We present the design of a diffraction limited, long working distance monochromatic objective lens for efficient light collection. Consisting of four spherical lenses, it has a numerical aperture of 0.29, an effective focal length of 36 mm and a working distance of 36.5 mm. This inexpensive system allows us to detect 8 · 10 4 fluorescence photons per second from a single cesium atom stored in a magneto-optical trap.

Using optical dipole forces we have realized controlled transport of a single or any desired small number of neutral atoms over a distance of a centimeter with sub-micrometer precision. A standing wave dipole trap is loaded with a prescribed number of cesium atoms from a magneto-optical trap. Mutual detuning of the counterpropagating laser beams moves the interference pattern, allowing us to accelerate and stop the atoms at preselected points along the standing wave. The transportation efficiency is close to 100 %. This optical "singleatom conveyor belt" represents a versatile tool for future experiments requiring deterministic delivery of a prescribed number of atoms on demand. PACS: 32.80.Lg, 32.80.Pj, 42.50.Vk Quantum engineering of microscopic systems requires manipulation of all degrees of freedom of isolated atomic particles. The most advanced experiments are implemented with trapped chains of ions [1,2,3]. Neutral atoms are more difficult to control because of the weaker interaction of induced electric or paramagnetic dipoles with inhomogeneous electromagnetic fields. Optical dipole traps [4] could provide a level of control similar to ion traps, since they store neutral atoms in a nearly conservative potential with long coherence times [5]. The variety of different dipole traps allows for an individual design, depending on the specific experimental demands [6].Here we use a time-varying standing wave optical dipole trap to displace atoms by macroscopic distances on the order of a centimeter with sub-micrometer precision [7]. A similar technique of moving optical lattices has been applied for the acceleration of large ensembles of atoms in [8,9]. Time-dependent magnetic fields can also be used for controlled transport of clouds of neutral atoms as has been demonstrated with a micro-fabricated device [10]. Recently, techniques have been developed to load a dipole trap with single atoms only [11,12]. In our case, we have combined controlled manipulation of the trapping potential with deterministic loading of a dipole trap with a prescribed small number of atoms [7]. These atoms are then transported with high efficiency over macroscopic distances and observed by positionsensitive fluorescence detection in the dipole trap. Here we describe the transportation technique in detail. We also analyze the dependency of the measured transportation efficiency on both the transportation distance and the acceleration. This is followed by a brief discussion of possible applications and alternative approaches. The standing-wave dipole trapOur dipole trap ( Fig.1) consists of two counter-propagating Gaussian laser beams with equal intensities and optical frequencies ν 1 and ν 2 producing a position-and time-dependent dipole potentialThe optical wavelength is λ = 2π/k, wis the beam radius with waist w 0 and the Rayleigh length z 0 = πw 2 0 /λ, and ∆ν = ν 1 −ν 2 ≪ ν 1 , ν 2 is the mutual detuning of the laser beams. The laser beams have parallel linear polarization and thus produce a standing wave interference pa...

We describe a simple experimental technique which allows us to store a small and deterministic number of neutral atoms in an optical dipole trap. The desired atom number is prepared in a magneto-optical trap overlapped with a single focused Nd:YAG laser beam. Dipole trap loading efficiency of 100 % and storage times of about one minute have been achieved. We have also prepared atoms in a certain hyperfine state and demonstrated the feasibility of a state-selective detection via resonance fluorescence at the level of a few neutral atoms. A spin relaxation time of the polarized sample of 4.2 ± 0.7 s has been measured. Possible applications are briefly discussed. 32.80Pj, 42.50VkNeutral atoms can conveniently and at very low kinetic energy be stored in a magneto-optical trap (MOT) [1], not only in large quantities but also in small and exactly known numbers of up to 20 single atoms [2,3]. For several applications, for instance in cavity quantum electrodynamics [4], it is of interest to use perfectly controlled or deterministic samples of atoms for further experiments involving quantum interactions of an exactly known number of atoms. Full control of all internal and external atomic degrees of freedom is necessary in such applications, but cannot be achieved in a MOT since due to its dissipative character all degrees of freedom are intimately mixed. In order to overcome this problem one can therefore combine the operating convenience of the MOT for isolated atoms with the advantages for quantum manipulation offered by the nearly conservative potential of optical dipole traps. The interest in optical dipole traps [5] as an elegant and simple way to store laser-cooled neutral atoms has rapidly increased within the last few years [6]. Far-off-resonance optical dipole traps [7] can confine atoms in all ground states for a long time with a very small ground-state relaxation rate [8]. Cold atoms can be localized in micropotentials of a 3D periodic potential created by interfering laser beams (so-called optical lattices [9]). Such experiments always operate with at least several thousands of atoms, the atom number can not be determined exactly and is controlled only on average.In the present work we load a small and exactly known atom number into an optical dipole trap with 100% efficiency, opening up a route to a novel kind of cold atom sources free of the indeterminism intrinsic to usual sources like atom beams. We have also demonstrated the feasibility of a state-selective detection at the level of a few neutral atoms and measured a long spin relaxation time of some seconds. Together with recently demonstrated Raman sideband cooling [10] and the generation of non-classical motional states of atoms in standing-wave dipole traps [11], this system promises to be a new basis for future experiments with full control of all atomic degrees of freedom. One of the most interesting possibilities would be long-time localization of more than one atom within a mode of a high finesse cavity.The relevant part of the apparatus is shown...

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We have prepared and detected quantum coherences with long dephasing times at the level of single trapped cesium atoms. Controlled transport by an "optical conveyor belt" over macroscopic distances preserves the atomic coherence with slight reduction of coherence time. The limiting dephasing effects are experimentally identified and are of technical rather than fundamental nature. We present an analytical model of the reversible and irreversible dephasing mechanisms. Coherent quantum bit operations along with quantum state transport open the route towards a "quantum shift register" of individual neutral atoms.PACS numbers: 32.80. Lg, 32.80.Pj, 42.50.Vk Obtaining full control of all internal and external degrees of freedom of individual microscopic particles is the goal of intense experimental efforts. The long lived internal states of ions and neutral atoms are excellent candidates for quantum bits (qubits), in which information is stored in a coherent superposition of two quantum states. Once sufficient control of individual particles is established, one of the most attractive perspectives is the engineered construction of quantum systems of two or more particles. Their coherent interaction is a key element for the realization of quantum gates and can be implemented by transporting selected qubits into an interaction zone [1]. In this context, ions have successfully been transported between distinct locations while maintaining internal-state coherence [2].In this letter we report on the coherence properties and on a quantum state transportation of neutral atoms in a standing wave dipole trap. We present a scheme of preparation and detection of the electronic hyperfine ground states, which has been applied to ensembles as well as to single atoms. Along with the demonstration of 1-qubit-rotations, our system is a promising candidate for storing quantum information, since the hyperfine ground states exhibit long (∼200 ms) coherence times. In particular, the coherence even persists while transporting the atoms over macroscopic distances. This opens a route towards quantum gates via cavity-mediated atom-atom coupling.We trap cesium atoms in a standing wave dipole trap (λ = 1064 nm) with a potential depth of U 0 = 1 mK, loaded from a high-gradient magneto-optical trap (MOT) [3,4]. The single-atom transfer efficiency between the two traps is better than 95% [5]. The MOT is also used to determine the exact number of trapped atoms by observing their fluorescence. By shifting the standing wave pattern along the direction of beam propagation, we can transport the atoms over millimeter-scale distances. This is realized by mutually detuning the frequencies of the dipole trap laser beams with acoustooptical modulators. Additionally, we use microwave radiation at ω hfs /2π = 9.2 GHz to coherently drive the | F = 4, m F = 0 → | F = 3, m F = 0 clock transition of the 6 2 S 1/2 ground state with Rabi frequencies of Ω/2π = 10 kHz.The initial state is prepared by optically pumping the atom into | F = 4, m F = 0 . For this purpo...