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arXiv:1606.06978 · Physical Review A

Suppression of single-cesium-atom heating in a microscopic optical dipole trap for demonstration of an 852-nm triggered single-photon source

Bei Liu, Gang Jin, Jun He, Junmin Wang

Published Jul 13, 2016• 9 citations
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Abstract

We investigate single-cesium-atom heating owing to the momentum accumulation process induced by the resonant pulsed excitation in a microscopic optical dipole trap formed by a strongly focused 1064-nm laser beam. The heating depends on the trap frequency, which restricts the maximum repetition rate of the pulsed excitation. We experimentally verify the heating of a single atom and then demonstrate how to suppress it with an optimized pulsed excitation and cooling method. The typical trap lifetime of a single cesium atom is extended from $108\ifmmode\pm\else\textpm\fi{}6\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{s}$ to $2536\ifmmode\pm\else\textpm\fi{}31\phantom{\rule{0.16em}{0ex}}\mathrm{ms}$, and the corresponding number of excitations increases from $\ensuremath{\sim}108$ to $\ensuremath{\sim}360\phantom{\rule{0.16em}{0ex}}000$. In applying this faster cooling method, we use the trapped single cesium atom as a triggered single-photon source at an excitation repetition rate of 10 MHz. The second-order intensity correlations of the emitted single photons are characterized by implementing a Hanbury Brown and Twiss setup, and a clear antibunching effect has been observed.

DOI
10.1103/physreva.94.013409

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European Quantum Electronics Conference (EQEC)•Jan 1, 2003•0 citations
We investigate single-cesium-atom heating owing to the momentum accumulation process induced by the resonant pulsed excitation in a microscopic optical dipole trap formed by a strongly focused 1064-nm laser beam. The heating depends on the trap frequency, which restricts the maximum repetition rate of the pulsed excitation. We experimentally verify the heating of a single atom and then demonstrate how to suppress it with an optimized pulsed excitation and cooling method. The typical trap lifetime of a single cesium atom is extended from 108\ifmmode±\else\textpm\fi6\ensuremathμs108\ifmmode\pm\else\textpm\fi{}6\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{s}108\ifmmode±\else\textpm\fi6\ensuremathμs to 2536\ifmmode±\else\textpm\fi31ms2536\ifmmode\pm\else\textpm\fi{}31\phantom{\rule{0.16em}{0ex}}\mathrm{ms}2536\ifmmode±\else\textpm\fi31ms, and the corresponding number of excitations increases from \ensuremath∼108\ensuremath{\sim}108\ensuremath∼108 to \ensuremath∼360000\ensuremath{\sim}360\phantom{\rule{0.16em}{0ex}}000\ensuremath∼360000. In applying this faster cooling method, we use the trapped single cesium atom as a triggered single-photon source at an excitation repetition rate of 10 MHz. The second-order intensity correlations of the emitted single photons are characterized by implementing a Hanbury Brown and Twiss setup, and a clear antibunching effect has been observed.
Single-atom-based single-photon source has several advantages, such as narrow bandwidth, wavelength matching with the absorption line of the same atomic ensemble, and insensitivity to the environment disturbing, and it is very important not only for basic researches in quantum optic field but also for applications in quantum information processing. In this paper, we report the generation of a 10-MHz-repetition-rate triggered single-photon source at 852 nm based on a trapped single cesium atom in a far-off-resonance microscopic optical dipole trap (FORT). To generate an optical dipole trap, a far-red-detuned 1064 nm laser beam is tightly focused by using a high numerical aperture lens, a typical trap depth is 2 mK and trap waist is 2.3 m. To obtain a maximum probability of pulsed excitation, the frequency of the pulsed laser should be resonant with the atomic energy levels and the trapped single atom must be excited with a -pulse. However, the interaction between the FORT laser and the atoms causes AC Stark shifts of the atomic energy levels. Thus, in order to demonstrate the resonant pulsed excitation, it is important to calculate and measure the shift of 6S1/2|Fg=4,mF=+4-6P3/2|Fe=5,mF=+5 cyclical transition in the FORT. For a two-level system, the probability of pulsed excitation can be described by Rabi oscillations with a characteristic Rabi frequency . With an optimized time sequence, we experimentally demonstrate the Rabi oscillation between the ground state and the excited state, and the peak power of -pulse laser is about 1.25 mW. We also measure the temporal envelope of single photons after a -pulse excitation. A gated pulsed excitation and cooling technique are used to reduce the possibility that atoms are heated by -pulse laser. The typical trapping lifetime of single cesium atom is extended from~108 ups to~2536 ms. The corresponding number of excitations is improved from 108 to 360000. The second-order intensity correlations of the emitted single-photon are characterized by implementing Hanbury Brown-Twiss setup. The statistics shows a strong anti-bunching with a value of 0.09 for the second-order correlation at zero delay. In the future, we will perform a Hong-Ou-Mandel two-photon interference experiment to analyze the indistinguishability of the single photons. We will also trap single atoms in a magic-wavelength optical dipole trap where the ground and the excited states have the same shift.
In our experiment, a single cesium atom prepared in a large-magnetic-gradient magneto-optical trap (MOT) can be efficiently transferred into a 1064-nm far-off-resonance microscopic optical dipole trap (FORT). The efficient transfer of the single atom between the two traps is used to determine the trapping lifetime and the effective temperature of the single atom in FORT. The typical trapping lifetime has been improved from ∼ 6.9 s to ∼ 130 s by decreasing the background pressure from ∼ 1 × 10 −10 Torr to ∼ 2 × 10 −11 Torr and applying one-shot 10-ms laser cooling phase. We also theoretically investigate the dependence of trapping lifetimes of a single atom in a FORT on trap parameters based on the FORT beam’s intensity noise induced heating. Numerical simulations show that the heating depends on the FORT beam’s waist size and the trap depth. The trapping time can be predicted based on effective temperature measurement of a single atom in the FORT and the intensity noise spectra of the FORT beam. These experimental results are found to be in agreement with the predictions of the heating model.
An optical tweezer composed of a strongly focused single-spatial-mode Gaussian beam of a red-detuned 1064-nm laser can confine a single-cesium (Cs) atom at the strongest point of the light intensity. We can use this for coherent manipulation of single-quantum bits and single-photon sources. The trapping lifetime of the atoms in the optical tweezers is very short due to the impact of the background atoms, the parametric heating of the optical tweezer and the residual thermal motion of the atoms. In this paper, we analyzed the influence of the background pressure, the trap frequency of optical tweezers and the laser intensity fluctuation of optical tweezers on the atomic trapping lifetime. Combined with the external feedback loop based on an acousto-optical modulator (AOM), the intensity fluctuation of the 1064-nm laser in the time domain was suppressed from ±3.360% to ±0.064%, and the suppression bandwidth in the frequency domain reached approximately 33 kHz. The trapping lifetime of a single-Cs atom in the microscopic optical tweezers was extended from 4.04 s to 6.34 s.
This work deals with the cooling and trapping of single cesium (Cs) atoms in a large-magnetic-gradient magneto-optical trap (MOT) and the confinement of single Cs atoms in a far-off-resonance optical dipole trap (FORT). The experiment setup is based on two large-numerical-aperture lens assemblies which allow us to strongly focus a 1064-nm TEM00-mode Gaussian laser beam to a 1/e2 radius of ~ 2.3 μm to form a microscopic FORT for isolating single atom with environment and to efficiently collect the laser-induced-fluorescence photons emitted by single atoms for detecting and recognizing single atom’s internal state. We have tried both of “bottom-up” and “top-down” loading schemes to confine single atoms in the microscopic FORT. In the “bottom-up” scheme, we have successfully prepared single Cs atoms in the MOT and transferred it into FORT with a probability of almost 100%. In the “top-down” scheme, we have achieved ~ 74% of single atom loading probability in the FORT using light-assisted collisions induced by blue detuning laser and with prepared many Cs atoms in the MOT. The relaxation time in hyperfine level of ground state of trapped single Cs atom is measured to be ~5.4 s. To coherently manipulate atomic quantum bits (qubit) encoded in the clock states (mF = 0 states in Fg = 3 and 4 hyperfine levels) of single Cs atom via the two-photon simulated Raman adiabatic passage (STIRAP), we have prepared two phase-locked laser beams with a frequency difference of ~ 9.192 GHz by optically injecting an 852-nm master laser to lock the +1-order sideband of a 9-GHz current-modulated slave diode laser. The two phase-locked laser beams are used to drive STIRAP process in the Λ-type three-level system consists of Cs |6S1/2 Fg = 4, mF = 0> and |6S1/2 Fg = 3, mF = 0< long-lived clock states and Cs |6S1/2 Fe = 4, mF = +1 > excited state with the single-photon detuning of ~ -20 GHz. Rabi flopping experiments are in progress.
Atoms trapped in magic-wavelength optical tweezer will have the same light shift for the desired ground and excited states. In this case the position-dependence differential light shift of the desired transition can be eliminated. For cesium 6S_1/2 (Fg = 4, mF = +4) - 6P_3/2 (Fe = 5, mF = +5) cycling transition (852 nm), the magic wavelength was calculated theoretically for a linearly-polarized optical tweezer, and also it was verified experimentally to be 937.6 nm. We have demonstrated 852-nm triggered single-photon sources based on single cesium atom trapped in linearly-polarized optical tweezers implemented by using of 1064-nm and 937.6-nm lasers, respectively. The photon statistics were characterized by using of the Hanbury Brown - Twiss (HBT) scheme based on Si single-photon detectors (SSPDs). Strong anti-bunching effect [g^2(t=0) = 0.09] was recognized, and it shown perfectly the single-photon characters. The Hong-Ou-Mandel (HOM) two-photon interference measurements based on SSPDs were employed to evaluate the photon indistinguishability. Our preliminary experimental results indicated that the indistinguishability of single photons has been improved ~ 20% for the case of magic-wavelength optical tweezer. References: [1] Phys. Rev. A 94 (2016) 013409; [2] Appl. Phys. Express 9 (2016) 072702; [3] Opt. Express 25 (2017) p.15861
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.