A diamond anniversary, fittingly spent harnessing the gem’s weird superpowers

Some people turn their noses up at synthetic diamonds, but they have a hidden value, far beyond sparkle and shine. Grown in a lab, this kind of diamond could be the ideal material for next-generation sensors.

Looking at a pink synthetic diamond is like glimpsing into the future. These diamonds, in particular, are full of atomic-scale flaws that can serve as building blocks for ultrasensitive quantum sensors. These quantum bits of information, known as qubits, can also be used to sense magnetic fields, temperature and electric currents with exquisite precision. But growing those diamonds perfectly, so that every atom is in the right place, is very challenging.

“We’re trying to make a crystal that doesn’t want to exist and chemistry that doesn’t want to happen,” said Alastair Stacey, who leads the Quantum Diamond Laboratory (QDL) at the U.S. Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL). “If you do this wrong, the diamond turns to graphite right in front of your eyes.” 

Graphite, which is found at the center of a pencil, doesn’t have the right atomic arrangement to be the foundation of this future tech. Graphite and diamond are both made of the same element – carbon – but the atoms are arranged differently. In graphite, the atoms are stacked in flat sheets that slide past each other easily. In diamond, however, the atoms are locked into a rigid, three-dimensional lattice, which is why it’s hard and transparent. It’s the same ingredient with radically different properties just because of how the atoms are arranged.

A model of the lattice of atoms that form diamond is shown above. The black balls represent carbon atoms. The blue and purple balls show a nitrogen atom with an adjacent vacant spot. This nitrogen-vacancy defect is what gives the diamond the properties that can make it an extraordinary sensor. (Photo credit: Michael Livingston / PPPL Communications Department)

How plasma expertise could shape the next tech race

Lyman Spitzer’s founding vision for PPPL rested on the idea that plasma can be understood, shaped and controlled with enough precision to generate net energy from fusing atoms. As the Lab celebrates 75 years in operation — its diamond anniversary — it still has the same fascination with plasma, but in addition to fusion, the Lab is also applying its expertise in new areas, including materials science. Researchers and engineers at QDL are using plasma to design and refine the next generation of high-tech materials, work that could strengthen U.S. competitiveness in industries from computing to defense.

“Diamond growth is barely a science,” Stacey said. “There’s so much richness in the complexity in the physics that goes into the growth and the plasma, and we can’t describe most of it.”

The stakes are not just economic. Diamond-based detectors are sensitive enough to pick up the faint magnetic signatures of submarines, guide aircraft when GPS is jammed and make a map of what is hidden beneath the ground. Whichever country masters these technologies first will hold a significant strategic edge. But none of it is possible without the right raw material. The challenge is growing diamond that is close enough to perfect for these purposes, without letting it turn into graphite along the way.

This new direction is part of Lab Director Steve Cowley’s goal to diversify the Lab, making it truly multipurpose. QDL is a part of the Lab’s Applied Materials and Sustainability Sciences (AMSS) Directorate. At the helm is Emily Carter, who was specifically recruited to be the inaugural senior strategic advisor and associate laboratory director for AMSS because of her pioneering work in the field and her extensive leadership experience. She is also the Gerhard R. Andlinger Professor in Energy and the Environment and a professor of mechanical and aerospace engineering and applied and computational mathematics at Princeton University. 

Growing diamond from seeds

Inside QDL, Stacey and his team use plasma reactors to generate plasmas for growing such diamond from a small piece called a seed. Atoms from the plasma interact with atoms from the diamond seed. As the atoms interact, new layers form on top of the seed, and the material grows in size. Researchers at PPPL test different combinations of gases (used to make the plasma), different plasma temperatures and different sample temperatures to see how each variable affects the atomic structure of the new material. 

So what’s so special about these lab-grown diamonds? Some of their atoms aren’t carbon at all. Instead, a nitrogen atom will take the place of one carbon atom, and an empty spot is left beside the nitrogen. This “flaw” is known as a nitrogen vacancy center, or NV center, and it can be manipulated as a qubit: the basic unit of information in the quantum world. It’s the quantum version of a bit: the zeros and ones that represent everything from digital documents and images to emails, apps and videos.

Scientists control qubits with flashes of laser light and pulses of microwaves. They read the answer by watching the diamond glow. Qubits are also very sensitive to their surroundings. A passing magnetic field or a slight change in temperature can nudge it in measurable ways, turning the diamond into a detector precise enough to map the electrical activity inside a single, living cell. 

There’s a very simple version of this that is reminiscent of something the 1980s TV character MacGyver might make. All that is needed is a laser pointer, a bar magnet and a pink diamond. Point the laser pointer at a pink diamond and it will glow. Then bring the bar magnet toward the diamond, and the glow will dim slightly. 

“The closer the bar magnet is, the dimmer it becomes,” explains Stacey.

Controlling qubits

Of course, scientists cannot rely on an office laser pointer and a bar magnet for the precision they need. But the concept is similar. They coax the NV center into a suspended, in-between state — called a superposition — using flashes of laser light and pulses of microwaves. While a qubit is held in a superposition, a passing magnetic field, a slight change in temperature or a faint electric current can nudge it. Researchers read those nudges by watching the diamond glow. 

The more qubits a diamond contains, the more powerful the sensor. You might think the fix is simple: just add more nitrogen. It is not. Every NV center needs an extra electron to become active, and right now that electron comes from the surrounding nitrogen in the crystal — the same nitrogen that creates the NV centers in the first place. It is a convenient arrangement, but it comes with a catch. The nitrogen introduces noise that ruins the quantum properties the researchers are trying to protect. The more qubits packed into the crystal, the worse the problem gets.

QDL’s answer is a strategy called co-doping, which simply means adding a second element to the mix. Instead of relying solely on nitrogen to supply electrons, researchers also add phosphorus. The NV centers remain, but the messy, noisy nitrogen is no longer doing double duty. The goal is a crystal where qubits can be packed more densely without sacrificing their quantum properties — and, eventually, placed at precise locations within the diamond rather than scattered at random.

“Normally, when you grow qubits in diamond, you just get random qubits in random places,” Stacey said. “We’re trying to be more sophisticated by adding what we call co-doping.”

Inside QDL: Cutting-edge equipment from Element Six

QDL is equipped with several industrial diamond reactors to use in its experiments, and the Lab’s capabilities continue to grow. Most notably, Element Six (E6) is providing QDL with diamond synthesis tools for experimentation. A world leader in synthetic diamond industrial supermaterials since 1946, E6 has more than 30 years of expertise in the design, development and production of engineered diamonds using a plasma process known as chemical vapor deposition. 

“Working with PPPL, we are accelerating the development of quantum‑grade materials that will underpin the next generation of these promising technologies,” said Daniel Twitchen, chief technologist at E6.

A tight collaboration between PPPL and Princeton University 

Perfecting the recipe for next-gen materials takes more than just great equipment. It takes teamwork, too. On the modeling and diagnostics side, QDL draws on PPPL plasma experts Igor Kaganovich and Yevgeny Raitses and on PPPL associated faculty David Graves from Princeton University, whose group studies plasma etching of diamond. Additional collaborations with Cornell University, Brookhaven National Laboratory, the City University of New York, Stony Brook University and physics-informed machine-learning specialists on the Princeton campus are in various stages of development.

At its core, the QDL program is built around a tight feedback loop between PPPL and Princeton University. The Lab currently focuses on diamond growth experiments, while the on-campus team focuses on measuring and analyzing that growth. 

“The new growth capabilities are conceived of and designed in the context of specific scientific questions. We’re not just trying to do random growth for growth’s sake,” said Nathalie de Leon, an associate professor of electrical and computer engineering at Princeton’s School of Engineering and Applied Science and — as the co-director of Princeton University’s Quantum Institute — the head of the school’s side of the collaboration. “Usually, what we’re trying to do is change something on the growth side that then allows us to see something on the measurement side, and then we have to have a very tight feedback loop between both parts of it.”

That kind of back-and-forth is unusual for the field, said de Leon, who is also an associated faculty member at PPPL. In most corners of the diamond-quantum world, she explained, measurement groups and materials groups ship samples back and forth and compare notes at conferences. 

The collaboration between PPPL and the University, however, is different. The teams routinely swap ideas and consider new approaches together. It was de Leon who originally proposed the idea of QDL, as she understood that national labs have the capabilities to do research no one else can do. The arrangement allows Princeton University to make good use of the Lab’s infrastructure.

Nathalie de Leon is an associate professor of electrical and computer engineering at Princeton’s School of Engineering and Applied Science, co-director of Princeton University’s Quantum Institute and associated faculty at PPPL. (Photo credit: Michael Livingston / PPPL Communications Department)

QDL is now also a partner in the Co-design Center for Quantum Advantage, a DOE-funded national effort led by Brookhaven National Laboratory, where the team is pushing for the next order-of-magnitude gain in the coherence of near-surface NV centers. The Lab also has a partnership with the Gemological Institute of America, the organization that grades engagement rings. The Institute has decades of experience looking at defects in bulk diamond, expertise that can also be applied to PPPL’s research-grade crystals. 

Looking ahead to a bright future in diamond

The near-term goal at QDL sounds almost mundane: making the diamond surface flat. Not smooth in any everyday sense, but flat at the scale of individual atoms. Think of it like a countertop with absolutely no scratches or grooves, right down to the last atom. Getting a surface like that matters because it gives researchers a stable, predictable foundation to work from so that the sensors can be as reliable as possible.

Right now, when the team exposes a diamond surface to a plasma, atoms land and lock into place somewhat randomly. A truly flat surface would change that. It would give the team a platform to start placing individual atoms at specific, chosen locations in the crystal rather than letting them settle wherever they land. That kind of precise control has already been achieved in silicon. For diamond, it remains the frontier.

“In the next 12 months, what we’re focusing on is getting that flat surface to give us a platform to start exploring,” Stacey said. “Ultimately, these flat surfaces could enable extremely reliable yet sensitive sensors that can detect tiny magnetic fields in electronic circuits or human cells. It could also be used to make devices that can measure electric fields, temperature and strain down to the nanometer. That’s sensing on a level we simply can’t do today, and it would mean a world of difference to the researchers who gather essential data using these tools.”

QDL was funded by the DOE’s Office of Fusion Energy Sciences and Basic Energy Sciences under contracts DE-AC02-09CH11466 and DE-SCL0000202. 

PPPL is mastering the art of using plasma — the fourth state of matter — to solve some of the world’s toughest science and technology challenges. Nestled on Princeton University’s Forrestal Campus in Plainsboro, New Jersey, our research ignites innovation in a range of applications, including fusion energy, nanoscale fabrication, quantum materials and devices, and sustainability science. The University manages the Laboratory for the U.S. Department of Energy’s Office of Science, which is the nation’s single largest supporter of basic research in the physical sciences. Feel the heat at https://energy.gov/science and https://www.pppl.gov.