The lowest energy of quantum system is set by the so-called quantum fluctuations. Usually, these fluctuations are difficult to observe in a quantum system containing many particles, since the energy is much larger due to the particles interaction. In recent experiments, a mixture of particles in two quantum states was used to turn off this interaction completely and thus we were able to observe the effects of quantum fluctuations clearly.

Experimentally, this was realized using so-called Bose-Einstein condensates, which are extremely cold and pure clouds of atoms. In these systems, the effect of quantum fluctuations was first predicted theoretically by Lee, Huang and Yang and is consequently called the LHY energy.

In the experiments, potassium atoms in two quantum states with well controlled atom numbers were used. By adjusting the interaction strength between the two states to be attractive, while the interactions between atoms in each state are repulsive, an effectively non-interacting system was realized.

Then the system was compressed to set it into motion and the resulting frequency of its oscillation was measured. This frequency depends critically on the state of the system, which is now governed only by the LHY energy. The measured frequency was indeed found to be in very good agreement with a complete simulation of the experiment, confirming that the systems behavior is dominated the LHY fluctuations.

These results pave the way for a deeper understanding of the quantum physics of many particle systems and for the use of the LHY energy in quantum simulation experiments.

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