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Einstein’s “spooky action” improves measurements

Entangled atoms for interferometry

Abbildung: Bernd Lücke Atoms, the building blocks of nature, can be used as extremely precise measurement devices. Since the 1960s, for example, our daily time is defined by the inner oscillation frequency of the cesium atom, and realized by so called atomic clocks. This inner oscillation frequency has to be converted to the clicks of a second. Even if the physical apparatus is not disturbed by technical or environmental noise, this frequency cannot be determined to arbitrary precision. There is a fundamental limit called shot noise limit if all atoms oscillate independently. Researchers at the Cluster of Excellence QUEST (Centre of Quantum Engineering and Space-Time Research) at Leibniz Universität Hannover, Germany, in collaboration with scientists from Spain, Italy and Denmark now report in the journal Science that they have overcome this shot noise limit (“Twin matter waves for interferometry beyond the classical limit”, Science, Published online 13 October 2011, 10.1126/science.1208798).

In an atomic clock, the atoms internally oscillate stepless between two inner states. To determine the elapse of time, the number of these oscillations within a given period has to be counted. This involves the determination of the inner state of each of the atom. In these measurements, all atoms behave like individual dice - despite the stepless oscillation - since the atoms can only be measured in one of the inner states, corresponding to odd or even results. If 100 dice are thrown simultaneously and the amount of odd and even numbers is counted, one would expect 50 even and 50 odd results. However, small deviations from this expectation, such as 48 even and 52 odd results, are frequently encountered, given the statistical nature and the discrete number of dice. These deviations from the expected value are called shot noise. They also appear when the inner state of the atoms is measured and thus limit the precision of the atomic clock. A measurement beyond the shot noise limit is feasible by utilizing the properties of quantum mechanics. Quantum mechanics predicts that two atoms may be “entangled” with each other. Such entangled pairs of atoms correspond to pairs of dice, which miraculously produce exactly one even and one odd result. If now 50 entangled dice pairs are thrown, always exactly 50 even and 50 odd numbers are obtained and the shot noise limit is beaten. This peculiarity of quantum mechanics has been disputed for a long time. Albert Einstein in particular questioned entanglement as “spooky action at a distance” and claimed “God does not throw dice”. Today, entanglement is understood as a fundamental part of nature and its existence has been proven in many physical experiments.

In the experiments in Hannover, it has been shown that such entangled pairs of atoms can be prepared if the atoms are extremely cold. For this purpose, the scientists cooled some ten thousand rubidium atoms close to the coldest possible temperature, below one millionth degree above the absolute zero point. The rubidium atoms behave like elementary magnets such that the inner states are defined by the magnet’s orientation. Initially prepared with horizontal orientation, the atoms form entangled pairs of up/down orientation which correspond to odd or even dice results described above. “In a series of measurements, we showed that these entangled atom pairs are indeed suited for high precision measurements beyond the shot noise limit”, says Dr. Carsten Klempt, physicist at the Institute of Quantum Optics at Leibniz Universität Hannover. “This process will enable future atomic clocks to benefit from entanglement, which Einstein called a spooky action”, Klempt continues.

The improvement of atomic clocks is beneficial for large variety of modern developments, including the Global Positioning System (GPS) and the precise synchronization of electricity networks or the internet. There is ongoing research for the refinement of other precision instruments, for example in Earth observation devices by measuring acceleration, rotation or gravity using atoms which may also be enhanced by quantum entanglement.

Notes for Editors

For further information, please contact Dr. Carsten Klempt (Institute of Quantum Optics), phone: +49 511 762 2238, e-mail or Dr. Ude Cieluch (QUEST communication and public relations), phone +49 511 762-17481; e-mail, who will be pleased to assist.

Presseinformation vom 13.10.2011


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