How can diamonds help us make sense of the world?

Posted: December 06, 2024

Diamond doesn't like to be diamond. All it wants is to be a lump of graphite, just like the grey material in your pencil. Yet, scientists are determined to master the art of growing diamond in the lab because, beyond its allure as a gemstone, the precious material holds extraordinary potential in advanced sensing technologies.

Diamond quantum sensors provide an unmatched combination of sensitivity, precision, and versatility for detecting and measuring phenomena at extremely small scales or under challenging conditions. "Diamond's really interesting for quantum technologies," Alastair Stacey, a physicist at the Princeton Plasma Physics Laboratory (PPPL), said at a PPPL seminar in March of this year. "We want to make sensors that see, measure and detect very important [things] in modern lives from phones to cars to medical uses. Most of these things are either really small and subtle or weak."

The better the sensors, the more accurately scientists can study the world's smallest details.

What is a quantum system?

A quantum system is a physical system that cannot be wholly understood or described simply by examining its components; its behavior and properties are irreducible. This means we cannot identify or track what each part is doing independently; instead, the system exhibits collective behaviors that the sum of its parts cannot explain.

This irreducibility leads to counterintuitive effects that differ significantly from our everyday experiences with larger objects, such as entanglement (where two or more particles act as a single system no matter the distance between them), superposition (where a system exists in multiple states simultaneously), and uncertainty (limits on how precisely we can measure certain properties).

Quantum systems do not follow the laws of classical physics and exhibit both particle-like and wave-like behavior_—a phenomenon known as wave-particle duality. This principle suggests that particles such as electrons and photons can behave as either particles or waves depending on how they are observed. Their behavior must be understood in terms of probabilities, distributions, and interactions.

Why diamond is so special

Diamond's exceptional physical properties make it indispensable for quantum and electronic applications. Its lattice, composed entirely of tightly packed carbon atoms, is exceptionally rigid and pure. These carbon atoms exhibit minimal movement at room temperature, allowing electrons to remain fixed to their host atoms. This stability enables scientists to embed controlled "quantum defects" into the lattice—intentional imperfections that act as quantum systems.

Unlike metals, where electrons are free to wander and disrupt quantum information, diamond's electrons stay put, creating a stable environment for quantum systems, a feature that other materials struggle to replicate. "No materials are the right material," Stacey said, “except diamond.”

Why is it hard to grow diamonds?

Despite its stability, diamond is not naturally permanent in its form. It is meta-stable, meaning it prefers to transition to graphite, its more thermodynamically stable form.

In nature, diamond forms deep within the Earth's mantle, approximately 87 to 120 miles beneath the surface, where temperatures range from 1,650 to 2,370 °F (900 to 1300 °C) and pressures exceed 45 to 60 kilobars (about 50,000 times atmospheric pressure).

These conditions are necessary for carbon atoms to bond in a tetrahedral structure to create the rigid lattice, but they are challenging to achieve in the lab in a controlled manner. They also limit the ability to integrate diamonds into modern devices. Silicon-based devices cannot withstand the extreme conditions required to create diamonds in situ, the high temperatures and pressures would destroy the silicon components. Instead, diamond is usually synthesized separately and then incorporated into devices, but this approach introduces challenges in maintaining compatibility and performance.

Scientists have long searched for ways to reduce the heat without sacrificing diamond quality. "If we want to implement diamond into silicon-based manufacturing, then we need to find a method of lower-temperature diamond growth," Yuri Barsukov, a computational research associate at PPPL told PPPL reporters. "This could open a door for the silicon microelectronics industry."

A better way to grow diamond

A common method for growing diamond is chemical vapor deposition. In the presence of hydrogen (H₂), carbon-rich gases like methane (CH₄) or acetylene (C₂H₂) break down under high temperatures into elemental carbon and hydrogen. Reactive carbon radicals then bond to a diamond seed crystal, starting diamond growth layer by layer.

Hydrogen helps to stabilize the diamond surface and prevent the formation of non-diamond carbon phases, such as graphite or soot. Atomic hydrogen can terminate dangling bonds on the diamond surface, which is useful in introducing chemical groups or molecules to modify its chemical or physical properties.

To make a diamond quantum sensor, scientists introduce controlled defects into the diamond lattice. Certain carbon atoms are replaced with nitrogen and empty spaces, creating nitrogen-vacancy (NV) centers, which give the material its unique optical properties.

This is often done by adding nitrogen gas to the mixture during chemical vapor deposition. After the diamond is formed, additional vacancies can be created by irradiating the diamond with high-energy particles, such as electrons.

NV centers can act like tiny magnets that respond to their environment, allowing scientists to measure very small changes in magnetic fields or to store and process information in quantum computers.

How does a diamond quantum sensor work?

NV centers create special sites that can trap electrons. When you shine green laser light on the diamond, it excites the electrons in the NV centers. This means that the electrons jump to a higher energy level. After being excited, the electrons eventually return to their original state. When they do this, they release energy in the form of light, which appears as red fluorescence.

The energy levels of the NV centers are sensitive to external magnetic fields. When a magnetic field is applied, the resonance frequencies of the NV center shift. Scientists measure changes in fluorescence intensity at these resonances to determine the strength of the magnetic field. They can also determine the magnetic field's direction by measuring across multiple NV center orientations within the diamond.

What can diamond measure?

One of the challenges scientists face in understanding how the world works is that many of the things they need to measure are extremely small, weak, or subtle, well beyond the capabilities of conventional sensors, Stacey explained at a PPPL seminar.

For instance, neurons in the brain produce very weak magnetic fields during activity. While researchers can view neurons under a microscope, detecting their low-energy signals is far more challenging. Similarly, tiny differences in temperature between different parts of a cell cannot be measured with traditional probes, as they are too large to fit inside a cell without disrupting it. Quantum sensors are powerful enough to detect weak magnetic fields produced by individual neurons or measure temperature differences within cells.

In space exploration, these sensors are integrated into satellites to monitor Earth's magnetic field, detect space debris, and enable better communication between systems. They are also useful in analyzing extreme environments, such as deep-earth exploration, where high pressures or intense radiation may destroy traditional sensors. In fusion energy research, quantum sensors can operate under plasma reactors' extreme temperatures and magnetic fields.

Once prized solely for their beauty, diamond quantum sensors are transforming how we observe and understand the universe at its smallest and most complex scales. As scientists continue to refine their methods for growing and using diamonds, these extraordinary crystals are becoming windows into the unseen forces that shape our world.