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A Digital Frequency Source for Movement of Ultracold Atoms by Acousto-Optic Deflection

Thomas McKellar

Published Jan 1, 2014• 1 citation
OpenAlex

Abstract

This thesis documents the development of a frequency source based on a field pro-grammable gate array (FPGA) and a direct digital synthesiser (DDS). The source was used to drive the two inputs of a dual-axis acousto-optic deflector (AOD), an integral component in a steerable optical tweezer unit designed for use with ultracold 87Rb and 40K. By synchronously changing the frequencies of the paired inputs to the AOD, an optical beam was able to be moved in two dimensions. The frequency source was used to trap and smoothly move samples of ultracold 87Rb over several millimetres, and by quickly toggling the inputs between multiple pairs of frequencies, multiple clouds were confined and moved in time-averaged potentials. The utility of the FPGA/DDS unit is shown, most notably by simultaneously evaporating four independent clouds through the Bose-Einstein Condensate (BEC) phase transition. ii Acknowledgements I’ve been very fortunate to have had such profound role models during my time in the

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This thesis documents the development of a frequency source based on a field pro-grammable gate array (FPGA) and a direct digital synthesiser (DDS). The source was used to drive the two inputs of a dual-axis acousto-optic deflector (AOD), an integral component in a steerable optical tweezer unit designed for use with ultracold 87Rb and 40K. By synchronously changing the frequencies of the paired inputs to the AOD, an optical beam was able to be moved in two dimensions. The frequency source was used to trap and smoothly move samples of ultracold 87Rb over several millimetres, and by quickly toggling the inputs between multiple pairs of frequencies, multiple clouds were confined and moved in time-averaged potentials. The utility of the FPGA/DDS unit is shown, most notably by simultaneously evaporating four independent clouds through the Bose-Einstein Condensate (BEC) phase transition. ii Acknowledgements I’ve been very fortunate to have had such profound role models during my time in the
We report on the implementation of an optical tweezer system for controlled transport of ultracold atoms along a narrow, static confinement channel. The tweezer system is based on high-efficiency acousto-optic deflectors and offers two-dimensional control over beam position. This opens up the possibility for tracking the transport channel when shuttling atomic clouds along it, forestalling atom spilling. Multiple clouds can be tracked independently by time-shared tweezer beams addressing individual sites in the channel. The deflectors are controlled using a multichannel direct digital synthesizer, which receives instructions on a submicrosecond time scale from a field-programmable gate array. Using the tweezer system, we demonstrate sequential binary splitting of an ultracold 87Rb cloud into 2(5) clouds.
This thesis describes the construction of a dual species setup using fermionic 40K and bosonic 87Rb with the goal of cooling to ultracold temperatures and quantum degeneracy, and also perfoming precision collision experiments using a novel optical collider. The setup is composed of a dual species magneto-optical trap, with transfer to an ultrahigh vacuum science cell achieved using a mechanical transport scheme. Atoms are cooled to ultracold temperatures using forced radio frequency evaporation in a Ioffe-Pritchard style magnetic trap. The optical collider utilises the optical dipole force to trap a sample of ultracold atoms and accelerate them towards a target sample using an acousto-optic modulator. The process of cooling 87Rb atoms in the |F = 2, mF = 2> hyperfine state to Bose-Einstein condensation is described, as well as progress towards production of ultracold 40K. Design, implementation, and characterisation of the optical collider is provided, showing that it is a precision metrology device.
Acousto-optic deflectors (AOD) enable spatiotemporal control of laser beams through diffraction at an ultrasonic grating that is controllable by radio-frequency (rf) waveforms. These devices are a widely used tool for high-bandwidth random-access scanning applications, such as optical tweezers in quantum technology. A single AOD can generate multiple optical tweezers by multitone rf input in one dimension. Two-dimensional (2D) patterns can be realized with two perpendicularly oriented AODs. As the acousto-optical response depends nonlinearly on the applied frequency components, phases, and amplitudes, and in addition experiences dimensional coupling in 2D setups, intensity regulation becomes a unique challenge. Guided by coupled-wave theory and experimental observations, we derive a compute-efficient model which we implement on a graphics processing unit. Only one-time sampling of single-tone laser-power calibration is needed for model parameter determination, allowing for straight-forward integration into optical instruments. We implement and experimentally validate an open-loop diffraction efficiency control system that enables programmable 2D multibeam trajectories with intensity control applied at every time step during digital signal generation, overcoming the limited flexibility, pattern-size constraints, and bandwidth limitations of methods using precalculation and precalibration of a predefined pattern set or closed-loop feedback. The system is capable of stable real-time waveform streaming of arrays with up to 50 x 50 tweezers with minimal time resolution of 1.4 ns (700 MS/s) and a peak latency below 257 microseconds for execution of newly requested patterns. Reactive, real-time 2D multibeam laser patterning and scanning with strict intensity matching will substantially benefit parallelization and increasing data rates in materials processing, microscopy, and optical tweezers.
Reconfigurable arrays of neutral atoms are a leading platform for quantum computing, quantum simulation, and quantum metrology. The most common method for atom reconfiguration using optical tweezers relies on frequency chirping of acousto-optic deflectors (AODs). However, chirp-induced acoustic lensing limits the speed of atom transport by deformation of the tweezer profile and warping of the tweezer trajectory. We use a three-dimensional acousto-optic deflector lens (3D-AODL) to mitigate both effects, a design predicted to halve current state-of-the-art long-range transport times. Additionally, we introduce fading-Shepard waveforms that bypass the finite AOD bandwidth and thus enable sustained axial displacement. We demonstrate unrestricted 3D motion within a cuboid volume of at least 200 μm × 200 μm × 136 μm, with tweezer velocities exceeding 4.2 m/s. The ability to move optical tweezers along arbitrary trajectories in 3D should enable rapid in-plane and out-of-plane rearrangement of atoms in 2D or 3D tweezer arrays and optical lattices, as well as omnidirectional trajectories and dynamical engineering of optical potentials. This technology has the potential to advance quantum control and atom manipulation in current atom-array quantum computers, boosting clock rates and enabling rapid sorting in geometries scalable to millions of qubits.
This thesis describes the design, construction, and testing of a radio frequency signal source for generating rapidly changing frequencies in a repeatable and phase continuous manner. The radio frequency (RF) signal is generated by a Direct Digital Synthesizer (DDS) which is controlled by a triggerable microprocessor unit which is itself remotely programmed through an Ethernet connection. Waveforms generated by the RF source can be used for direct RF evaporation or RF manipulation of magnetic trapping potentials for ultra-cold atoms on an atom chip. The ultimate objective of the project is to mix the RF signal with higher frequency carrier waves to generate microwaves for the manipulation of hyperfine states of ultra-cold atoms of potassium and rubidium.