A large team of researchers from MIT, Harvard University, and Sandia National Laboratories has scored a major advance toward building practical quantum computers. The work, which is described in the current Nature Communications, offers a new pathway toward using diamonds as the foundation for optical circuits—computer chips based on manipulating light rather than electric current, basically.
Pushing beyond the quantum computing hype and, perhaps, misinformation, we're still faced with a largely theoretical technology. Engineering a real quantum computer is hard because it should be hard. What we're attempting to do is harness a highly strange and even more so fragile property of the quantum world, which is the ability of particles to occupy seemingly contradictory physical states: up and down, left and right, is and isn't.
If we could just have that property in the same sense that we can have a basic electronic component like a transistor, we'd be set. But maintaining and manipulating qubits, the units of information consisting of simultaneous contradictory particle states, is really hard. Just looking at a quantum system means disrupting it, and, if that system happened to be encoding information, the information is lost.
The almost-perfect lattice structure of atoms in a diamond offers a promising foundation for a quantum circuit. Here, a qubit is stored within a "defect" within the diamond. Every so often within the neatly ordered confines of a diamond, an atom will be missing. In this vacancy, another atom might sneak in to replace the missing carbon atom. This diamond defect may in turn have some free electrons associated with it, and it's among these particles that information is stored (while information is transmitted around the diamond as photons, or light particles).
Crucially, this little swarm of electrons naturally emits light particles that are able to mirror the quantum superposition (the particle or particle system in multiple states). This is then a way of retrieving information from the qubit without disturbing it.
The challenge is in finding and implementing the ideal replacement for the carbon atom in the diamond lattice. This replacement is known as a dopant. This is where the new study comes in.
The most-studied dopant for diamond-defect optical circuits is nitrogen. It's stable enough to maintain the requisite quantum superposition, but is limited in the frequencies of light that it can emit. It's like having a perfect encryption system that can nonetheless only represent like a quarter of the alphabet.
The dopant explored in the new research is silicon. Silicon atoms embedded into a diamond lattice are able to emit much narrower wavelength bands. It's like they have a higher-resolution. But the cost of being able represent information with more precision are more precarious quantum states. Consequently, the diamonds have to be kept at very near absolute-zero temperature. Nitrogen states, meanwhile, can withstand heat up to about four degrees above absolute zero. In either case, we're not exactly talking about quantum laptops.
The researchers were able to implant silicon defects into diamonds via a two-step process involving first blasting the diamond with a laser to create vacancies and then heating the diamond way up to the point that the vacancies start to move around the lattice and bond with silicon atoms. The result is a lattice with an impressively large number of embedded silicon atoms that are exactly where they should be within the structure.
The result is a promising pathway toward reliable fabrication of "efficient light–matter interfaces based on semiconductor defects coupled to nanophotonic devices." The stuff of a quantum computer, in other words.