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Investigating the effect of external magnetic fields and polarisation of intense light fields on Ba+ ions

Buijsman, S.F. (2016) Investigating the effect of external magnetic fields and polarisation of intense light fields on Ba+ ions. Bachelor's Thesis, Physics.

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The Ba+ ion is well suited for investigating effects of atomic parity violation (APV). Furthermore it can be exploited to build an atomic clock in the optical frequency range. Optical spectroscopy of the 6s2S1/2 - 6p2P1/2, 5d2D3/2 - 6p2P1/2 and 6s2S1/2 - 5d2D3/2 transition is employed in the work towards measurement of APV on a single trapped Ba+ ion. In preparation to this, experimental parameters affecting the 5d2D3/2 - 6p2P1/2 transition are investigated. The magnitude and direction of the magnetic field B as a function of current I through sets of Helmholtz coils at the position of the ion was estimated by measuring the field at two locations close to a vacuum chamber. A non-linear dependence of Bz on I was observed. This effect is not yet fully explained. Laser light polarisation was controlled using a half-waveplate. This waveplate is inserted in a motorised rotation optical mount. The polarisation direction of this waveplate was found to be in the vertical direction when the rotation mount was put at an angle of 18(1) degrees. The polarisation angles of blue and red laser light w.r.t. the vertical direction are determined to be theta_Blue = 0.5110(19) degrees and theta_Red = 56.41(80) degrees. The degree of circularity of the blue and red laser light is found to be phi_Blue = 1.158(38) degrees and phi_Red = 43.8(4.2) degrees. An uncertainty in these values may be present as a consequence of the light passing through a dichroic beamsplitter. The transmittance and reflectance through this beamsplitter might depend on the polarisation of the light. This would result in a systematic error. Spectroscopy measurements have been conducted for two polarisation angles. One in the direction of a PMT to observe atomic transitions and one in the vertical direction. Two more measurements were performed for a different direction of the magnetic field, which is equivalent to rotating the polarisation. Matlab code was used to fit the recorded spectrum to a lineshape. The values obtained for polarisation and circularity are yet insufficiently precise to draw an unambiguous conclusion on the polarisation and circularity that the ion 'sees'. Deviations of the model from previous measurements may be caused by a differing choice of coordinates. The polarisation was determined relative to the lab vertical axis, defined as the z-axis. The z-axis for the ion is defined by the quantisation axis, which is in the direction of the magnetic field. This direction does not in general correspond to the lab vertical axis. Suggestions for improved measurements are given. More measurements can be made to arrive at a distribution of polarisations, which is expected to peak at the value closest to the values the ion `sees'. Some improvements to the model may include accounting for motion of the ion or improving knowledge of the magnetic field or light field intensity.

Item Type: Thesis (Bachelor's Thesis)
Degree programme: Physics
Thesis type: Bachelor's Thesis
Language: English
Date Deposited: 15 Feb 2018 08:14
Last Modified: 15 Feb 2018 08:14

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