Ask the Expert

Artificial atoms, also called quantum dots, are actually made of dozens of atoms, but they act very much just like a single atom in one important respect: when you provide them with the right amount (quanta) of energy, they will emit light with a very distinct color. This property makes them useful in applications as diverse as data communications and medical testing.

To understand how an artificial atom emits light, first consider how a real atom does it. In a real atom, electrons surround a dense nucleus and occupy different “energy levels.” When a particle of light called a photon strikes the atom, an electron will take that energy to temporarily “leap” to a higher energy level. When that “excited” electron comes back “down,” it will release that energy to emit a new photon with a particular frequency, or color. Each different kind of atom (i.e. each element in the periodic table) emits a unique combination of light colors because of the limited number of ways its electrons can change or leap from one state to another. The rules of physics at the size scale of atoms, called quantum mechanics, are what account for these rules or limitations.

When we combine dozens of atoms to make a quantum dot, it is still small enough for these quantum rules to hold. In my lab, we typically make quantum dots out of semiconducting elements, such as silicon or even molecules such as indium arsenide. When we illuminate a quantum dot with the right amount of energy, just like an atom it will absorb that energy and then emit photons of only particular wavelengths. The wavelengths are determined by the size of the dot, which depends on the number of atoms that compose the dot. By controlling the number of atoms in the dots, we are producing an “artificial periodic table,” in which the energy levels of the atoms determines how the electrons are arranged and therefore how they can leap between levels in our artificial atom.

So what good are artificial atoms? In my research we use them to create lasers or to amplify light. We can make quantum dots that emit desired colors of light when they are exposed to electrical currents in a process is similar to that at play in an LED. The current is a flow of negatively charged electrons going one way and holes (gaps in a material where electrons should be, but aren’t) flowing the opposite direction. When the electrons meet the holes, they give off a photon. Properly added to a material, quantum dots can make this happen efficiently, because they can concentrate the flow of electrons and holes much like the drain in a shower. We’ve been able to make artificial atom lasers and amplifiers to improve how we use light in data communications and computer chips.

Other researchers use quantum dots in medical testing. Let’s say you wanted to see whether five particular genes were present in somebody’s DNA. One way is to create five “tags” that will link up just to those five genes. When the tags are exposed to special lights, those tags will then shine if they’ve found and linked to their targets. The trick has been to make good tags. Traditionally, people have used molecules that light up, but each kind of molecule needs to be exposed to its own color of light to work. That’s difficult to arrange because the molecules emit over a wide range of colors that begin to overlap with each other.

Quantum dots, by design, can all be stimulated by the same color of light, however, the colors they subsequently emit don’t overlap. So with just one light source, you can see exactly the colors you expect if the tags have found their gene targets.

We still have a lot to learn about quantum dots, the key challenges today are making them precisely the size we want and placing them exactly where we want them in electronic and photonic devices. We’d like better control of where they end up.

Despite the challenges, quantum dots are proving useful as engineering research tools and are making their way into commercial use. Their structure is artificial, but their value is real.