Precision Qubit Program

The Precision Qubit Program aims to position, control and read out the electron spin on a single P atom in silicon which acts as a quantum bit ot qubit. By integrating single electron transistors as read-out devices and microwave strip lines for coherent control we are able to both read-out and manipulate the electron spin. The major challenge gong forwards is to scale these devices to large qubit numbers whilst maintaining coherence of the electron spins.

Previous results have shown that we can position single dopant atoms in silicon with atomic precision (shown left) with a scanning probe microscope. Subsequent encapsulation by silicon molecular beam epitaxy activates the dopant in a substitutional site allowing the free electron to be used as the spin qubit. Patterning a nearby single electron transistor allows us to read out the spin state of this electron using spin to charge conversion.

Importantly the ability to control dopant location with atomic precision within a silicon crystal allows us to engineer devices completely out of regions of doped and undoped epitaxial silicon with no dielectric interfaces and far away from surface states. Simply by creating regions of high doping, separated by regions of no doping we can create nanowires down to ~1.5 nm widths that still exhibit ohmic behaviour. These wires are used as in plane gates to the single dopant devices. The resistance of tunnel gaps can also be controlled over many orders of magnitude simply by engineering the device dimensions.

The strong and abrupt lateral and vertical quantum confinement leads to dramatic changes in the silicon bandstructure. This leads to the observation of complex valley and spin physics in these atomic-scale devices, where despite the extremely small length scales created very small energy level splittings in the stability plots are observed (shown left).

Working closely with theoretical modelling groups at Madison Wisconsin, Purdue University, Sydney University, Liverpool University and the University of Melbourne we aim to understand all aspects of our devices from understanding the transport data, the surface chemistry, the complex spin behaviour and novel device architectures for scale up to large numbers of qubits.

Our future goals are to measure single shot read-out of an individual P donor in silison and extract the spin lifetimes, and to control the exchange interaction between pairs of spins to demonstrate a fully functional 2-qubit quantum logic gate. In parallel we will investigate the optimal method of qubit transport either by coherent transport by adiabatic passage, spin bus, or charge shuttling configurations. The ultimate goal is to demonstrate all the components of an in-plane all epitaxial 10 qubit architecture (shown left).