The paper presents the results of the development of a laser-optical system for atomic interferometer based on rubidium atoms cooled to sub-Doppler temperatures. The laser-optical scheme was created to ensure the processes of cooling, pumping, and detecting atoms involved in interferometric measurements of free-fall acceleration. The system utilizes frequency doubled fiber lasers and broadband fiber electro-optical modulators. The choice of fiber lasers is driven by their high efficiency, narrow spectral line, low phase noise levels, as well as ease of operation and reliability. Laser frequency stabilization was carried out using modulation transfer spectroscopy and optical phase-locking methods. Experimental setups were described that allow the generation of multichromatic radiation and minimize spontaneous scattering by detuning from excited levels. It was noted that the design provides a full set of optical frequencies necessary for Raman spectroscopy. The amplitudes of frequency fluctuations of the cooling and pumping lasers were studied. It was shown that the implemented laser-optical system can ensure continuous operation of an atomic interferometer based on clouds of cold rubidium atoms, and an assessment of the fundamental sensitivity limit was conducted.
The paper presents the results of the development of a laser-optical system for atomic interferometer based on rubidium atoms cooled to sub-Doppler temperatures. The laser-optical scheme was created to ensure the processes of cooling, pumping, and detecting atoms involved in interferometric measurements of free-fall acceleration. The system utilizes frequency doubled fiber lasers and broadband fiber electro-optical modulators. The choice of fiber lasers is driven by their high efficiency, narrow spectral line, low phase noise levels, as well as ease of operation and reliability. Laser frequency stabilization was carried out using modulation transfer spectroscopy and optical phase-locking methods. Experimental setups were described that allow the generation of multichromatic radiation and minimize spontaneous scattering by detuning from excited levels. It was noted that the design provides a full set of optical frequencies necessary for Raman spectroscopy. The amplitudes of frequency fluctuations of the cooling and pumping lasers were studied. It was shown that the implemented laser-optical system can ensure continuous operation of an atomic interferometer based on clouds of cold rubidium atoms, and an assessment of the fundamental sensitivity limit was conducted.
The free-fall atomic clock has shown considerable potential for long-term stability and mobility, yet reports on its design and implementation remain limited. Herein, we introduce a simple integrated optical system for free-fall cold Rb atomic clocks. A distributed feedback (DFB), frequency-doubled fiber laser, and a 780 nm DFB laser diode are utilized as the cooling and repumping light source, respectively. Both lasers feature a broad mode-hop-free tuning range and reduced sensitivity to acoustic noise. Benefiting from a high signal-to-noise ratio of modulation transfer spectroscopy (MTS), the linewidth of the fiber laser is measured to be 42 kHz. The power and frequency management of the cooling beams is implemented by two 110-MHz acousto-optic modulators (AOMs), enabling a frequency detuning as large as −92 MHz. The power-frequency management module is integrated into a single side bench of 316 mm × 270 mm × 52 mm. The power fluctuation of the probe beam is measured to be 0.8% (RMS) over 24 hours measurement. Using an ultra-high vacuum system, the functionality of the optical system is preliminarily verified by the time-of-flight (TOF) signal of the cold atom cloud.
Atom interferometry has hugely benefitted from advances made in cold atom physics over the past twenty years, and ultra-precise quantum sensors are now available for a wide range of applications [1]. In particular, cold atom interferometers have shown excellent performances in the field of acceleration and rotation measurements [2,3], and are foreseen as promising candidates for navigation, geophysics, geo-prospecting and tests of fundamental physics such as the Universality of Free Fall (UFF). In order to carry out a test of the UFF with atoms as test masses, one needs to compare precisely the accelerations of two atoms with different masses as they fall in the Earth’s gravitational field. The sensitivity of atom interferometers scales like the square of the time during which the atoms are in free fall, and on ground this interrogation time is limited by the size of the experimental setup to a fraction of a second. Sending an atom interferometer in space would allow for several seconds of excellent free-fall conditions, and tests of the UFF could be carried out with precisions as low as 10-15 [4]. However, cold atoms experiments rely on complex laser systems, which are needed to cool down and manipulate the atoms, and these systems are usually very sensitive to temperature fluctuations and vibrations. In addition, when operating an inertial sensor, vibrations are a major issue, as they deteriorate the performances of the instrument. This is why cold atom interferometers are usually used in ground based facilities, which provide stable enough environments. In order to carry out airborne or space-borne measurements, one has to design an instrument which is both compact and stable, and such that vibrations induced by the platform will not deteriorate the sensitivity of the sensor. We report on the operation of an atom interferometer on board a plane carrying out parabolic flights (Airbus A300 Zero-G, operated by Novespace). We have constructed a compact and stable laser setup, which is well suited for onboard applications. Our goal is to implement a dual-species Rb-K atom interferometer in order to carry out a test of the UFF in the plane. In this perspective, we are designing a dual-wavelength laser source, which will enable us to cool down and coherently manipulate the quantum states of both atoms. We have successfully tested a preliminary version of the source and obtained a double species magneto-optical trap (MOT).
The laser cooling of atoms to ultracold temperatures has propelled many groundbreaking advances in fundamental research and precision measurement, through such applications as quantum simulators and interferometric sensors. Laser cooling remains, however, highly species-selective, and techniques for its application to molecules are still in their infancy. This thesis broadly concerns the development of laser cooling schemes, based on sequences of coherent optical pulses, which can in principle be applied to a wide range of species. We describe a cooling scheme, in which a velocity-selective impulse analogous to that in Doppler cooling is generated by a light-pulse Ramsey interferometer, and present a proof-of-concept demonstration of the scheme using ultracold rubidium-85 atoms as a test-bed. We realise an interferometer for the atoms, as they are in free-fall after release from a magneto-optical trap, by inducing stimulated Raman transitions between their ground hyperfine states. We provide a comprehensive characterisation of these Raman light-pulse interferometer optics, where particular attention is paid to light shift effects. Raman pulses, and indeed coherent operations in any quantum control system, unavoidably suffer from systematic errors in the control field intensity and frequency, and these lead to reductions in pulse fidelity and readout contrast. In parallel to the work on interferometric cooling in this thesis, we report our preliminary investigations into composite pulses, whereby 'naive' single pulses are replaced by sequences of rotations with tailored durations and phases, for improving pulse fidelity in the presence of inhomogeneities. We find that composite pulses can indeed be highly effective in our cold atom system, and propose that their application in such devices as interferometric sensors is a promising prospect.
We have developed a modified Michelson interferometer type Raman laser system to manipulate cold 87 Rb atoms to interfere. A frequency modulated continuous wave technique was introduced to determine the optical path difference, thus compensating it to zero to minimize the effects of common mode noise. The linewidth (full width at half maximum) of the beat signal at 6.834 GHz was measured to 1Hz limited by the resolution bandwidth of the spectral analyzer. The measured rms phase variance of the phase noise at 166 MHz was 0.015 rad2, mainly restricted by our poor performance radio frequency microwave source. With modest improvements, we plan to apply this laser system to form an atom interferometer for acceleration and rotation measurements.
Bose-Einstein condensates are coherent matter waves, produced by cooling gaseous atomic clouds to ultra-low temperatures. For applications in atom interferometry and precision measurements, Bose-condensed sources present an intriguing alternative to thermal atoms. Although the current sensitivity achievable with interferometers using coherent atoms is not comparable to thermal beam machines (mainly due to the lower flux), there are promising ways to utilise the potential of Bose-condensed sources for atom interferometry. Among those is the low momentum width of Bose-Einstein condensates, which can generally be well controlled and is advantageous for increased interferometric sensitivities by implementing large momentum transfer beam splitters. As part of this thesis, experimental and theoretical investigations are presented to investigate the potential of Bose-Einstein condensates for such applications. We shall present the quantum projection noise limited performance of a Ramsey interferometer operating on the atomic clock transition of a freely expanding cloud of Bose-condensed rubidium 87 atoms. The results include Ramsey fringes of high visibility, not measurably affected by atomic interaction-induced dephasing effects. The achievement and detection of the quantum projection noise limit rely critically on the precision and accuracy of both the imaging setup and the coupling scheme in the interferometric beam splitters. The stabilisation of the beam splitters via an optical Sagnac interferometer is the basis for the quantum projection noise limited performance of the interferometer presented. For an increase of bandwidth and flux in atom interferometric measurements, it is advantageous to use a continuous atomic beam. A truly continuous coherent atom source has not been realised to date, and we present results on a pumping mechanism in this thesis, as a decisive step towards a continuous atom laser. By the investigation of different momentum resonances, we find that the pumping scheme relies on a Raman superradiance-like process. Finally, the thesis demonstrates two interaction measurements in rubidium. The strong mean field interactions due to the high densities in Bose-Einstein condensates are used to probe the potential of a rubidium 87 condensate with an atom laser. The measurement allows a determination of the scattering length between the two atomic states involved. In addition to this two-body scattering scheme, we present a measurement of three-body loss coefficients, extracted from loss curves in rubidium 85 Bose-Einstein condensates. The measurement provides new upper bounds on the three-body loss coefficients at the scattering lengths considered.
Presented in this thesis are results of research on two topics in the field of cold atoms. These topics are connected by the fact they both employ laser induced forces caused by momentum transfer from photons to atoms. In both cases the laser induced forces change the velocity distribution of the atomic ensemble. In the first part of the thesis a new way to implement a synthetic Lorentz force into a cold atomic gas is presented. The synthetic Lorentz force (SLF) is based on radiation pressure and the Doppler effect, making it straightforward to implement in a large volume, for a broad range of velocities, and can be extended to different geometries. The force is perpendicular to the velocity of an atom, and zero for an atom at rest. This SFL is experimentally demonstrated in a system of cold rubidium atoms in two scenarios: first, by observing the center-of-mass motion of a cold atomic cloud and second, by observing the angular deflection of a rotationally asymmetrical cloud when released from a magneto-optical trap. The introduction of synthetic magnetism into the system of cold thermal atoms makes it an excellent candidate to emulate numerous complex classical systems, for example a tokamak fusion reactor or a star. In the second part of the thesis, the possibility of laser cooling with a frequency comb (FC) is explored. For this purpose a scheme for full stabilization of a fiber based FC that does not require traditional self-referencing is developed and implemented; the repetition frequency is locked to a stable microwave reference while the offset frequency is indirectly stabilized by referencing the frequency comb to a continuous wave laser that is stabilized by polarization spectroscopy in rubidium vapor. The FC stabilized in this way is used to cool rubidium atoms on a dipole-allowed transition at 780 nm to sub-Doppler temperatures. Temperatures as low as 55 μK were measured in a one-dimensional FC cooling geometry using the time-of-flight method. Laser cooling with FCs could enable achieving sub-Doppler temperatures for atoms with dipoleallowed transitions in the vacuum ultraviolet. This can significantly improve the precision of optical frequency standards, enable measurements of fundamental constants with unprecedented accuracy, and open up the possibility to reach quantum degeneracy with atoms that have optical transitions unreachable by continuous wave lasers such as hydrogen, deuterium and antihydrogen.