Could Diamonds Unlock Improved Qubits for Quantum Computing?

These properties enable QPUs to solve highly complex problems much faster than classical computers by exploring multiple solutions simultaneously.

“The advantage of qubits is that they can hold much more information than regular bits can. This means that they can also give us much more information about their environment, making them extremely valuable as sensors, for example.”

Alastair Stacey- Managing principal research physicist and head of quantum materials and devices at PPPL.

However, qubits are extremely fragile, and measuring their properties is not an easy task.

So what if we instead counted on one of the hardest materials on earth – diamond – to perform tasks in our most advanced computer? This is the vision of researchers at Princeton University, who published recently in Diamond And Related Materials, under the title “Quantum chemistry model of surface reactions and kinetic model of diamond growth: Effects of CH3 radicals and C2H2 molecules at low-temperatures CVD”.

This joins the works of other researchers at the University of Melbourne and Princeton University, published under the title “Methods for Color Center Preserving Hydrogen-Termination of Diamond.”

Growing Diamonds On Demand

Diamonds, historically only a natural stone, are mostly manufactured from raw carbon these days. However, this process requires very intense heat and pressure, so it cannot be combined with other materials like silicon used in computer chips. For this, low-temperature diamond manufacturing is needed.

Some methods have already been explored, like using acetylene and a technique called “plasma-enhanced chemical vapor deposition”.

The problem with it is that while it can grow microscopic diamonds, it also deposits a lot of soot, which can grow on top of the diamond and inhibit performance for optics, sensors, and chips. Until now it was not clear why the soot formed instead of diamonds.

Goldilocks Temperature & Hydrogen

The researchers found that there is a precise temperature at which the process creates a diamond. Above this critical temperature, acetylene contributes mostly to diamond growth. Below this critical temperature, it contributes mostly to soot growth.

Source: Diamond And Related Materials

Another factor is the activity of hydrogen atoms near the surface of the diamond. With more hydrogen near the surface, more diamonds can form, even at lower temperatures.

“Hydrogen atoms don’t fuel diamond growth directly, but hydrogen dissociation, or breakdown, is crucial for transforming methane into acetylene and transporting atomic hydrogen to the diamond growth surface. These are both important for diamond growth,”

Alexander Khrabry – Princeton University Research Scholar

Together, these insights in diamond formation open the way for reliably creating microscopic diamonds directly inside silicon semiconductors without damaging the rest of the material with high temperatures or creating unwanted soot.

Quantum Diamonds

Simple diamonds made only on carbons could have some applications in optics and sensors. But more advanced forms of diamonds could be even more useful.

For example, quantum diamonds are made when some of the carbon atoms forming the diamond are replaced by other atoms, like for example nitrogen, and some other carbon atoms are just removed. This creates a so-called nitrogen-vacancy (NV).

In such a diamond, the electrons inside start to follow quantum rules instead of classic physics, which could be used to build qubits.

“The electrons in this material don’t behave according to the laws of classical physics as heavier particles do. Instead, like all electrons, they behave according to the laws of quantum physics.”

Alastair Stacey- Managing principal research physicist and head of quantum materials and devices at PPPL.

Perfecting The Diamond Cookbook

Until now, the method of using plasma to create diamonds has been far from precise. It was using a lot of trial and error, as the theory of what exactly happens at the surface of the diamond is not well understood.

Ideally, plasma could also be used to add a mono-atomic layer of hydrogen on top of the diamond. But in the case of quantum diamonds, the high temperature would destroy the nitrogen-vacancy.

So the researchers built an elaborate analytical system (using photoluminescence spectroscopy) to judge what works best for creating a hydrogen layer on NV diamonds.

Source: Advanced Materials Interface

They found that 2 new methods could be used, although each with its own drawbacks for now.

Source: Advanced Materials Interface

• Forming gas annealing, which uses a mixture of hydrogen molecules and nitrogen gas, worked but required very pure hydrogen gas without any oxygen, something difficult to achieve at low temperatures.
• Cold plasma termination, which uses hydrogen plasma indirectly, did not damage the NV center and was easier to implement, but created a lower quality of hydrogen layer on the diamond.