It takes only a slight disturbance for a pencil standing on its tip to fall in one direction or another. In the quantum world it is possible in principle for particles of a system to fall both left and right at the same time. Differentiating this "and" state -- the quantum entanglement of particles -- from the classical "or" is an experimental challenge. Scientists from Heidelberg University's Kirchhoff Institute for Physics have now devised a novel and universal method that enables entanglement verification for states of large atomic systems. The results of their research in the field of quantum metrology were published in Science.
In their experiments, the team headed by Prof. Dr. Markus Oberthaler used a classically unstable state of an ultracold atomic gas known as a Bose-Einstein condensate. This condensate is an extreme aggregate state of a system of indistinguishable particles, most of which are in the same quantum mechanical state. The Heidelberg researchers used a gas of approximately 500 atoms at a temperature of 0.00000001 Kelvin above absolute zero. After a short time a system with a high degree of quantum entanglement emerged. To be able to experimentally verify this "and" state and its unique quantum mechanical properties, the team had to create a large number of these atomic systems under the same conditions and with different settings of the lab setup. "This process required measurements over several weeks, during which the fluctuations of the magnetic field applied had to be 10,000 times smaller than the magnetic field of the earth," explained the study's primary author, Helmut Strobel.
Another challenge was to correctly analyse the measurements, which required the development of new statistical concepts. The goal was to extract the metrologically relevant information from the measured data. This so-called Fisher information, named after geneticist and statistician Ronald A. Fisher, explicitly and universally quantifies the sensitive dependence of a given quantum mechanical state on the metrologically relevant parameters. According to Markus Oberthaler, conventional methods simply do not work in an atomic Bose-Einstein condensate of this size. Furthermore, the novel method can be used for even larger systems. "We can use it to verify the suitability of any experimental quantum state for precision measurements beyond what can be done with a classical state," continues Prof. Oberthaler. "This is a hot topic in the field of quantum metrology."
Markus Oberthaler heads the Synthetic Quantum Systems working group at the Kirchhoff Institute for Physics. Researchers from the Quantum Science and Technology in Arcetri (QSTAR) research centre and the European Laboratory for Non-Linear Spectroscopy (LENS) also contributed to the work.
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