For Quantum Computing, New Hardware Platform Based on 2D Materials

To harness the capability of quantum computing, hardware must be developed that can access, measure, and manipulate individual quantum states. In response to this need, researchers at the University of Pennsylvania have demonstrated a new hardware platform based on isolated electron spins in a 2D material. The electrons are trapped by defects in sheets of one-atom-thick semiconductor material (hexagonal boron nitride, or h-BN).

The researchers were able to optically detect the system’s quantum states. They identified and characterized individual quantum emitters in h-BN with spin-dependent optical properties and demonstrated that some quantum emitters in h-BN exhibit room-temperature, magnetic-field-dependent photoluminescence.

Researchers at the University of Pennsylvania’s School of Engineering and Applied Science have demonstrated a new hardware platform based on isolated electron spins in a two-dimensional material. The researchers were able to optically detect the system’s quantum states. Courtesy of Ann Sizemore Blevins.

For its work, the team used a system of electron spins in diamonds. The spins were trapped at the defects in the diamond’s regular crystalline pattern. The defects acted like isolated atoms or molecules and interacted with light in a way that enabled their spin to be measured and used as a qubit.

Systems involving electron spins in diamonds can operate at room temperature, unlike other prototypes based on ultracold superconductors or ions trapped in a vacuum. Working in 2D provides better control over a bulk diamond-based system, said the researchers.

“One disadvantage of using spins in 3D materials is that we can’t control exactly where they are relative to the surface,” said professor Lee Bassett. “Having that level of atomic scale control is one reason to work in 2D ... When the spins are confined to a single atomic plane, you enable a host of new functionalities.”

The researchers looked for a 2D material that would be most like a flat analog of bulk diamond and chose h-BN over graphene. Professor Annemarie Exarhos at Lafayette University said, “Graphene behaves like a metal, whereas diamond is a wide-bandgap semiconductor and thus acts like an insulator. Hexagonal boron nitride, on the other hand, has the same honeycomb structure as graphene, but, like diamond, it is also a wide-bandgap semiconductor and is already widely used as a dielectric layer in 2D electronics.”

The researchers focused on the defects in the honeycomb lattice of h-BN that can emit light and found that for some defects, the intensity of the emitted light changed in response to a magnetic field.

“We shine light of one color on the material and we get photons of another color back,” Bassett said. “The magnet controls the spin and the spin controls the number of photons that the defects in the h-BN emit. That’s a signal that you can potentially use as a qubit.”

While the researchers conducted an extensive survey of h-BN defects to discover ones that have special spin-dependent optical properties, the exact nature of those defects is still unknown. Next steps for the team will include investigating what makes some, but not all, defects responsive to magnetic fields, and then re-creating those useful defects.

Having a quantum machine’s qubits on a 2D surface could enable other applications, beyond computation, that depend on proximity.

“Quantum systems are supersensitive to their environments, which is why they’re so hard to isolate and control,” Bassett said. “But the flip side is that you can use that sensitivity to make new types of sensors. In principle, these little spins can be miniature nuclear magnetic resonance detectors, like the kind used in MRIs, but with the ability to operate on a single molecule.”

The research was published in Nature Communications (https://doi.org/10.1038/s41467-018-08185-8).