Quantum computing – University of Copenhagen

Condensed Matter Theory > Research > Quantum computing

Quantum computing

The fundamental unit in a quantum computer is the quantum mechanical equivalence of 0 and 1, the so-called qubit. It is two-level system such as for example a single electron spin, which indeed is one of the candidates for an information carrier in a solid state implementation of quantum computers.

We study, for example, ways to control the quantum state of spin-qubits by electrical means. This can be done is systems in spin-orbit interaction and in the presence of a magnetic field, that splits the electrical nature of the two time-reversed states, called a Kramers pair. Various systems are considered for this kind of control, namely carbon nanotubes and semiconducting wires.

A large part of our activity now involves systems for topological quantum computing, which requires quantum state protected by topology. Examples are Majorana Fermions in superconducting structures. In recent years, it has been realized that one can design materials with induced p-wave superconducting order, which is an exotic superconducting state with broken time-reversal  known to have localized Majorana Fermions as end state. The notion of Majorana Fermions makes an interesting connection to particle physics, where there is a long-standing debate whether such particles, being their own antiparticles, can exist.

Majorana fermions are particles that are their own antiparticles and therefore only constitute half a degree of freedom. The other half is can be far away and therefore to read of their mutual state non-local measurements are needed. Since noise is local in nature, this property makes the quantum states of a Majorana particles, represented by the parity of fermion number, topologically protected and therefore potentially useful for quantum computation purposes. 

We investigate designs of strong spin-orbit materials connected to conventional superconductors, with the aim of controlling and studying the properties of Majorana fermions. For the manipulation of their quantum state, we investigate the possibility of interfacing with "conventional" mesoscopic quantum systems, such as quantum dots.