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 internal oscillation frequency of the cesium atom, and realized by so-called atomic clocks. This internal 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 smoothly oscillate between two internal states. To determine the elapse of time, the number of these oscillations has to be counted. This involves the determination of the internal state of each of the atoms. In these measurements, all atoms behave like individual dice - despite the stepless oscillation - since the atoms can only be measured in one of the internal 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 internal 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 was skeptical in general, saying “God does not play 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 internal 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.
Entangled dice. Entanglement can be visualized by throwing magic dice. If the dice are entangled, they form pairs (indicated by the dice’s color), while each individual die can give any possible result. However, if one of the paired dice is even, the other is bound to be odd. Einstein called this effect the “spooky action at a distance”.
Measurement of the number of atoms. The ultracold cloud of atoms is produced with horizontal magnet orientation (central atomic cloud). The entangled atoms are then produced with either upward (left cloud) or downward (right cloud) orientation. The three clouds are initially prepared at the same position. By applying a magnetic field, the atoms split into the three clouds according to their orientation. The number of atoms is then represented by the volume of the three peaks. The number of atoms in the right peak is almost perfectly equal to the number of atoms in the left peak, corresponding to the equal number of odd and even dice, and they are entangled.
