Manipulating individual atoms

In an incredible experiment, a six-member team from the University of Otago, Dunedin (New Zealand), has successfully isolated and observed the interactions of three individual atoms in the process of molecular bonding. This chemical activity was zoomed and recorded on a special camera and it offers unprecedented detail about molecular processes.

The experiment as described in Direct Measurements of Collisional Dynamics in Cold Atom Triads involved trapping and cooling three atoms of rubidium down to nearly absolute zero. The research team consisted of Mikkel Anderson, Luke Reynolds, Marvin Wayland, Eyal Schwartz, Ulrich Ebling and Joachim Brand.  

Rubidium molecules contain two atoms but a minimum of three atoms is required to start the chemical process of molecular bonding. When a molecule is formed, all three atoms may receive “a kick of energy” in what’s called a three-body recombination.

Prior to this experiment, scientists have often modelled the interactions of large “clouds” of atoms to understand the processes of molecular formation. So this process has been studied in statistical terms, and there’s data when it involves interactions of many atoms. But new details were revealed in this study of the interactions of the minimum number of three atoms.  

The three cold atoms were trapped with laser beams known as optical tweezers within a vacuum chamber, about the size of a toaster. Optical tweezers as they are called are “single beam gradient force traps”. A highly focussed laser beam is used to hold and manipulate microscopic objects.

The laser beam creates a field, which holds the target particle. The shorter the wavelength of the laser, the smaller the objects it can be used to trap and manipulate. Optical tweezers have been used to trap individual molecules before, and their use in the studying of bio-molecules is common. Optical tweezers have been used to isolate, and study individual viruses for example.

This phenomenon — that laser beams can trap small objects — was discovered in the early 1970s by Arthur Ashkin, then a scientist at Bell Labs. Ashkin and his team at Bell Labs succeeded in building the first optical tweezers in 1986.  In 2018, Ashkin was awarded the Nobel Prize for physics.

This experiment in New Zealand was the first time individual atoms had been trapped and forced to interact in a “few-body regime”. After the atoms had been cooled to near absolute zero, three separate traps were used to isolate the three atoms inside the vacuum chamber. Then the three traps were brought very close together to allow the atoms to interact. Two of the traps were removed, leaving the three atoms to collide within the remaining trap.

This was done multiple times with the collision dynamics being observed on each occasion. The results varied from a three-body recombination (with all the atoms being scattered from the trap by the energy “kick”), to the formation of a two-atom molecule with no kick of energy imparted to the third, leaving the unpaired atom in the trap.

The experiment also apparently measured a longer-than-predicted recombination rate. The data indicates it took much longer than expected to form a molecule compared to theoretical calculations. This discrepancy cannot be explained, which means that it could certainly be a rich area of research.

Rubidium is an alkali metal with an atomic number of 37. It’s highly reactive, catching fire in atmosphere, which means it is never found isolated in nature. The most stable isotope, Rubidium-85 is quite abundant. It is a favourite element in quantum mechanics experiments. Rubidium has already been extensively studied in Bose Einstein Condensates (BEC). BECs are a special state of matter created when bosons (a type of particle named after Satyendra Nath Bose) are cooled to very low temperatures.

The existence and behaviour of BECs was predicted in 1924 by Bose and Einstein but the existence was only confirmed in 1995 when a BEC of Rubidium-87 (another isotope of Rubidium) was first created. That demonstration led to the 2001 Nobel for Physics.

According to Bose-Einstein statistics, a BEC sees many particles dropping into the same low–energy state in unison. Since 1995, BEC behaviour involving Rubidium has been studied in detail, leading to an understanding of “many-body” molecular processes involving the element.

But as the new Otago University paper explains, “there is currently no reliable theory for quantitatively describing three-body processes in an optical tweezer trap”. The team captured the three-body interaction, and realised that the atoms did not unite as quickly as the theoretical models predicted. This is presumably a quantum effect but explaining it satisfactorily will involve developing new insights.

Apart from the theoretical implications and insights provided by this experiment, the demonstration shows that it is possible to manipulate individual atoms. That could have huge applications in nanotechnology in the future.

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