



Video: Exotic matter free fall

A capsule of exotic matter is readied for the big drop (Image: Institute for Quantum Optics/University of Hannover)

What do you get if you hurl a cloud of rubidium atoms, together with computer-controlled lasers and a magnetic coil, down an empty elevator shaft?

The answer is an exotic and fragile form of matter that is in just the right state to put a key assumption of Albert Einstein’s theory of general relativity to the test.


Researchers recently succeeded in creating this fragile state, known as a Bose-Einstein condensate (BEC), on a 110-metre drop down what is essentially an elevator shaft with a vacuum at its centre. It’s the first time a BEC has been created in free fall.

Elevators have long helped elucidate physics. Einstein’s “equivalence principle”, which underpins general relativity, says that if you stand in a falling elevator, your acceleration should effectively cancel out the pull of gravity, leaving you unable to determine whether you are in free fall or whether there is simply no gravity present at all.

Fragile state

Determining whether quantum systems are also subject to this equivalence principle might help pin down why quantum theory has so far resisted any merger with general relativity – a mystery that haunts physics. But finding a way to test the equivalence principle at the quantum scale has proved tough.

A BEC might help. This is an ultracold form of matter dreamed up by Albert Einstein and Satyendra Nath Bose that exists at the boundary between quantum and classical physics. The atoms in a BEC lose their identities and behave as a single quantum object, yet they take on macroscopic dimensions.

Unfortunately, putting a fragile BEC into microgravity – which is what is needed to test the equivalence principle – is no easy matter. Now Ernst Rasel of the University of Hannover in Germany and colleagues have done just that, during a 4.7-second plunge.

Their trick was to create a device consisting of computer-controlled lasers and magnetic coils that can turn a cloud of rubidium atoms into an ultracold BEC “on the fly”.

Candid camera

The atoms are initially cooled by laser, while trapped by a magnetic field and laser beams, before being transferred to a photon-free magnetic trap for the final stage of cooling, which involves kicking out the warmest atoms until only the coldest 0.1 per cent remain. The result is a BEC, in which all the atoms adopt a single wave pattern and so form a single quantum object.

This device was loaded into a 2-metre-long capsule and sent down a 110-metre-long vacuum shaft. It was programmed to create the fragile BEC 1 second after release – once the capsule has stopped vibrating.

The team knew their experiment had worked because just after the BEC formed it was released from the trap and floated into a space between a detection laser and camera on board the capsule. The laser shone on the condensate, producing a shadow about 0.1 millimetres long that was captured by the camera.

An instant after that, around 3 seconds after the capsule was released, it hit the ground, coming to rest in a vat of styrofoam balls 8 metres deep. “It sounds amazing when it’s crashed,” says team member Waldemar Herr.

Interference pattern

Ignazio Ciufolini of the University of Lecce in Italy says the experiment lays the groundwork for “probing the boundary between general relativity and quantum mechanics and helping to shed light on one of the main problems in physics nowadays: the merging of general relativity with quantum mechanics”. He describes it as an “outstanding” piece of work.

To test whether quantum states obey the equivalence principle, the researchers will need to hone their device further so that it splits the Bose-Einstein condensate. This will allow them to send it along two separate paths and then determine if there is any differences between the two waves when they are reunited.

Rasel says his team is already working to create such an interferometer to drop down the shaft.

Journal reference: Science, DOI: 10.1126/science.1189164