There are many different ways in which one could realize a quantum bit (qubit), the fundamental building block for quantum computers. One possibility is to use a single electron spin inside a semiconductor as a qubit. In the nanowire team of QT we do just that. By creating quantum dots inside a nanowire we can confine single electrons with which we can then form spin qubits.
Nanowires are a type of system that make possible the creation of defect-free crystalline structures. Moreover, it allows the material composition to be easily and precisely varied during growth. The one-dimensionality of nanowires could also make them a natural candidate for realising spin qubit registers. Our nanowires are made of the III-V semiconductor materials InAs or InSb, which have large g-factors and spin-orbit coupling, enabling fast control over the electron spins. Moreover, the spin-orbit coupling enables us to use microwave electric fields to control the spin, rather than magnetic fields. Using this principle, we recently demonstrated full and selective control over a single spin in an InAs nanowire double quantum dot . This was enabled by earlier work in our group that helped distinguish the signatures of the spin-orbit interaction from those of the hyperfine interaction with the nuclear spins .
Realising a quantum computer will require not only the control of a single qubit, but also to create entanglement between two qubits. The next step in our research will be to realise this entanglement, for example through the development of a controlled phase (CPHASE) gate .
 S. Nadj-Perge et al., Nature 468, 1084-1087 (2010)
 S. Nadj-Perge et al., Physical Review B 81, 201305 (2010)
 T. Meunier et al., Physical Review B 83, 121403 (2011)
Majorana fermions are a type of exotic particles whose anti-particles are themselves. It is extensively searched as an elementary particle but has never been observed yet. Recently it has been proposed that by bringing semiconductor nanowires with strong spin-orbit interaction in contact with a superconductor, Majorana fermions can emerge at the end of the nanowires as quasi-particles.
In our group, we try to create Majorana fermions in InSb nanowires combined with a superconductor, to first observe the key signature of Majorana fermions in solid state system.
Carbon nanotubes combine several properties that make them promising for electrical and mechanical devices. Because the dominant carbon isotope, 12C, has no nuclear spin, electron spin qubits defined in nanotubes can avoid the effects of hyperfine decoherence. At the same time, spin-orbit coupling offers a route to all-electrical spin manipulation. We are working to create a spin qubit based on a nanotube quantum dot device.
Nanotubes also have outstanding mechanical properties. We have developed mechanical resonators based on suspended carbon nanotubes – so called nano-guitar strings. Using devices in which the nanotube is grown as the final step of fabrication, very high mechanical quality factors can be achieved, even at GHz frequncies. This combination of high frequency (implying a large quantum level spacing) and large quality factor (implying a long lifetime) is promising for attaining control of a mechanical resonator at the single-quantum level. The figure shows a typical suspended device, together with an electrically detected resonance at 39 GHz.