They get for the first time the quantum control of a molecule

German and North American researchers have managed to control the quantum states of molecules, opening the door to a new technology that will help the processing of quantum information and increase the control of chemical reactions. They have achieved this by joining a molecule to an atom, which they used to manipulate the molecule.

More than twenty years ago, scientists learned to control the quantum parameters of atoms and other elementary particles such as the electron or the photon, enabling the construction of atomic clocks or calculations related to the pursued quantum computer.

Now, a German-American team has discovered the system by which to quantumly control molecules. This type of control of molecular ions – several atoms linked together and carrying an electrical charge – could lead to more sophisticated architectures for processing quantum information, amplifying signals in basic physical research, such as measuring “roundness” of the shape of the electron and increase the control of chemical reactions.

In comparison to atoms, molecules are more difficult to control because they have more complex structures that involve many electronic levels of energy, vibrations and rotations. The molecules can consist of many different numbers and combinations of atoms and be as large as the strands of DNA, more than a meter long.

Like atoms, molecules also have quantum parameters. However, molecules are more fragile, in quantum terms, than atoms: their manipulation and measurement often leads to the destruction of their coherence and therefore to the loss of information and characteristics that could be useful for a calculation or measurement .The new method finds the quantum state (electronic, vibratory and rotary) of the molecular ion transferring the information to an atomic ion, which can be cooled by laser and controlled with previously known techniques. The quantum state of the molecule is controlled indirectly, through the atomic ion.

Researchers have trapped two calcium atoms in an optical cavity some micrometers in diameter and put them in a vacuum chamber at room temperature. Then they introduced gaseous hydrogen until a calcium ion reacted to form a molecular ion of calcium hydride (CaH +), formed by a calcium ion and a hydrogen atom linked together.

Like a pair of pendulums that are coupled by a spring, the two ions can develop a shared movement due to their physical proximity and the repulsive interaction of their electric charges.

The researchers used a laser to cool the atomic ion, cooling the molecule to the lowest energy state. At room temperature, the molecular ion is also in its lowest electronic and vibratory state, but it remains in a mixture of rotational states.

Using laser pulses at different moments of the experiment, they managed to orient the state of rotation of the molecule in a desired direction and thus control its quantum state.

According to the researchers, this technique can be applied to all molecules and thus provide much more quantum information than that given by atoms, thus making it much more powerful, for example, to a quantum computer.

Preparation and coherent manipulation of pure quantum states of a single molecular ion. Nature 545, 203–207 (11 May 2017) doi:10.1038/nature22338

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