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Optics Letters

Fluorescence collection efficiency of atoms in dipole traps

Guang-Jie Chen, Jun-Jie Wang, Yanan Lv, Hong-Jie Fan, Zhu-Bo Wang, Gang Li, Chun‐Hua Dong, Yan‐Lei Zhang, Guang‐Can Guo, Chang‐Ling Zou

Published Aug 16, 2024• 1 citation
Publisher

Abstract

The fluorescence collection from single atoms and emitters has been extensively utilized in quantum information and quantum optics research. Here, we investigated the collection efficiency of an objective lens by drawing an analogy between the free-space beam (FSB) and a waveguide mode. We explored how efficiency is influenced by their thermal motion within a dipole trap. Furthermore, we introduce an effective energy fraction ratio to quantify potential imperfections in the focusing of the objective lens. Our results provide valuable insights for optimizing the fluorescence collection in single-atom experiments and highlight the importance of considering realistic experimental conditions when estimating achievable efficiencies.

DOI
10.1364/ol.537054

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The fluorescence collection from single atoms and emitters has been extensively utilized in quantum information and quantum optics research. Here, we investigated the collection efficiency of an objective lens by drawing an analogy between the free-space beam (FSB) and a waveguide mode. We explored how efficiency is influenced by their thermal motion within a dipole trap. Furthermore, we introduce an effective energy fraction ratio to quantify potential imperfections in the focusing of the objective lens. Our results provide valuable insights for optimizing the fluorescence collection in single-atom experiments and highlight the importance of considering realistic experimental conditions when estimating achievable efficiencies.
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.
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.
Fluorescence collection from individual emitters plays a key role in state detection and remote entanglement generation, fundamental functionalities in many quantum platforms. Planar photonics have been demonstrated for robust and scalable addressing of trapped-ion systems, motivating consideration of similar elements for the complementary challenge of photon collection. Here, using an argument from the reciprocity principle, we show that far-field photon collection efficiency can be simply expressed in terms of the fields associated with the collection optic at the emitter position alone. We calculate collection efficiencies into ideal paraxial and fully vectorial focused Gaussian modes parameterized in terms of focal waist, and further quantify the modest enhancements possible with more general beam profiles, establishing design requirements for efficient collection. Toward practical implementation, we design, fabricate, and characterize two diffractive collection elements operating at λ = 397 nm; a forward emitting design is predicted to offer 0.25% collection efficiency into a single waveguide mode, while a more efficient reverse-emitting design offers 1.14% collection efficiency, albeit with more demanding fabrication requirements. Close agreement between simulated and measured emission for both designs indicates practicality of these collection efficiencies, and we indicate avenues to improved devices approaching the limits predicted for ideal beams. We point out a particularly simple integrated waveguide configuration for polarization-based remote entanglement generation enabled by integrated collection.
Single photon emitters coupled to optical fibers are becoming important as sources of non-classical light and nano-scale sensors. At present it is not possible to efficiently interface single photon emitters with the optical fiber platform, and there are particular challenges associated with the need to ensure highly efficient collection and delivery of emitted photons. To model single particle emission, we have considered the coupling of a dipole to an optical fiber mode as a function of orientation and position with respect to the core-cladding interface. Our model shows that it is possible to significantly enhance the collection efficiency into the guided modes as a result of modifications to the dipole emission pattern and power resulting from the surrounding fiber environment. For certain geometries the fiber-dipole coupling can result in a factor of 2.6 increase in the power emitted by the dipole.
Abstract 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%.