to change in spatial structure in such a way as to be enhanced (suppressed) around the impurity, while slightly declining (growing) in a far region from the impurity. Such behavior of the quantum depletion of the condensate can be understood by decomposing the impurity-induced change in the profile of the excited component with respect to the radial and azimuthal quantum number. A significant role of the centrifugal potential and the "hole" excitation level is thus clarified.

Prospects for strongly coupled atom-photon quantum nodes

We discuss the trapping of cold atoms within microscopic voids drilled perpendicularly through the axis of an optical waveguide. The dimensions of the voids considered are between 1 and 40 optical wavelengths. By simulating light transmission across the voids, we find that appropriate shaping of the voids can substantially reduce the associated loss of optical power. Our results demonstrate that the formation of an optical cavity around such a void could produce strong coupling between the atoms and the guided light. By bringing multiple atoms into a single void and exploiting collective enhancement, cooperativities ~400 or more should be achievable. The simulations are carried out using a finite difference time domain method. Methods for the production of such a void and the trapping of cold atoms within it are also discussed.

Realizing time crystals in discrete quantum few-body systems

The exotic phenomenon of time translation symmetry breaking under periodic driving - the time crystal - has been shown to occur in many-body systems even in clean setups where disorder is absent. In this work, we propose the realization of time-crystals in few-body systems, both in the context of trapped cold atoms with strong interactions and of a circuit of superconducting qubits. We show how these two models can be treated in a fairly similar way by adopting an effective spin chain description, to which we apply a simple driving protocol. We focus on the response of the magnetization in the presence of imperfect pulses and interactions, and show how the results can be interpreted, in the cold atomic case, in the context of experiments with trapped bosons and fermions. Furthermore, we provide a set of realistic parameters for the implementation of the superconducting circuit.

Ultrasmall Optical Vortex Knots Generated by Spin‐Selective Metasurface Holograms

AbstractTying isolated vortex knots in complex optical fields has been recently realized using spatial light modulators (SLMs). These traditional devices suffer from low efficiency, narrow bandwidth, and they cannot be integrated to nanophotonic systems. Here, the design and realization of metasurface holograms (MHs) is reported to efficiently generate the optical vortex knots in a broad spectral region. In comparison with conventional optical devices, the thicknesses and pixels of MHs are both in subwavelength scale, and the size of created optical vortex knot can be six orders of magnitude smaller than those produced by SLMs. Such extremely little vortex knots may find widespread applications in templating knotted micro/nano‐structures in photosensitive materials and 3D optical trapping of cold atoms in complex topologies at microscale. In particular, it is found that the spatial position of the generated off‐axis optical vortex knot is able to be symmetrically reversed by just changing the helicity of the incident light. This spin‐selective vortex knot generator can be regarded as a new type of metasurface device for manipulating the topological structure of optical vortex lines, which has potential applications in the field of singular optics.

Observation of Ultrastrong Spin-Motion Coupling for Cold Atoms in Optical Microtraps.

We realize a mechanical analogue of the Dicke model, achieved by coupling the spin of individual neutral atoms to their quantized motion in an optical trapping potential. The atomic spin states play the role of the electronic states of the atomic ensemble considered in the Dicke model, and the in-trap motional states of the atoms correspond to the states of the electromagnetic field mode. The coupling between spin and motion is induced by an inherent polarization gradient of the trapping light fields, which leads to a spatially varying vector light shift. We experimentally show that our system reaches the ultrastrong coupling regime; i.e., we obtain a coupling strength that is a significant fraction of the trap frequency. Moreover, with the help of an additional light field, we demonstrate the insitu tuning of the coupling strength. Beyond its fundamental interest, the demonstrated one-to-one mapping between the physics of optically trapped cold atoms and the Dicke model paves the way for implementing protocols and applications that exploit extreme coupling strengths.

Cold-atom-based implementation of the quantum Rabi model

The interaction of a two-level system (TLS) with a single bosonic mode is one of the most fundamental processes in quantum optics. Microscopically, it is described by the quantum Rabi model (QRM). Here, we propose an implementation of this model based on single trapped cold atoms. The TLS is implemented using atomic Zeeman states, while the atom's vibrational states in the trap represent the bosonic mode. The coupling is mediated by a suitable fictitious magnetic field pattern. We show that all important system parameters, i.e., the emitter-field detuning and the coupling strength of the emitter to the mode, can be tuned over a wide range. Remarkably, assuming realistic experimental conditions, our approach allows one to explore the regimes of ultra-strong coupling, deep strong coupling, and dispersive deep strong coupling. The states of the bosonic mode and the TLS can be prepared and read out using standard cold-atom techniques. Moreover, we show that our scheme enables the implementation of important generalizations, namely, the driven QRM, the QRM with quadratic coupling as well as the case of many TLSs coupled to one mode (Dicke model). The proposed cold-atom based implementation will facilitate experimental studies of a series of phenomena predicted for the QRM in extreme, so far unexplored physical regimes.

The Emerging Field of Quantum Based Measurements for Pressure, Vacuum and Beyond

The future of pressure, vacuum and even temperature measurement will employ lasers, Fabry-Perot optical cavities, cold atom traps and lots of quantum physics. For pressure measurement of a gas, photons interact at the quantum level such that light travels at a slower speed in gas than it does in vacuum. For extreme vacuum measurements, cold atom traps will be used to detect single collisions between gas trapped cold atoms enabling the number density of the gas to be measured. For temperature measurement is performed using silicon photonics to detect the small changes in refractive index in micro machined siliconphontoic cavities coupled to optical fibers. For dynamic pressure, NIST is developing a method where the unique quantum mechanical characteristics of the molecules are themselves the standard for pressure, making it consistent with the quantum-SI. Our approach is to use independent molecular spectroscopy as a dynamic measurement of pressure, where the pressure and temperature is ascertained by measuring time-resolved pressure-broadened spectra of CO molecules. This paper briefly reviews the status of these projects currently underway at the NIST Thermodynamic Metrology Group.