Quantum measurement and feedback control of nano-mechanical systems and atomic spin-ensembles

  • Liam Stuart Walker

Student thesis: Doctoral Thesis


In recent years, considerable developments have been made in controlling quantum systems through a combination of measurement and feedback. All measurements naturally disturb the system in question, as they necessitate some level of interaction in order to extract information. However, if we can characterise the resulting disturbance and correctly interpret the information - especially in the case of weak measurements - then we can determine how to feed back the measurement in such a way as to drive desired evolution, preparing potentially highly non-classical states. In this thesis, we investigate using feedback to prepare and manipulate quantum states of motion of levitated nano-particles, as well as for the preparation of many-body squeezed states in atomic ensembles. We first consider a possible route to ground state cooling with a levitated nanoparticle, magnetically trapped by a strong permanent magnet. The trap frequency of this system is much lower than those involving trapped ions or in many other nano-mechanical resonators. Minimisation of environmental heating is therefore challenging as it requires control of the system on a timescale comparable to the inverse of the trap frequency. We show that these traps are an excellent platform for performing optimal feedback control via real-time state estimation, and that they may also be an ideal testing ground for quantum collapse models when operating in the free particle limit. We go on to explore a separate system, considering applications of feedback for preparing collective pseudo-spin states in a dilute cloud of atoms. We model how information in typically discarded measurement channels can be used to stabilise noise in order to produce enhanced levels of spin squeezing. In these projects we make use of quantum trajectory techniques alongside analytical models, to explore and simulate realistic parameter regimes for current or near-future experiments. Throughout, we develop ideas for creating non-classical states in a new generation of quantum technologies.
Date of Award11 Nov 2020
Original languageEnglish
Awarding Institution
  • University Of Strathclyde
SponsorsEPSRC (Engineering and Physical Sciences Research Council)
SupervisorGordon Robb (Supervisor) & Andrew Daley (Supervisor)

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