By Zeeya Merali
A quantum trick could provide sharper ticks for the atomic clocks of the future. A set of studies demonstrate that quantum entanglement can make the measurement technique used by atomic clocks even more precise.
The "ticks" of the current standard atomic clocks are marked by the regular vibrations of an ensemble of cesium atoms, which vibrate 9.2 billion times every second. However, noise inherent in the system means that there is a fundamental "classical limit" to how accurately the clocks can measure those vibrations. Now two groups, one led by Markus Oberthaler at the University of Heidelberg in Germany, and the other by Philipp Treutlein, then at the Ludwig-Maximilians University of Munich, Germany, have shown that this classical limit can be breached using a quantum twist.
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Both teams used rubidium atoms that had been super-cooled to form a quantum phase of matter known as a Bose-Einstein condensate (BEC). According to quantum mechanics, atoms can exist in a "superposition" in which they inhabit multiple states simultaneously; the atoms snap permanently into a state only when their state is measured. In this case, both teams put each atom in a superposition of two energy states.
To understand how noise gives rise to the classical limit, you need to repeatedly measure the BEC after the atoms are in a superposition, explains Oberthaler. Each time, you will find roughly 50 percent of the atoms in each level, but one run it may be 49:51, the next 55:45, and so on, he says. "It is this fluctuation around the mean that acts as 'noise', making your measurements imprecise."
In a bind
Both teams aimed to reduce this noise by carefully controlling the collisions between the atoms in the BEC. To do this, they suppressed interactions between atoms in different energy states. "Only interactions between atoms in the same state remain, thereby entangling these atoms together and dampening fluctuations between levels," says Treutlein.
Treutlein's group manipulated BEC atoms trapped on an "atom chip," fabricated from gold wires and housing a rudimentary atomic clock. Under normal conditions, all atoms can collide with each other. However, the team used a microwave pulse to push the two energy levels farther apart so that only atoms in the same energy state could collide. "The idea is to tune the atoms so that they only 'see' other atoms in the same level," says Treutlein.
After entangling the atoms, the team measured their energy levels and found that the noise was 2.5 decibels, or 44 percent, lower than the classical limit.
Oberthaler's group squeezed the noise down even farther in a different system--this time for BEC atoms trapped in an 'optical lattice' by a network of laser beams. The team applied a magnetic pulse to the BEC atoms to strengthen collisions between atoms in the same state, and weaken those between atoms in different states. In so doing, the team reduced the noise to 8.2 decibels lower than the classical limit--or about 85 percent.
Both teams publish their work in the April 1 issue of Nature.
Gentle squeeze
Eugene Polzik, an expert on entanglement-assisted measurement at the Niels Bohr Institute in Copenhagen, says that both sets of results are "spectacular."
Because Oberthaler and his colleagues used BECs trapped in an optical lattice, Polzik thinks that their technique could be useful for the next generation of atomic clocks, which exploit vibrations in the optical range. By comparison, the cesium standard used by many atomic clocks operates at microwave frequencies. "Optical clocks have the highest potential at the moment for taking over as the standard," says Polzik. This is because optical vibration frequencies are much larger than microwave frequencies, allowing time to be sliced into smaller intervals.
The key advantage of Treutlein's technique is that it is chip-based. "It may not give world-record precision, but it will have a whole range of other applications based on its portability," says Polzik. For instance, it could help improve the accuracy of the clocks used in Global Positioning System satellites.
However, Polzik notes that BECs are not the only way to improve the precision of atomic clocks. His group has recently squeezed down the noise in a caesium microwave clock by passing a gentle laser pulse over a cloud of 100,000 ultracold cesium atoms. The pulse generates entanglement, but is too weak to break the atoms' superposition--a process called 'quantum non-demolition' (QND). This squeezing technique has also been demonstrated by a team led by Vladan Vuletić at the Massachusetts Institute of Technology in Cambridge. Polzik is in discussions with the Paris Observatory, which uses caesium standards, about trying to implement his technique in its state-of-the-art atomic clocks.
"We don't yet know which techniques--BECs or QND--will ultimately be the best for atomic clocks," says Treutlein. "But we are all demonstrating the basic ingredients to beat the standard limits."
