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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Nearly deterministic loading of a single cesium atom in a magneto-optical trap and in a microscopic optical tweezer by feedback control

Bei Liu, Junmin Wang, Wen-Ting Diao, Jieying Wang, Gang Y Jin, Jun He

Published May 1, 2014• 0 citations
Publisher

Abstract

Based on feedback control techniques and our realization of nearly complete transferring cold cesium (Cs) atoms from a magneto-optical trap (MOT) to a far-off-resonance microscopic optical tweezer, we investigated the possibility for nearly deterministic loading of a single Cs atom in a MOT and in a microscopic optical tweezer. We combined feedback controls on the gradient of the MOT quadrupole magnetic field (QMF) and on blue-detuned light-assisted collisions (LAC) of confined cold atoms in the tweezer. Using active feedback on QMF of the MOT, we have achieved ~ 98% of probability of single atom loading in a MOT. In a microscopic optical tweezer, by combining the feedback controls on the QMF and the LAC, we finally achieved ~ 95.2% of probability of single atom loading in the tweezer. This two-path feedback control scheme may be extended to load a small-size 2D tweezer array with exact single atom trapped in each site simultaneously. This is very important and promising to implement an addressable multiple-qubit system for demonstrating quantum register and quantum processor.

DOI
10.1117/12.2052085

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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE•Apr 30, 2010•2 citations
Based on feedback control techniques and our realization of nearly complete transferring cold cesium (Cs) atoms from a magneto-optical trap (MOT) to a far-off-resonance microscopic optical tweezer, we investigated the possibility for nearly deterministic loading of a single Cs atom in a MOT and in a microscopic optical tweezer. We combined feedback controls on the gradient of the MOT quadrupole magnetic field (QMF) and on blue-detuned light-assisted collisions (LAC) of confined cold atoms in the tweezer. Using active feedback on QMF of the MOT, we have achieved ~ 98% of probability of single atom loading in a MOT. In a microscopic optical tweezer, by combining the feedback controls on the QMF and the LAC, we finally achieved ~ 95.2% of probability of single atom loading in the tweezer. This two-path feedback control scheme may be extended to load a small-size 2D tweezer array with exact single atom trapped in each site simultaneously. This is very important and promising to implement an addressable multiple-qubit system for demonstrating quantum register and quantum processor.
A single atom in a magneto–optical trap (MOT) with trap size (hundreds of micrometers) can be transferred into an optical microscopic tweezer with a probability of ~ 100%. The ability to transfer a single atom into two traps back and forth allows us to study the loading process. The loading probability is found to be insensitive to the geometric overlap of the MOT and the tweezer. It is therefore possible to perform simultaneously loading of a single atom into all sites of the tweezer array for many qubits. In particular, we present a simulation of the one-dimensional and two-dimensional arrays of an optical microscopic tweezer. We find the same qualitative behavior for all of the trap parameters.
Efficient loading of single atoms into tightly confined traps is crucial for advancing quantum information processing and exploring atom-photon interactions. However, directly loading atoms from a magneto-optical trap (MOT) into static tweezers in cavity-based systems and hybrid atom-photon interfaces remains a challenge. Here, we demonstrate atom loading in a tightly confined optical tweezer 0.6mm away from MOT by an optical conveyor belt. By employing real-time feedback control of the atom number in the overlapping region between the conveyor belt and the tweezer, we enhance a single-atom loading probability to 77.6%. Our technique offers a versatile solution for deterministic single-atom loading in various experimental settings and paves the way for diverse applications based on hybrid photonic-atom structures.
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.
We report the realization of a deterministic single-atom preparation by the method of all-optical feedback. Using a fast-real-time feedback, the light-induced atom desorption effect and blue detuned light-induced atom collision process can increase a success probability of single-atom preparation up to more than 99%. We investigate the dynamics of loading single atom trapped in a trap with a size of hundreds of micrometers into a pair of microscopic tweezers. The detailed experimental results show that the feedback loading is spatially insensitive, which implies that it is possible to use the feedback protocol to simultaneously implement the loading of large number of qubits arrays.
Using the light-assisted-collisions (LAC) and the feedback controlling loop on a quadrupole magnetic field, we have realized high probability of single atoms in the far-off-resonance trap (FORT). We analyzed the principle of LAC irradiated by a red-detuning laser or by a blue-detuning laser. And we also experimentally proved that using the red-detuned laser (the blue-detuned laser) we can realize 50% (80%) of single atom probability in the FORT. Using the feedback controlling loop, we realized 95% of single atom probability in the FORT, which opens a way for a two-dimensional FORT array. When the number of atom was zero, we decreased the gradient of the quadrupole magnetic field to quickly load atoms, and when we had more than one atom in the FORT, we switched on the blue-detuned laser to irradiate the atoms to play LAC. We measured the second-order coherence degree of the fluorescence photons emitted by the atom trapped in the FORT by using HBT scheme and found it was g(2)(τ=0)=0.08.
Single cesium atom prepared in a large-magnetic-gradient magneto-optical trap (MOT) has been efficiently loaded into a microscopic far-off-resonance optical trap (FORT, or optical tweezer), and the atom can be transferred back and forth between two traps with high efficiency. The intensity noise spectra of tweezer laser are measured and the heating mechanisms in optical tweezer are analyzed. To prolong the lifetime of single atom trapped in optical tweezer, laser cooling technique is utilized to decrease atom's kinetic energy, and the effective temperature of single atom in tweezer is estimated by the release-and-recapture (R&R) method. Thanks to laser cooling, typical lifetime of ~ 130.6 ± 1.8 s for single atom in tweezer is obtained. These works provides a good starting point for coherent manipulation of single atom.