Metasurface optical trap array for single atoms.

Metasurfaces made of subwavelength silicon nanopillars provide unparalleled capacity to manipulate light, and have emerged as one of the leading platforms for developing integrated photonic devices. In this study, we report on a compact, passive approach based on planar metasurface optics to generate large optical trap arrays. The unique configuration is achieved with a meta-hologram to convert a single incident laser beam into an array of individual beams, followed up with a metalens to form multiple laser foci for single rubidium atom trapping. We experimentally demonstrate two-dimensional arrays of 5 × 5 and 25 × 25 at the wavelength of 830 nm, validating the capability and scalability of our metasurface design. Beam waists ∼1.5 µm, spacings (about 15 µm), and low trap depth variations (8%) of relevance to quantum control for an atomic array are achieved in a robust and efficient fashion. The presented work highlights a compact, stable, and scalable trap array platform well-suitable for Rydberg-state mediated quantum gate operations, which will further facilitate advances in neutral atom quantum computing.

Scale Invariance of a Spherical Unitary Fermi Gas.

A unitary Fermi gas in an isotropic harmonic trap is predicted to show scale and conformal symmetry that have important consequences in its thermodynamic and dynamical properties. By experimentally realizing a unitary Fermi gas in an isotropic harmonic trap, we demonstrate its universal expansion dynamics along each direction and at different temperatures. We show that as a consequence of SO(2,1) symmetry, the measured release energy is equal to that of the trapping energy. We further observe the breathing mode with an oscillation frequency twice the trapping frequency and a small damping rate, providing the evidence of SO(2,1) symmetry. In addition, away from resonance when scale invariance is broken, we determine the effective exponent γ that relates the chemical potential and average density along the BEC-BCS crossover, which qualitatively agrees with the mean field predictions. This Letter opens the possibility of studying nonequilibrium dynamics in a conformal invariant system in the future.

High-Precision Atom Interferometer-Based Dynamic Gravimeter Measurement by Eliminating the Cross-Coupling Effect.

A dynamic gravimeter with an atomic interferometer (AI) can perform absolute gravity measurements with high precision. AI-based dynamic gravity measurement is a type of joint measurement that uses an AI sensor and a classical accelerometer. The coupling of the two sensors may degrade the measurement precision. In this study, we analyzed the cross-coupling effect and introduced a recovery vector to suppress this effect. We improved the phase noise of the interference fringe by a factor of 1.9 by performing marine gravity measurements using an AI-based gravimeter and optimizing the recovery vector. Marine gravity measurements were performed, and high gravity measurement precision was achieved. The external and inner coincidence accuracies of the gravity measurement were ±0.42 mGal and ±0.46 mGal after optimizing the cross-coupling effect, which was improved by factors of 4.18 and 4.21 compared to the cases without optimization.

Dependence of the ellipse fitting noise on the differential phase between interferometers in atom gravity gradiometers.

Ellipse fitting is widely used in the extraction of the differential phase between atom interferometers amid substantial common phase noise. This study meticulously examines the dependency of extraction noise on the differential phase between atom interferometers during ellipse fitting. It reveals that the minimum extraction noise can manifest at distinct differential phases, contingent upon the dominance of different noise types. Moreover, the outcomes are influenced by whether the interferometers undergo simultaneous detection or not. Our theoretical simulations find empirical validation in a compact horizontal atom gravity gradiometer. The adjustment of the differential phase significantly enhances measurement sensitivity, culminating in a differential gravity resolution of 1.6 × 10-10 g @ 4800 s.

Accuracy Improvement of a Compact 85Rb Atom Gravimeter by Suppressing Laser Crosstalk and Light Shift.

We design and implement a compact 85Rb atom gravimeter (AG). The diameter of the sensor head is 35 cm and the height is 65 cm; the optical and electronic systems are installed in four standard 3U cabinets. The measurement accuracy of this AG is improved by suppress laser crosstalk and light shift. In addition, the angle of the Raman laser reflector is adjusted and locked, and the attitude of the sensing head is automatically adjusted, and the vibration noise is also compensated. The comparison measurement results between this AG and the superconducting gravimeter indicate that its long-term stability is 0.65 µGal @50000 s.

Feedback control of atom trajectories in a horizontal atom gravity gradiometer.

The coincidence between the atom trajectory and the Raman pulse sequence is very important for an intersection type atom interferometer. Here we present a feedback control technique for the atom trajectories in our horizontal gravity gradiometer, which improves the stabilities of the trajectories by about 2 orders of magnitude. Through the further study of the dependence of the interferometer contrasts on the atom trajectories, we lock the trajectories at optimal positions. And by this technique, the sensitivity of the gravity gradiometer is improved from 982 E/Hz1/2 to 763 E/Hz1/2, while the long-term stability is enhanced more significantly and reaches 8.9 E after an integration time of 6000 s. This work may provide hints to other experiments based on intersection type atom interferometers.

Defect-Free Arbitrary-Geometry Assembly of Mixed-Species Atom Arrays.

Optically trapped mixed-species single atom arrays with arbitrary geometry are an attractive and promising platform for various applications, because tunable quantum systems with multiple components provide extra degrees of freedom for experimental control. Here, we report the first demonstration of two-dimensional 6×4 dual-species atom assembly of ^{85}Rb (^{87}Rb) atoms with a filling fraction of 0.88 (0.89). This mixed-species atomic synthesis is achieved via rearranging initially randomly distributed atoms by a sorting algorithm (heuristic heteronuclear algorithm) which is designed for bottom-up atom assembly with both user-defined geometries and two-species atom number ratios. Our fully tunable hybrid-atom systems with scalable advantages are a good starting point for high-fidelity quantum logic, many-body quantum simulation, and single molecule array formation.

1200x broadband modal converter using a subwavelength self-focusing structure.

Efficient modal interconversion between optical manipulation of cold atoms in free space and transmitted light within an integrated waveguide remains a challenge in the area of integrated atomic photonics. Here, a 1200x modal converter with a footprint on the order of millimeters is proposed based on a Si3N4 subwavelength self-focusing structure. The 2.8µm×1.7µm subwavelength structure enables efficient single modal conversion. The transmission efficiency is 84% at a wavelength of 830 nm with a working bandwidth of 240 nm. The device can work in dual polarization states.

Modular-assembled laser system for a long-baseline atom interferometer.

The Zhaoshan long-baseline Atom Interferometer Gravitation Antenna (ZAIGA) is a new, to the best of our knowledge, type of large-scale atom interferometer facility under construction for the study of gravitation and related problems. To meet the different requirements of the laser system for the atom interferometer using various atoms (including 85Rb, 87Rb, 87Sr, and 88Sr), we design and implement a modular assembled laser system. By dividing the laser system into different basic units according to their functions and modularizing each unit, the laser system is made highly scalable while being compact and stable. Its intensity stability is better than 0.1% in 102s and 0.5% in 104s. We test the performance of the laser system with two experimental systems, i.e., an 85Rb-87Rb dual-species ultracold atom source and an 85Rb atom interferometer. The 85Rb-87Rb dual-species magneto-optical trap and the 85Rb atom interference fringes are realized by using this laser system, indicating that its technical performance can meet the major experimental requirements.

All acousto-optic modulator laser system for a 12 m fountain-type dual-species atom interferometer.

We design and implement a laser system for 85Rb and 87Rb dual-species atom interferometers based on acousto-optic frequency shift and tapered amplifier laser technologies. We use eight-pass acousto-optic modulators to generate repumping lasers for 85Rb and 87Rb atoms. The maximum frequency shift of the laser is 2.8 GHz, and the diffraction efficiency is higher than 20%. We use high-frequency acousto-optic modulators to generate the Raman lasers. This laser system uses only two seed lasers to provide the various frequencies required by 85Rb and 87Rb dual-species atom interferometers, which greatly improves laser usage. The laser system is applied in the equivalence principle test experiment using an 85Rb and 87Rb dual-species atom interferometer. The signal of atoms launched to 12 meters is successfully observed, and the resolution of gravity differential measurement is improved from 8×10-9 g to 1×10-10g.

Effect of an echo sequence to a trapped single-atom interferometer with photon momentum kicks.

We investigate a single-atom interferometer (SAI) in an optical dipole trap (ODT) with photon momentum kicks. An echo sequence is used for the SAI. We find experimentally that interference visibilities of a counter-propagating Raman type SAI decay much faster than the co-propagating case. To understand the underlying mechanism, a wave-packet propagating simulation is developed for the ODT-guided SAI. We show that in state dependent dipole potentials, the coupling between external dynamics and internal states makes the atom evolve in different paths during the interfering process. The acquired momentum from counter-propagating Raman pulses forces the external motional wave packets of two paths be completely separated and the interferometer visibility decays quickly compared to that of the co-propagating Raman pulses process. Meanwhile, the echo interference visibility experiences revival or instantaneous collapse which depends on the π pulse adding time at approximate integer multiples or half integer multiples of the trap period.

Balanced Coherence Times of Atomic Qubits of Different Species in a Dual 3×3 Magic-Intensity Optical Dipole Trap Array.

We construct a polarization-mediated magic-intensity (MI) optical dipole trap (ODT) array, in which the detrimental effects of light shifts on the mixed-species qubits are efficiently mitigated so that the coherence times of the mixed-species qubits are both substantially enhanced and balanced for the first time. This mixed-species magic trapping technique relies on the tunability of the coefficient of the third-order cross term and ground state hyperpolarizability, which are inherently dependent on the degree of circular polarization of the trapping laser. Experimentally, polarization of the ODT array for ^{85}Rb qubits is finely adjusted to a definite value so that its working magnetic field required for magic trapping amounts to the one required for magically trapping ^{87}Rb qubits in another ODT array with fully circular polarization. Ultimately, in such a polarization-mediated MI-ODT array, the coherence times of ^{87}Rb and ^{85}Rb qubits are, respectively, enhanced up to 891±47 ms and 943±35 ms. Moreover, we reveal that the noise of the elliptic polarization causes dephasing effect on the ^{85}Rb qubits but it could be efficiently mitigated by choosing the dynamical range of active polarization device. We also show that light shifts seen by qubits in an elliptically polarized MI-ODT can be more efficiently compensated due to the decrease in the ground state hyperpolarizability. It is anticipated that the novel mixed-species MI-ODT array is a versatile platform for building scalable quantum computers with neutral atoms.

Ground-State Phase Diagram of a Spin-Orbital-Angular-Momentum Coupled Bose-Einstein Condensate.

By inducing a Raman transition using a pair of Gaussian and Laguerre-Gaussian laser beams, we realize a ^{87}Rb condensate whose orbital angular momentum (OAM) and its internal spin states are coupled. By varying the detuning and the coupling strength of the Raman transition, we experimentally map out the ground-state phase diagram of the system for the first time. The transitions between different phases feature a discontinuous jump of the OAM and the spin polarization, and hence are of first order. We demonstrate the hysteresis loop associated with such first-order phase transitions. The role of interatomic interaction is also elucidated. Our work paves the way to explore exotic quantum phases in the spin-orbital-angular-momentum coupled quantum gases.

Realization of a compact one-seed laser system for atom interferometer-based gravimeters.

A simple and compact design of the laser system is important for realization of compact atom interferometers (AIs). We design and realize a simple fiber bench-based 780-nm laser system used for 85Rb AI-based gravimeters. The laser system contains only one 780 nm seed laser, and the traditional frequency-doubling-module is not used. The Raman beams are shared with one pair of the cooling beams by using a liquid crystal variable retarder based polarization control technique. This laser system is applied to a compact AI-based gravimeter, and a best gravity measurement sensitivity of 230 µGal/Hz1/2 is achieved. The gravity measurements for more than one day are also performed, and the long-term stability of the gravimeter is 5.5 µGal.

High-Fidelity Single-Qubit Gates on Neutral Atoms in a Two-Dimensional Magic-Intensity Optical Dipole Trap Array.

As a conventional approach, optical dipole trap (ODT) arrays with linear polarization have been widely used to assemble neutral-atom qubits for building a quantum computer. However, due to the inherent scalar differential light shifts (DLS) of qubit states induced by trapping fields, the microwave-driven gates acting on single qubits suffer from errors on the order of 10^{-3}. Here, we construct a DLS compensated ODT array based upon a recently developed magic-intensity trapping technique. In such a magic-intensity optical dipole trap (MI-ODT) array, the detrimental effects of DLS are efficiently mitigated so that the performance of global microwave-driven Clifford gates is significantly improved. Experimentally, we achieve an average error of (4.7±1.1)×10^{-5} per global gate, which is characterized by randomized benchmarking in a 4×4 MI-ODT array. Moreover, we experimentally study the correlation between the coherence time and gate errors in a single MI-ODT with an optimum error per gate of (3.0±0.7)×10^{-5}. Our demonstration shows that MI-ODT array is a versatile platform for building scalable quantum computers with neutral atoms.

Characterization and optimization of a tapered amplifier by its spectra through a long multi-pass rubidium absorption cell.

We present a method to characterize and optimize a tapered-amplifier laser system (TALS) by its spectral quality through a long multi-pass rubidium absorption cell. A thermal vapor cell is used to measure the non-resonant spectrum of TALS, including the broadband amplified spontaneous emission (ASE), which is its main spectral noise. This method gives a simple quantified measurement to optimize various working parameters of a TA, including current and temperature online. It can as well be used to compare various TA chips based on their usage time during our precision measurement experiments. The results of this method are compared and found in sync with traditional methods of Fabry-Perot cavity and beat measurements. Such characterization and optimization are important for noise control in atom interferometers, atomic clocks, and other atomic manipulations. It can very well be used for investigating nonlinearity and ASE inside amplifying chips and can be utilized in other applications of ASE using bioimaging.

High-resolution ex vacuo objective for cold atom experiments.

We present a versatile and cost-efficient objective with a five-lens configuration, which consists completely of commercial singlets. The home-built objective has a numerical aperture (NA) of 0.44 and a long working distance of 35.9 mm, making it suitable for ex vacuo utilization. A diffraction-limited resolution of 1.08 µm and a field of view of about 210 µm are achieved when a 780 nm light passes through a 5 mm thick vacuum window. Moreover, such a design can be well adapted to a broad range of laser wavelengths (560-1000 nm) and vacuum window thicknesses (0-6 mm) by simply modifying one lens spacing, while maintaining a NA of above 0.43. The characteristics of the objective are evaluated experimentally, which are in good agreement with the simulations. Also, the objective has been successfully used for single-atom trapping and detecting in experiments. We believe that it will find more applications in various cold atom experiments.

Compact portable laser system for mobile cold atom gravimeters.

We have designed and realized a compact portable laser system for high-sensitivity mobile cold atom interferometers. The laser system is mounted on a single module with dimensions of 45 cm×45 cm×16 cm and emits lights directly on up to 13 fiber ports for a two-dimensional magneto-optical trap, atom fountain, Raman transition, and normalized detection. A double-sided optical structure and mounts without kinematic adjustment are designed to achieve high-level integration and stability. The laser system is applied to a mobile atom gravimeter and achieves a sensitivity of 28 µGal/Hz1/2. Experiencing 1200-km-long truck transportation, the laser system could be restored to good operating status without internal realignment.

Measurement and control of the sideband to carrier ratio of an electro-optic modulator used in atom interferometers.

The sideband and carrier of an electro-optic modulator (EOM) are usually used as Raman lasers in atom interferometry. To eliminate AC-Stark shift in atom interferometry, the stability of sideband to carrier ratio (SCR) is of great significance. We present a beating method to accurately measure and control the SCR. The influence of imperfect frequency response of the beating system is avoided by chirping the reference laser with half chirping rate of the modulation frequency. Making use of this method, we performed a SCR locking by feedback to the modulation depth. The locked SCRs' variation is averaged to be less than 0.1% within 20 MHz chirping span, and the according error for gravity measurement with 180 ms free evolution time is 6.2×10-11g. Thus both the SCR variation and the estimated gravity measurement error are reduced by about 2 orders. This work may provide hints to other EOM involving experiments.

Entangling Two Individual Atoms of Different Isotopes via Rydberg Blockade.

We report on the first experimental realization of the controlled-not (cnot) quantum gate and entanglement for two individual atoms of different isotopes and demonstrate a negligible cross talk between two atom qubits. The experiment is based on a strong Rydberg blockade for ^{87}Rb and ^{85}Rb atoms confined in two single-atom optical traps separated by 3.8 µm. The raw fidelities of the cnot gate and entanglement are 0.73±0.01 and 0.59±0.03, respectively, without any corrections for atom loss or trace loss. Our work has applications for simulations of many-body systems with multispecies interactions, for quantum computing, and for quantum metrology.