Rebecca Han

Rebecca graduated from MIT in 2014 with a Bachelor of Science degree in physics and chemical engineering. While she continues her graduate education at Georgia Tech in the fall, she hopes to have more time to play squash, learn German, and acquire a pet tarantula. After a long day her favourite way to unwind is by watching Beavis and Butthead, Superjail, or the Mighty Boosh.

Sir Arthur C. Clarke, the famous science fiction author wrote, "In the long run, there are no secrets in science." There are moments when an idea is ready to be discovered, when great minds thinking alike separately converge to a triumphant "Eureka!"—literally, "I have found it!" This is such a story.

In the late 2000s a new material called quantum dots was trending as the latest "hot topic" in engineering and science. An iconic image of the quantum dots made its way onto popular science magazine covers: a row of vials on black background, each filled with different luminous colors, like a rainbow distilled. This is a story of how two physicists separately looked at that rainbow, saw a great new idea for neutrino studies, and are now working to make that idea a reality.

Quantum dots are nanocrystals between 5-10 nanometers in diameter—about the distance your fingernail grew in the time it took you to read this sentence. These "dots" have a spherical core made of a blend of semiconducting materials, like the ones in solar cells or your iPhone's computer chips. Because of their tiny size, they exhibit quantum mechanical properties, like the ability to absorb UV light and re-emit it as visible light. The size of the crystal regulates the wavelength, and hence the color, of the re-emitted light. Changing a quantum dot's diameter by just a few nanometers is what allowed scientists to create the dramatic colors radiating from those vials: larger dots are redder, smaller dots are bluer.

The late 2000s was also when the smartphone industry was just taking off. Electronic display companies poured money into quantum dot research, and the fever spread quickly. Computer scientists started arranging arrays of dots for faster quantum computation. Biologists found ways to tag antibiotics with quantum dots and follow their real-time journey through the human body.

Physicists Lindley Winslow (at the time an MIT post-doctoral assistant) and Taritree Wongjirad (a Duke University graduate student) had also caught onto the excitement, but not for the same reasons as the rest of the world. Their instincts led them independently to a similar hunch. Could quantum dots give clues to the universe's "missing" antimatter?

Most of the matter in the universe was created during the Big Bang. According to conservation laws, antimatter and matter should have originated in identical amounts. The problem is, particles and antiparticles that come in contact vanish in a burst of energy. Yet a universe of un-annihilated matter exists, meaning there's a "missing" universe's worth of antimatter. This imbalance calls into question the completeness of present theories.

There is an alternative possibility, one that Winslow and Wongjirad are hoping to prove. In 1928, physicist Ettore Majorana was studying in Rome when he proposed that the neutrino could be its own antiparticle. Shortly after Majorana published the mathematical model that provided the basis for his idea, he disappeared while travelling to Naples, Italy, lending further intrigue to the "Majorana particle" hypothesis.

If neutrinos are their own antiparticles, that might lead to a theory that explains a good chunk of the universe's missing antimatter. That would also mean that neutrinos could annihilate themselves. To test this theory, Winslow and Wongjirad plan to use quantum dots to detect the creation and sudden disappearance of two neutrinos. This phenomenon, known as neutrinoless double beta decay, would strongly suggest self-annihilation.

Detecting neutrinoless double beta decay requires two key components. First, physicists need a radioactive source that produces two neutrinos at once. Sometimes unstable heavy metals want to decay (to emit subatomic particles) and settle into a more stable state. Imagine pushing an overflowing shopping cart very fast: at first some groceries fall off the top, until the cart is now well-balanced although still heavy. Elements tend to radioactively decay in the same patterns (the same types of particles "fall off"). For example, in beta decay, a neutron in the atom's nucleus turns into a proton and emits an electron and a neutrino in the process. Of the elements that beta decay, a few actually undergo double beta decay—they simultaneously decay twice to reach their most stable form. This produces two electrons and, more importantly, two neutrinos! Fortuitously, two of these metals (cadmium and selenium) are exactly the materials used to make quantum dots.

A second requirement for studying neutrinoless double beta decay is a detection system sensitive enough to discriminate between real decay events and false background signals. One clever method uses what is known as a "liquid scintillation tank." To reduce background noise, these are often set up in mine shafts deep underground. The tank is a large round sphere between 10 to 20 meters across filled with liquid that sparkles, or scintillates, when charged particles (like those from radioactive decay) go through it. The two stray electrons from a double beta decay cause the liquid to sparkle twice as much. Theoretically, sparkling from a double beta decay will be brighter then the light created by a neutrinoless double beta decay. This detection system achieves very good sensitivity because the liquid scintillator can be filtered until it is very pure—in this way, experimentalists can be reasonably certain that the only scintillation they measure comes from the metal they added.

However liquid scintillation tanks are not perfect. One problem is that the heavy metals capable of producing double beta decay do not dissolve well in the liquid. Here is where quantum dots come to the rescue! The dots are crystals grown from heavy metals. This process happens in a solvent which itself is a good scintillating liquid. Therefore, dots are easily added to scintillators, much like pouring sugar water into tea. Another problem is that the initial "sparkle" is UV light. Not only can we not see UV, the usual light detectors we use are best at detecting light between 360-460 nanometers in wavelength, which is visible light. Again, quantum dots come to the rescue! Not only can quantum dots absorb UV light and reemit it as visible light, physicists can also control the color of a quantum dot by changing its size. Thus, physicists can make dots that will glow precisely in the detectors' optimal wavelength range—these look cyan or cobalt to us.

In 2011, after some trial and error (mostly trying different brands of commercial quantum dots) Wongjirad finally detected scintillation in his laboratory setup. His experiment was not designed to look for neutrinoless double beta decay yet, merely to verify the concept. Meanwhile, Winslow successfully tried several types of quantum dots (purchased from different vendors) and characterized their properties. She found that the greatest current challenges are consistency and stability of commercially available quantum dots; not all quantum dots in a batch will have perfectly uniform size or efficiency, and the nanocrystals lose efficiency as they age.

Nevertheless, Winslow and Wongjirad have reason to be optimistic. As quantum dots are rapidly integrated into customer electronics—Sony boasts the world's first quantum dot television screen, Amazon wants to use quantum dots in their Kindles, and it is widely rumored that the iPhone 6 screen will have quantum dot technology—companies like Sony and Samsung will certainly find ways to make larger quantities of higher-quality dots more cheaply.

Winslow and Wongjirad have now teamed up on the NuDot experiment. Perhaps even within this decade, they will have another "Eureka!" moment—this time, the discovery of neutrinoless double beta using a quantum dot detector.