Towards improving the quantum coherence in Ion microtraps

  • Mariam Sara Akhtar

Student thesis: Doctoral Thesis

Abstract

Ion traps have a number of applications in optical atomic clocks, quantum metrology and quantum information processing. Quantum coherence is essential in these applications, yet motional decoherence of ions remains a significant limitation. Experiments towards improving the quantum coherence of ions confined in microfabricated traps are presented. Surface contamination, noise on DC sources and instabilities in magnetic field are all potential sources of decoherence that are investigated. Spectroscopy on a single ion aswell as a two ion string using 88Sr+ is then demonstrated. Hydrocarbon contamination on electrode surfaces is a possible sources of electric-field noise that may result in motional heating of the ion and therefore decoherence. A capacitively-coupled RF micro discharge was generated in situ with energies suited to selective removal of surface contamination. The plasma parameters needed for the calculation of the ion bombardment energy, namely the electron density and the gas temperature, were determined using optical emission spectroscopy. For the range of operating parameters tested, the mean ion energies between 0.3 eV and 4.1 eV were calculated. While these energies are below the sputtering threshold for hydrocarbon contamination (12 eV), calculations show that the high energy tail of the ion energy distribution should remove two adsorbate monolayers in as little as 1 min. Furthermore, calculations show that during this time, the distribution is insufficiently energetic to have a significant effect on the Au electrode surface. The results presented here suggest that the microplasma surface processing is suited to in situ selective removal of surface adsorbates from ion microtrap electrodes. If electrical noise present on the electrodes of the trap is resonant with the motion of the ion, ion motional heating can occur and result in a reduced ion coherence time.Therefore it is essential to minimise the electrical noise at the motional frequencies of the ion. A system was created for versatile control of the DC potentials on the ion electrodes. Therefore it is essential to minimise the electrical noise at the motional frequencies of the ion. A system was created for versatile control of the DC potentials on the ion electrodes. Filtering of the DC signals such that the noise at the motional frequencies of the ion are attenuated, was implemented with a pair of interchangeable filter boards. For heating rate measurements a 2nd order RC filter board was designed with an attenuation of 192 dB at 1 MHz and a cut-off frequency of 6 Hz. A second filter board was made for ion shuttling; a 3rd order Butterworth filter with 59 dB attenuation at 1 MHz and a cut-off frequency of 100 kHz. Within this work the Δmj = -2 Zeeman component of the optical qubit transition in 88Sr+ is used. This transition is beneficial since, given the geometry of the experimental apparatus used here, it allows for a higher coupling strength relative to the other Zeeman components. However this transition also has a higher sensitivity to magnetic field fluctuations. The separation of the energy levels that need to be addressed are dependent on the magnetic field applied. Therefore magnetic field fluctuations lead to dampening of the phase relation between the states; i.e. decoherence. A high-precision stabilisation system was implemented for the control of the currents to the coils that generate the magnetic field the ion experiences. A derived stability of the current applied to the coils was expected to result in a magnetic field stability of 3 Δ10-7 G over 1000 s. However when measured directly, the magnetic field stability was limited by the drift of the ion pump magnet.The characterisation of this drift and the methods for reducing it are presented. As atoms are evaporated towards the trap, the atomic flux can adsorb onto the electrode surfaces an
Date of Award4 Jun 2019
Original languageEnglish
Awarding Institution
  • University Of Strathclyde
SponsorsUniversity of Strathclyde & Defence Science and Technology Laboratory DSTL MoD
SupervisorErling Riis (Supervisor) & Paul Griffin (Supervisor)

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