Laser polarization and manipulation of spins of atomic nuclei in two-dimensional materials

At 23:00 Beijing time on August 15, 2022, the team of Professor Li Tongzang of Purdue University and his collaborators published a new study entitled “Nuclear spin polarization and control in hexagonal boron nitride” in the journal Nature Materials.

The research group realized the laser polarization, quantum coherence control and detection of the spin of the nitrogen nucleus in hexagonal boron nitride by introducing a spin quantum defect that can emit light in the two-dimensional material hexagonal boron nitride. The work provides a new direction for quantum precision measurements and quantum simulations using two-dimensional materials. The corresponding author of the paper is Li Tongzang; The first author is Gao Xingyu.

Spin quantum defects in solids are one of the important systems for quantum information storage and quantum precision measurement. These spin quantum defects are very sensitive to environmental changes at the nanoscale and can be used to detect magnetic fields, temperatures, and pressures at the microscopic scale. Spin quantum defects in three-dimensional solid systems such as diamond have been studied in depth and used in microscopic NMR. Two-dimensional hexagonal boron nitride as a two-dimensional van der Waals material with a structure similar to graphene, because of its unique characteristics different from three-dimensional solid systems, provides new possibilities for further improving the resolution and sensitivity of quantum sensors. Electron spin quantum defects in the material were first discovered in 2019 and have since gained widespread attention at home and abroad. On the other hand, the spin of the atomic nucleus generally has a longer coherence time than the electron spin due to its weak coupling to the noise of the environment. Both the nitrogen atoms and boron atoms in hexagonal boron nitride have non-zero nuclear spins. If the nuclear spins in hexagonal boron nitride can be effectively polarized and manipulated, these nuclear spins can provide greater sensitivity for quantum precision measurements or be used for quantum simulation studies. However, due to the relatively small spin magnetism of nuclear spin and the high difficulty of its control, the nuclear spin of two-dimensional materials has not been fully studied.

In this work, Li Tongzang’s team based on the study of the electron excited spin level structure of the VB-spin defect in hexagonal boron nitride, through the Zeman effect of the magnetic field, to achieve the simplification of the electron spin energy levels ms=0 and ms=-1 in the VB-defect, and thus transfer the electron polarization degree in the VB-spin defect to the nuclear spin of the surrounding nitrogen atoms with the help of the ultra-fine coupling between the electron and the nucleus. Due to the self-energy level nature of the VB-defect, its electron spin can be polarized by continuous excitation using a 532nm laser. This method realizes the polarization of the spin of the nitrogen nucleus by laser at room temperature, and increases the polarization to more than 50%. This polarization is increased by more than four orders of magnitude compared to the nuclear spin polarization of traditional NMR.

Figure 1: Polarization of nearby nitrogen nuclei using laser and hexagonal boron nitride VB-spin quantum defects

Figure 2: Polarization of nitrogen nuclei at different magnetic fields and laser light intensities

Through the polarization of nuclear spins, optical detection of the nuclear magnetic resonance spectrum near the VB-defect in hexagonal boron nitride can be realized. After polarizing the VB-electron spin and the nearby nitrogen nucleus spin with a 532nm laser, the electron spin and the nitrogen spin can be manipulated separately by applying microwave and radio frequency signals, and finally the nuclear spin state can be read out again by using the 532nm laser. This signal is obtained by recording the number of photons collected after the 532nm laser excitation VB-defect. On this basis, the NMR spectrum of nitrogen nuclei can be obtained by changing the frequency of the RF signal and recording the corresponding number of photons.

Figure 3: Optical detection of the NMR spectrum of three nitrogen nuclei near a VB-spin quantum defect

Finally, by changing the length of time the RF signal is manipulated, a signal that changes with the time period can be obtained, that is, the rabbi oscillation signal of the nuclear spin. The period of the signal is proportional to the strength of the RF signal in direct proportion to the root number. At the same time, due to the coupling of electrons and nitrogen nuclei, the effective spin-magnetic ratio of nitrogen nuclei is enhanced by two orders of magnitude, and the speed of radio frequency manipulation is also accelerated by two orders of magnitude. This enables electronic spins in VB-defects based on hexagonal boron nitride, which are polarized and read out by lasers and manipulated by radio frequency signals nearby nitrogen nucleus spins.

Figure 4: Quantum coherence manipulation of three nitrogen nuclei near a VB-spin quantum defect

In this work, Li’s team and collaborators also found that the ultra-fine coupling between electrons and nuclei can increase the spin-magnetic ratio of nitrogen nuclei near VB-spin defects by 350 times, greatly improving the speed of quantum coherence manipulation of nucleus spin. In addition, electron spin can be used as a mediator to increase the nuclear spin coupling between two atomic nuclei by a factor of 100,000, and it is expected to be used for quantum manipulation and quantum simulation of multiple nuclear spins in the future. (Source: Science Network)

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