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Physics Seminar - Dr Stuart Szigeti

Monday, 4 September 2017

Physics Department

Dr Stuart Szigeti

Research Fellow

The University of Queensland

"Measurement-based feedback control of quantum gases: Why bother and why now?"

The high controllability of ultracold atomic gases makes them ideal platforms for the detailed study of many-body quantum physics. To date, research has predominantly focussed on coherent (i.e. open-loop) control of the gas via direct control of optical and magnetic potentials, with considerable success. An alternative approach is measurement-based feedback control. Here a continuous weak measurement of the gas - perhaps via phase-contrast or Faraday imaging - is used to obtain an estimate of the quantum field (or some subset of moments, such as mean position). This knowledge can then be used to effect feedback via trapping potentials. Unlike coherent control, feedback control is robust to uncharacterised sources of error, and is particularly advantageous in situations where there is an uncertain preparation condition or complicated dynamics that are too difficult to model. In this talk, I will focus on the specific example of feedback cooling and stabilising a Bose-Einstein condensate (see figure), and use this to illustrate (1) how to design an effective feedback control, and (2) how to correctly incorporate the effect of measurement backaction in any theoretical model. I will also discuss the main issue which has prevented the realisation of any feedback-controlled quantum gas: the inability of theorists to convince experimentalists that this is a worthwhile use of time and resources! Unlike evaporative cooling, feedback cooling is number conserving and independent of specific atomic collisional properties, allowing for larger degenerate sources and the direct cooling of traditionally uncoolable systems (e.g. Fermi gases). Other more speculative applications of feedback control include the precise manipulation and creation of vortices and the creation of turbulence via nonlinear forcing. Given this, alongside recent advances in minimally-destructive imaging and potential manipulation (e.g. via DMDs), there has never been a more exciting time to feedback control your quantum gas.

WHEN: Monday 4 September 2017
WHERE: Room 314, Science 3 Building
TIME: 3.00 pm–4.00 pm

All interested are welcome to attend

Light refreshments to follow in Common Room