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Near-deterministic preparation of single atoms and Raman sideband cooling

Y. H. Fung

Published Jan 1, 2016• 0 citations
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Abstract

In this thesis, we present our methods and results towards the development of deterministic preparation of single neutral atoms with a known quantum state in a single trap potential. Firstly, we experimentally study the dynamics of two atoms that undergo light-assisted collisions, where we can quantify different two-atom loss channels. We find that a combination of repulsive light-assisted collisions and in-trap laser cooling can result in only one of the two colliding atoms escaping the trap with near unity probability. This forms the essential basis in our single atom preparation method. With optimized repulsive light-assisted collisions and laser cooling parameters, we can prepare single neutral atoms with a preparation efficiency of 91%. When similar methods are applied in collisional blockade loading of single atoms in a tight trap, the preparation efficiency can be enhanced from the previously observed limit of around 50% to 80%. In a separate experiment, we also demonstrate an in-trap fluorescence detection method to count the number of atoms in a high density trap from 1 to more than 100 atoms with sub-Poissonian precision. Finally, we perform Raman sideband cooling on the prepared single atom with a single pair of Raman transition beams that couples both of the radial vibrational dimensions of the trapped atom. We show that the atom occupies the vibrational ground state in the two radial dimensions with a probability of 84%. This gives a 69% fidelity for the preparation of a pure atomic quantum state for a 2-D system.

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In this thesis, we present our methods and results towards the development of deterministic preparation of single neutral atoms with a known quantum state in a single trap potential. Firstly, we experimentally study the dynamics of two atoms that undergo light-assisted collisions, where we can quantify different two-atom loss channels. We find that a combination of repulsive light-assisted collisions and in-trap laser cooling can result in only one of the two colliding atoms escaping the trap with near unity probability. This forms the essential basis in our single atom preparation method. With optimized repulsive light-assisted collisions and laser cooling parameters, we can prepare single neutral atoms with a preparation efficiency of 91%. When similar methods are applied in collisional blockade loading of single atoms in a tight trap, the preparation efficiency can be enhanced from the previously observed limit of around 50% to 80%. In a separate experiment, we also demonstrate an in-trap fluorescence detection method to count the number of atoms in a high density trap from 1 to more than 100 atoms with sub-Poissonian precision. Finally, we perform Raman sideband cooling on the prepared single atom with a single pair of Raman transition beams that couples both of the radial vibrational dimensions of the trapped atom. We show that the atom occupies the vibrational ground state in the two radial dimensions with a probability of 84%. This gives a 69% fidelity for the preparation of a pure atomic quantum state for a 2-D system.
In this thesis we study: the dynamics leading to a near-deterministic preparation of single atoms in a microscopic optical dipole trap; the non destructive imaging and counting of trapped atoms; and a system for sorting single atoms between optical dipole traps using a high numerical aperture lens and a spatial light modulator. \n \nWe near-deterministically prepare trapped single Rubidium atoms by exposing them to a tailored light field which induces light assisted collisions. By preparing pairs of Rubidium atoms in an optical micro-trap we can study the outcome of light assisted collisions at the single event level. Using this knowledge we tailor the light such that each collision releases only enough energy for one atom of a colliding atom-pair to escape the trap. This thereby yields an 83% single atom loading efficiency. \n \n \nWe demonstrate a method to count small numbers of atoms held in a deep, microscopic optical dipole trap by collecting fluorescence from atoms exposed to a standing wave of light that is blue-detuned from resonance. While scattering photons, the atoms are cooled by a Sisyphus mechanism that results from the spatial variation in light intensity. The use of a small blue detuning limits the losses due to light assisted collisions, thereby making the method suitable for counting several atoms in a microscopic volume. \n \nWe create two overlapping one-dimensional optical lattices using a single laser beam, a spatial light modulator and a high numerical aperture lens. These lattices have the potential to trap single atoms, and using the dynamic capabilities of the spatial light modulator may shift and sort atoms to a minimum atom-atom separation of 1.52 µm. We show how a simple feedback circuit can compensate for the spatial light modulator's intensity modulation.
Optical lattices induced by light detuned far from the frequency of any atomic resonance transition are ideal systems in which to develop techniques for the coherent control of atomic motional and internal states. Decoherence arising from spontaneous emission can be eliminated to an arbitrary degree by varying the detuning of the optical field. The atoms trapped in these lattices are to a large extent isolated from environmental disturbances and from each other. The first step towards the coherent control of atoms in a far-detuned lattice is their preparation in a single motional state. In this thesis the preparation of atoms in the ground vibrational state of a two-dimensional far-detuned optical lattice via resolved-sideband Raman cooling is presented. This sideband-cooling scheme involves stimulated Raman transitions between bound vibrational states of a pair of magnetic ground state sublevels, followed by an irreversible step due to optical pumping, resulting in a net loss of one quantum of vibrational energy per cooling cycle. This process provides efficient cooling in two-dimensions and leads to the accumulation of a large fraction of atoms in the 2-D ground vibrational state of a potential well associated with a single Zeeman substate. Experiments aimed at improving and monitoring the characteristics of the lar-detuned lattice and the sideband-Raman cooling efficiency are also described. Parametric excitation experiments and modelling are employed to investigate the degree of anharmonicity of the optical potential and the importance of heating induced by laser intensity noise, whilst Zeeman-state analysis of the sample is performed for monitoring the distribution of atoms over different magnetic sub- states. Finally, spin-polarization experiments are carried out in order to study the paramagnetic properties of the lattice and lead to an evaluation of a spin-temperature for the sample.
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.
We combine near-deterministic preparation of a single atom with Raman sideband cooling, to create a push-button mechanism to prepare a single atom in the motional ground state of tightly focused optical tweezers. In the two-dimensional (2D) radial plane, we achieve a large ground-state fidelity for the entire procedure (loading and cooling) of $\ensuremath{\sim}0.73$, while the ground-state occupancy is $\ensuremath{\sim}0.88$ for realizations with a single atom present. For 1D axial cooling, we attain a ground-state fraction of $\ensuremath{\sim}0.52$. The combined 3D cooling provides a ground-state population of $\ensuremath{\sim}0.11$. Our Raman sideband cooling variation is indifferent to magnetic field fluctuations, allowing widespread unshielded experimental implementations. Our work provides a pathway towards a range of coherent few-body experiments.
This dissertation reports on quantum state preparation of cesium atoms in a two-dimensional optical lattice, by resolved-sideband Raman cooling. An optical lattice is a periodic potential produced by the light shift interaction between an atom and light field. Laser cooled atoms can become strongly localized about the bottom of potential wells in an optical lattice, where they occupy a discrete spectrum of bound vibrational energy levels. The distribution over vibrational levels of atoms in the lattice is characterized by the mean vibrational excitation, n . In an optical lattice, absorption and emission of photons from lattice beams causes n to increase in time. This source of heating is always present, but its rate can be greatly reduced in a lattice detuned far from the atomic resonance. Sideband cooling is an efficient means of transferring atoms from higher into lower-lying vibrational levels and, thus, it reduces n for the ensemble. If the sideband cooling rate is much greater than the heating rate, then n approaches zero and virtually all atoms are in the lowest vibrational level in their potential wells. Our sideband cooling scheme involves stimulated Raman transitions between bound states in the potential wells of a pair of magnetic sublevels, followed by optical pumping, for a net loss of one quantum of vibration per cooling cycle. The process accumulates 98% of atoms in the ground vibrational level of a potential well associated with a single Zeeman substate. Each atom in the lattice is then very close to a pure state. For two-dimensional lattice with sideband cooling we find nx≈ny≈0.008 &parl0;16&parr0; . Various issues related to state preparation and sideband cooling are also discussed in the context of a one dimensional lin ⊥ lin optical lattice. These include improvement of laser cooling in a near resonance lattice by application of weak magnetic fields, transfer of atoms from near into far off-resonance lattices, and heating rates in far off-resonance lattices.
Summary form only given. Complete control of individual atoms trapped in far-off resonance optical tweezers is vital for gaining a better understanding of the microscopic world. It will provide a platform with unprecedented flexibility for studying few-body physics, and might lead to new quantum technologies. By combining near-deterministic preparation of single atoms [1-3] with Zeeman-insensitive Raman sideband cooling [4], we present a push button method to prepare a single 85Rb atom in its centre of mass ground state with high fidelity. Our Raman sideband cooling scheme works efficiently in an environment with magnetic field fluctuations that were too large for other Raman sideband cooling variations [5,6]. In 2D, we achieve a fidelity for the entire loading procedure of ~0.7 for a single atom in the ground state of the optical tweezers. In 3D, the fidelity drops to 0.1 since the cooling is less efficient for the weakly confined tweezer axis.