Direct Observation of Quantum Backaction in a Bose Condensate

The principal tenet of quantum technologies has two sides: quantum measurement and quantum control. We interpret the Bose-Einstein condensate (BEC) imaging mechanism as a quantum measurement process in which the environment ``measured'' the scattered photons but this information was not passed on to the observer. BECs offer multiple minimally destructive imaging methods, which are weak measurement techniques that yield a controlled reservoir and consequently allow time-resolved study of the system evolution paving the way for real-time control of quantum gases. We employ phase-contrast imaging (PCI) technique — a nondestructive detection method — to acquire repeated measurements of the same BEC. In this measurement paradigm, by performing a pair of back-to-back PCI measurements of the same BEC, we directly observe the quantum backaction of the first measurement as correlated excitations in the second measurement. To this end, I will describe our versatile high-resolution ultracold atom microscope: a combined hardware/software system that recovers near-diffraction limited performance and minimizes the information loss. Our high-fidelity digital correction technique reduces the contribution of photon shot noise to density-density correlation measurements which would otherwise contaminate the quantum projection noise signal in weak measurements [1]. I will discuss the experimental characterization of the measurement process via the PCI technique and the measurement induced overall heating of the condensate.

[1] E. Altuntas, and I. B. Spielman, Physical Review Research, 3, 043087 (2021).