24MgH+ (orange) and 25Mg+ (green) together in a linear ion trap. An oscillating dipole force changes the motional state according to the rotational state of the molecular ion.

In the quantum world, it is sometimes difficult to measure something without destroying it in the process. A research team based in Germany has learned how to measure a particular quantum state of a trapped molecular ion without ruining it (Nature, doi:10.1038/nature16513).
In this case, the group of scientists at the Physikalisch-Technische Bundesanstalt (PTB) wanted to investigate the rotational quantum state of a molecular ion. Some atoms reveal their internal quantum states under laser illumination, but most molecules do not unless scientists break them up with laser light.
The researchers, led by Piet O. Schmidt of PTB's QUEST Institute, placed a molecular ion, 24MgH+, and an atomic ion, 25Mg+, together in a laser-cooled linear Paul trap. The ions, which exhibit strong Coulomb coupling with each other, oscillate in a one-dimensional optical lattice. The team then cooled the ions down to their ground state and probed the qubits with beams from a frequency-quadrupled Raman fiber laser system. Because the molecular and atomic ions are coupled, changes in the internal quantum state of one affects the internal quantum state of the other. Thus, scientists can measure the quantum state of the atomic ion and use it to calculate the state of the co-trapped molecule.
In regular practice, the entire non-destructive process of determining the internal state of a molecular ion could take less than 10 ms to accomplish. The scientists believe this technique could apply to other species of molecular ions and could lead to high-precision optical clocks based on such ions.