Chinese scientists achieve first control of nuclear spin quantum bits in ultra-thin hBN

Two-dimensional arrays of electron and nuclear spin quantum bits open up a new frontier in quantum science.

By using photon and electron spin quantum bits to control nuclear spin in two-dimensional materials, researchers will enable applications such as atomic-level nuclear magnetic resonance spectroscopy and the ability to read and write nuclear spin quantum information in two-dimensional materials. on Aug. 15, a Purdue University research team used electron spin quantum bits as atomic-level sensors to achieve the first experimental control of nuclear spin quantum bits in ultrathin hexagonal boron nitride (hBN) The first experimental control of nuclear spin quantum bits in ultra-thin hexagonal boron nitride (hBN) has been achieved. The related results were published in Nature Materials under the title "Nuclear spin polarization and control in hexagonal boron nitride" [1].

Tongcang Li

It is worth mentioning that Tongcang Li, the corresponding author of this paper, graduated from the University of Science and Technology of China and is now an associate professor of physics, astronomy, electrical and computer engineering at Purdue University and a member of the Institute for Quantum Science and Engineering at Purdue [2].

"This is the first work to demonstrate optical initialization and coherent control of nuclear spins in two-dimensional materials," said Tuzang Li, "Now we can use light to initialize nuclear spins, and with this control, we can write and read quantum information with nuclear spins in two-dimensional materials. This approach can have many different applications in quantum storage, quantum sensing and quantum simulation."

The team used light and electron spin quantum bits to control nuclear spin in two-dimensional materials, opening up new frontiers in quantum science and technology. Both nitrogen (blue) and boron (green) atoms have non-zero nuclear spin.

Quantum technology relies on the quantum bit, which is a quantum version of the classical computer bit. Similar to silicon transistors, quantum bits are usually composed of atoms, subatomic particles or photons. In an electron or nuclear spin quantum bit, the familiar binary "0" or "1" state of a classical computer bit is represented by spin: spin is a property similar to magnetic polarity, which means that spin is sensitive to electromagnetic fields. To perform any task, the spin must first be controlled and kept coherent or persistent.

Spin quantum bits can be used as sensors, for example to probe the structure of a protein or the temperature of a target with nanoscale resolution: electrons trapped in 3D diamond crystal traps yield imaging and sensing resolution in the 10-100 nm range.

Quantum bits embedded in monolayers or 2D materials can get closer to the target sample, providing higher resolution and stronger signals. The first electron spin quantum bit in hexagonal boron nitride was constructed in 2019 by removing a boron atom from the atomic lattice and capturing an electron in its place, paving the way for this goal. The so-called "boron vacancy electron spin quantum bit" also provides an attractive way to control the nuclear spin of nitrogen atoms around each electron spin quantum bit in the lattice.

In this work, Tuzang Li's team has created an "interface" between photons and nuclear spin in ultrathin hexagonal boron nitride.

The nuclear spin can be optically initialized by surrounding quantum bits of electron spin - set to a known spin. Once initialized, the RF can be used to change the nuclear spin quantum bits, essentially "writing" information, or measuring changes in the nuclear spin quantum bits, or "reading" information. They do this by using three nitrogen nuclei at once, with a coherence time that is more than 30 times that of a room-temperature electron quantum bit. The two-dimensional material can be layered directly onto another material to create a built-in sensor.

"The two-dimensional nuclear spin lattice will be suitable for large-scale quantum simulations," said Tuzang Li, "which can operate at higher temperatures than superconducting quantum bits."

To control the nuclear spin quantum bits, the scientists first removed a boron atom from the lattice and replaced it with an electron. The electron is now located at the center of three nitrogen atoms. At this point, each nitrogen nucleus is in a random spin state: possibly -1, 0 or +1. Next, the electron is pumped with a laser to a spin state of 0, which has a negligible effect on the spin of the nitrogen nucleus. Finally, hyperfine interactions between the excited electrons and the three surrounding nitrogen nuclei force the spin of the nucleus to change. When the cycle is repeated several times, the spin of the nucleus reaches the +1 state, which remains constant regardless of the repeated interactions. Setting all three nuclei to the +1 state, they can thus be used as three quantum bits.

Ultimately, the team experimentally demonstrated fast coherent control of nuclear spins, enabling multiple quantum bit manipulation; manipulating nuclear spins in van der Waals materials opens new avenues for quantum information science and technology.

Reference links：

[1]https://www.nature.com/articles/s41563-022-01329-8

[2]https://www.physics.purdue.edu/people/faculty/tcli.php