Physics

Are Diamonds Really Forever? Quantum Mechanics says yes

By Allison Kubo Hutchison

Synthetic diamond created using vapour deposition process. Steve Jurvetson, Apollo synthetic diamond, CC BY 2.0

In late 1940, the Debeers Diamond company started using the slogan “Diamonds are forever” to popularize diamond engagement rings. What they didn’t know is that in terms of quantum mechanics that might be true. Diamonds are formed of pure carbon with the atoms arranged tetrahedrally in a strong, rigid crystal structure. They are the hardest known material meaning that they have a hardness of 70-150 GPa in the Vickers Hardness test. This hardness is inherited from its strong crystal structure and is exactly what makes diamond forever.

Green indicates carbon atoms within a diamond crystal structure. When a nitrogen a vacancy can form, basically an empty hole in the crystal. NIST, Nitrogen-vacancy center, marked as public domain.

All materials have specific quantum states which are extremely fragile. They must be isolated from everything to be measured but even the measurement can change them. However, diamonds due to their structure can maintain quantum states long enough for measurements. Which is basically forever. Specifically, atom scale defects in the diamond, a nitrogen atom where a carbon atom should be, can be measured and used for quantum mechanics applications. One specific defect in the crystal structure which would be undesirable in a wedding ring but excellent for physics applications is the nitrogen-vacancy (NV). The spin quantum numbers of the NV defect can be measured by shining specific wavelengths of green light on the defect. The amount of light emitted by the defect depends on the ground-state spin. Moreover, when the green light is applied the electrons will cycle through spin state and eventually reach ms = 0 and then can be manipulated in further experiments. This is a major breakthrough because the behavior is well known and can be seen at room temperature, a luxury in many quantum mechanics fields. Where most materials exhibit fleeting quantum states, diamonds hold theirs.

One of the main goals of this work is to use diamonds, specifically the NV site, as the basis of the quantum computer. The defects are incredibly small, the size of two atoms, but are also incredibly sensitive to magnetic fields. Changing the magnetic field as much as one nanotesla will change the luminosity of the NV site indicating a change in spin. so they could be used to make very small sensors. Imagine a quantum computer made with diamond circuits where each nanometer scale defect stores one “qubit”, a quantum computer bit. However, in order to harness this, we must create incredibly precise defects within synthetic diamonds.
Diamonds, once valued for their beauty and clarity, are now valued for their defects.

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