Holding a Single Atom
Remember middle school physics? You were told it was impossible to see an atom with an optical microscope because they are smaller than the wavelength of light. Maybe you are in middle school right now taking physics, and you've just recently been taught this.
Dr. Mikkel Andersen, an associate professor of physics and leader of the atomic physics department at the University of Otago in New Zealand, was taught the same thing. But, instead of just accepting this as fact, he used his childhood curiosity to fight the odds and create a technique that could capture, isolate and take a picture of a single atom.
I recently got to speak with him about his research, but before we go into how he did it, let's find out why observing an atom with light seems like such an impossible task.
Why we are told it's impossible?
The first thing we always hear is that the wavelength of light is much larger than an atom. The way optical microscopes work is by bouncing light waves off the specimen so that the lens can pick up the resulting refracted light. Similar to the way we see with our eyes.
Since atoms are so much smaller than the wavelength of light, it's hard to get light waves to hit and bounce of the atom.
Another problem is “catching” the atom. Not only are atoms really small, but they also move really fast. With atoms whizzing around at speeds near the speed of sound, it's really hard to catch a group of atoms, let alone a single one.
This brings us to the last problem. Atoms like to travel together. It's very rare that you will be able to catch a single atom and move it around.
Despite these challenges, Dr. Andersen and his team were able to get around them. Let's see how they did it.
Using Lasers to Cool Things Down?
To solve the problem of fast-moving atoms, we need to slow them down. Using Laser Cooling, a technique utilized to trap microscopic specimens by former US secretary of energy Steven Chu, Dr. Andersen was able to create “Optical tweezers” which could slow down and hold individual atoms.
While light waves don't accelerate, they do have momentum. This means when they interact with an object, a momentum exchange occurs between the object and the electromagnetic field from which the light wave originated. This results in an “optical force” or force resulting from the light.
This force affects every object that light hits, even you and me. We just don't notice it because we and the everyday objects we see are so large that the effect is negligible.
As we get down to the nanoscale, however, the effect becomes a lot more prominent. This force from light is very prevalent on atoms due to their size. Dr. Andersen brings up the example of comets and their tails. The tail forms due to the light waves from the sun applying an optical force to the microscopic materials on the comet.
Laser cooling is the process of using this effect to slow atoms down, bringing them to a temperature one-millionth of a degree above absolute zero. By pointing a high-powered laser beam opposite to the direction of the atom’s motion, the light waves from the laser can slow down the atom enough for observations to take place.
It's important to note that the frequency of the laser being used must be the same frequency at which the atom vibrates. This heavily influenced Dr. Anderen’s choice of using the element Rubidium, as its frequency is the same as the lasers used to read CDs in a DVD player making the cost of obtaining the lasers very low.
From 50 to 1
Now I know I was talking in terms of singular atoms but in the process Dr. Andersen uses, they actually start off with a grouping of about 50 atoms. After using laser cooling to slow down the group, they then have to “pop” off atoms until only one remains.
By applying another beam of light with quasi-resonant light waves (waves that are close to being resonant with the frequency of the atoms but not quite), they are able to create collisions within the group which creates enough energy to pop a single atom off. The process is done repeatedly until one atom remains. Dr. Andersen’s group has been able to isolate a single atom 90% of the time.
Other research groups have been able to achieve this before, but only with an accuracy of around 50%. I asked Dr. Andersen what makes his group’s method more deterministic and he says it comes down to the amount of energy created when the light collides with the aggregate.
The atoms in the group usually have a tendency to pop off in pairs of two. This is why other groups only reach an accuracy of 50%. Because there is about a 50/50 chance that the amount of atoms they initially start off with is odd. An odd amount results in one atom at the end while an even amount would result in none as the last two atoms leave together.
Dr. Andersen's group has solved the problem by figuring out a way to have more control over the amount of energy released in the collision to where there is enough for one atom to bounce but not enough for it to take another atom with it.
Deep Space Tech for Nanoscale Science
So now we finally got our atom in position, how do we actually see it. Well, first we need a lens, the type of lens used for space telescopes to look into deep space.
By trapping our atom in the focal point of this lens, we can send a beam of quasi-resonant light at it causing light to be emitted from the particle in a process known as fluorescence imaging. Because of how small an atom is, very few photons of light are emitted. This is why we need the lens from a deep space telescope, as they are made specifically for detecting little amounts of photons in a sea of darkness.
From here all you got to do is snap the pic. We have now slowed down, trap, isolate and take a photo of a single atom.
Combining these Atoms Together
The photo above was taken in 2010. Since then they have been doing research to increase the amount of control they can get over individual atoms and discover the underlying processes that control chemical reactions.
They have been able to use optical tweezers to control two individual particles, combine them together and observe the energy released in the process. Recently they have bumped that number up to three to manually form a molecular triad.
When I asked Dr. Andersen what they are working on now, he told me about their goals to try and manually entangle atoms. Quantum entanglement at a high level is when the outcome of one particle determines the outcome of the entangled counterpart despite the distance between them.
Given more control over individual atoms through laser cooling, this has opened up doors to manipulate two atoms in a way to manually get them entangled with each other.
All of this is very exciting stuff but I'm sure you're wondering, what can we do with these findings?
The Future of Atomic Precision
When asked about the possible applications, Dr. Andersen talked about the use of this research for developing quantum computers. While that is not the goal of their research they see a benefit in being able to control single atoms when trying to create qubits for quantum computing.
Individual control of atoms would also give us the ability to create transistors the size of a couple of atoms allowing us to reach the physical limit for transistor size.
Ultimately, this research is a stepping stone on the way to finding out how atoms interact in an isolated environment, how they bond together, and how we could control the process to create molecules that we can’t form through self-assembly.
Despite the work that has been done though, creating commercial quantum qubits or products regularly with atomic precision is still many years away. I asked Dr. Andersen why this is and what research needs to be done before this can become a reality.
Dr. Andersen doesn’t try to hide that the work they are doing is very basic in comparison to full-scale control of atomic systems. One of the main problems comes from how fragile the current systems are. The process of isolating a single atom takes a lot of energy and costs, and as more atoms get introduced, the process becomes more complex. Also, using different elements can increase the cost as they have different resonant frequencies requiring different types of lasers. Many of the formations we want to make aren’t chemically stable as well, and a lot of energy is required to form them and prevent them from bonding with the surrounding particles.
The other issue he brought up was that the application of the technique must justify using it. We need to get to a point where this method of manufacturing either becomes better than our current ways either in cost or efficiency or is necessary to create a product that can’t be created with conventional means.
Closing Thoughts
While these obstacles seem daunting, Dr. Andersen finds his research as well as research similar to his, important as it will help give future generations the tools and knowledge needed to solve these problems.
Being part of this future generation, it was inspiring to know that there are people out there working on an idea that seems crazy when you think about it. Everything that we see is made up of atoms and people are out there trying to figure out how to manipulate them to create things with the same precision the universe uses.
Maybe we should use this experiment as a lesson. Dr. Andersen was consistently told you couldn't see an atom with light and then he did just that. Next time you are skeptical about something your science teacher says, dive deeper into it. You might just be on the track to a breakthrough.
If you liked this article, please check out Dr. Andersen’s research group with this link.
Want to reach out? You can email me at williamslater2003@gmail.com and check out my recent Newsletter here.
TL;DR
- In middle school, most of us are told we can't see a single atom with an optical microscope
- An associate professor from the University of Otoga in New Zeland, Dr. Mikkel Andersen, leads a research group that did just that
- Using Laser cooling they can slow the atom down
- By controlling the amount of energy coming from a laser beam, they can break a group of atoms apart until there is one left
- Using a deep space telescope lens, they can detect the little number of photons coming off of the single atom
- Further research the group has achieved involves combining two and even three atoms together with full control
- The process could be used in the future to create atomically precise products but right now, the cost and fragility of the systems make this a goal for the distant future.
