Affiliation: The University of New South Wales

Building Quantum Matter Atom by Atom

Atomic-scale engineering reached a level of control where single-atom devices can be reproducibly fabricated. This talk focuses on single dopant atom placement in the context of engineered matter for quantum simulation and computation. Silicon offers an interesting platform for engineered quantum matter because when isotopically purified it acts as a “semiconductor vacuum” for spins. After a general introduction of quantum simulation and computation a first step towards engineered Hamiltonians for Fermionic systems in the form of atomic chains will be presented. Here strongly interacting dopants were employed to simulate a two-site Hubbard Hamiltonian at low effective temperatures with single-site resolution which allows the quantification of the entanglement entropy and Hubbard interaction strengths. To scale this approach to larger systems in-situ multi-electrode devices have been fabricated by a scanning probe hydrogen depassivation and decoration technique. Spatially resolved gated single-electron spectroscopy maps obtained ion these devices n ultra-high vacuum will be presented. Such quantum-state images of two-donor devices led to a donor based two qubit gate design that is robust in regard to variability in dopant placement. In addition to the work on donors I will also present work on single defects in silicon with a spin-orbit interaction for electrical manipulation and coupling.

Affiliation: Physical Measurement Laboratory, National Institute of Standards and Technology

A microscopic view of the quantum Hall effect in graphene with atomic force spectroscopy*

Scanning tunneling spectroscopy (STS) has proved to be a powerful tool to study the quantum Hall physics in graphene; examples include spectroscopic studies of charge puddles, energy dispersion, Landau levels, etc. Recently, we added the ability to perform simultaneous atomic force microscopy (AFM) and quantum transport measurements at mK temperatures for detailed measurements of 2D devices in-operando. An advantage of utilizing the AFM in the Kelvin probe force microscopy (KPFM) mode is that it allows chemical potential variations to be measured without the effect of tip gating the two-dimensional electron system. In this talk I show that we can perform Landau level spectroscopy of graphene devices in high magnetic fields with KPFM measurements as a function of back gate potential with the same fidelity as STS measurements. In particular, symmetry breaking states can be resolved at filling factors ν = ±1 inside the N=0 Landau level manifold, showing the lifting of the graphene four-fold degeneracy due to spin and valley. A strong enhancement in chemical potential signatures are observed at integer filling, suggesting enhanced interactions. We show using KPFM that we can map the dispersion of the Landau levels across the quantum Hall edge boundary as a function of density and spatial position, including resolving the ν = ±1 edge modes. The dispersion of the broken symmetry ν = ±1 edge modes are consistent with an antiferromagnetic ground state with gapped edge modes at the boundary.

*In collaboration with S. Kim, J. Schwenk, D. Walkup, F. Ghahari, Y. Zeng, C. Dean, J. Berwanger, F. Giessibl, K. Watanabe, T. Taniguchi, and N. Zhitenev.