Physicists at Cambridge University in the UK have developed a new way to make quantum electronic devices using an atomic force microscope. The technique, known as erasable electrostatic lithography, allows researchers to create and change quantum devices and circuits during an experiment – a feat that was hitherto impossible. Erasable lithography could be used in experiments to study quantum phenomena and may also provide a route to the fabrication of a solid-state quantum computer (R Crook et al. 2003 Nature 424 751).
Rolf Crook and colleagues used an atomic force microscope to define quantum electronic components – such as quantum wires and dots – on the surface of a gallium arsenide wafer. The tip of the microscope, which was biased to –6 volts, “drew” patterns of charge on the surface, and these patterns depleted electrons from a two-dimensional “sheet” of electrons in a layer of aluminium gallium arsenide beneath the surface (see figure). The experiments were carried out at 20 mK.
The technique allows the team to tailor properties of the components, such as their size and shape, and to link individual components into more complex circuits. The patterns can be erased by going over them again with the tip biased to +3 volts, or the whole surface can be erased by illuminating it with red light.
“The most exciting feature of erasable electrostatic lithography (EEL) is the complete freedom to change device geometry during the experiment,” Crook told PhysicsWeb. “For example, we could change the shape of a quantum dot from square to triangular because EEL is performed in the same low-temperature high-vacuum environment as the measurement. This is simply not possible using any other lithographic technique.” The Cambridge team now plans to improve the resolution of the technique by using a smaller probe, or by moving the electron sheet closer to the surface, and to investigate phenomena such as quantum decoherence and fractals in quantum dots.
Crook and colleagues also believe that EEL could be used to make a quantum computer based on arrays of quantum dots. “A quantum computer would require an array of almost identical quantum dots,” he said, “but inherent material defects make it hard to see how this could be achieved with using conventional techniques such as electron-beam lithography. Erasable electrostatic lithography would vastly simplify the fabrication of such devices.”
