A brake for spinning molecules

The precise control of the rotational temperature of molecular ions opens up new possibilities for laboratory-based astrochemistry

March 13, 2014

Chemical reactions taking place in outer space can now be more easily studied on Earth. An international team of researchers from the University of Aarhus in Denmark and the Max Planck Institute for Nuclear Physics in Heidelberg, discovered an efficient and versatile way of braking the rotation of molecular ions. The spinning speed of these ions is related to a rotational temperature. Using an extremely tenuous, cooled gas, the researchers have lowered this temperature to about -265 °C. From this record-low value, the researchers could vary the temperature up to -210 °C in a controlled manner. Exact control of the rotation of molecules is not only of importance for studying astrochemical processes, but could also be exploited to shed more light on the quantum mechanical aspects of photosynthesis or to use molecular ions for quantum information technology.

Crystal size and shape control the heating of molecular ions

The physicists increase the micro-motion velocity of the molecular ions by varying the shape and size of the ion crystal in the trap: they knead the crystal as it were by means of the alternating voltage which is applied to the trap electrodes. The alternating field that the electrodes produce is equal to zero only along the trap axis. The further the molecular ions are located away from this axis, the more they feel the oscillating force of the field and the more violent is their micro-motion. Part of the kinetic energy of the swirling molecular ions is absorbed by the helium atoms in collisions, and these atoms in turn transfer it to the rotational motion of the ions, thus raising their rotational temperature.

For the Danish-German collaboration, the ability to control the rotation of the molecular ions not only enables the manipulation of the micro-motion, and thus the rotational temperature, but also the quantum-mechanical measurement of this temperature. The scientists do this by exploiting the fact that the rotational motion of the molecules is quantised. Put simply: the quantum states of a molecule correspond to certain speeds of its rotation.

At very cold temperatures the molecules occupy only very few quantum states. The researchers remove the molecules of one quantum state from the crystal by means of laser pulses whose energy is matched to that particular state. They determine how many ions are lost in this process, in other words how many ions take on this particular quantum state, from the size of the crystal remaining. They determine the rotational temperature of the molecular ions by thus scanning a few quantum states.

Accurate control of quantum states is a prerequisite for many experiments

“Being able to control the rotation of the molecular ions and thus the quantum state so accurately is important for many experiments,” says José Crespo. Scientists can therefore recreate in the laboratory chemical reactions that take place in space if they can bring the reactants into the same quantum state in which they drift through interstellar space. Only in this way can one quantitatively understand how molecules are formed in space, and ultimately explain how interstellar clouds, the hotbeds of stars and planets, evolve both physically and chemically.

This speed control knob for rotating molecules could also contribute to a better understanding of the quantum physics of photosynthesis. In photosynthesis, plants use the chlorophyll in their leaves to collect sunlight, whose energy is ultimately used to form sugars and other molecules. It is not yet entirely clear how the energy required for this is quantum mechanically transferred within the chlorophyll molecules. To understand this, the researchers must once again very accurately control and measure the quantum states and the rotation of the molecules involved. The findings thus obtained could serve as the basis for imitating or optimising the photosynthesis at some time in the future in order to supply us with energy.

Last but not least, this control is a prerequisite for quantum simulations as well as for many concepts of universal quantum computations. In quantum simulations physicists mimic a quantum mechanical system that is difficult, or even impossible, to examine directly with another quantum system that is well-known and controllable. In universal quantum computers which physicists are trying to develop, the aim is to process information extremely quickly using the quantum states of particles. Molecules are possible candidates for this, their chances now growing as molecular rotation can be quantum mechanically controlled.

“Our method for the cooling of the rotation of molecules opens up new possibilities in a variety of different fields,” says José Crespo. His team, too, will now use the new method to investigate open questions about the quantum mechanical world.

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CryPTEx – a trap for cold ions

CryPTEx, the Cryogenic Paul Trap Experiment, is a cryogenically cooled trap setup developed and built by the team of José R. Crespo López-Urrutia at the Max Planck Institute for Nuclear Physics (MPIK) in Heidelberg based on a trap design by collaborator Michael Drewsen of Aarhus University (AU) in order to investigate highly charged ions (HCI). Production, trapping and spectroscopy with HCI are the fields of expertise of the Max Planck group, which uses various ultrahigh vacuum cryogenic settings for their investigation. These specific conditions required for HCI studies are also very beneficial for the study of molecular ions. The Heidelberg team then moved CryPTEx to Aarhus and commissioned the apparatus there together with the local team. Trapping and manipulation of molecular ions is the specialty of the Aarhus group, which has pioneered many of the laser-based techniques now used in the field. Drewsen saw the novel opportunities for cooling molecular ions in the cryogenic setting, including the application of an ultra-tenuous helium buffer gas. Thus, CryPTEx stayed in Aarhus for one year, where the young scientists from both groups carried out long series of experiments and tested new ideas. During those experiments, ion crystallisation and buffer gas cooling could be achieved simultaneously over a wide range of effective temperatures, down to the lowest ever recorded for a molecular ion.

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