In the 'Silicon Qubit Environment and Interface' program we develop atomistic techniques that probe donor and quantum dot qubits in Si. This expertise is based on direct electrical transport, noise, and high-frequency measurements and UHV scanning tunneling spectroscopy at low temperatures. The goal of the program is to understand the impact of the environment on the qubit, i.e. the understanding of the physics that determines the electronic spectrum and relaxation processes of the orbital and spin degree of freedom.

The control over the electron wavefunction requires interfaces which lead to the loss of bulk properties of the qubit due to physical processes like the valley-orbit coupling, exchange, many body effects in coherent coupling. An atomistic understanding of the impact of the environment on the qubit is essential for quantum computation since it leads to optimal coherence times and maximal robustness of the quantum gates.

This program focus on the physics of single-donor and quantum-dot devices. Different fabrication processes are a compared, such as commercially fabricated FinFETs, bottom-up fabricated devices at UNSW which are implanted at the Melbourne node, as well as UNSW bottom-up devices. In the near future we need to resolve the following problems: the nature of the two electron state and decoherence mechanisms. Traditionally, both problems are understood in the bulk. However, recent experiments show that a donor in a nano-device, i.e. coupled to a gate and a reservoir, does not show simple bulk behavior. A related problem appears for spins in motion in a CTAP or surface based transport scheme. On a technical level, we need to understand our contacts and control surfaces on an atomistic level to determine their interaction with the donors.

Working with top-down as well as bottom-up fabricated devices and their comparison with commercially fabricated with CMOS structures will bring a depth of understanding to the origin and physical mechanism of decoherence. We use the 4K STM to determine the electronic properties of the fully fabricated bottom-up device in situ. The local determination of the density of states in the contacts and actual wavefunction of the linked donors allows direct comparison with theory. This atomistic input of the local electronic properties will enhance the required understanding in conjunction with the transport experiments. Questions which are now hard difficult to answer, e.g. the electronic structure of the leads, can be directly resolved.

An optical interface to solid-state qubits is desirable in a quantum computer architecture since it allows for simple long distance coupling schemes. However, the indirect band-gap of Si makes this not trivial. We work on an optical interface to localized electron states, donor or dot, based on cavity enhancement and excitonic states.