Solid state materials having quantum properties that can be used, at room temperatures, for storing qubits are leading candidates for use in practical quantum computers. Nitrogen-vacancy centers in diamonds are one promising possibility that has received a lot of attention. However, the right kind of diamond crystals are not easy to grow.
Now another material has been identified that may be well-suited for quantum information applications and also has had a number of industrial uses for many years, so there’s extensive experience with its fabrication into large crystals – silicon carbide. Not only has it been used for over 100 years as an industrial abrasive (carborundum), but it’s also a semiconductor already used extensively in the electronics industry.
The research team discovered that silicon carbide contains crystal imperfections that can be controlled at a quantum mechanical level. …
In conventional semiconductor-based electronic devices, crystal defects are often deemed undesirable because of their tendency to immobilize electrons by “trapping” them at a particular crystal location. However, the UCSB team discovered that electrons that become trapped by certain imperfections in silicon carbide do so in a way that allows their quantum states to be initialized, precisely manipulated, and measured using a combination of light and microwave radiation. This means that each of these defects meets the requirements for use as a quantum bit, or “qubit,” which is often described as the quantum mechanical analog of a transistor, since it is the basic unit of a quantum computer.
The type of defects that are of interest in silicon carbide are quite similar to nitrogen-vacancy centers in diamond. They consist of missing silicon and carbon atoms in adjacent positions in the crystal lattice. These “divacancies” involve multiple-electron systems that have a net spin, so they can represent qubits where the spin is parallel or antiparallel to an applied magnetic field. The quantum state of a defect can be manipulated with infrared laser beams, and it can be read from florescence emitted by the divacancy after a laser pulse.
However, there are many problems that must be solved before either silicon carbide or diamond crystals can be employed in a practical quantum computer. A commentary that accompanies the research article in Nature points out some of the “challenges” that remain before the research results on silicon carbide can be used in practice:
Although silicon carbide qubits offer enticing prospects for quantum computing, a number of challenges for this new technology remain. First, the qubit operations reported by Koehl and colleagues were performed on a large ensemble of qubits, so the next step will be to demonstrate control and measurement of a single qubit. More significantly, technologies must be developed to ‘engineer’ thousands of individually addressable divacancy qubits, rather than merely identifying accidentally located defects. Engineering will also be needed to configure pairs of adjacent qubits reliably, to enable controlled two-qubit operations — another vital requirement for quantum computation.