The present Diploma thesis was motivated by the need for a versatile radio frequency source to be used in experiments with cold atoms. The working principles and the design of said source are presented including its characterization and an example of its evaluation in an experiment. The desired experimental uses impose a set of strict requirements on the design such as highly stable output phase and frequency as well as reproducible and precisely controllable profiles of phase, frequency and output power. The device developed in this thesis features output frequencies from 5MHz up to 500MHz, phasecontinuous operation, fast linear frequency sweeps with very small frequency jumps, phase synchronized output of two frequencies and arbitrary sweep profiles with update rates of up to 8 kHz. All parameter and profile controls can be are accessed and adjusted using a graphical user interface that is integrated into the experiment control framework. Validating the features experimentally, RubidiumBose-Einstein condensates were transported over distances of a few centimeters in a moving interference pattern of two counterpropagating laser beams. The frequency difference of the laser beams was controlled by the device, permitting precise and reproducible positioning of the atoms. Further possible applications include generation of radio frequency sweeps to drive Landau-Zener transitions or generation of Ramsey type pulses. The required precise control of timing and phase of multiple pulses is achievable with the present design approach.
The present Diploma thesis was motivated by the need for a versatile radio frequency source to be used in experiments with cold atoms. The working principles and the design of said source are presented including its characterization and an example of its evaluation in an experiment. The desired experimental uses impose a set of strict requirements on the design such as highly stable output phase and frequency as well as reproducible and precisely controllable profiles of phase, frequency and output power. The device developed in this thesis features output frequencies from 5MHz up to 500MHz, phasecontinuous operation, fast linear frequency sweeps with very small frequency jumps, phase synchronized output of two frequencies and arbitrary sweep profiles with update rates of up to 8 kHz. All parameter and profile controls can be are accessed and adjusted using a graphical user interface that is integrated into the experiment control framework. Validating the features experimentally, RubidiumBose-Einstein condensates were transported over distances of a few centimeters in a moving interference pattern of two counterpropagating laser beams. The frequency difference of the laser beams was controlled by the device, permitting precise and reproducible positioning of the atoms. Further possible applications include generation of radio frequency sweeps to drive Landau-Zener transitions or generation of Ramsey type pulses. The required precise control of timing and phase of multiple pulses is achievable with the present design approach.
A radio-frequency (RF) source designed for cold atom experiments is presented. The source uses AD9858, a direct digital synthesizer, to generate the sine wave directly, up to 400 MHz, with sub-Hz resolution. An amplitude control circuit consisting of wideband variable gain amplifier and high speed digital to analog converter is integrated into the source, capable of 70 dB off isolation and 4 ns on-off keying. A field programmable gate array is used to implement a versatile frequency and amplitude co-sweep logic. Owing to modular design, the RF sources have been used on many cold atom experiments to generate various complicated RF sequences, enriching the operation schemes of cold atoms, which cannot be done by standard RF source instruments.
Cold-atom interferometers have matured into a powerful tool for fundamental physics research, and they are currently moving from realizations in the laboratory to applications in the field. A radio frequency (RF) generator is an indispensable component of these devices for controlling lasers and manipulating atoms. In this work, we developed a compact RF generator for fast switching and sweeping the frequencies and amplitudes of atomic-interference pulse sequences. In this generator, multi-channel RF signals are generated using a field-programmable gate array (FPGA) to control eight direct digital synthesizers (DDSs). We further propose and demonstrate a method for pre-loading the parameters of all the RF pulse sequences to the DDS registers before their execution, which eliminates the need for data transfer between the FPGA and DDSs to change RF signals. This sharply decreases the frequency-switching time when the pulse sequences are running. Performance characterization showed that the generated RF signals achieve a 100 ns frequency-switching time and a 40 dB harmonic-rejection ratio. The generated RF pulse sequences were applied to a cold-atom-interferometer gyroscope, and the contrast of atomic interference fringes was found to reach 38%. This compact multi-channel generator with fast frequency/amplitude switching and/or sweeping capability will be beneficial for applications in field-portable atom interferometers.
The generation and manipulation of ultracold atomic ensembles in the quantum regime require the application of dynamically controllable microwave fields with ultra-low noise performance. Here, we present a low-phase-noise microwave source with two independently controllable output paths. Both paths generate frequencies in the range of 6.835 GHz ± 25 MHz for hyperfine transitions in 87Rb. The presented microwave source combines two commercially available frequency synthesizers: an ultra-low-noise oscillator at 7 GHz and a direct digital synthesizer for radio frequencies. We demonstrate a low integrated phase noise of 480 µrad in the range of 10 Hz to 100 kHz and fast updates of frequency, amplitude, and phase in sub-µs time scales. The highly dynamic control enables the generation of shaped pulse forms and the deployment of composite pulses to suppress the influence of various noise sources.
We present a radio frequency source based on the AD9914 direct digital synthesizer chip, with a frequency range up to 1.4 GHz. The phase noise at an output frequency of 1 GHz is lower than −110 dBc/Hz at a 10 kHz offset. Through an optimized design of a field-programmable gate array, we have achieved the theoretical maximum modulation rate of AD9914 and developed multiple functional modules for cold atom experiments to implement modulation of frequency, phase, and amplitude. The modulation update rate reaches 146 MHz. The phase modulation response latency is 158 ns, and the amplitude modulation response latency is 95.1 ns. We have applied the RF source to a sideband Pound–Drever–Hall locking experiment and achieved frequency stabilization, demonstrating its suitability for cold atom experiments that require precise and complex laser control.
The ongoing drive to transfer quantum technologies from research laboratories to practical, portable devices requires the development of multiple key enabling technologies. The field of cold atoms, which includes high-precision metrology and quantum simulation, has various real-world applications including time-keeping and navigation. A crucial component in these systems is a cold-atom source that generates a flux of low-velocity atoms suitable for subsequent capture. A large flux enables fast repetition rates, high duty cycles, long measurement times, and resistance to long-term performance degradation with a good signal-to-noise ratio, all of which are essential to produce cold-atom devices and technologies that are capable of out-performing the classical alternatives. This thesis presents the design, fabrication and characterisation of a novel cold-atom source using a compact pyramidal magneto-optical trap (MOT) with a unique adjustable-aperture mechanism. This allowed an investigation of the dependence of the flux characteristics on the aperture size, a parameter space that has not previously been explored. The measurements showed a strong dependence of the flux on the aperture size, with a negligible flux below 0.4mm, then an approximately linear increase up to 1.0mm, at which point the relationship showed possible signs of saturation. Using 87Rb, a flux of 1.18×1010 atoms/s was produced using a 1.27mm aperture, for which the mean velocity of the atoms was 21.6(3) m/s with a FWHM of 11.2(2) m/s, and the FWHM divergence of the beam was 86(2) mrad. This is the highest recorded flux from a pyramid or 2D+-MOT source, and amongst the highest reported fluxes from any MOT sources, and avenues for improvement still remain. This work fulfils the aim of demonstrating a compact and robust cold-atom source generating a high flux of laser-cooled atoms for real-world quantum technology applications.
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.