Cold atom samples, at temperatures of the order 100 uK, are useful for a wide reaching array of new and exciting technological and scientific endeavours. Atoms are conventionally cooled by Doppler cooling, which relies on the continuous absorption and reemission of photons in a closed optical cycle. This requirement is diffcult to achieve when there are many allowed decay paths for the excited atom, making Doppler cooling only feasible for a handful atoms with simple energy level structures. More exotic energy level structures, such as those found in molecules, are notoriously diffcult to cool. Coherent cooling schemes in comparison, offer advantages such as insensitivity to frequency detuning or a higher number of photon momenta which can be imparted for each spontaneous emission event making them promising candidates for the optical cooling of particles with more general energy level structures. In order to demonstrate these schemes, we have explored the coherent manipulation of an atomic cloud of Rubidium cooled using a two photon Raman resonance. Despite the long spontaneous decay times of such systems, we find a significant decay in the fidelity of the coherent manipulations, which we have characterised using the techniques of Raman spectroscopy, Rabi oscillations, Ramsey interferometry and spin echo. We have found the minimum time constant for the decay in the decoherence to be 2.1fi0:2 ms, which is a result of non-radiative and partially non-stochastic dephasing mechanisms. Due to a high level of decoherence during the spin-echo experiments further investigation is required to determine the exact ratio of stochastic to non-stochastic dephasing
Cold atom samples, at temperatures of the order 100 uK, are useful for a wide reaching array of new and exciting technological and scientific endeavours. Atoms are conventionally cooled by Doppler cooling, which relies on the continuous absorption and reemission of photons in a closed optical cycle. This requirement is diffcult to achieve when there are many allowed decay paths for the excited atom, making Doppler cooling only feasible for a handful atoms with simple energy level structures. More exotic energy level structures, such as those found in molecules, are notoriously diffcult to cool. Coherent cooling schemes in comparison, offer advantages such as insensitivity to frequency detuning or a higher number of photon momenta which can be imparted for each spontaneous emission event making them promising candidates for the optical cooling of particles with more general energy level structures. In order to demonstrate these schemes, we have explored the coherent manipulation of an atomic cloud of Rubidium cooled using a two photon Raman resonance. Despite the long spontaneous decay times of such systems, we find a significant decay in the fidelity of the coherent manipulations, which we have characterised using the techniques of Raman spectroscopy, Rabi oscillations, Ramsey interferometry and spin echo. We have found the minimum time constant for the decay in the decoherence to be 2.1fi0:2 ms, which is a result of non-radiative and partially non-stochastic dephasing mechanisms. Due to a high level of decoherence during the spin-echo experiments further investigation is required to determine the exact ratio of stochastic to non-stochastic dephasing
Cold atom samples, at temperatures of the order 100 uK, are useful for a wide reaching array of new and exciting technological and scientific endeavours. Atoms are conventionally cooled by Doppler cooling, which relies on the continuous absorption and reemission of photons in a closed optical cycle. This requirement is diffcult to achieve when there are many allowed decay paths for the excited atom, making Doppler cooling only feasible for a handful atoms with simple energy level structures. More exotic energy level structures, such as those found in molecules, are notoriously diffcult to cool. Coherent cooling schemes in comparison, offer advantages such as insensitivity to frequency detuning or a higher number of photon momenta which can be imparted for each spontaneous emission event making them promising candidates for the optical cooling of particles with more general energy level structures. In order to demonstrate these schemes, we have explored the coherent manipulation of an atomic cloud of Rubidium cooled using a two photon Raman resonance. Despite the long spontaneous decay times of such systems, we find a significant decay in the fidelity of the coherent manipulations, which we have characterised using the techniques of Raman spectroscopy, Rabi oscillations, Ramsey interferometry and spin echo. We have found the minimum time constant for the decay in the decoherence to be 2.1fi0:2 ms, which is a result of non-radiative and partially non-stochastic dephasing mechanisms. Due to a high level of decoherence during the spin-echo experiments further investigation is required to determine the exact ratio of stochastic to non-stochastic dephasing
The production of dense samples of atoms whose translational velocity can be parameterized\nby temperatures in the microkelvin range has revolutionized the fields of spectroscopy,\nmetrology, quantum computing and sensitive tests of quantum mechanics.\nSuch ultracold temperatures may be reached by Doppler cooling which uses a velocitysensitive\nscattering force. This technique relies upon atoms which have closed electronic\ntransitions between two states so that the atoms may continuously absorb photon momenta\nand do not spontaneous decay into a dark state. Very few atoms fulfil this condition\nand attempts to cool molecules are inhibited by their extra degrees of freedom,\nvia rotation and vibration, which add manifolds of extra states.\n\nThis thesis describes the early experimental stages of investigation into coherent laseratom\ninteractions which may be used as a general all-optical method to impart momentum\nto atoms and molecules and thus manipulate their velocity. The thesis covers the\nconstruction and operation of stable diode lasers, a magneto-optical trap to produce cold\nsamples of the test species Rubidium and a high-power, phase and intensity, controllable\nlaser to induce Raman transitions. Studies into the spectroscopy of Rubidium and the\nnature of coherent Raman interactions in multilevel atoms is also covered. Experimental\nresults shows that coherent Raman transitions between the 5S1/2 ground states has been\nachieved in the form of sinc-squared lineshapes and Rabi-flopping.
The range of atoms which can be cooled by lasers is limited to those which have a closed two level structure. Several schemes have been proposed which aim to extend this range by using coherent control of the particle momenta, but none have yet been demonstrated. We hope to implement these and other coherent manipulation schemes, and we begin with a system which is well understood and over which we can exert precise control. This thesis covers the design and construction of an experiment to demonstrate coherent manipulation of cold rubidium atoms collected in a magneto-optical trap. The lower hyperfine levels of these cold atoms very closely mimic the ideal two-level atom, and we use carefully crafted laser pulses to prepare, manipulate, and read their quantum state. The hyperfine levels are coupled using two fields whose frequency difference is equal to the hyperfine splitting. The way in which these Raman coupled levels can be used to emulate a two-level atom is explored, and the experimental apparatus used to create and control the driving fields is described in detail. The amplitude, frequency and phase of these fields is programmable, and complex manipulation schemes can be implemented merely by programming a computer. We have observed Raman transitions in the cold rubidium atoms, and the experimental methods used to detect these features amidst large experimental noise are discussed. Although we have not yet seen Rabi oscillations, we are confident that we can now have sufficient control to begin to implement simple interferometric sequences. However, there remain significant challenges if we are to coherently manipulate the momentum, and the prospects for such manipulation are discussed.
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 report the 1D cooling of ^{85}Rb atoms using a velocity-dependent optical force based upon Ramsey matter-wave interferometry. Using stimulated Raman transitions between ground hyperfine states, 12 cycles of the interferometer sequence cool a freely moving atom cloud from 21 to 3 μK. This pulsed analog of continuous-wave Doppler cooling is effective at temperatures down to the recoil limit; with augmentation pulses to increase the interferometer area, it should cool more quickly than conventional methods and be more suitable for species that lack a closed radiative transition.
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.