Single atom as a highly sensitive force sensor

Synchronization with external vibrations makes atoms extremely sensitive to forces

Atoms © Forschungszentrum Jülich
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A "sensor" of only one atom even responds to forces of just five quadrillionths of Newton, as shown by a now published in the journal "Physical Review Letters" experiment. This was achieved by a special treatment of a magnesium ion in an ion trap and the measurement of synchronization with an external field. In the future, this technique could be used, for example, to measure the magnetic field of individual molecules or to verify fundamental theories.

Already in the 17th century, the Dutch physicist Christiaan Huygens noticed that two pendulums can exactly synchronize, if they have the opportunity to influence each other. Interestingly, this interaction does not have to be strong at all, in the example of the pendulum it can be enough if both are hung on the same wall. Today it is known that a huge variety of oscillatory systems show this synchronization characteristic, from organ pipes to lasers to electronic oscillators. A research group at the Max Planck Institute for Quantum Optics has now succeeded in observing this technically important phenomenon even in a single extremely cold atom.

Ion in the electrode trap

The starting point for this experiment is a single magnesium ion, which in a so-called

Paul trap is stored. The alternating electric fields of electrodes arranged in a square hold the ion at a certain point in the trap, while a vacuum system ensures that it can swing as undisturbed as possible. Now two lasers come into play, which are carefully tuned to vibrate the ion stably with an amplitude of about a tenth of a millimeter back and forth. High-resolution optics and a sensitive camera allow this oscillation to be registered using the scattered light.

In order to investigate the synchronization of this optically excited oscillator with an external source, another weak alternating electric field - a radio signal - is applied to a nearby electrode and the oscillation of the ion observed stroboskopisch. The frequency of the external field differs only slightly from the vibration frequency of the ion. When the amplitude of the radiated radio signal is large enough, the oscillation of the ion synchronizes and it oscillates in phase with the radio signal. display

Reaction already at five quadrillionths of Newton

A careful determination of the forces exerted by the incident alternating field on the ion shows that synchronization takes place even with extremely weak excitations of only five Yoctonewtons - that is five quadrillionths of a Newton. By this force, the ion would only be displaced by about one nanometer from its rest position. It would scarcely be possible to prove this without the Tricks of the researchers used here, because the ion alone swings back and forth with an amplitude of 5, 000 nanometers due to its temperature alone.

The now proven high sensitivity of a single atom could now be used, for example, to determine the magnetic field of a single molecule with the help of such an atomic sensor and thus to verify fundamental theories. The work presented here is a promising first step in this direction.

(Max Planck Institute of Quantum Optics, 06.08.2010 - NPO)