Crossed vortex bottle beam trap for single-atom qubits

Miniature atom bottle traps enabled by chiral doughnut light

We highlight what we believe to be a novel optical set-up which enables the confinement of cold atoms in a finite set of sub-wavelength bottle traps. This involves two counter-propagating vortex beams with the same winding number ℓ = ±1 and the same circular polarization ($\sigma =\mp 1)$. Strong focusing generates significant longitudinal field components which become responsible for an on-axis standing wave enabling the axial confinement of far blue-detuned atoms. The off-axis radial confinement is provided by the optical potential due to the transverse components of the light. The trap characteristics are illustrated using experimentally accessible parameters and are tunable by changing the power, focusing and ellipticity of the light. Atoms trapped in such a set-up are useful for applications, including quantum simulation and quantum information processing.

Enhanced optical vector bottle beams with obscured nodal surfaces

Optical bottle beams, characterized by their unique three-dimensional dark core, have garnered substantial interest due to their potential applications across multiple domains of science and technology. This paper delves into the current methods used to create these beams and provides a method to obscure their nodal planes through coaxial non-interfering orthogonally polarized beams to generate bottle beams with enhanced uniformity. Experimental and theoretical results show the enhanced vector bottle beam maintains a smaller, more spherically uniform potential well and interesting quasi-particle polarization characteristics.

In-plane subwavelength optical capsule for lab-on-a-chip nano-tweezers

In this Letter, we propose a new, to the best of our knowledge, proof-of-concept of optical nano-tweezers based on a pair of dielectric rectangular structures that are capable of generating a finite-volume in-plane optical capsule. Finite-difference time-domain simulations of light spatial distributions and optical trapping forces of a gold nanoparticle immersed in water demonstrate the physical concept of an in-plane subwavelength optical capsule integrated with a microfluidic mesoscale device. It is shown that the refractive index of and the distance between the two dielectric rectangular structures can effectively control the shape and axial position of the optical capsule. Such an in-plane mesoscale structure provides a new path for manipulating absorbing nano-particles or bio-particles in a compact planar architecture, and should thus lead to promising perspectives in lab-on-a-chip domains.

Robust 17 W single-pass second-harmonic-generation at 532 nm and relative-intensity-noise investigation

We demonstrate a 17 W single-frequency, low-intensity-noise green source at 532 nm, by single-pass second-harmonic generation of a 50 W continuous-wave fiber laser in a 30 mm MgO-doped periodically-poled stoichiometric lithium tantalate crystal. The maximum conversion efficiency is about 37%. A nearly Gaussian beam (M²<1.15 at 15 W) and low wavefront distortion are obtained. The system shows stable behavior over 100 h of uninterrupted operation. The evolution of the relative-intensity-noise transfer from the fundamental to the second harmonic is theoretically and experimentally investigated with high resolution.

Triply Magic Conditions for Microwave Transition of Optically Trapped Alkali-Metal Atoms

We report the finding of "triply magic" conditions (the doubly magic frequency-intensity conditions of an optical dipole trap plus the magic magnetic field) for the microwave transitions of optically trapped alkali-metal atoms. The differential light shift (DLS) induced by a degenerate two-photon process is adopted to compensate a DLS associated with the one-photon process. Thus, doubly magic conditions for the intensity and frequency of the optical trap beam can be found. Moreover, the DLS decouples from the magnetic field in a linearly polarized optical dipole trap, so that the magic condition of the magnetic field can be applied independently. Therefore, the triply magic conditions can be realized simultaneously. We also experimentally demonstrate the doubly magic frequency-intensity conditions as well as the independence of the magnetic field. When the triply magic conditions are fulfilled, the inhomogeneous and homogeneous decoherences for the optically trapped atom will be dramatically suppressed, and the coherence time can be extended significantly.

Measurement of magic-wavelength optical dipole trap by using the laser-induced fluorescence spectra of trapped single cesium atoms

Based on the multi-level model, we have calculated light shifts for Zeeman states of hyperfine levels of cesium (Cs) 6S_1/2 ground state and 6P_3/2 excited state. The magic-wavelength linearly-polarized optical dipole trap (ODT) for Cs 6S_1/2 |F = 4, m_F = + 4ñ - 6P_3/2 |F^' = 5, m_F = + 5ñ transition is experimentally constructed and characterized by using the laser-induced fluorescence spectra of trapped single Cs atoms. The magic wavelength is 937.7 nm which produces almost the same light shift for 6S_1/2 |F = 4, m_F = + 4ñ ground state and 6P_3/2 |F^' = 5, m_F = + 5ñ excited state with linearly-polarized ODT laser beam. Compared to undisturbed Cs 6S_1/2 |F = 4, m_F = + 4ñ - 6P_3/2 |F^' = 5, m_F = + 5ñ transition frequency in free space, the differential light shift is less than 0.7 MHz in a linearly-polarized 937.7 nm ODT, which is less than 1.2% of the trap depth. We also discussed influence of the trap depth and the bias magnetic field on the measurement results.

A technique for individual atom delivery into a crossed vortex bottle beam trap using a dynamic 1D optical lattice

We describe a system for loading a single atom from a reservoir into a blue-detuned crossed vortex bottle beam trap using a dynamic 1D optical lattice. The lattice beams are frequency chirped using acousto-optic modulators, which causes the lattice to move along its axial direction and behave like an optical conveyor belt. A stationary lattice is initially loaded with approximately 6000 atoms from a reservoir, and the conveyor belt transports them 1.1 mm from the reservoir to a bottle beam trap, where a single atom is loaded via light-assisted collisions. Photon counting data confirm that an atom can be delivered and loaded into the bottle beam trap 13.1% of the time.

Coherence Preservation of a Single Neutral Atom Qubit Transferred between Magic-Intensity Optical Traps

We demonstrate that the coherence of a single mobile atomic qubit can be well preserved during a transfer process among different optical dipole traps (ODTs). This is a prerequisite step in realizing a large-scale neutral atom quantum information processing platform. A qubit encoded in the hyperfine manifold of an ^{87}Rb atom is dynamically extracted from the static quantum register by an auxiliary moving ODT and reinserted into the static ODT. Previous experiments were limited by decoherences induced by the differential light shifts of qubit states. Here, we apply a magic-intensity trapping technique which mitigates the detrimental effects of light shifts and substantially enhances the coherence time to 225±21  ms. The experimentally demonstrated magic trapping technique relies on the previously neglected hyperpolarizability contribution to the light shifts, which makes the light shift dependence on the trapping laser intensity parabolic. Because of the parabolic dependence, at a certain "magic" intensity, the first order sensitivity to trapping light-intensity variations over ODT volume is eliminated. We experimentally demonstrate the utility of this approach and measure hyperpolarizability for the first time. Our results pave the way for constructing scalable quantum-computing architectures with single atoms trapped in an array of magic ODTs.

Controllable light capsules employing modified Bessel-Gauss beams

We report, in theory and experiment, on a novel class of controlled light capsules with nearly perfect darkness, directly employing intrinsic properties of modified Bessel-Gauss beams. These beams are able to naturally create three-dimensional bottle-shaped region during propagation as long as the parameters are properly chosen. Remarkably, the optical bottle can be controlled to demonstrate various geometries through tuning the beam parameters, thereby leading to an adjustable light capsule. We provide a detailed insight into the theoretical origin and characteristics of the light capsule derived from modified Bessel-Gauss beams. Moreover, a binary digital micromirror device (DMD) based scheme is first employed to shape the bottle beams by precise amplitude and phase manipulation. Further, we demonstrate their ability for optical trapping of core-shell magnetic microparticles, which play a particular role in biomedical research, with holographic optical tweezers. Therefore, our observations provide a new route for generating and controlling bottle beams and will widen the potentials for micromanipulation of absorbing particles, aerosols or even individual atoms.

Three-dimensional cooling of a single atom by a pair of counter-propagating tightly focused beams

A light beam tightly focused by a high numerical-aperture lens system contains longitudinal components with polarization parallel to the propagation axis. By numerically analyzing the polarization distribution around the focal region in one pair of confocally aligned counter-propagating tightly focused light beams with orthogonal linear polarizations, we found that there exists a three-dimensional polarization gradient pattern similar to that used in cooling neutral atoms. This can be used to three-dimensionally cool atoms trapped in a far-off-resonant trap with only one pair of counter-propagating beams in one dimension. This new cooling scheme can be used to individually cool single atoms in an addressable two-dimensional single-atom array for quantum information processing and be applied to perform readouts of qubit encoded in these atoms without losing them.

Characterization of x-ray phase vortices by ptychographic coherent diffractive imaging

We have characterized the x-ray phase vortices generated at the focal spot of various spiral Fresnel zone plates with an outermost zone width of Δr=50 nm. The complex-valued wavefields of phase vortices as small as 50 nm in size (FWHM) and several topological charges were reconstructed using ptychographic coherent diffractive imaging. The reconstructed focal spots demonstrate good agreement with the theoretically expected wavefields and diffraction-limited focusing.

Suppressing phase decoherence of a single atom qubit with Carr-Purcell-Meiboom-Gill sequence

We experimentally demonstrate the strong suppression of dephasing of a qubit stored in a single 87Rb atom in an optical dipole trap by using Carr-Purcell-Meiboom-Gill (CPMG) pulse sequences. Regarded as a repetition of spin echo, CPMG sequence is an excellent choice for suppressing both inhomogeneous and homogeneous phase decoherence mechanisms. In comparison with atomic ensembles, the dephasing due to atomic collisions disappears for individual atoms. Thus, CPMG suppression effect is efficient with a few π-pulses. In our trap with 830 nm wavelength and 0.7 mK potential depth, the reversible inhomogeneous dephasing time is T2*=1.4 ms. We obtain the homogeneous dephasing time of T'2 = 103 ms in the spin echo process. By employing CPMG sequence with pulse number n = 6, the homogeneous dephasing is further suppressed by a factor of 3, and its corresponding coherence time is extended to T'2 = 304 ms.

Generating a three-dimensional dark focus from a single conically refracted light beam

We report here the generation of a three-dimensional (3D) dark focus from a single focused monochromatic Gaussian beam that undergoes conical refraction when it propagates along one of the optic axes of a biaxial crystal. We study the resulting ring intensity pattern behind the crystal as a function of the ratio between the ring radius and the beam waist and derive the particular parameter values for which a 3D dark focus with null intensity at the ring center is formed. We have performed experiments with a KGd(WO(4))(2) biaxial crystal, reporting the generation of a bottle beam in full agreement with our theoretical investigations.

Optical vault: a reconfigurable bottle beam based on conical refraction of light

We employ conical refraction of light in a biaxial crystal to create an optical bottle for photophoretic trapping and manipulation of particles in gaseous media. We show that by only varying the polarization state of the input light beam the optical bottle can be opened and closed in order to load and unload particles in a highly controllable manner.

Self-imaging generation of plasmonic void arrays

A plasmonic device is proposed to produce a self-imaging surface plasmon void array (2D surface bottle beam array) by the interference of two nondiffracting surface beams, namely, cosine-Gauss beams. The self-imaging surface voids are shown by full-wave calculations and then verified experimentally with an aperture-type near-field scanning optical microscope. We also demonstrate that the void array can be adjusted with flexibility in terms of the pattern and the number of voids.