We propose and demonstrate a scheme for strong radial confinement of a single 87 Rb atom by a bichromatic far-off resonance optical dipole trap (BFORT). The BFORT is composed of a blue-detuned Laguerre-Gaussian LG01 beam and a red-detuned Gaussian beam. The atomic oscillation frequency measurement shows that the effective trapping dimension is much sharper than that from a diffraction-limited microscopic objective. Theory shows that the added scattering rate due to imposing blue-detuned light is negligible when the temperature of the single atoms is close to ground state temperature. By carrying out sub-Doppler cooling, the mean energy of single atoms trapped in the BFORT is reduced to 15 ± 1 μK. The corresponding mean quantum number of radial vibration n is about 1.65, which satisfies the Lamb-Dicke regime. We conclude that the BFORT is a suitable Lamb-Dicke trap for further cooling a single neutral atom down to the ground state and for further application in quantum information processing.
We propose and demonstrate a scheme for strong radial confinement of a single 87 Rb atom by a bichromatic far-off resonance optical dipole trap (BFORT). The BFORT is composed of a blue-detuned Laguerre-Gaussian LG01 beam and a red-detuned Gaussian beam. The atomic oscillation frequency measurement shows that the effective trapping dimension is much sharper than that from a diffraction-limited microscopic objective. Theory shows that the added scattering rate due to imposing blue-detuned light is negligible when the temperature of the single atoms is close to ground state temperature. By carrying out sub-Doppler cooling, the mean energy of single atoms trapped in the BFORT is reduced to 15 ± 1 μK. The corresponding mean quantum number of radial vibration n is about 1.65, which satisfies the Lamb-Dicke regime. We conclude that the BFORT is a suitable Lamb-Dicke trap for further cooling a single neutral atom down to the ground state and for further application in quantum information processing.
We have demonstrated preparing and rotating single neutral rubidium atoms in an optical ring lattice generated by a spatial light modulator, inserting two atoms into a single microscopic optical potential efficiently by dynamically reshaping the optical dipole trap, trapping single atoms in a blue detuned optical bottle beam trap, and confining single atoms into the Lamb-Dicke regime by combining red and blue detuned optical potentials. In combination with the manipulation of internal states of single atoms, the study is opening a way for research in the field of quantum information processing and quantum simulation. In this paper we review the past works and discuss the prospects.
The theory of an atom dipole trap composed of a focused, far red-detuned, trapping laser beam, and a pair of red-detuned, counterpropagating, cooling beams is developed for the simplest realistic multilevel dipole interaction scheme based on a model of a (3+5)-level atom. The description of atomic motion in the trap is based on the quantum kinetic equations for the atomic density matrix and the reduced quasiclassical kinetic equation for atomic distribution function. It is shown that when the detuning of the trapping field is much larger than the detuning of the cooling field, and with low saturation, the one-photon absorption (emission) processes responsible for the trapping potential can be well separated from the two-photon processes responsible for sub-Doppler cooling atoms in the trap. Two conditions are derived that are necessary and sufficient for stable atomic trapping. The conditions show that stable atomic trapping in the optical dipole trap can be achieved when the trapping field has no effect on the two-photon cooling process and when the cooling field does not change the structure of the trapping potential but changes only the numerical value of the trapping potential well. It is concluded that the separation of the trapping and cooling processes in a pure optical dipole trap allows one to cool trapped atoms down to a minimum temperature close to the recoil temperature, keeping simultaneously a deep potential well.
It was recently shown that a single atom can efficiently scatter photons out of a focused coherent light beam [1, 2, 3]. The scattering probability is strongly dependent on a thermal motion of the atom and can be maximized if the atom is well localized at a focus. To achieve that, we implement a Raman sideband cooling technique that is commonly used in ion traps [4]. Our trap, formed by focused Gaussian light beam at 980nm, has characteristic frequencies of ντ= 55 kHz and νl= 7 kHz corresponding to transverse and longitudinal confinements. A single87Rb atom is loaded into the trap from an optical molasses. Two Raman beams couple the motional states of |F = 2〉 and |F = 1〉 manifolds with a Lamb-Dicke parameter η = 0.084 (Figure 1). The Raman beams are oriented such that momentum transfer occurs only along the strong confinement of the trap with ντ= 55 kHz. The cooling sequence consists of following steps: (1) initial preparation of the atom in |F = 2,mF= −2〉 Zeeman state, (2) Raman transfer between the motional states |F = 2,mF= −2,N〉 and |F = 1,mF= −1,N − 1〉. (3) recycling the atomic population back to |F = 2,mF= −2〉 state via an optical pulse resonant to |5P3/2, F = 2〉 state thus removing a phonon via spontaneous emission.
This thesis reports on the design, setup, and operation of a new atom trap apparatus, which is developed to confine few rubidium atoms in ultrahigh vacuum by means of taylored and dynamically variable light potentials (dipole traps), thus making the atoms available for spectroscopy and controlled manipulations. To maintain low background pressure, atoms of a vapour cell are transferred into a cold atomic beam by laser cooling techniques, and accumulated by a magneto-optic trap (MOT) in a separate part of the vacuum system. The laser cooled atoms are then transferred into dipole traps made of focused far-off-resonant laser fields in single or crossed-beam geometry, which are superimposed with the center of the MOT. Gaussian as well as hollow Laguerre-Gaussian (LG01) beam profiles are used with red-detuned and blue-detuned light. Microfabricated dielectric phase objects allow the efficient and robust mode conversion of Gaussian into Laguerre-Gaussian laser beams. Trap geometries can easily be changed due to the highly flexible experimental setup. The dipole trap laser beams have powers up to several hundred milliwatts and are focused to around 10 m. At a detuning of several tens of nanometers from the atomic resonance, the resulting traps are a few millikelvin deep, and the trapped atoms exhibit motional frequencies near 30 kHz, trap light scattering rates of a few hundred photons per second per atom, and storage times of several seconds. The number of dipole-trapped atoms ranges from more than ten thousand to below ten. The dipole-trapped atoms are detected either by a photon counting system with efficient straylight discrimination, or by recapture into the MOT, which is imaged onto a sensitive photodiode and a CCD-camera. Due to the strong AC-Stark shift imposed by the high-intensity trapping light, energy-selective resonant excitation and detection of the stored atoms is possible. The measured energy distribution is consistent with a harmonic potential shape and allows for the determination of temperatures and heating rates. From initial measurements, the thermal energy is found to be about 10 % of the trap depth. Using a crossed-beam geometry with red-detuned laser light, efficient transfer of atoms between the beams is observed. Optimum transfer occurs when the two beams cross at a radial offset, which can qualitatively be understood when the particle energy and geometrical properties of the two-beam trapping potential are considered. Numerical simulations reproduce the general features of the measured transfer efficiency vs. radial beam offset. This experiment is a first step towards dynamically variable potentials. Atoms have been radially confined in a blue-detuned hollow beam, and their decay time has been determined. This configuration is currently extended to a three-dimensionally confining blue-detuned dipole trap. For advanced laser cooling, state manipulation and spectroscopy of trapped atoms, a system of two diode lasers has been set up which are phase-coherently locked with a difference frequency near 6.834 GHz, to drive Raman transitions between the hyperfine-split ground states of 87Rb atoms. Resonances of such a transition have been recorded in a buffer gas loaded rubidium vapour cell with linewidths below 100 Hz.
We propose a laser cooling scheme that allows to cool a single atom confined in a harmonic potential to the trap ground state ∣0>. The scheme assumes strong confinement, where the oscillation frequency in the trap is larger than the effective spontaneous decay width, but is not restricted to the Lamb-Dicke limit, i.e. the size of the trap ground state can be larger than the optical wavelength. This cooling scheme may be useful in the context of quantum computations with ions and Bose-Einstein condensation.
We propose a novel three-dimensional diffraction-limited far-off-resonance optical dipole trap (DFORT) for neutral atoms operating in the Lamb–Dicke regime. Such a microscopic DFORT is generated by the diffracted LP01 mode of a hollow-core optical fiber near the fiber facet. For 87Rb and 133Cs atoms, we estimate that with a 100-mW trap-laser power, a cigar-shaped far-off-resonance optical dipole trap provides potential depth U0≃10 mK, radial (axial) trap frequency fr≃100 kHz(fz≃10 kHz), and trap volume Vtrap≃104λ3. The DFORT has the unique feature of a tightly focused trap with a large trap volume and convenient loading and cooling of the precooled atoms, which may be useful for optical Bose–Einstein condensation. A DFORT may be also operated as an elongated one-dimensional optical trap.