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arXiv:1703.07084 · Journal of Physics B Atomic Molecular and Optical Physics

Fluorescence detection of single lithium atoms in an optical lattice using Doppler-cooling beams

Hyok Sang Han, Hyun Gyung Lee, Seokchan Yoon, D. Cho

Published Jun 14, 2018• 1 citation
arXivHTMLPDF

Abstract

Abstract We demonstrate in situ fluorescence detection of 7 Li atoms in a 1D optical lattice with single atom precision. Even though illuminated lithium atoms tend to boil out, when the lattice is deep, red-detuned probe beams without extra cooling retain the atoms while producing sufficient fluorescent photons for detection. When the depth of the potential well at an antinode is 1.6 mK, an atom remains trapped for longer than 20 s while scattering probe photons at the rate of 5.3 × 10 4 s −1 . We propose a simple model that describes the dependence of the lifetime of an atom on well depth. When the number of trapped atoms is reduced, a clear stepwise change in integrated fluorescence is observed, indicating the detection of a single atom. The presence or absence of an atom is determined within 300 ms with an error of less than 5 × 10 −4 at a photon-collecting efficiency of 1%, which is limited by the small numerical aperture.

DOI
10.1088/1361-6455/aacca9

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Abstract We demonstrate in situ fluorescence detection of 7 Li atoms in a 1D optical lattice with single atom precision. Even though illuminated lithium atoms tend to boil out, when the lattice is deep, red-detuned probe beams without extra cooling retain the atoms while producing sufficient fluorescent photons for detection. When the depth of the potential well at an antinode is 1.6 mK, an atom remains trapped for longer than 20 s while scattering probe photons at the rate of 5.3 × 10 4 s −1 . We propose a simple model that describes the dependence of the lifetime of an atom on well depth. When the number of trapped atoms is reduced, a clear stepwise change in integrated fluorescence is observed, indicating the detection of a single atom. The presence or absence of an atom is determined within 300 ms with an error of less than 5 × 10 −4 at a photon-collecting efficiency of 1%, which is limited by the small numerical aperture.
We report fluorescence detection of a single 7 Li atom in a magneto-optical trap (MOT). We use a double MOT system, which allows us to control the loading rate of the second MOT by simply turning on or off the first MOT and to use a modest magnetic field gradient of 46 G/cm. A single atom is trapped for longer than 100 s. The fluorescence from the trapped atoms is imaged onto an electron-multiplying charge-coupled device with a unit magnification factor by a pair of aspheric lenses. The numerical aperture of the objective lens is 0.22 with a collecting efficiency of 1.3%. Accumulated fluorescence is extracted by integrating the signal over a region of interest of 500×350 μ m 2 . For a 200-ms exposure time, the fluorescence count from a single atom is 8.8×10 3 and the background count from scattered MOT beams is 6.8 × 10 4 . We actively stabilize the trap beam power to reduce the noise and the measured noise is two times the statistical noise. Overall signal-to-noise ratio of the single-atom detection is 37/√ Hz. We also describe a modification of the system that is adapted to an experimental configuration for quantum manipulation of single atoms in a 1D optical lattice.
We demonstrate single-site and single-atom-resolved fluorescence imaging of a bosonic Mott insulator of 7Li^{7}\mathrm{Li}7Li atoms in an optical lattice. The fluorescence images are obtained by implementing Raman sideband cooling on a deep two-dimensional square lattice, where we collect scattered photons with a high numerical aperture objective lens. The square lattice is created by a folded retroreflected beam configuration that can reach a 2.5 mK lattice depth from a single laser source. The lattice beam is elliptically focused to have a large area with deep potential. On average, 4000 photons are collected per atom during 1 s of Raman sideband cooling and the imaging fidelity is over 9595%95 in the central $80\ifmmode\times\else\texttimes\fi{}80$ lattice sites. As a first step to study correlated quantum phases, we present the site-resolved imaging of a Mott insulator. By tuning the magnetic field near the Feshbach resonance, the scattering length can be increased to 680aB680{a}_{B}680aB​ and we are able to produce a large-sized unity filling Mott insulator with 2000 atoms at low temperature. Our work provides a stepping stone to further in-depth investigations of intriguing quantum many-body phases in optical lattices.
In the experiment, single Rubidium-87 atoms are loaded out of a molasses of Doppler-cooled atoms into the focus of a Gaussian laser beam created by a single mode laser diode with a wavelength of 856 nm. A confocal microscope outside our vacuum chamber is used to focus the trapping beam to a minimal waist of 3.5 /spl mu/m (thus creating a trap depth of 1-2 mK) and to collect the fluorescence light from the center of the dipole trap. The collected fluorescence light is coupled into a single-mode optical fiber for spatial filtering, and the photons are detected with an avalanche photodiode. With this setup the overall detection efficiency of photons emitted from the trap region is about 0.2%. First experimental results show sub-Poissonian occupation statistics of the dipole trap: only single atoms are observed inside our trap, never two ore more. The measured 1/e lifetime of atoms inside the dipole trap in the presence of cooling light is about 1-2 seconds. To prove that we really observe single atoms inside the trap, a Hanbury-Brown-Twiss measurement of the fluorescence light from the trap will be shown. The trapping of single atoms inside our dipole trap marks the first step towards the investigation of the properties of an entangled atom photon state. Progress in preparing and analyzing single atoms in well defined hyperfine ground states will be reported.
Neutral atoms trapped in optical lattices are promising candidates for quantum information processing and quantum simulation. Over the last decades, elegant tools for the manipulation of the internal and external states of optically trapped atoms have been developed. The crucial capability of scalable internal state readout in these systems, however, still relies on destructive methods. In spite of the important role of near-resonant illumination for the manipulation and detection of atoms in the lattice, there also exists a significant lack of studies on the heating and cooling dynamics of optically trapped atoms interacting with near-resonant light. An in-depth understanding of the heating and cooling processes is essential to finding the conditions of illumination that enable the non-destructive internal state readout of multiple atoms. This work presents an experimental system to cool, trap, manipulate, and detect the internal and external states of a small ensemble of 87Rb neutral atoms trapped in a one-dimensional optical lattice. A high photon detection efficiency in our experimental system allows for fast fluorescence imaging with acquisition times of 20 ms and fast position determination of atoms in the optical lattice with an accuracy of ∼ 40 nm. Using this experimental system, we investigate the heating dynamics of a neutral atom trapped in a standing wave dipole trap illuminated by a single near-resonant laser beam. A theoretical description to describe our measurements is provided in two experimentally relevant regimes. First, we consider the case of a weak near-resonant beam and later the case of off-resonant illumination. From this analysis, we find settings for the illumination light which allows an atom to scatter many photons before it is expelled from the trap. Building on these results we demonstrate simultaneous, non-destructive determination of the internal state of spatially resolved atoms trapped in a one-dimensional optical lattice with a fidelity of 98.6 ± 0.2% and a survival probability of 99.0 ± 0.2%. During the readout process, less than 2% of the atoms change their initial ground state. In order to determine the state of atoms that are not spatially resolved, a novel image analysis technique is presented. The technique uses Bayesian methods, which include the statistics of the detected photons as well as the response from the EMCCD camera. The Bayesian method is implemented on experimental data for atoms trapped in a one-dimensional optical lattice and its accuracy is tested by numerical simulations. In addition, an extension of this algorithm for atoms trapped in two-dimensional lattices is provided. Finally, the non-destructive state detection method is utilized as a tool for the state determination following the coherent control of the internal and external states of atoms in the optical trap. Here Raman sideband cooling is implemented and utilized in an atomic compression sequence for the creation of a small and dense atomic ensemble. These techniques will play an important role in experiments studying the collective light interaction of the atomic ensemble in a recently added optical fiber cavity.
We explore the feasibility of single atom detection on an atom chip by using a tiny fluorescence detector mounted on the chip. Resonant fluorescence from a trapped ultracold atom will be collected with a miniature aspheric lens and taken out of a vacuum chamber through a fiber. During detection, the atom can be held at the focus of the detector with a dipole trapping beam introduced through the same fiber. We have experimentally determined the optical performance of such a detector, taking into account effects such as stray light from the dipole trapping beam and chromatic aberration. The collection efficiency for isotropically emitted radiation is experimentally obtained to be 2.5%. From this, it is estimated that the fluorescence emitted from a single Rb atom will produce a photon count rate of 4.7x10(4) Hz, which is much larger than the shot noise limited background fluctuations.