Light tailored graphene for quantum technology

All electronic devices around us, including the one you are using to read this, utilize the charge of an electron for storing and processing information. All operations performed by these electronic devices from playing a movie in Netflix to solving a complex math problem are performed in units of 1 and 0 or equivalently on and off, which can be seen as two different states of an electron. A typical silicon chip in these devices performs one billion operations per second and reaches an upper limit of performance in the range of gigahertz. Improving the performance in a conventional way seems not feasible.

Similar to the charge, the electron can also have another degree of freedom: valley pseudospin, which determines the valley that the electron occupies. Valleys are local minima in the energy bands of solids. Similar to 1 and 0, two valleys can be seen as two units of operations. Not only that, operations in between the two units, i.e., the superposition of  1 and 0, can also be realised using two valleys. The superposition principle is an essential ingredient for quantum technology. Therefore, these valleys may be used to encode, process, and store quantum information at room temperature – A holy grail for quantum computing. Present quantum computers at Google, IBM, and Microsoft are operating at ultralow temperatures (-321°F or less).

Realisation of atomically thin graphene offers to bring revolution in digital electronics. Not only graphene promises to miniaturise the devices but also to improve their operational speed as graphene has exceptional transport and thermal properties.  A monolayer graphene consists of six carbon atoms in a hexagonal pattern and exhibits inversion symmetry. A unit cell of graphene, a basic structural unit, has two carbon atoms. This translates into two identical valleys as a consequence of inversion symmetry of graphene. Identical valleys mean identical units of operations and not 1 and 0. This makes graphene unsuitable for valley operations despite having other wonderful properties. It was an accepted belief that valley operations are not feasible in graphene.

Several similar materials with two different kinds of atoms have been synthesised so that these materials do not possess inversion symmetry and have two distinct valleys. However, these materials are not as good as graphene when it comes to other properties.

In a recent work published in Optica, authors from India and Germany suggest an idea that allows one to realise valley polarisation in a single layer of graphene – an important step for quantum technology. Asymmetry between the two valleys is achieved by tailoring the polarization of light to the symmetry of graphene’s triangular lattice. This allows one to break the symmetry between neighbouring carbon atoms and exploit the anisotropic band structure in the regions close to the valleys, inducing valley polarization.

Not only the flashes of light control the asymmetry between the two valleys, but they also wiggle the electron several hundred trillion times in one second. This opens a door to perform valleytronics at a petahertz rate - a million times faster than the conventional speed today. By exploiting the light driven valley-operations in graphene, quantum computers operating at ambient temperature, just like ordinary computers, might one day become possible.

Visualization of valley polarization: Short flashes of laser (shown in red) shine on a single layer of hexagonal graphene (shown by the hexagonal plane in grey). Due to the light-matter interaction, graphene exhibits different electronic populations at the alternate corners of the hexagon (indicated by the cones at the corners). These different populations, known as valley polarization, can be treated as 0 and 1: the basis of a qubit.

Original publication

Light-induced valleytronics in pristine graphene

M. S. Mrudul, Álvaro Jiménez-Galán, Misha Ivanov, and Gopal Dixit

Optica Vol. 8, Issue 3, pp. 422-427 (2021)