Quantum science has reached a new frontier with the 2D array of electron and nuclear spin qubits.

Using photons and electron spin qubits to manipulate nuclear spins in a two-dimensional material, researchers have opened up a new path in quantum science and industry. This will allow for applications such as nuclear magnetic resonance spectroscopy on an atomic scale and the capacity to read and write quantum information using nuclear spins in 2D materials.

The Purdue University research team utilized electron spin qubits as atomic-scale sensors and as the first experimental controllers of nuclear spin qubits in ultrathin hexagonal boron nitride. “This is the first study to demonstrate optical initialization and coherent control of nuclear spins in 2D materials,” said Tongcang Li, an associate professor of physics and astronomy and electrical and computer engineering at Purdue and a member of the Purdue Quantum Science and Engineering Institute.

“Now that we can initiate nuclear spins with light, we can write and read quantum information using nuclear spins in two-dimensional materials. This technique has numerous applications in quantum memory, quantum sensing, and quantum simulation.

Quantum technology relies on the qubit (quantum bit), the quantum equivalent of a conventional computer bit. Instead of a silicon transistor, atoms, subatomic particles, or photons are frequently used to construct qubits. In an electron or nuclear spin qubit, the familiar binary “0” or “1” state of a traditional computer bit is represented by spin, a feature that is loosely similar to magnetic polarity; thus, spin is responsive to an electromagnetic field. Before any activity can be completed, the spin must be regulated and coherent, or enduring.

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The spin qubit can therefore be utilized as a nanoscale-resolution sensor to examine, for instance, the structure of a protein or the temperature of a target. Electrons trapped in the flaws of three-dimensional diamond crystals have enabled imaging and sensing resolution in the range of 10 to 100 nanometers.

However, qubits implanted in single-layer or two-dimensional (2D) materials can reach closer to a target sample, providing even higher resolution and a stronger signal. The first electron spin qubit in hexagonal boron nitride, which may exist in a single layer, was constructed in 2019 by removing a boron atom from the lattice of atoms and trapping an electron in its stead. This achievement paved the path for the achievement of the stated objective. In addition, so-called boron vacancy electron spin qubits gave a fascinating opportunity to alter the nuclear spin of the nitrogen atoms surrounding each electron spin qubit in the lattice. In this study, Li and his colleagues created an interface between photons and nuclear spins in ultrathin hexagonal boron nitride crystals.

The nuclear spins can be optically initialized, or set to a known spin, using the electron spin qubits that surround them. Once initialized, a radio frequency can be used to alter nuclear spin qubits, or “write” information, or to measure changes in nuclear spin qubits, or “read” information. Their approach utilizes three nitrogen nuclei simultaneously, with coherence periods that are more than 30 times longer than those of electron qubits at ambient temperature. And the 2D material can be directly stacked onto another material to create a sensor.

Li stated, “A 2D nuclear spin lattice will be appropriate for large-scale quantum simulation.” It can operate at temperatures higher than superconducting qubits. To manage a nuclear spin qubit, scientists began by substituting an electron for a boron atom in the lattice. The electron is currently located in the middle of three nitrogen atoms. At this moment, the spin state of each nitrogen nucleus is random and may be -1, 0 or +1.

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Next, the electron is pumped to a spin-state of 0 by laser light, which has no discernible influence on the nitrogen nucleus’ spin. A hyperfine interaction between the excited electron and the three surrounding nitrogen nuclei causes the nucleus’ spin to shift. When the cycle is performed numerous times, the spin of the nucleus reaches +1 and remains there independent of further interactions. With all three nuclei in the +1 state, they can function as a qubit trio.

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