NItrogen-Vacancy Spin Defects in Diamond
At the nanoscale, isolated spins can be exceedingly stable and precisely controlled. This holds the promise for their use as quantum bits, the basic building block of a quantum computer. Their natural stability also renders them sensitive magnetic, electric, and thermal probes. Recent experimental advances have enabled the control and manipulation of individual quantum mechanical spins --- Nitrogen-Vacancy (NV) defects --- in diamond. In each defect, a nitrogen atom and an adjacent vacancy substitute for two carbon atoms in the diamond lattice. The NV harbors a spin triplet electronic ground state that can be polarized, manipulated and optically detected. Our group is interested in studying the magnetic properties of layered two dimensional materials and in pressure driven phase transitions using both single and ensemble NV magnetometry.
One exciting direction we are pursuing in conjunction with our colleagues in the Ultrafast Nano-Optics Group is to use shallow NV centers to probe exciton spin relaxation and coherence dynamics in van der Waals heterostructures. Recent measurements on spin relaxation and coherence times using optical Kerr spectroscopy have demonstrated extremely long electron spin lifetimes. Our setup consists of back-gated monolayer transition metal dichalcogenides exfoliated on a diamond substrate with shallow NVs implanted within a few nanometers of the diamond surface.
Since the NV electronic spin couples directly to both DC and AC magnetic fields, the energy splitting between the NV's Zeeman sub-levels behave as a sensitive probe of any spin polarization of the TMD excitons. In particular, we plan to use wide-field imaging to directly map out the vector magnetic field generated by circularly polarized excitation of the TMD. This is enabled by the fact that excitons in the K and K' valleys selectively couple to left and right circularly polarized light. In combination with strong spin-orbit coupling in the valence band, this enables one to excite both spin and valley polarized excitons by using a combination of polarization and frequency selection. The ultimate sensitivity of our approach is determined by a combination of spin coherence time and spin projection noise. We will utilize Ramsey spectroscopy, which maps the magnetic field generated by the excitons into a relative phase difference accumulated by the NV's electronic spin states. By alternating the TMD excitation between left and right circular polarization, we can perform spin-echo spectroscopy, which significantly enhances the NV sensitivity.