Laser cooling and electromagnetic trapping of single atoms and lattices has led to a revolution in atomic physics, because they can be exploited for coherent quantum control of one atom by electromagnetic pulses and by each other. The control is via combinations of microwave pulses to control the spin, and optical pulses to control the Rydberg state. Some silicon qubit gate schemes are exactly analogous and while spin control requires similar microwave pulses, the “trapping” must be by atomic scale lithography and the optical Rydberg transitions occur in the THz region of the spectrum. At the present time the ADDRFSS team is the only group capable of unifying THz orbital control with deterministic single atom placement.
The programme is divided into three projects, aiming respectively to overcome the engineering challenge of multi-species atomistic lithography; to produce scientific understanding of the coupling and control within patterned lattices of dopant atoms; and, at the interface between these two science and engineering challenges, to make electronic and opto-electronic devices based on individual impurity atoms or coherent control of impurity ensembles.
Project 1 “Manufacturability”
We are developing semiconductor engineering at the atomic scale to produce pairs, ordered 2-D arrays and ordered 3-D ‘lattices’ of single and multiple dopant atom species, embedded in Group IV semiconductors, using deterministic doping. We are developing schemes to scale these structures to the dimensions of commercial wafers, and we are working with Zyvex Labs who are providing us with their lithography control hardware and software. Deterministic dopant incorporation solves the issue of transistor faults due to random straggle at the edges of implant regions, a crucial problem when the channel length is a few tens of lattice spacings. Our primary goal is to understand how the properties can be modified and controlled through precise coupling to one or more neighbours in a variety of different geometries. The tools we use are a combination of low temperature scanning tunnelling microscopy (STM) and ab-initio density function theory (DFT) calculations.
Project 2 “Crystal traps’
We are translating cold-atom physics into the solid state, to develop new quantum technology paradigms. In recent years, lattices of ultracold trapped atoms have provided an unprecedented level of quantum control, based on the control of the lattice geometry, but to achieve the degenerate (quantum) limit the heavy atoms must be cooled so that their thermal de Broglie wavelengths exceed this spacing. Hence the need for ultralow (nK) temperatures, a vacuum environment, and a complex infrastructure involving multiple lasers. Here we aim to realize similar physics in the solid state, at more accessible temperatures and in a relatively robust solid-state environment, using precisely defined arrays of dopant atoms. The centre-of-mass of the atoms will be fixed for the duration of the experiment; control will be realized instead through electrical gating as well as optical and microwave fields produced by a variety of sources including free-electron lasers (FELIX) and solid state devices.
Project 3 “solotronic devices”
We are constructing highly reproducible single atom and single molecule devices (“solotronics”) in silicon at the ultimate limits of size scale, such as single atom p-n junctions. We will produce single molecule silicon equivalents to the organic single molecule LED, for single photon emission both in the near-IR with an n-p molecule (i.e. a donor-acceptor pair such as P-B) and in the THz regime with n(+)-n (e.g. P-Bi). In reverse bias these structures will act as single photon detectors. On a larger scale, arrays of single atom structures also have extremely attractive properties. In periodic arrays, defects can produce new bandstructures in the host with different transport, e.g. with longer mean free path.