We present a method for determining the atom number distribution of few atoms in a tight optical tweezer from their fluorescence distributions. In the tight tweezer regime, the detection light causes rapid atom loss due to light-assisted collisions. This in turn leads to non-Poissonian and overlapping fluorescence distributions for different initial atom numbers, and commonly used threshold techniques fail. We use maximum likelihood estimation algorithms to fit model distributions that account for the atom loss. This gives accurate atom number distributions for relatively few experimental runs (about 600 is sufficient) to sample a photon number distribution. We show that the method can be extended to situations when the photon number distributions for known initial atom numbers cannot be modeled, at the cost of requiring a higher number of experimental runs.

We capture individual rubidium-85 atoms in steerable optical tweezers with high efficiency to study atomic interactions with a known number of atoms. These tweezers allow us to capture and move individual atoms, and to prepare them in a specific quantum states. We show the usefulness of this platform to study atomic interactions like individual molecule formation and spin changing collisions.

Tightly focused laser beams form optical tweezers that can hold and manipulate individual atoms. They give superb control over microscopic quantum systems and have paved the way for bottom up assembly of few-atom systems. Such assembled systems provide an ideal starting point for many fundamental studies of atomic interactions and few-atom phenomena. Here we review the present stage of these fields, as well as some of the basic experimental techniques required for these experiments Figure from [74].

We present a self-locking laser system that does not require operator interventions. The system automatically finds a desired atomic transition and subsequently locks to it. Moreover, it has the ability to automatically detect if the laser is out of lock and activate the re-locking process. The design was implemented on two different diode lasers, a distributed Bragg reflector (DBR) diode laser and a Fabry Perot (FP) diode laser, used as a repump laser for a magneto-optical trap in a laser cooling experiment and a Raman laser for a four-level Raman transition experiment, respectively. The design relies on frequency modulation transfer spectroscopy to obtain a sub-Doppler atomic spectrum of rubidium-85. This spectrum is then demodulated to obtain zero-crossing linear slopes at the exact points of each atomic and crossover transition. The frequency modulation, the signal analysis, and the automatic locking and re-locking of the lasers are all implemented using an Arduino Due microcontroller. The lock loop has a bandwidth of 7 kHz. The lasers used for the design are characterized, and the robustness of the lock is analyzed. The achieved linewidths of DBR and FP lasers are 1.4 and 5.5 MHz, respectively. The frequency drifts of both lasers are a few 100 kHz over a course of days. The capture range of the locking system is up to 4.9 GHz for the DBR laser and 725 MHz for the FP laser. Both lasers performed well under actual experimental conditions.

We present a classical approximation for the peaks of survival resonances occurring when diffracting matter waves from absorption potentials. Generally, our simplified model describes the absorption-diffraction process around the Talbot time very well. Classical treatments of this process are presently lacking. For purely imaginary potentials, the classical model duplicates quantum-mechanical calculations. The classical model allows for simple evolution of phase-space probability densities, which in the limit of the effective Planck constant going to zero allows for a compact analytical expression of the survival probability as a function of remaining parameters. Our work extends the range of processes that can be described through classical analogs.

We investigate the photoassociation dynamics of exactly two laser-cooled $^{85}\mathrm{Rb}$ atoms in an optical tweezer and reveal fundamentally different behavior to photoassociation in many-atom ensembles. We observe nonexponential decay in our two-atom experiment that cannot be described by a single rate coefficient and find its origin in our system's pair correlation. This is in stark contrast to many-atom photoassociation dynamics, which are governed by decay with a single rate coefficient. We also investigate photoassociation in a three-atom system, thereby probing the transition from two-atom dynamics to many-atom dynamics. Our experiments reveal additional reaction dynamics that are only accessible through the control of single atoms and suggest photoassociation could measure pair correlations in few-atom systems. It further showcases our complete control over the quantum state of individual atoms and molecules, which provides information unobtainable from many-atom experiments.

We investigate a scheme that enhances survival resonances in a delta-killed system through actively recycling lost atoms. The process causes atoms to dissipate into superpositions of momentum states with sustained survival when exposed to kicks of a dissipative optical standing wave. The recycling process causes momentum redistribution, which gives the atoms multiple chances to enter a long-surviving mode. The survival resonance peak height shows an improvement of a factor of 2.4. This technique can increase the sensitivity and precision of atomic interferometers based on survival resonances.

The introduction of optical tweezers for trapping atoms has opened remarkable opportunities for manipulating few-body systems. Here, we present the first bottom-up assembly of atom triads. We directly observe atom loss through inelastic collisions at the single event level, overcoming the substantial challenge in many-atom experiments of distinguishing one-, two-, and three-particle processes. We measure a strong suppression of three-body loss, which is not fully explained by the presently availably theory for three-body processes. The suppression of losses could indicate the presence of local anticorrelations due to the interplay of attractive short range interactions and low dimensional confinement. Our methodology opens a promising pathway in experimental few-body dynamics.

The complex collisional properties of atoms fundamentally limit investigations into a range of processes in many-atom ensembles. In contrast, the bottom-up assembly of few- and many-body systems from individual atoms offers a controlled approach to isolating and studying such collisional processes. Here, we use optical tweezers to individually assemble pairs of trapped 85 Rb atoms, and study the spin dynamics of the two-body system in a thermal state. The spin-2 atoms show strong pair correlation between magnetic sublevels on timescales exceeding one second, with measured relative number fluctuations 11.9 ± 0.3 dB below quantum shot noise, limited only by detection efficiency. Spin populations display relaxation dynamics consistent with simulations and theoretical predictions for 85 Rb spin interactions, and contrary to the coherent spin waves witnessed in finite-temperature many-body experiments and zero-temperature two-body experiments. Our experimental approach offers a versatile platform for studying two-body quantum dynamics and may provide a route to thermally robust entanglement generation.

Bottom-up assembly of few- and many-body systems from individual atoms could deliver robust entanglement, which is a key resource for quantum technologiescite{Isenhower2010, Regal2015, Endres2016}. Furthermore, it allows quantum processes to be observed at the individual event level, revealing information concealed by ensemble-averaged measurementscite{Toschek1986, Andersen2010} and providing insights into molecular processescite{Andersen2013, Liu2018}, thermalizationcite{Greiner2016}, and quantum thermodynamicscite{Rosnagel:2016}. Here, we study the spin dynamics of a two-body system consisting of individually-assembled pairs of boldsymbol{$^{85}$}Rb atoms, whose collisional properties prevent investigation in the many-body regime. The thermal spin-2 atoms show perfect pair correlation between magnetic sublevels on timescales exceeding one second, with measured relative number fluctuations boldsymbol{$11.9pm0.3$} dB below quantum shot noise (QSN), limited only by detection efficiency. Both microscopic simulations and experiments display relaxation dynamics, contrary to the coherent spin waves witnessed in finite-temperature many-body experimentscite{Lett2013} and zero-temperature two-body experimentscite{Bloch2005}. The relative relaxation rates are consistent with theoretical predictions of the boldsymbol{$^{85}$}Rb spin-dependent interaction strengthscite{Greene2001}. Our experiment is a versatile platform for studying two-body quantum dynamics and may provide a route to thermally-robust entanglement generation.

We present an optical system for the formation of arbitrary spatial polarization and phase patterns with a single spatial light modulator (SLM). Any such complex light beam can be generated by using three interactions with a computer controlled SLM. We achieved this by bouncing the beam three times in different areas on the SLM surface, and the cumulative interactions provide for any desired polarization and phase state imprinted on the output beam. The system can deliver high output power, since the only light loss arises from imperfect optical elements.

We investigate laser-cooled atoms periodically driven by pulsed standing waves of light tuned close to an open atomic transition. This nonunitary system displays survival resonances for certain driving frequencies. The survival resonances emerge as a result of the matter-wave Talbot-Lau effect, similar to the Talbot effect causing quantum resonances in the atom optics delta-kicked rotor. Since the Talbot-Lau effect occurs for incoherent waves, the survival resonances can be observed using thermal atoms. A microlensing effect can enhance the height and incisiveness of the resonances. This may find applications in precision measurements.

We do a proof-of-principle demonstration of an atomic gravimeter based on survival resonances of dissipatively driven atoms. Exposing laser-cooled atoms to a sequence of near-resonant standing-wave light pulses reveals survival resonances when the standing-wave interference pattern accelerates. The resonant accelerations determine the local gravitational acceleration and we achieve a precision of 5 ppm with a drop distance less than 1 mm The incisiveness of the resonances scales with the square of the drop time. Present results indicate that an appropriately designed atomic gravimeter based on survival resonances might be able to reach a precision of 1 mu Gal with a 10-cm-high fountain. The relatively simple experimental construction of this technique may be of interest for a compact absolute atomic gravimeter.

Summary form only given. Complete control of individual atoms trapped in far-off resonance optical tweezers is vital for gaining a better understanding of the microscopic world. It will provide a platform with unprecedented flexibility for studying few-body physics, and might lead to new quantum technologies. By combining near-deterministic preparation of single atoms [1-3] with Zeeman-insensitive Raman sideband cooling [4], we present a push button method to prepare a single 85Rb atom in its centre of mass ground state with high fidelity. Our Raman sideband cooling scheme works efficiently in an environment with magnetic field fluctuations that were too large for other Raman sideband cooling variations [5,6]. In 2D, we achieve a fidelity for the entire loading procedure of ~0.7 for a single atom in the ground state of the optical tweezers. In 3D, the fidelity drops to 0.1 since the cooling is less efficient for the weakly confined tweezer axis.

The interaction blockade phenomenon isolates the motion of a single quantum particle within a multiparticle system, in particular for coherent oscillations in and out of a region affected by the blockade mechanism. For identical quantum particles with Bose statistics, the presence of the other particles is still felt by a bosonic stimulation factor root N that speeds up the coherent oscillations, where N is the number of bosons. Here we propose an experiment to observe this enhancement factor with a small number of bosonic atoms. The proposed protocol realizes an asymmetric double-well potential with multiple optical tweezer laser beams. The ability to adjust bias independently of the coherent coupling between the wells allows the potential to be loaded with different particle numbers while maintaining the resonance condition needed for coherent oscillations. Numerical simulations with up to three bosons in a realistic potential generated by three optical tweezers predict that the relevant avoided level crossing can be probed and the expected bosonic enhancement factor observed.

We combine near--deterministic preparation of a single atom with Raman sideband cooling, to create a push button mechanism to prepare a single atom in the motional ground state of tightly focused optical tweezers. In the 2D radial plane, we achieve a large ground state fidelity for the entire procedure (loading and cooling) of $\sim$0.73, while the ground state occupancy is $\sim$0.88 for realizations with a single atom present. For 1D axial cooling, we attain a ground state fraction of $\sim$0.52. The combined 3D cooling provides a ground state population of $\sim$0.11. Our Raman sideband cooling variation is indifferent to magnetic field fluctuations, allowing wide--spread unshielded experimental implementations. Our work provides a pathway towards a range of coherent few body experiments.