By using photons and electron spin qubits to control nuclear spins in a two-dimensional (2D) material, researchers at Purdue University have opened up a new frontier in quantum science and technology. This could lead to applications like atomic-scale nuclear magnetic resonance spectroscopy, and to reading and writing quantum information using nuclear spins in 2D materials.
As they report in a paper in Nature Materials, the researchers used electron spin qubits as atomic-scale sensors, and to achieve the first experimental control of nuclear spin qubits in ultrathin hexagonal boron nitride. “This is the first work showing optical initialization and coherent control of nuclear spins in 2D materials,” said corresponding author Tongcang Li, an associate professor of physics and astronomy and electrical and computer engineering at Purdue University, and a member of the Purdue Quantum Science and Engineering Institute. “Now we can use light to initialize nuclear spins and, with that control, we can write and read quantum information with nuclear spins in 2D materials. This method can have many different applications in quantum memory, quantum sensing and quantum simulation.” Quantum technology depends on the qubit, which is the quantum version of a classical computer bit. It is often built using an atom, subatomic particle or photon instead of a silicon transistor. In an electron or nuclear spin qubit, the familiar binary ‘0’ or ‘1’ state of a classical computer bit is represented by spin, a property that is loosely analogous to magnetic polarity – meaning the spin is sensitive to an electromagnetic field. To perform any task, the spin must first be controlled and coherent, or durable. The spin qubit can then be used as a sensor, probing, for example, the structure of a protein or the temperature of a target with nanoscale resolution. Electrons trapped in the defects of 3D diamond crystals have produced imaging and sensing resolution in the 10–100nm range. But qubits embedded in single-layer, or 2D, materials can get closer to a target sample, offering even higher resolution and a stronger signal. Paving the way to that goal, the first electron spin qubit in hexagonal boron nitride, which can exist as a single layer, was built in 2019 by removing a boron atom from the lattice of atoms and trapping an electron in its place. So-called boron vacancy electron spin qubits also offer a tantalizing path to controlling the nuclear spin of the nitrogen atoms surrounding each electron spin qubit in the lattice. In this study, Li and his team established an interface between photons and nuclear spins in ultrathin hexagonal boron nitrides. They showed that the nuclear spins can be optically initialized – set to a known spin — via the surrounding electron spin qubits. Once initialized, a radio frequency can then be used to change the nuclear spin qubit, essentially ‘writing’ information, or to measure changes in the nuclear spin qubits, to ‘read’ information. This method harnesses three nitrogen nuclei at a time, and these nuclei have more than 30 times longer coherence times than electron qubits at room temperature. The 2D material can also be layered directly onto another material, creating a built-in sensor. “A 2D nuclear spin lattice will be suitable for large-scale quantum simulation,” Li said. “It can work at higher temperatures than superconducting qubits.” To control the nuclear spin qubit, the researchers began by removing a boron atom from the lattice and replacing it with an electron. The electron now sits in the center of three nitrogen atoms. At this point, each nitrogen nucleus is in a random spin state, which may be -1, 0 or +1. Next, the electron is pumped to a spin-state of 0 with laser light, which has a negligible effect on the spin of each nitrogen nucleus. Finally, a hyperfine interaction between the excited electron and the three surrounding nitrogen nuclei forces a change in the spin of the nucleus. When this cycle is repeated multiple times, the spin of the nucleus reaches the +1 state, where it remains regardless of repeated interactions. With all three nuclei set to the +1 state, they can be used as a trio of qubits. This story is adapted from material from Purdue University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.