A single ytterbium atom, cooled down to extreme temperatures and manipulated with laser beams, could reveal how gravity affects quantum objects.
The way gravity affects the quantum realm has so far remained mysterious. But an experiment that uses lasers as a pair of tweezers could let researchers assess how Earth’s gravitational pull affects an atom that ticks like a clock.
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| Tweezers made from laser beams can hold and move a single atom Aepsilon/Shutterstock |
At extremely cold temperatures – think billionths of a degree above absolute zero – quantum effects make atoms behave like “matter waves” rather than particles. Physicists have long taken advantage of this: by colliding different matter waves and measuring the resulting ripples, they can identify the forces influencing the atoms, a process called atom interferometry.
Yoav Sagi and Ilan Meltzer at Technion-Israel Institute of Technology came up with an intriguing interferometry experiment that could determine how Albert Einstein’s ideas about gravity work in the quantum realm. If gravity can change a quantum state, there could be far-reaching implications. For example, it might help explain why a quantum particle can exist in multiple places at once, a state called superposition, but a large and heavy object cannot.
The test relies on a single atom of ytterbium. Such atoms have previously been used in extremely precise atomic clocks, which control ultracold atoms through the electromagnetic forces arising from laser beams. Laser light can push electrons within the atom to rapidly oscillate between two specific quantum states, with each oscillation acting like the tick of a clock. And laser light can also control the atom’s position as if it were held in tweezers. The interferometry experiment would start with the same set-up.
Because the cold atom is behaving like a wave, it can exist in a superposition: if researchers placed a second set of laser tweezers near the first, the atom-wave could end up existing in two states, with the atom located in both tweezers at the same time. The researchers would then separate the tweezers by several millimetres before bringing them together again. The recombination would cause the two parts of the superposition state to clash, producing ripples like in any interferometry experiment.
This would all be done with one atom that is also acting as a clock, so the experiment would be sensitive to anything odd that may happen to time – like gravity slowing it down, says Sagi. Einstein’s general theory of relativity predicts such an effect, asserting that time slows down in areas that experience a stronger gravitational pull. For example, a clock at sea level runs very slightly slower than one on top of a mountain, and thus further from Earth’s gravitational influence.
Consequently, the researchers could implement a quantum version of Einstein’s twin paradox. This thought experiment typically refers to time passing differently for a pair of twins, one staying on Earth and the other travelling on a spaceship moving so fast that time slows down. In the new case, the twins would be the atom’s quantum states – each held in a different pair of tweezers – and the experiment would be orientated so that these states are stacked vertically, one above the other. Because the two tweezers would be at different heights and therefore experience different gravitational potentials, gravity’s pull could make the atom-turned-clock in one of the tweezers tick very slightly more slowly than the other. If the researchers could find signatures of this effect after bringing the tweezers together, it would indicate that gravity changes the states of quantum objects, making the two parts of the superposition differ.
Hendrik Ulbricht at the University of Southampton in the UK says that similar protocols have been implemented in experiments before but were not sensitive enough to definitively determine how gravity changes the atom’s quantum state or the way it experiences time. Using optical tweezers could help, but “experiments will need to prove if this is indeed the case”, he says.
Sagi is confident that they will. “I’m not by any means claiming that this experiment will be easy, but we propose something which is at least possible to do with current technology,” he says. His team has already performed some preliminary experiments but hasn’t built the full ytterbium set-up yet.
Journal reference:
Physical Review A, in press
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