One of the fundamental properties of subatomic particles is their ‘spin’, a form of angular momentum associated with these particles.
Several quantum technologies rely on the orientation of these spins. One particularly promising approach is by embedding impurity atoms in a wafer of silicon, then encoding information in the spin of its nucleus or associated electron. This can be used to store quantum information, or as a sensor in quantum sensing devices that can carry out tasks such as determining the elemental composition of molecules.
However, detecting the orientation of these spins is not straightforward.
“If you look at the spins of electrons in silicon,” says Cheuk Lo, lead author of the study, “the existing way of measuring them is a technique called ‘spin-dependent tunnelling’ that physically pulls the electron away from its atom. However, for this to work it requires very low temperatures – a fraction of a degree above absolute zero – and a powerful magnetic field. These conditions can only be achieved with very specialised equipment, which limits the applications you can use with spin-based silicon quantum technology.”
The team’s solution is to use a laser to help read the electron spins.
Silicon is the raw material of computer chips, thanks to its peculiar electrical properties. The team used a wafer of silicon with phosphorus impurities, which provides an extra electron, whose spin is to be measured.
When a silicon lattice is bombarded with light from a laser, it creates ‘bound excitons’, where an extra pair of electron and hole sits at the site of the phosphorus impurity. When these electron-hole pairs combine together, they release energy that kicks the remaining electron out of the impurity altogether. This does not directly allow the spin to be measured – the only thing that can be detected is that an electron has been released, producing a tiny electrical current.
However, the light can be tuned in a way which creates a bound exciton which selects which spin of electron to eject.
“Our technique relies on the slightly different energy associated with electrons with different spins,” says Lo. “By finely tuning the wavelength of the laser light to provide just the right amount of energy, we can selectively eject only ‘up’ or ‘down’ electrons from the impurities. By detecting whether a current is produced when different wavelengths are shone upon the experiment, we can determine what the spin of the electron was.”
Since the mechanism works entirely thanks to the energy coming from the laser light, the experiment can be carried out with little or no magnetic field. This in turn allows electron spins to be measured in environments in which magnetic fields would be problematic, something which was not possible until now.
Professor John Morton, director of UCL Quantum Science & Technology Institute, a co-author of the research, said: “This hybrid method brings together the advantages of optical addressing, including hyperpolarisation and high selectivity, with electrical detection that can be miniaturised in nano-devices.”