Diamonds seeding advances quantum sensors

A new facility at the University of Melbourne to create deliberately flawed diamonds will position Australia as a world leader in the rapidly growing field of quantum sensors.

Heading up this research at the University is Professor Lloyd Hollenberg, who says the field of quantum sensors is growing exponentially, and diamond-based sensors are leading the charge.

Quantum sensors are exquisitely sensitive to changes in the environment, including magnetic, temperature, chemical and electronic changes. They can detect the strength of these variations and also the direction from which the changes come.

At the same time, they are portable and can operate at room temperature.

These features create a wide range of potential application in defence. Quantum sensors can be used to detect minute traces of chemicals that might be associated with certain weapons. They can help monitor the movements of objects such as ships or vehicles through effects on magnetic fields. They can even be used for navigation that doesn’t rely on satellite-based GPS.

Professor Hollenberg and colleague Dr David Simpson are directing the building of a new diamond-based quantum sensor fabrication facility in Melbourne, with funding from the Australia Research Council (ARC) in partnership with RMIT, UNSW, the Australian National University and Deakin University. The facility will be the first of its kind in Australia.

From the inside out

Professor Hollenberg and Dr Simpson have been working in the field of quantum technology for decades. They began with quantum computers, where the aim is to shield the quantum computing units, or qubits, from their environment as much as possible to avoid interference.

To do that, they needed to understand how qubits responded to their physical and chemical surroundings. That work has informed development of diamond-based quantum sensors.

Quantum sensing typically turns efforts to protect systems from environmental interference “inside out”, Professor Hollenberg says.

“It uses all of that knowledge and ability to control that quantum system to deliberately expose it to an environment around it. This allows us to glean, through quantum measurement and control, aspects of the environment that you might not normally be able to detect.”

Their quantum sensors consist of a single diamond. The normally pure carbon lattice of the diamond is seeded with a deliberate defect called a nitrogen vacancy centre: a single nitrogen atom with an atom-sized space next to it.

When that nitrogen vacancy centre inside the diamond is bombarded with microwave radiation, it puts it into a quantum state.

Once in that state, any change in its surrounding environment, for example, a change in magnetic field, will alter the quantum mechanical properties of the atomic defect inside the diamond lattice. That change in the quantum mechanical properties can then be measured by shining a fluorescent green light onto the diamond and looking at how that light is altered.

Quantum sensors are not only sensitive, but they’re also incredibly small. “Because it’s so small, you can actually get it very close to the action that you’re interested in,” Professor Hollenberg says.

One way to create nitrogen vacancy centres is by blasting nitrogen atoms at a diamond in an ion accelerator to knock carbon atoms out of the lattice. The higher the energy of the accelerator, the deeper the nitrogen atoms will go in the diamond.

For the most part, those nitrogen vacancy centres will be close to the diamond’s surface, which works well if the material to be sensed can be placed directly against the diamond.

Advanced sensors

Embedding a larger number of nitrogen atoms deeper into the diamond is a more advanced process. To do this, the diamonds are grown synthetically in a process called chemical vapour deposition, and this is the approach that will be taken by the new processing reactor.

A seed diamond is placed in a plasma of methane with a small amount of nitrogen, and the carbon in the methane is ripped apart so it coalesces in layers onto the seed diamond, along with the occasional atom of nitrogen.

This process can also be tweaked to achieve nitrogen vacancy centres that are all aligned in the same direction to enable directional sensing and improve the sensitivity of the sensors.

“That’s really useful for a lot of the defence applications, for example we can monitor the presence and movement of metallic objects by detecting tiny changes in the earth’s magnetic field due to the movement of these magnetic objects,” Dr Simpson says. These directional magnetic sensors can also be used as navigation devices, based on geographic variations in the Earth’s magnetic field.

The new reactor will also enable precise control over the carbon isotopes that go into the diamond. The diamonds currently used consist of carbon 12 with around one percent carbon 13. However, those carbon 13 atoms have their own magnetic field, which creates ‘noise’ in the sensor readings. The new reactor will be able to create diamonds that are 99.99% pure carbon 12, improving the sensitivity and accuracy of new sensors.

“It’s really important for Australia to have sovereign capability over the production of these quantum diamond materials, so we can go end to end from fundamental research through to a commercialised product,” Dr Simpson says.