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Ultracold matter

Ultracold matter

Bose–Einstein condensate is made onboard the International Space Station

17 Jun 2020
Cooling at CAL
Cooling at CAL: this illustration shows six finely tuned lasers used inside NASA's Cold Atom Lab to slow down atoms, lowering their temperature. This is step one in a three-step cooling process. (Courtesy: NASA/JPL-Caltech)

Looking to escape the clutches of gravity for as long as possible, physicists in the US have made a Bose–Einstein condensate onboard the International Space Station (ISS). The orbiting lab does not yet exceed the performance of the coldest atom experiments on Earth but could in future be the ideal place to run quantum-mechanical gravimeters and carry out the most precise tests of the equivalence principle.

A Bose–Einstein condensate (BEC), known as the fifth state of matter, is a dilute gas of bosonic atoms whose temperature is so low that their wavelength becomes comparable to the distance between one atom and the next. In these circumstances the atoms all occupy the same quantum state and act in unison as a superfluid – so bringing otherwise microscopic wavelike properties into the macroscopic realm.

Physicists usually make BECs by confining a gas of bosonic atoms in a magnetic trap and firing laser beams at the particles to cool them down. The snag is having to release the condensate to study it. Once free, the atoms repel one another and quickly spread out if they are not cold enough – making the gas too tenuous to be detectable. But gravity also poses a major problem, its downward tug causing the atoms to collide with the bottom of the experimental apparatus within a fraction of a second.

Drop towers and parabolic flights

Researchers have carried out a variety of experiments to extend the lives of BECs by putting them in free fall and so temporarily removing the effect of gravity. One option is to drop condensates from the top of a tall, evacuated tower. Alternatively, they can be flown onboard aircraft following a parabolic trajectory or placed on sounding rockets – one such experiment in Sweden having reached a height of over 240 km and achieved free fall for 6 min.

But the best place to carry out such experiments is in orbit. Objects there are in continuous free fall, meaning that they create perpetual zero-gravity conditions. Not only does this in principle allow much more time for experiments, it also means that before atoms are released from their trap the magnetic fields confining them can be gradually turned down – allowing the atoms to spread out slowly and cool down to even lower temperatures.

The new research has been carried out using the “Cold Atom Lab” (CAL), launched by NASA in 2018 and housed inside the US Destiny module on the ISS. Operated remotely, the $70m lab occupies just 0.4 m3 but contains lasers, magnets and all the other instruments needed to trap, cool and control an atomic gas. The atoms are initially held at the centre of a vacuum chamber, before being transferred to an “atom chip” at the top of the chamber that uses radio waves to siphon off the fractionally hotter atoms and leave the remainder at less than a billionth of a kelvin.

Distinctive features

Robert Thompson, David Aveline and colleagues of the Jet Propulsion Laboratory at the California Institute of Technology used CAL to create BECs from atoms of rubidium-87. The condensates were detectable for up to 1.18 s and have a number of distinctive features compared to their terrestrial cousins. In particular, the researchers noted that some of the rubidium atoms used in the experiment remained separate from the condensates and instead formed a halo shape around them. Held very weakly in the trap via a phenomenon known as the second-order Zeeman effect, these atoms would on Earth simply fall to the floor.

According to Brynle Barrett of the Institut d’Optique d’Aquitaine in France, the lifetime of CAL’s condensates is comparable with those produced by the best terrestrial facilities. He points out that the 6 min of free fall time achieved with sounding rockets comprised many separate experiments, none of which lasted more than 300 ms. The fact that Thompson and colleagues have not gone much beyond a second, he explains, is not due to gravity or other inertial effects but instead technical constraints – such as the challenge in getting to ever lower temperatures and the need to reduce residual magnetic forces that disturb the condensates.

The real advantage of being in orbit, says Barrett, is that potentially years of free fall should allow researchers to continually refine their experimental parameters. As such, he believes the latest research “represents a significant step toward performing high-precision experiments with quantum gases in space”.

Atom interferometry

Among the experiments that could be carried out include using the atoms’ halo formation to produce ultra-cold gases with extremely low densities. Another could involve creating a BEC in the (gravity-defying) shape of a bubble. But perhaps the most eagerly awaited experiments will involve atom interferometry. This entails making very accurate measurements of gravity by recording the interference fringes generated when cold atoms placed in a quantum superposition follow two very slightly different paths through a gravitational field.

Such interferometers could potentially be used to carry out exacting tests of the universality of free fall – the idea that inertial and gravitational masses are one and the same – or to enable sensitive environmental monitoring and mineral prospecting from space. However, several technical challenges, including leaks from the atom chip, led the NASA researchers to delay installing additional equipment needed for interferometry. But following the launch of fresh supplies in December, they did that in January this year and a month later were again generating BECs.

Looking further ahead, Barrett says there are several proposals to launch a dedicated satellite that would use cold atoms to make fundamental tests of gravity – free from the vibrations that occur on the ISS. “This decade could see several of these exciting proposals become a reality,” he says.

The research is described in Nature.

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