Spin Qubits for Quantum Information Technologies

The search for coherent quantum bits in scalable solid-state environments is an active field of research. One of the milestones in the field has been the coherent manipulation of the single nitrogen-vacancy (NV) defect spin in diamond. However, inherent difficulties in growing and controlling the lattice of C diamond pose limitations to the use of the NV center for scalable quantum technologies. In close collaboration with experiments, we search for analogs to this defect in silicon carbide and oxide materials. We also investigate the fundamental properties of other quantum materials (e.g. thin films, 2D materials and molecular qubits) within the quantum sciences research areas at the University of Chicago. Part of our work is supported by AFOSR (QISpin), by the Chicago MRSEC, by Q-NEXT hub and the NSF project Qubbe.

Electronic and coherence properties of spin defects

We investigate the electronic and coherence properties of point defects in condensed and molecular systems for the realization of optically addressable two-level systems to be used as qubits. The objective is to realize optical transitions from ground to an excited state, followed by a spin-selective decay path with nonradiative transitions between states of differing spin multiplicity, and to create coherent states. By combining a variety of first principles techniques we have studied the electronic and coherence properties of numerous defects in bulk SiC, AlN, diamond, as well as in selected oxides and molecular systems. Our goal is to interpret & guide experiments with fundamental insights and eventually use predictions for design purposes.

Spin defects in 2D materials

Using spin Hamiltonians and a cluster expansion method, we investigated the electron spin coherence of defects in two-dimensional (2D) materials, including delta-doped diamond layers, thin Si films, MoS2, and h-BN. We showed that isotropic purification is much more effective in 2D than in three-dimensional materials, leading to an exceptionally long spin coherence time of more than 30 ms in an isotopically pure monolayer of MoS2. We also studied the electronic structure of carbon-based spin defects in h-BN.

Spin defects in functional ionic crystals

To date, defect qubits have only been realized in materials with strong covalent bonds. We recently introduce a strain-driven scheme to rationally design defect qubits in functional ionic crystals. Using a combination of state-of-the-art ab-initio calculations based on hybrid density functional and many-body perturbation theory, we predicted that the negatively charged nitrogen vacancy center in piezoelectric aluminum nitride exhibits spin-triplet ground states under realistic uni- and bi-axial strain conditions; such states may be harnessed for the realization of qubits.

  • "Strain-controlled design of defect spins in piezoelectric aluminum nitride for solid-state hybrid quantum technologies", Hosung Seo, Marco Govoni, and Giulia Galli, Sci. Rep. 6, 20803 (2016).
  • "Designing defect spins for wafer-scale quantum technologies", William F. Koehl, Hosung Seo, Giulia Galli, and David D. Awschalom, MRS Bulletin 40, 1146 (2015).

Surface defects for atomic scale electronics

Surface defects created and probed with scanning tunneling microscopes are a promising platform for atomic-scale electronics and quantum information technology applications. Using first-principles calculations we demonstrate how to engineer dangling bond (DB) defects on hydrogenated Si(100) surfaces, which give rise to isolated impurity states that can be used in atomic-scale devices. In particular, we show that sample thickness and biaxial strain can serve as control parameters to design the electronic properties of DB defects. While in thick Si samples the neutral DB state is resonant with bulk valence bands, ultrathin samples (1-2 nm) lead to an isolated impurity state in the gap; similar behavior is seen for DB pairs and DB wires. Strain further isolates the DB from the valence band, with the response to strain heavily dependent on sample thickness. These findings suggest new methods for tuning the properties of defects on surfaces for electronic and quantum information applications. Finally, we present a consistent and unifying interpretation of many results presented in the literature for DB defects on hydrogenated silicon surfaces, rationalizing apparent discrepancies between different experiments and simulations.

Boron: a frustrated element

All elements, except for helium, appear to solidify into crystalline forms at zero temperature, and it is generally assumed that the introduction of lattice defects results in an increase in internal energy. By using lattice Monte Carlo techniques combined with ab initio calculations, we find that β-Boron is stabilized by a macroscopic amount of intrinsic defects that are responsible not only for entropic effects but also for a reduction in internal energy. These defects enable the conversion of two-center to three-center bonds and are accompanied by the presence of localized, nonconductive electronic states in the optical gap. The ab initio Ising model describing the partial occupancy of β-boron has macroscopic residual entropy, suggesting that boron is a frustrated system analogous to ice and spin ice.

  • "β-Rhombohedral Boron: At the Crossroads of the Chemistry of Boron and the Physics of Frustration", Tadashi Ogitsu, Eric Schwegler and Giulia Galli, Chem. Rev. 113, 3425 (2013)
  • "Geometrical frustration in an elemental solid: An Ising model to explain the defect structure of rhombohedral β-boron", T. Ogitsu, F. Gygi, J. Reed, M. Udagawa, Y. Motome, E. Schwegler and G. Galli, Phys. Rev. B 81, 020102(R) (2010)
  • "Imperfect crystal and unusual semi-conductor: Boron,a frustrated element", T. Ogitsu, F. Gygi, J. Reed,Y. Motome, E. Schwegler and G. Galli, J. Amer. Chem. Soc. 131, 1903 (2009)

BaHfN2: a superconductor?

We have examined the electronic and vibrational structure of the ternary nitride BaHfN2 within density-functional theory. We find that BaHfN2 has chemical and electronic similarities with high Tc metallochloronitrides MNCl’s M=Ti,Hf,Zr, so its candidacy as another high Tc superconducting nitride is plausible. The basic electronic and vibrational properties of the undoped insulating phase provide a basis for an understanding of the behavior of BaHfN2 upon doping.

  • "First-principles study of electronic and vibrational properties of BaHfN2", A. Kaur, E. R. Yivisaker, Y. Li, G. Galli and W. Pickett, Phys. Rev. B. 85, 155125 (2010)