Ultracold molecules are promising for applications in new quantum
technologies. Unfortunately, these molecules are destroyed upon colliding
with each other. Researchers at Harvard University, MIT, Korea University
and Radboud University have demonstrated that these collisional losses can
be prevented by guiding the interaction between molecules using microwaves
in such a way that they repel each other and, therefore, do not come close
to each other during collisions. Their paper was published in Science on 13
August.
Upcoming quantum technologies such as quantum computing and quantum
simulation are all the hype right now. Huge leaps are made towards their
realization in various platforms such as trapped ions and Rydberg atom
arrays. Ultracold molecules are another promising platform. Unfortunately,
collisions between the molecules lead to loss as if they were chemically
reactive, which has limited the ability to cool molecules over the last
decade. A team of researchers has now demonstrated these collisional losses
can be suppressed by engineering repulsive interactions between the
molecules using microwaves.
Eliminating collisional losses and boosting elastic collisions will enable
cooling molecules to a quantum gas and bring their application in new
quantum technologies within reach. A unique perk of ultracold molecules is
that interactions between molecules can be tuned and controlled by the turn
of a knob in the lab, using external fields. For example, when the molecules
are exposed to microwaves, their dipole moments will oscillate along with
the microwaves. In this way we can control interactions between the
molecular dipole moments.
Rather than following the microwave field, the dipole moments can also
interlock with one another, which can cause either attraction or repulsion
between the molecules. Repulsion between the molecules can prevent them from
coming close together. "In this way we can shield the molecules from
collisional losses," explains Tijs Karman of Radboud University, who
proposed this method and whose calculations guided the experiment.
Experimental realization
For the first time, microwave shielding has been demonstrated experimentally
in the lab of Professor John Doyle at Harvard University. This experiment
uses calcium monofluoride molecules (CaF) that are cooled to a temperature
of 100 µK using a technique called laser cooling. These molecules are then
stored in individual traps made by focused-down laser light, which are
called optical tweezers. Two of these tweezers, each containing a single
molecule, are then merged to study collisions between exactly two molecules.
To shield the molecules, they are exposed to microwaves from an array of
antennas. In this way, physicists engineered repulsive interactions between
the molecules that shield them from collisional loss. The loss rate has been
reduced by a factor of six.
Cooling to a quantum gas of molecules
In addition to suppressing collisional losses, the repulsion between
molecules when they are far apart leads to fast elastic collisions. Here
elastic collisions are boosted by a factor 17. These elastic collisions are
important for thermalisation. Fast thermalisation and slow loss is exactly
what is needed for further cooling of molecules by evaporation, a
long-standing milestone in the field. Therefore, the shielding demonstrated
here is major step towards creating a quantum gas of ultracold molecules and
realizing future quantum technologies such as quantum computing and quantum
simulation.
Reference:
Loïc Anderegg et al, Observation of microwave shielding of ultracold
molecules, Science (2021).
DOI: 10.1126/science.abg9502
Tags:
Physics