Diamonds and Ion Qubits

There are three major competing Quantum computer technologies for qubits. They are nitrogen-vacancy (NV) centers in diamond lattices, trapped ions, and superconducting qubits.

The benefit of trapped-ion systems is that the ions are inherently going to affect one another. They’re easily entangled, plus scientists can turn that interactivity on and off easily so that individual qubits don’t engage with unwanted thermal fluctuations or heat that would cause the system to decohere. IonQ and Honeywell are companies investing in and developing trapped ion quantum computers.

Trapped ion qubits are clean and have a long coherence time. As a system, they are really mature. Every system does have a problem with scaling past a certain point. Ion trapping has a viable path to scaling.

Electromagnetic field and lasers are needed to trap and manipulate the ions.

nitrogen-vacancy in Diamond

In the mid-2000s there was a small diamond mined from the Ural Mountains. Is was called the ‘magic Russian sample. The diamond was extremely pure—almost all carbon, which isn’t common but with a few impurities that gave it strange quantum mechanical properties.

Now anyone can go online and buy a $500 quantum-grade diamond for an experiment—from the company Element Six, owned by De Beers. The diamonds Element Six sells have nitrogen impurities—but what Schloss’s group needs is a hole right next to it, called a nitrogen vacancy.

Russian “magic diamonds” can hold qubits in place and thus act the same way that a trapped-ion rig does. They replace a single carbon atom in a diamond’s atomic lattice with a nitrogen atom and leaving a neighboring lattice node empty, engineers can create what’s called a nitrogen-vacancy (NV) center. This is generally inexpensive since it’s derived from nature.

A team of scientists demonstrated, in a paper published in Optica in 2019, the success of the new method to create particular defects in diamonds known as nitrogen-vacancy (NV) color centers. These comprise a nitrogen impurity in the diamond (carbon) lattice located adjacent to an empty lattice site or vacancy. The NV centers are created by focusing a sequence of ultrafast laser pulses into the diamond, the first of which has an energy high enough to generate vacancies at the centre of the laser focus, with subsequent pulses at a lower energy to mobilise the vacancies until one of them binds to a nitrogen impurity and forms the required complex.

The new research was carried out by a team led by Prof Jason Smith in the Department of Materials, University of Oxford, and Dr Patrick Salter and Prof Martin Booth in the Dept of Engineering Science, University of Oxford, in collaboration with colleagues at the University of Warwick.

this method might ultimately be used to fabricate centimetre-sized diamond chips containing 100,000 or more NV centres as a route towards the ‘holy grail’ of quantum technologies, a universal fault-tolerant quantum computer.