Graphene for quantum computing

Quantum computing is a new paradigm in computing that utilizes the benefits of quantum mechanics to enhance the computing experience. Quantum computers will no longer rely on binary digits (0 and 1 states), that computers have relied on since the early beginnings, but will instead use quantum bits, which can be in a superposition of states. Quantum bits, or qubits, have the advantage of being in many states at once, offering parallel computing advantages. For example, they have long been regarded as far superior to classical computers for applications in data encryption.

Although the concept of quantum computers has been known for several decades, practical realizations are still lacking. The main limiting factor has been the critical influence of the environment on a qubit. Most physical systems need to be in perfectly controlled conditions in order to remain in the superposition state, whereas any interaction (mechanical, thermal, or other) with the environment perturbs this state and ruins the qubit. Such perturbation is termed “decoherence” that has plagued many potential qubit systems.

Graphene, having spurred research into numerous novel directions, is naturally also considered as a candidate material host for qubits. For example, back in 2013, a team of researchers from MIT found that graphene can be made into a topological insulator – meaning that electrons with one spin direction move around the graphene edges clockwise, whereas those that have the opposite spin move counterclockwise. They made this happen by applying two magnetic fields: one perpendicular to the graphene sheet, to make the electrons flow at sheet edges only, and another parallel to the sheet, that separates the two spin contributions. Electron spin has long been considered a candidate qubit, because it is inherently a quantum system that is in a superposition of states. In graphene, the spins move along the sheet edges robustly, without much decoherence. Furthermore, the same research showed switching the spin selection on and off, an important feature of q-bit transistors. Nevertheless, extreme conditions such as strong magnetic fields and temperatures near absolute zero are required for this effect in graphene, raising questions about real-world applicability.

Image: Graphene spin qubit, MIT.

This year, the same group discovered a new kind of quantum state that appears when graphene is sandwiched between two superconductors. In this situation the electrons in graphene, formerly behaving as individual, scattering particles, instead pair up in “Andreev states” — a fundamental electronic configuration that allows a conventional, non-superconducting material to carry a “super-current,” an electric current that flows without dissipating energy. Andreev states, like the spin qubits, have very little decoherence, due to their paired configuration. These states are predicted to give rise to Majorana fermions, exotic particles that can be used for quantum computing. Although this experiment is also performed at low temperatures, it is an important proof-of-concept that should in the future open doors towards practical realizations of quantum computing.

Most recently, a group from EPFL in Switzerland devised a new way to use graphene in quantum electronics. In a layered capacitor structure, where graphene forms the capacitor parallel plates and boron nitride makes the insulating layer, quantum capacitance gives rise to novel nonlinear electronic phenomena. In this system small changes in, for example, the intensity of an incident laser beam, give rise to large changes in the measured capacitance of the device. The researchers calculate that one single incident photon could be enough to change qubit states, which is an ideal case of a qubit. Again, low temperatures are required for operation, however a significant advantage of this design is that there is no need for external magnetic fields, rendering this solution a step closer to practical applications.

To summarize, there are several different proposals to use graphene in quantum computers. From spin qubits, to Majorana fermions, to nonlinear capacitors, each has their own advantage. One common theme is that all these solutions are highly novel and innovative, and that the marriage of 2D materials and quantum computing is inevitable in the long run.