Physicists from the Lawrence Berkeley National Laboratory and the University of California have recorded the weakest force ever measured — just 42 yoctonewtons. This is equivalent to an influence billions of times weaker than the weight of a speck of dust. The experiment brought scientists closer to the fundamental barrier of precision known as the standard quantum limit.
The measurement was based on a mechanical oscillator made of 1200 rubidium atoms cooled almost to absolute zero. The cloud of atoms was held in an optical trap made of laser beams. An external force was applied by modulating one of the lasers, and the response was registered by a probe. This design made it possible to isolate the system from external noise and to minimise the back-action of the measurement on the oscillator itself.
The standard quantum limit arises from Heisenberg's uncertainty principle: the very act of measurement inevitably disturbs the object. In ordinary instruments this creates a "back-action fluctuation" that masks weak signals. The California team overcame part of this barrier by using only 1200 atoms and laser isolation, which yielded a sensitivity four times higher than previous records.
The achievement opens the way to more precise detection of gravitational waves — those very "ripples" of space-time predicted by Einstein. Modern interferometers cannot yet confidently register the weakest signals from the early Universe. The new sensitivity could change that.
The authors note that further improvement of the optical detector will allow them to come even closer to the theoretical limit. Theorists predicted the possibility of such a measurement back in 1980, but the actual experiment required thirty additional years.
The work has been published in the journal Science and shows that quantum limitations are not absolute — they can be circumvented with elegant engineering solutions.
