One step closer to quantum computing

The first steps towards a novel quantum computer based on standard electronics technology have been taken by a multi- disciplinary team using the Dutch free electron laser FELIX at the FOM Institute Rijnhuizen near Utrecht.

The results bring the reality of a workable quantum computer one step closer, suggesting for the first time that it is possible to make these computers in silicon rather than a vacuum, which has been the focus of previous research.

Quantum computers have the potential to solve some problems much faster than ordinary computers by storing information in the internal states of atoms. However, for quantum computers to work, the atoms must be kept isolated from external influences so allowing them to move in an undisturbed quantum wave. This can hold much more information than a normal computer bit, but is so delicate that it is easily destroyed, and therefore must be kept isolated. However, in order to program the computer we have to intervene and modify and control the atoms.

Previous research has succeeded in creating some building blocks for a quantum computer by using atoms trapped in a vacuum. This isolates them well, but makes them difficult to control. Atoms trapped in a silicon crystal, are less well isolated but this research team – involving scientists from University College London, The University of Surrey, Heriot-Watt University, The University of Glasgow and The Infrared User Facility FELIX at FOM Rijnhuizen – have shown that the quantum states last long enough for computer operations. Work is now in progress to improve the control of the atomic states.

Prof Marshall Stoneham said “Present day computing is possible because of the ingenious ways that silicon technology lets us manipulate electrical properties. What our new work does is show excited electronic states have the right properties for use in new ways that might make quantum computers possible.”

The researchers used the ‘free electron laser’ FELIX in the Netherlands to carry out the work which has been published in Proceedings of the National Academy of Sciences of the USA.


The electronics industry, which has grown up over the last 50 years, is almost completely dependent on the electronic properties of Silicon. Computers and iPods, mobile ‘phones and dishwashers, credit cards and digital watches all rely on the motion of electrons in Si for their good behaviour, and, indeed they usually behave very well.

Now in many devices the electrons whose behaviour is so important, are donated by Phosphorus which is incorporated into the structure of the Si, usually in such large oncentrations that the extra electrons the P donates can move freely throughout the material. At smaller concentrations each P atom holds on to its electron, and behaves like an isolated atom, with a set of excited states at definite excitation energies. Once an electron is in the excited state it will decay back to the ground state at a characteristic rate.

This has been known for many years – in particular, the structure of the P donors has been measured by absorption spectroscopy. Light of a wavelength which corresponds exactly to the excitation energies of the P electrons is strongly absorbed; light with a nearby wavelength less so, so that each excited state leaves its trace as an absorption line centred at a particular wavelength, and with a certain width. The minimum width that an absorption line can have is proportional to the decay rate, but usually the linewidth is much larger than this minimum because of imperfections in the Si host, which distort the states. This makes it difficult to determine the lifetimes of the excited states.

However, a team with members from FOM Rijnhuizen, The Advanced Technology Institute of the University of Surrey, The  Dept of Electronics and Electrical Engineering, University of Glasgow, the Dept of Physics Herriot-Watt University, and the LCN, at UCL have succeeded in measured the lifetime of the first excited state of Phosphorus in
Silicon (Si:P).


Fig. 1 - Schematic of experiment: The free-electron laser excites the P donors, which then decay back with a lifetime of 200 ps

The team used a pump-probe technique in which an intense pulse of infra-red light of the correct wavelength from the Dutch free-electron laser FELIX excites the P donors, which then become transparent to a following weak pulse – the type that is normally absorbed – also provided by FELIX. By altering the delay between the strong pump and weak probe the way in which the transmission of the weak pulse falls as the excited state decays back to the ground state can be followed. They found that the first excited state has a lifetime of 200 ps.

There are some surprises. The first surprise is that, with so much known about Si:P, the lifetimes of the excited states were unknown – this is the first time the lifetime has been measured. However, there is an even greater surprise. Although the lifetime of the excited state was not previously known, the corresponding linewidth was. If we translate our lifetime into the minimum possible linewidth, we find, as we expect, that it is much less than the linewidth measured in most Si samples; that is, most Si introduces distortion. However, the best Si samples have a linewidth which is only slightly bigger than the minimum these measurements imply, so that the best hosts produce very little distortion. This was unexpected, and fortunate. It means that the Si can hold the P so gently that it will not interfere with anything we may want to do to the P donor electrons, and thus allows us to control them precisely. Indeed, we expect to control the quantum behaviour of these electrons – the most fundamental level of control available to us, and so to create a new generation of solid-state quantum electronics. Experiments to extend these ideas are currently under way at FELIX.

Journal link:   Proceedings of the National Academy of Science August 5 2008 vol 105 no 31 pp 10649-10653 

Notes for Editors:

About the London Centre for Nanotechnology
The London Centre for Nanotechnology is an interdisciplinary joint enterprise between University College London and Imperial College London. In bringing together world-class infrastructure and leading nanotechnology research activities, the Centre aims to attain the critical mass to compete with the best facilities abroad. Research programmes are aligned to three key areas, namely Planet Care, Healthcare and Information Technology and bridge together biomedical, physical and engineering sciences. Website:

About the Infrared User Facitity FELIX
The Infrared User Facility FELIX of the FOM Institute for Plasma Physics 'Rijnhuizen' in Nieuwegein, The Netherlands provides continuously tunable infrared radiation in the spectral range of 40-2500 cm-1 (4-250 mm), at peak powers ranging up to 100 MW in (sub)-picosecond pulses. The infrared radiation is available to the international user community for scientific research in the fields of biology, chemistry and physics.

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