If you’re shopping for an engagement ring, you’re probably going to want a diamond with little to no discernible flaws. But researchers at Princeton University have discovered that implanting flaws in diamonds could be a key to preserving quantum communications information over long distances.
In theory, quantum information networks could allow quantum computers to work together to complete problems that are currently unsolvable. They would also be extremely secure. On the downside, however, the information they contained would take the form of fragile quantum bits, also known as qubits.
Scientists have looked to adapt a method used by standard communications networks, in which devices called repeaters briefly store and re-transmit signals in order to allow them to travel greater distances. A problem with the idea of quantum repeaters, though, is that “nobody knew how to build them,” according to Nathalie de Leon, an assistant professor of electrical engineering at Princeton University.
The key challenge is finding a material that could both store and transmit qubits, which are typically encoded in particles of light known as photons. Optical fibers can be used to transmit information via photons, but qubits can travel only short distances in a fiber before their quantum properties are lost and information is scrambled. Trapping and storing photons is challenging because, by definition, they move at the speed of light.
Researchers have looked instead to solids such as crystals. Theoretically, qubits could be transferred within a crystal from photons to electrons, which are easier to store. The best place to carry out that transfer would be a color center within the crystal, and in the case of a diamond, an impurity representing an element other than carbon trapped within the carbon lattice. These impurities produce colors, a phenomenon which for researchers presents an opportunity to manipulate light and create a quantum repeater.
Led by de Leon, the Princeton team partnered with Element Six, an industrial diamond manufacturing company, to construct electrically-neutral “vacancies” within the diamonds - flaws in which a different element is used to replace carbon atoms. They focused on silicon, which in theory should be electrically neutral but can be influenced by the presence of other nearby impurities. In order to minimize that influence, they added boron atoms to crowd out other impurities; heated the diamonds to high temperatures to remove additional impurities; and even performed analyses in collaboration with scientists at the Gemological Institute of America (GIA), the originator of the “4Cs of Diamond Quality,” with which anyone doing shopping for a wedding proposal is bound to be familiar.
The neutral silicon vacancy turned out to be good at both transmitting quantum information using photons and storing quantum information using electrons, key ingredients in creating the quantum property known as entanglement, which allows particle pairs to stay correlated even if they become separated. Entanglement is the reason quantum networks are inherently secure - a message recipient can compare measurements of an entangled pair to see if an eavesdropper has corrupted one of the messages.
The next step is to build an interface between the neutral silicon vacancy and the photonic circuits, which will bring the photons from the network into and out of the color center.
The research appears in the July 6 edition of Science.