Optical Dipole Trap Research Articles

Time-domain optics for atomic quantum matter

We investigate time-domain optics for atomic quantum matter. Within a matter-wave analog of the thin-lens formalism, we study optical lenses of different shapes and refractive powers to precisely control the dispersion of Bose–Einstein condensates. Anharmonicities of the lensing potential are incorporated in the formalism with a decomposition of the center-of-mass motion and expansion of the atoms, allowing to probe the lensing potential with micrometer resolution. By arranging two lenses in time formed by the potentials of an optical dipole trap and an atom-chip trap, we realize a magneto-optical matter-wave telescope. We employ this hybrid telescope to manipulate the expansion and aspect ratio of the ensembles. The experimental results are compared to numerical simulations that involve Gaussian shaped potentials to accommodate lens shapes beyond the harmonic approximation.

Effects related to the temperature of atoms in an atom interferometry gravimeter based on ultra-cold atoms

The temperature of atoms, coupled to several effects, plays an important role in high precision atom interferometry gravimeters. In this work, we present an ultra-cold 87Rb atom interferometry gravimeter, in which the atom source is produced by evaporative cooling in an all optical dipole trap to investigate the effects related to atom temperature. A condensate containing 4 × 104 atoms can be prepared within 3.2 s through an all-optical dipole trap composed of two reservoirs and a dimple. The fringe contrast of our atom interferometry gravimeter reaches up to 76(4)% due to the advantage of ultra-cold atom source even at a free evolution time of T=80 ms. A resolution of 6 μGal (1 μGal=1×10-8 m/s2) after 3000 s integration time with a sampling rate of 0.25 Hz is achieved in this atom gravimeter. The relationship between the fringe contrast and the atom temperature in the atom gravimeter is studied; in addition, the wavefront aberration effect in the atom gravimeter is also investigated by varying the temperature of atoms.

Fast, simple, all-optical production of sodium spinor condensates

We present an experiment for quickly producing Bose–Einstein condensates (BECs) of 23Na atoms in a simple crossed dipole trap. The atoms were collected in a magneto-optical trap from a zero-crossing Zeeman slower. The atoms are then transferred to a tightly focused crossed optical dipole trap, and a pure F = 1 BEC of ∼1.1 × 105 sodium atoms was created by a fast forced evaporation. Our system reliably generates the condensate at a rate exceeding 105 atoms every ∼8 s. We also compare the lifetimes of the mixed F = 1 condensate and the spin components (m F = 0, m F = +1).

Collective self-trapping of atoms in a cavity

We study experimentally the optical dipole trapping of a cloud of cold atoms in a high-finesse cavity in the parameter regime where the atomic back-action on the cavity mode is significant. Back-action based effects lead to state selective optical manipulation schemes. We identify a parameter range where the collective back action of the atoms is needed for the trapping, i.e. a single atom would not be trapped under the same laser drive conditions. The collective self-trapping is demonstrated by the observation of a significant increase of the trapping time as a function of the atom number. The atomic back action on the cavity field gives rise to a simultaneous real-time monitoring of the number of trapped atoms. This is used to show a non-exponential collapse of the atom trap.

Laser refrigeration of optically levitated sodium yttrium fluoride nanocrystals.

Solid state laser refrigeration can cool optically levitated nanocrystals in an optical dipole trap, allowing for internal temperature control by mitigating photothermal heating. This work demonstrates cooling of ytterbium-doped cubic sodium yttrium fluoride nanocrystals to 252 K on average with the most effective crystal cooling to 241 K. The amount of cooling increases linearly with the intensity of the cooling laser and is dependent on the pressure of the gas surrounding the nanocrystal. Cooling optically levitated nanocrystals allows for crystals prone to heating to be studied at lower pressures than currently achievable and for temperature control and stabilization of trapped nanocrystals.

Interactions of Ions and Ultracold Neutral Atom Ensembles in Composite Optical Dipole Traps: Developments and Perspectives

Ion–atom interactions are a comparatively recent field of research that has drawn considerable attention due to its applications in areas including quantum chemistry and quantum simulations. In first experiments, atomic ions and neutral atoms have been successfully overlapped by devising hybrid apparatuses combining established trapping methods, Paul traps for ions and optical or magneto-optical traps for neutral atoms, respectively. Since then, the field has seen considerable progress, but the inherent presence of radiofrequency (rf) fields in such hybrid traps was found to have a limiting impact on the achievable collision energies. Recently, it was shown that suitable combinations of optical dipole traps (ODTs) can be used for trapping both atoms and atomic ions alike, allowing to carry out experiments in absence of any rf fields. Here, we show that the expected cooling in such bichromatic traps is highly sensitive to relative position fluctuations between the two optical trapping beams, suggesting that this is the dominant mechanism limiting the currently observed cooling performance. We discuss strategies for mitigating these effects by using optimized setups featuring adapted ODT configurations. This includes proposed schemes that may mitigate three-body losses expected at very low temperatures, allowing to access the quantum dominated regime of interaction.

Dynamics of a spin qubit in an optical dipole trap

We present a theoretical investigation of coherent dynamics of a spin qubit encoded in hyperfine sublevels of an alkali-metal atom in a far off-resonant optical dipole trap. The qubit is prepared in the "clock transition" utilizing the Zeeman states with zero projection of the spin angular momentum. We focus on various dephasing processes such as the residual motion of the atom, fluctuations of the trapping field and its incoherent scattering and their effects on the qubit dynamics. We implement the most general fully-quantum treatment of the atomic motion, so our results remain valid in the limit of close-to-ground-state cooling with low number of vibrational excitations. We support our results by comparison with an experiment showing reasonable correspondence with no fitting parameters.

Large-area Rb87 Bose-Einstein condensate in a clipped-Gaussian optical dipole trap

We demonstrate a production of large-area $^{87}$Rb Bose-Einstein condensates (BECs) using a non-Gaussian optical dipole trap (ODT). The ODT is formed by focusing a symmetrically truncated Gaussian laser beam and it is shown that the beam clipping causes the trap geometry elongated and flattened along the beam axis direction. In the clipped-Gaussian ODT, an elongated, highly oblate BEC of $^{87}$Rb is generated with length and width of approximately $470~\mu\textrm{m}$ and $130~\mu\textrm{m}$, respectively, where the condensate healing length is estimated to be $\xi\approx 0.25~\mu\textrm{m}$ at the trap center. The ODT is characterized to have a quartic trapping potential along the beam axis and the atom density of the condensate is uniform within 10% over $1000\xi$ in the central region. Finally, we discuss the prospect of conducting vortex shedding experiments using the elongated condensate.

Loading and spatially resolved characterization of a cold atomic ensemble inside a hollow-core fiber

We present a thorough experimental investigation of the loading process of laser-cooled atoms from a magneto-optical trap into an optical dipole trap located inside a hollow-core photonic bandgap fiber, followed by propagation of the atoms therein. This, e.g., serves to identify limits to the loading efficiency and thus optical depth which is a key parameter for applications in quantum information technology. Although only limited access in 1D is available to probe atoms inside such a fiber, we demonstrate that a detailed spatially-resolved characterization of the loading and trapping process along the fiber axis is possible by appropriate modification of probing techniques combined with theoretical analysis. Specifically, we demonstrate the loading of up to $2.1 \times 10^5$ atoms with a transfer efficiency of 2.1 % during the course of 50 ms and a peak loading rate of $4.7 \times 10^3$ atoms ms$^{-1}$ resulting in a peak atomic number density on the order of $10^{12}$ cm$^{-3}$. Furthermore, we determine the evolution of the spatial density (profile) and ensemble temperature as it approaches its steady-state value of $T=1400$ $\mu$K, as well as loss rates, axial velocity and acceleration. The spatial resolution along the fiber axis reaches a few millimeters, which is much smaller than the typical fiber length in experiments. We compare our results to other fiber-based as well as free-space optical dipole traps and discuss the potential for further improvements.

Rapid generation of metastable helium Bose-Einstein condensates

We report the realisation of Bose-Einstein condensation (BEC) of metastable helium atoms using an in-vacuum coil magnetic trap and a crossed beam optical dipole trap. A novel quadrupole-Ioffe configuration (QUIC) magnetic trap made from in-vacuum hollow copper tubes provides fast switching times while generating traps with a 10G bias, without compromising optical access. The bias enables in-trap 1D doppler cooling to be used, which is the only cooling stage between the magneto-optic trap (MOT) and the optical dipole trap. This allows direct transfer to the dipole trap without the need for any additional evaporative cooling in the magnetic trap. The entire experimental sequence takes 3.3 seconds, with essentially pure BECs observed with $\sim 10^6$ atoms after evaporative cooling in the dipole trap.

Loss of atoms from a near-resonance hollow dipole trap

The lifetime of a gas of 6Li atoms in a large hollow optical dipole trap formed by radiation with a frequency detuned by 4 or 2 GHz upward from resonance is measured. The trap has the shape of a thin-walled cylinder with flat bases and a volume of ∼1 mm3. The main mechanism responsible for the loss of atoms is heating due to Rayleigh scattering. The influence of collisions of atoms with the background gas and with each other on the measured lifetime is negligible.

Production of Degenerate Fermi Gases of 6Li Atoms in an Optical Dipole TrapSupported by the National Key Research and Development Program of China (Grant No. 2016YFA0301503), the National Natural Science Foundation of China (Grant Nos. 11674358, 11434015, and 11974384), the Chinese Academy of Sciences (Grant No. YJKYYQ20170025), and K. C. Wong Education Foundation (Grant No. GJTD-2019-15).

We report the experimental production of degenerate Fermi gases of 6Li atoms in an optical dipole trap. The gray-molasses technique is carried out to decrease the atomic temperature to 57 μK, which facilitates the efficient loading of cold atoms into the optical dipole trap. The Fermi degeneracy is achieved by evaporative cooling of a two-spin mixture of 6Li atoms on the Feshbach resonance. The degenerate atom number per spin is 3.5 × 104, and the reduced temperature T/T F is as low as 0.1, where T F is the Fermi temperature of the non-interacting Fermi gas. We also observe the anisotropic expansion of the atom cloud in the strongly interacting regime.

Hybrid evaporative cooling of 133Cs atoms to Bose-Einstein condensation.

The Bose-Einstein condensation (BEC) of 133Cs atoms offers an appealing platform for studying the many-body physics of interacting Bose quantum gases, owing to the rich Feshbach resonances that can be readily achieved in the low magnetic field region. However, it is notoriously difficult to cool 133Cs atoms to their quantum degeneracy. Here we report a hybrid evaporative cooling of 133Cs atoms to BEC. Our approach relies on a combination of the magnetically tunable evaporation with the optical evaporation of atoms in a magnetically levitated optical dipole trap overlapping with a dimple trap. The magnetic field gradient is reduced for the magnetically tunable evaporation. The subsequent optical evaporation is performed by lowering the depth of the dimple trap. We study the dependence of the peak phase space density (PSD) and temperature on the number of atoms during the evaporation process, as well as how the PSD and atom number vary with the trap depth. The results are in excellent agreement with the equation model for evaporative cooling.

Cavity-assisted preparation and detection of a unitary Fermi gas

We report on the fast production and weakly destructive detection of a Fermi gas with tunable interactions in a high finesse cavity. The cavity is used both with far off-resonant light to create a deep optical dipole trap, and with near-resonant light to reach the strong light–matter coupling regime. The cavity-based dipole trap allows for an efficient capture of laser-cooled atoms, and the use of a lattice-cancellation scheme makes it possible to perform efficient intra-cavity evaporative cooling. After transfer in a crossed optical dipole trap, we produce deeply degenerate unitary Fermi gases with up to 7 × 105 atoms inside the cavity, with an overall 2.85 s long sequence. The cavity is then probed with near-resonant light to perform five hundred-times repeated, dispersive measurements of the population of individual clouds, allowing for weakly destructive observations of slow atom-number variations over a single sample. This platform will make possible the real-time observation of transport and dynamics as well as the study of driven-dissipative, strongly correlated quantum matter.

Production of 23Na Bose–Einstein condensates in the F = 2 state using D2 gray molasses

We report on the production of Bose–Einstein condensates of sodium atoms in the hyperfine state | F = 2 , m F = 2 ⟩ in a crossed optical dipole trap using D 2 gray molasses. We use the gray molasses sub-Doppler cooling technique to obtain an atomic sample of 3 × 10 8 at 56 µK. After the radio frequency (RF) evaporation cooling in an optically plugged magnetic trap, we transfer the atoms sample of 2.5 × 10 6 at 5.7 µK to a crossed optical dipole trap where a pure condensate with an atom number of 2 × 10 5 and lifetime of 6.3 s is obtained by further evaporation. We compare the cooling process of sodium atoms in the | 2 , 2 ⟩ and the | 1 , 1 ⟩ states by evaporation in the optical trap, and also observe the different three-body loss rates by the lifetime measurement.

Preparation of a two-state mixture of ultracold fermionic atoms with balanced population subject to the unstable magnetic field* *Project supported by the National Key Research and Development Program of China (Grant Nos. 2016YFA0301602 and 2018YFA0307601), the National Natural Science Foundation of China (Grant Nos. 12034011, 92065108, 11704234, 11804203, 11974224, 12022406, and 12004229), and the Fund for Shanxi “1331 Project” Key

We report a novel method to prepare a mixture of 40K Fermi gas having an equal population of the two ground magnetic spin states confined in an optical dipole trap, in the presence of an noisy quantization (magnetic) field. We realize the equal population mixture by applying a series of RF pulses. We observe the dependence of the population distribution between two spin states on the number of the applied RF pulses and find that the decoherence effects leading to the population fluctuations are overcome by the high number of RF pules. Our demonstrated technique can be potentially used in the precision measurement experiments with ultracold gases in noisy environments.

Production of dual species Bose–Einstein condensates of 39K and 87Rb* *Project supported by the National Key R&D Program of China (Grants Nos. 2016YFA0301602 and 2018YFA0307601), the National Natural Science Foundation of China (Grant Nos. 11974224, 11704234, 11804203, 12034011, 12022406, 12004229, and 92065108), the Fund for Shanxi “1331 Project” Key Subjects Construction, and the Program of Youth Sanjin Scholar.

We report the production of 39K and 87Rb Bose–Einstein condensates (BECs) in the lowest hyperfine states | F = 1,mF = 1 〉 simultaneously. We collect atoms in bright/dark magneto-optical traps (MOTs) of 39K/87Rb to overcome the light-assisted losses of 39K atoms. Gray molasses cooling on the D1 line of the 39K is used to effectively increase the phase density, which improves the loading efficiency of 39K into the quadrupole magnetic trap. Simultaneously, the normal molasses is employed for 87Rb. After the microwave evaporation cooling on 87Rb in the optically plugged magnetic trap, the atoms mixture is transferred to a crossed optical dipole trap, where the collisional properties of the two species in different combinations of the hyperfine states are studied. The dual species BECs of 39K and 87Rb are obtained by further evaporative cooling in an optical dipole trap at a magnetic field of 372.6 G with the background repulsive interspecies scattering length a KRb = 34 a 0 (a 0 is the Bohr radius) and the intraspecies scattering length a K = 20.05a 0.

Implementation of Single-Qubit Quantum Gates Based on a Microwave Transition in a Single Rubidium Atom in an Optical Dipole Trap

The results of experiments on the implementation of single-qubit quantum gates with a single 87Rb atom in an optical dipole trap with a wavelength of 850 nm are presented. The trap is formed by a long focal-length objective lens located outside the vacuum chamber of a magneto-optical trap. An atom is detected using a resonance fluorescence signal with an sCMOS video camera. The experiments involved the trapping and confinement of a single atom at times up to 50 s, optical pumping by polarized laser radiation, microwave transitions between two hyperfine sublevels of the ground state, and the measurement of the quantum state of the atom by pushing it from the trap. Rabi oscillations are observed during the “clock” microwave transition 5S1/2(F = 2, MF = 0) → 5S1/2(F = 1, MF = 0) between two operating qubit levels at a frequency of up to 4.2 kHz, a contrast of up to 95%, and a coherence time of up to 3 ms. These oscillations correspond to the implementation of two basic single-qubit quantum operations (Hadamard gate, NOT gate) from various initial qubit states with an average fidelity of 95.2 ± 3%.

Trapping, detection and manipulation of single Rb atoms in an optical dipole trap using a long-focus objective lens

Single alkali-metal atoms in arrays of optical dipole traps represent a quantum register that can be used for quantum computation and simulation based on short-term Rydberg excitations, which switch the interactions between qubits. To load single atoms into optical dipole traps and then detect them by resonance fluorescence, lenses with a large numerical aperture (NA > 0.5) inside a vacuum chamber and expensive EMCCD cameras are commonly used. We present our recent experimental results on demonstrating the trapping of single 87Rb atoms using a long-focus objective lens with a low numerical aperture (NA = 0.172) placed outside the vacuum chamber, and detecting single atoms with a low-cost sCMOS camera. We also present our current results on implementing a single-qubit gate based on optical pumping and subsequent microwave transition between two hyperfine sublevels of a single 87Rb atom with fidelity near 95%.

Characterizing the temperature dependence of Fano-Feshbach resonances of ultracold polarized thulium

Recent studies demonstrated anomalous temperature shifts for some Fano-Feshbach resonances of thulium atoms. These anomalies were explained by the variation in light intensity in the optical dipole trap, which accompanied changes in temperature. In addition, a temperature-related transformation of the statistics of the inter-resonance spacing was demonstrated [Aikawa et al., Phys. Rev. Lett. 108, 210401 (2012)]. Here, we analyze the shifts of isolated $s$- and $d$-type Fano-Feshbach resonances of ultracold thulium atoms with temperature for a fixed depth of an optical dipole trap. The measurements are consistent with the three-body recombination-based theory of the temperature-related resonance shift and enable the extraction of the resonance parameters, particularly the magnetic moments of closed channel states. This parameter and the known polarizability of the open channel enable us to separate the contributions of the temperature and Stark shift to the overall shift of the resonances and show the dominant role of the Stark effect in the overall shift.