Measurement-enhanced determination of BEC phase transitions

Abstract

We demonstrate how dispersive atom number measurements during evaporative cooling can be used for enhanced determination of the parameter dependence of the transition to a Bose-Einstein condensate (BEC). In this way shot-to-shot fluctuations in initial conditions are detected and the information extracted per experimental realization is increased. We furthermore calibrate in-situ images from dispersive probing of a BEC with corresponding absorption images in time-of-flight. This allows for the determination of the transition point in a single experimental realization by applying multiple dispersive measurements. Finally, we explore the continuous probing of several consecutive phase transition crossings using the periodic addition of a focused "dimple" potential.Quantum effects have become increasingly important in the development of modern technologies and have led to the need for accurate quantum simulation [1,2]. Various approaches, including cold atoms [3-5] and ions [6], superconducting circuits [7], and photons [8] are used to perform such simulations. In particular, many quantum simulators aim to pin down the boundaries between discrete phases of many-body systems, such as Ising spin transition points [4,6] or magnetic phases [5], with the highest possible accuracy. Current simulations focus on providing tight experimental bounds for benchmarking theoretical models, such as finite temperature bosonic superfluids in optical lattices [9].In this article we investigate the potential for using dispersive probing of ultracold atom clouds in two distinct toymodel settings to enhance future quantum simulations. First, we demonstrate that benchmark probes of thermal clouds during forced evaporative cooling can be used to enhance the accuracy in determining the critical point, at which the transition to a Bose-Einstein condensate (BEC) occurs. Second, we investigate multiple, in-situ probing of BECs during evaporation as a means of single-shot mapping of a phase transition.Since the pioneering work on dispersive probing [10,11], it has been employed to investigate the relative repulsion of BECs and thermal clouds [12] and to monitor the formation of a BEC [13,14] as well as the appearance and dynamics of solitons [15,16]. In-sequence benchmark measurements of cold atomic clouds have recently been applied as a tool to moderately reduce classical fluctuations in atom number [17]. Using active feedback, atom number stabilization of cold clouds to the 10 −3 level was demonstrated in Ref. [18]. However, to date no experiments have explored benchmark-measurement enhanced evaluation of the BEC atom number. Efficient suppression of classical fluctuations could enable the settlement of long-standing theoretical disputes concerning the nature * M. G. Bason and R. Heck contributed equally to this work † Electronic address: sherson@phys.au.dk of fundamental fluctuations arising at the BEC phase transition [19]. In recent years, theoretical studies on dispersive lightmatter interaction based Quantum Non-Demolition (QND) measure...

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“…An experimental cycle is initiated by collecting a cloud of 87 Rb atoms in a 3D magnetooptical trap. It is subsequently optically pumped into the |F = 2, m F = 2 state and magnetically transported to an intermediate vacuum chamber where forced microwave evaporation is performed [22]. Thereafter the cloud is loaded into a focused single beam optical dipole trap at a wavelength of 1064 nm, with a 1/e 2 waist of 45 µm [23].…”

“…The main advantage of minimally-destructive dispersive imaging methods is the option to take multiple images in a single realisation. This enables us to investigate dynamics in the atomic system [22,24].…”

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“…The experimental system is described in Refs. [25,26] and only its main features are described here.…”

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