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Science Background

(Top) Condensate atom cloud imaged in the IR with decreasing temperature. (Bottom) Temperature contour plot showing the atom cloud.
Over the past three decades, much advancement has been made in Earth-based laboratories in reducing the temperature of Bose Einstein Condensate (BEC) to below the condensate temperature. Inherent to these experiments is the application of an intense magneto-optical trap to hold the atoms in place to obtain the required cooling, due to the pull of gravity. Drop tower experiments have also been performed, which is a high quality microgravity environment, but interaction times are limited to less than 1 second. Formation of BECs in space-based experiments can therefore significantly increase interaction time and reduce perturbations that come from applied fields. Specifically, longer observation time for unconfined atoms. Such a space-based laboratory could lead to exploration of unknown quantum mechanical phenomena and the understanding that comes with it.

What is a Bose Einstein Condensate (BEC)

Transition from a particle to wave nature with decreasing temperature.
Satyendra Nath Bose and Albert Einstein first proposed Bose Einstein Statistics in 1924. They theorized that there are two classes of fundamental particles in the universe, Bosons, and Fermions. Fermions cannot occupy the same quantum state, and therefore follow the Pauli Exclusion Principle. However, Bosons can occupy the same quantum state and therefore can exhibit macroscopic behavior. If a population of Bosons is reduced to a temperature below their condensate temperature, a new state of matter, called a Bose Einstein Condensate (BEC), is formed. Where the population of atoms takes on a wave like nature, eventually the same wave function, and a macroscopic matter wave is observable, as shown in Figure 1. In this state, a BEC exhibits macroscopic quantum behavior. This was proposed by Bose and later created in ground based laboratory experiments by C. Tannoudji, S. Chu, W.D. Phillips, E. A. Cornell, W.G. Ketterle and C. E. Wieman. All these scientists won Nobel prizes for their discoveries and the novel techniques to obtain the BECs.

Formation of the Condensate

The process of laser cooling is summarized in the image below. The species of interest is exposed to a photon flux tuned to a particular resonance frequency. At resonance the photons impart momentum to the atoms. If the photon frequency is Doppler red-shifted from resonance then only atoms coming towards the laser beams will be affected. Those moving away from the laser will be unaffected by the photon flux. If laser beams are such that they are coming from all directions the atoms will be cooled from all directions. This laser cooling, lowers the atom population temperature to ~100 microKelvin, still above the condensate temperature.

Laser cooling:Formation of the condensate uses a combination of laser and evaporative cooling and adiabatic expansion.

The next stage of cooling is evaporative cooling with an applied Radio Frequency (RF) field. Another unique property of atoms is that for atoms above a certain energy level, when exposed to an RF field, they can be excited and essentially removed from the population, leaving behind only those at a lower energy and therefore population temperature. This is called evaporative cooling and brings the temperature of the population to much below one microKelvin.

The final stage of cooling is adiabatic expansion. The atoms are held and compressed on an integrated circuit with a precisely tuned magnetic field. When the field is turned off the cloud expands, and cools further. The final stage brings the population to below the nK range, and in the space environment, to the pK range. The condensate is formed and can live on the order of 20 seconds in microgravity where it can be exposed to other magnetic fields, electric fields, species of condensate, and imaged.