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Atomic clocks' ticks tamed by 3,000 entangled atoms

Boffins break record for entanglement with mirrors and lasers

By Richard Chirgwin, 30 Mar 2015

MIT and University of Belgrade boffins have taken a big step in applying the quantum property of entanglement to macro systems: they're claiming to have roped together more than 2,900 rubidium atoms with a single photon.

In what lead author Vladan Vuletić says is a “new class” of entangled states, in work that they believe will help create even more precise atomic clocks (because a clock that will drift by less than a second over the lifetime of the Sun is still too inaccurate).

MIT's canned release explains that entangling lots of atoms is one way to improve clock accuracy.

A classical-physics atomic clock is based on the oscillations of atoms in a cloud, and is proportional to the square root of the number of atoms (nine times as many atoms provides a clock three times as accurate).

By applying entanglement to the cloud, MIT says, that square-root relationship can be turned into a linear relationship – so (for example) a tripling of accuracy needs only three times as many atoms. Entangling nearly 3,000 atoms is therefore a big deal in the world of atomic timekeeping.

To create the “mutual entanglement” among their ensemble of atoms, the atoms were first cooled to close to absolute zero, and held in place with a laser trap. The researchers then sent a weak laser pulse through the cloud, with a detector set up to look for a particular photon in the beam.

As Physicsworld explains:

“The experiment involves an optical cavity – two opposing imperfect mirrors – containing about 3100 rubidium-87 atoms that are cooled to a temperature of near absolute zero. Light is shone into one side of the cavity and allowed to bounce back and forth between the mirrors. Some of the light will eventually escape through the opposite side of the cavity, where it is captured by a detector. A magnetic field is applied to the atoms, which causes them to align their spins along the length of the cavity. However, the probabilistic nature of quantum mechanics means that the spins are not all aligned and their directions will fluctuate about the magnetic field.”

MIT's release explains Vuletić's reasoning thus: “if a photon has passed through the atom cloud without event, its polarisation … would remain the same. If, however, a photon has interacted with the atoms, its polarisation rotates just slightly”.

The change to the photon's polarisation indicates that the photon has been affected by quantum noise in the atoms' ensemble.

Vuletić says when an exiting photon's polarisation is perpendicular to its polarisation on the way in, “we know that it must have been caused by the atomic ensemble … that detection generates a very strongly entangled state of the atoms.”

MIT says the largest number mutual entanglement previously was 100 atoms, but its press release declines to identify the authors of the previous work (The Vuletić paper refers to an entanglement between 100 photons in this 2013 paper, which given the general quality of science communications might be what the press office meant).

The research has been published in Nature (abstract here). ®

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