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Optical control of single neutral atoms

Wolfgang Alt

Published Jan 1, 2004• 10 citations
OpenAlex

Abstract

This thesis presents experiments concerning the preparation and manipulation of single neutral atoms in optical traps. The experimental setup as well as the properties of the optical dipole trap are described. The examination and control of all trapping parameters and heating effects is a prerequisite for the future realization of quantum gates. A magneto-optical trap captures and cools down a few cesium atoms. By efficiently detecting their fluorescence, we are able to determine their exact number. They are then transferred without loss into a standing-wave optical dipole trap. The temperature of the atoms in this trap is measured using two methods which are devised to work with small numbers of atoms: By adiabatically lowering the trap depth, an energy-selective loss of atoms is obtained, which yields the energy distribution of the atoms in the trap. Alternatively, the temperature is inferred from the size of the cloud and from the oscillation frequencies of the atoms in the dipole trap. The oscillation frequencies are determined using resonant and parametric excitation. Various intrinsic as well as technical heating mechanisms in the dipole trap are examined experimentally and theoretically. Finally, the setup of a miniature ultra-high finesse optical resonator is presented, which will be used to couple two trapped atoms via the exchange of a photon. The optical resonance frequency of the resonator is successfully stabilized to the atomic transition by an electronic servo loop.

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Single atoms in optical traps

S. Bergamini, Benoît Darquié, Antoine Browaeys +1 more

We present a brief overview of optical trapping experiments of individual neutral atoms. Then we describe in more details an experiment using a very small optical dipole trap, that is designed to store and manipulate individual atoms. Due to the very small dipole trap volume, a "collisional blockade" mechanism locks the average number of trapped atoms on the value 0.5 over a large range of loading rates. We study this regime experimentally, and we describe methods to measure the oscillation frequencies and the temperature of a single atom in the trap with a high accuracy.

Journal de Physique IV (Proceedings)•Oct 1, 2004•1 citation
This thesis presents experiments concerning the preparation and manipulation of single neutral atoms in optical traps. The experimental setup as well as the properties of the optical dipole trap are described. The examination and control of all trapping parameters and heating effects is a prerequisite for the future realization of quantum gates. A magneto-optical trap captures and cools down a few cesium atoms. By efficiently detecting their fluorescence, we are able to determine their exact number. They are then transferred without loss into a standing-wave optical dipole trap. The temperature of the atoms in this trap is measured using two methods which are devised to work with small numbers of atoms: By adiabatically lowering the trap depth, an energy-selective loss of atoms is obtained, which yields the energy distribution of the atoms in the trap. Alternatively, the temperature is inferred from the size of the cloud and from the oscillation frequencies of the atoms in the dipole trap. The oscillation frequencies are determined using resonant and parametric excitation. Various intrinsic as well as technical heating mechanisms in the dipole trap are examined experimentally and theoretically. Finally, the setup of a miniature ultra-high finesse optical resonator is presented, which will be used to couple two trapped atoms via the exchange of a photon. The optical resonance frequency of the resonator is successfully stabilized to the atomic transition by an electronic servo loop.
Cavity quantum electrodynamics (cavity QED) describes electromagnetic fields in a confined space and the radiative properties of atoms in such fields. The simplest example of such system is a single atom interacting with one mode of a high-finesse resonator. Besides observation and exploration of fundamental quantum mechanical effects, this system bears a high potential for applications quantum information science such as, e.g., quantum logic gates, quantum communication and quantum teleportation. In this thesis I present an experiment on the deterministic coupling of a single neutral atom to the mode of a high-finesse optical resonator. In Chapter 1 I describe our basic techniques for trapping and observing single cesium atoms. As a source of single atoms we use a high-gradient magneto-optical trap, which captures the atoms from background gas in a vacuum chamber and cools them down to millikelvin temperatures. The atoms are then transferred without loss into a standing-wave dipole trap, which provides a conservative potential required for experiments on atomic coherence such as quantum information processing and metrology on trapped atoms. Moreover, shifting the standing-wave pattern allows us to deterministically transport the atoms (Chapter 2). In combination with non-destructive fluorescence imaging of individual trapped atoms, this enables us to control their position with submicrometer precision over several millimeters along the dipole trap. The cavity QED system can distinctly display quantum behaviour in the so-called strong coupling regime, i.e., when the coherent atom-cavity coupling rate dominates dissipation in the system. This sets the main requirements on the resonator's properties: small mode volume and high finesse. Chapter 3 is devoted to the manufacturing, assembling, and testing of an ultra-high finesse optical Fabry-Perot resonator, stabilized to the atomic transition. In Chapter 4 I present the transportation of single atoms into the cavity and their coupling to the cavity mode. The strong coupling manifests itself in a strong reduction of the cavity transmission probed by a weak external laser. The atoms remain trapped and coupled to the cavity mode for several seconds until we move them out of the cavity for final analysis of their number and position.
This thesis presents novel techniques to realize controllable quantum systems of neutral atoms. Besides the preparation of the exact number of atoms, we manipulate all physical degrees of freedom of the trapped particles. The first part (Chapters 2, 3) reports on a deterministic source of single atoms, which overcomes the limitations imposed by statistical arrival in conventional sources. Single cold cesium atoms prepared in a magneto-optical trap are transferred into a standing wave optical dipole trap, made of two counter-propagating red-detuned Nd:YAG laser beams. Mutual detuning of the laser beam frequencies moves the standing wave pattern, allowing us to accelerate and stop an atom at preselected points along the standing wave. This "optical conveyor belt" can transport one atom over a maximum distance of 10~mm. The second part (Chapters 4, 5) reports on the investigation of the coherence times of the Cs hyperfine ground states using microwave transitions. Using Ramsey spectroscopy techniques we measured coherence times of up to 100 ms. 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. Finally, we prove that controlled transport by the optical conveyor belt over macroscopic distances preserves the atomic coherence with slight reduction of coherence time.
Single atoms are a promising candidate for the creation of a scalable quantum computer. Neutral atoms trapped by optical fields are especially encouraging due to the full controllability of the trap potential. Single neutral atoms trapped within a fibre-fibre cavity allow us to strongly couple atoms and photons. This creates an efficient atom-photon interface which can compensate for the short state transfer distance inherent of neutral atoms. A spatial light modulator allows for the creation and manipulation of an array of these single atom traps.\n\nIn this research we create a setup to cool and trap neutral rubidium-87 atoms by a combination of a magneto-optical trap (MOT) and a dipole trap. The dipole trap uses a strongly focused, far red-detuned laser, which enables us to trap single atoms via the collisional blockade regime. These traps are realised within a novel glass cell, allowing for great optical access.\n\nThe MOT in this setup is able to trap (4.27 +/- 0.10) * 10^3 atoms with a density of (7.89 +/- 0.56) * 10^9 atoms cm^-3 . The MOT temperature is in the order of 100 μK. We also show the manipulation of these properties, allowing us to tailor the MOT. The dipole trap is created with a trap waist of 37 μm, allowing us to trap a cloud of atoms. With the dipole trap we achieve a lifetime of atoms of (34.8 +/- 2.0) ms.
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 present a brief overview of optical trapping experiments of individual neutral atoms. Then we describe in more details an experiment using a very small optical dipole trap, that is designed to store and manipulate individual atoms. Due to the very small dipole trap volume, a "collisional blockade" mechanism locks the average number of trapped atoms on the value 0.5 over a large range of loading rates. We study this regime experimentally, and we describe methods to measure the oscillation frequencies and the temperature of a single atom in the trap with a high accuracy.