Measurement induced dynamics and defect stabilization in spinor condensates*

Weakly measuring many-body systems and allowing for feedback in real time can simultaneously create and measure new phenomena in quantum systems. We study weak measurement and classical feedback in spinor Bose-Einstein condensates, focusing on the trade-off between usable information obtained from measurement and quantum backaction. As a prototype example, we consider the dynamics of a domain wall in a two-component BEC and show that quantum backaction due to measurement causes two primary effects: domain wall diffusion and overall heating. The system dynamics and signal-to-noise ratio depend on the choice of measurement observable. We describe a feedback protocol to create and stabilize a domain wall in the regime where domain walls are unstable, giving a prototype example of Hamiltonian engineering using measurement and feedback. Finally, we discuss extensions of this idea to higher spin systems.