Science & Technology (Commonwealth Union) – Imagine a hammer hitting a bell. The bell rings, then the sound slowly fades. Scientists at the University of Chicago have just worked out a missing piece of how a certain kind of tiny quantum “bell” fades, and one of the ways it fades turns out to matter far more than anyone expected.
First, what are we talking about?
Some solid materials, like diamond, contain microscopic flaws. A typical flaw might be a spot where an atom should be but isn’t. These flaws are called “spin defects,” and despite the unflattering name, scientists find them very useful. They can act as qubits, the basic building blocks of quantum technology. One promising use is quantum sensing, which means measuring things with extraordinary precision.
When you shine a laser on one of these flaws, it soaks up energy and becomes “excited.” Think of a kid after too much sugar. Eventually the flaw has to calm down and get rid of that energy. To build dependable technology from these flaws, researchers need to know exactly how that happens, and how fast.
Three ways to calm down
According to the researchers, an excited flaw can release its energy in three main ways:
- It gives off a flash of light. The energy leaves as a tiny particle of light, called a photon.
- It flips its “spin.” Spin is a quantum property that works a bit like a tiny compass needle. The flaw can swap from one spin state to another and shed energy in the process.
- It shakes the atoms around it. Instead of light, the energy turns into vibrations, like a faint hum running through the material. Scientists call this internal conversion.
Physicists could already predict the first two reasonably well using the known rules of quantum mechanics. The third was the troublemaker. The rules are known, but applying them is brutally difficult. A computer needs to key in factors such as how the flaw engages with many of atoms that are close, at the same time. A large proportion of math that until now had no proper way of carrying out.
The team at the University of Chicago Pritzker School of Molecular Engineering, led by professor Giulia Galli, decided to tackle the problem. Her group had developed a number of different calculation tools in the past. They combined them and worked out a new mathematical formulation for internal conversion.
In simple terms, their method allows a computer to follow the links between the energy states of each of the flaw, and the wobbling of a lot of surrounding atoms together, instead of checking one wobble at a time. That is the difference between listening to a complete orchestra and listening to the musicians one by one.
The team integrated the method into WEST, a free, open-source simulation program that Galli’s group has been developing for about a decade. It’s open source and optimised for graphics processors (the same kind of chips used for video games and AI), so other research labs can start using it now.
The surprise
The team first applied the method to a well-understood flaw in diamond. The predictions matched recent laboratory measurements. The results also confirmed scientists’ earlier assumptions: for some energy states, internal conversion is of little importance, but for others it is the main route of energy exit.
Then they tried a different flaw, found in a material called silicon carbide, and things got interesting. The calculations reveal that internal conversion plays an important role in the lifetime of one of the excited states of the flaw. Scientists hadn’t seen this before. It also fixed a long-standing mismatch between what theory predicted and what experiments actually measured.
“We expected an effect, but one of smaller magnitude,” said Galli. Thus, the shaking-atoms route is more important than the team had expected.
This is important for anyone trying to make reliable qubits from these flaws. You misjudge how a qubit loses energy, you misjudge how well it works.
Why you should care
One of those flaws you’re unlikely to get your hands on any time soon. But learning more about them could enable engineers to make better quantum sensors, and eventually better quantum computers.
And there is a bonus. Internal conversion is more than a curiosity of quantum defects. You see the same kind of energy loss in materials used for renewable energy, including energy conversion and storage systems. What makes the work so exciting is the overlap, Galli said. A technique developed for quantum defects could come in handy in designing cleaner energy technology.
The snag
Running the new method is not cheap. It requires a lot of computing power and Galli warned that this is a real limitation for now. The team is exploring adding machine learning to speed the calculations, said Stefano Villani, postdoctoral researcher and first author of the study. They are also adapting the method to defects in silicon and other energy materials.
The research appeared in the journal Physical Review Letters .
