How to watch a Bose–Einstein condensate for a very long time

28 November 2013 3:36 PM

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How to watch a Bose–Einstein condensate for a very long time

A new way of making measurements on Bose–Einstein condensates (BECs) has been proposed by physicists in the UK and Australia. Calculations made by the team suggest that the technique could allow researchers to monitor BECs over much longer timescales than the several seconds possible today. If successfully implemented in the lab, the process could increase the use of BECs in practical applications such as atomic clocks and accelerometers.

A BEC is created by holding atomic gas in a magnetic trap and cooling the gas to nanokelvin temperatures. This causes all of the atoms (sometimes tens of thousands) to settle into a single coherent quantum state. When this occurs, the condensate is a macroscopic system that is described by the same wavefunction. BECs can then be studied to gain further insights into quantum mechanics or be used to create practical devices such as atomic clocks or extremely sensitive accelerometers that could be used for navigation and other applications.

An important challenge facing anyone trying to create and use a BEC is that quantum coherence extending over thousands of ultracold atoms is extremely fragile and the tiniest disturbance can destroy the BEC. This makes it difficult to monitor the properties of a condensate once it has been made. Indeed, most experiments involve taking just one image of a BEC, which destroys it. Other methods do allow the BEC to be monitored for a few seconds, but developing techniques to monitor a BEC over longer periods of time has proven elusive.

A promising way of imaging a BEC is to use "off-resonance" light – light at a frequency that does not correspond to any atomic transitions in the condensate. Because this light will not be strongly absorbed by the BEC, it should not cause much disruption – at least in principle. In practice, however, physicists have found that BECs quickly evaporate when exposed to off-resonance light.

Now, Michael Hush of the University of Nottingham together with colleagues at the University of Queensland and the Australian National University have developed new computer-modelling techniques that provide fresh insights into how a BEC interacts with the light used to image it. This new information has inspired the team to create with a new protocol that – if it works in the lab – could allow multiple images to be taken over several minutes or even indefinitely.

To gain a better insight into how BECs interact with light, Hush and colleagues used a simulation technique called the "number-phase Wigner (NPW) particle filter". The NPW aspect of the technique refers to the phase-space representation of the BEC used in the calculations, while the particle filter is a numerical technique that provides an optimal estimate of the quantum state of the BEC. "The simulations allow us to see heating effects that others had not predicted," says Hush. "These effects could be the cause of atom loss when a BEC is imaged," he adds.

Armed with this new knowledge, the researchers developed a feedback scheme to remove this heat (see figure). In their system, the BEC would be bathed in a standing wave of off-resonant light, which would be used to monitor its density. Heat absorbed from the light will cause the atoms in the BEC to vibrate and these vibrations would be detected by the light. A feedback system would use this information to adjust the parameters of the magnetic trap so that these vibrations are dampened. While the scheme has not yet been tested, Hush is hopeful that it could be implemented in the lab.

Jacob Scherson of Aarhus University in Denmark, who was not involved in the research, described the work as "a great contribution to the field", saying that as an experimentalist he would love to implement the ideas in the lab. While Scherson points out that certain experimental challenges would have to be overcome to implement the scheme, he adds that he is "convinced that the ideas put forth [by Hush and colleagues] can help extend the duration over which BECs can be monitored".


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