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All the titles of the theses involved contain the words ‘nanowire’ and ‘quantum dot’. Apparently, this is a hot domain of research. But what’s it all about? Professor-emiritus Hans Mooij (Applied Sciences) explains that various forms of quantum bits are being investigated by the Quantum Transport research group at the Kavli institute for Nanoscience. Quantumbits, or qubits for short, are units of information capable of not just containing a value ‘0’ or a ‘1’, like a normal digital bit, but also capable of containing the values ‘0’ and ‘1’ at the same time. A qubit is said to contain a ‘superposition’ of states. Physically, qubits can be as diverse as an atom trapped in a laser beam, a superconducting coil, a pair of confining electrodes on a semiconductor (the quantum dot), or a nanowire made of semiconductor material or carbon nanotube. What they all have in common is the isolation and the malleability of a quantum property, such as spin, charge, current or polarisation.
PhD student, Maarten van Kouwen, worked with semiconducting nanowires (30-40 nanometres wide and a few micrometers long) containing a quantum dot. His most remarkable result, according to his PhD supervisor, professor Leo Kouwenhoven (AS), is the coupling between the optical and electrical properties in a quantum dot. After a laser beam has hit the quantum dot, it emits light itself in a process known as photoluminescence. The weird thing is that the colour of the emitted light can be tuned by varying an electric voltage over the quantum dot. This phenomenon will enable researchers to discover and use new quantum properties, says Kouwenhoven: “You may be able to couple or decouple photons and electrons just by turning a dial.”

Another PhD student, Maarten van Weert, has used the device to achieve a quantum coupling between the polarisation of a single photon (elementary light particle) and the spin of a single electron. This means that the luminescence light coming from the quantum dot carries information about the electron spin state. In other words, you can optically sense the state that the qubit is in by measuring the polarisation of the light coming off it. “This is important”, says Kouwenhoven. “It enables us to measure the electron spin from a distance.”
While electron spin states ‘up’ and ‘down’ neatly correspond with light polarisations ‘right’ and ‘left’, a superposition of electron spin states has not yet been converted to a superposition of polarisations. Kouwenhoven and his group posit schemes on ways of getting that done. Just imagine: quantum communication over glass fibre.

One of the first researchers to work with the semiconducting nanowires was PhD student, Juriaan van Tilburg. He studied the external influences on the electron spin of a single electron captured in a quantum dot on the wire. He discovered, among other things, that the influence of a magnetic field varies with its orientation with regard to the direction of the nanowire. Kouwenhoven believes this new property may be useful to create superpositions of electron spins: “It gives us a new button to play with.”

Even experts like Van Kouwenhoven are sometimes perplexed by the behaviour of the quantum devices. His PhD student, Georg Götz, succeeded in putting two electrons on a single carbon nanotube (an open nanowire made of a rolled-up carbon sheet), and this earned him a publication in Nature Nanotechnology. The device made it possible for the first time to measure the force between two electrons a micrometer apart. “You have two fundamental forces competing: the classical electrostatic repulsion and quantum mechanical uncertainty principle formulated by Heisenberg. That’s not so easy to understand.”
After four years into the research of solid state quantum information processing (SSQIP), an interim score has been reached. The follow-up will be aimed at the optical transfer of electron spin superpositions and trying to demonstrate quantum entanglement on an electronic chip. 

Maarten van Kouwen, ‘Opto-electronics on Single Nanowire Quantum Dots’, 28 June 2010
Maarten van Weert, ‘Quantum dots in vertical nanowire devices’, 29 June 2010
Juriaan van Tilburg, ‘Electron spins in nanowire quantum dots’, 2 July 2010
Georg Götz, ‘Single Electron-ics with Carbon Nanotubes’, 2 July 2010