Coherent Optical and Microwave Physics for Atomic-Scale Spintronics in Silicon (COMPASSS)

Silicon and the technology developed with it has revolutionised industry, entertainment and communication, and there is always interest in new ways to remember and process information. The challenge is to find methods of encoding information in the smallest possible volume and manipulating it in the most complex ways with the lowest energy cost and the highest speed. In this programme, we shall develop methods for encoding information in a single electron, orbiting a single impurity atom in a silicon crystal. We shall develop the technology for manipulating that information with terahertz speed by its magnetic connection with adjacent impurity electrons.At the same time our work will produce a new way of studying atomic physics, which, at its most exciting, requires that single atoms are trapped in a vacuum by complex combinations of laser beams and electro-magnetic fields. Recent interest in atomic physics has centred around the detailed control of quantum states in atoms. Quantum physics allows an electron to be in two places or two states at once, and also allows two electrons to be in a single entangled state where examination of one electron gives up full knowledge of the other. Experiments with pulsed visible lasers on atoms trapped in vacuum have demonstrated new phenomena based on this principle. Similar physics applies to the nucleus, and has resulted in the technology of NMR and MRI. Impurities in crystals, if they have substituted one of the host atoms, can sometimes be thought of just in terms of their extra charge. For example, phosphorus impurities in silicon crystals look like silicon atoms with an extra proton in the nucleus and an extra electron. The extra electron orbits around the extra proton charge in a very similar way to the orbit of the electron around a proton in a hydrogen atom. The energies of the allowed orbits follow the same pattern (the Rydberg series) except that the transitions are in the far-infrared, as opposed to the visible/X-rays for hydrogen. Our programme will extend the analogy between the atom trap and the impurity in a crystal. For example we will demonstrate that electrons can be put into two states at once - an electronic version of the trigger that caused Schrodinger's famous cat to be both alive and dead at the same time-and that they can live in this superposition for long times. Recently, atomic physicists have been trying to minaturise their traps using silicon technology to manipulate the atoms in free space just above an atom chip . In a sense, we are working on a similar problem with the atoms held just below the surface instead of just above it. This has the advantage that the impurities are permanently fixed, that complex and new molecule states can be made with adjacent impurities, and that we can take full advantage of the processing technologies in the world's best developed material. The experiments are all enabled by the Free-Electron Laser FELIX facility near Utrecht. This kind of laser (which does not yet exist in the UK) is analogous to the pulsed visible lasers used for the atomic physics experiments, but gives out far-infrared pulses suitable for impurity atoms in silicon. Our proposal takes advantage of the fact that FELIX provides very high power, very short pulsed light that is coherent and tunable. The magnetic interactions between adjacent impurity electrons will be produced by exciting them with FELIX, but to measure the resulting changes we need microwave pulses and a magnet to perform electron paramagnetic resonance (EPR). We will install an EPR spectrometer at FELIX for this purpose. The equipment will also have enormous potential for applications in biology, since EPR is used extensively for measuring the configuration of large bio-molecules, while far-infrared laser pulses can induce controlled changes in conformation. Although we will concentrate on the atomic physics, a spin-out programme in biology will result.