Get a Charge, Get a Quantum Dot

Hank Hogan

Researchers at Harvard University in Cambridge, Mass., have demonstrated carbon nanotube-based quantum dots that can be defined and controlled electrostatically and could be used for quantum computing systems.

Previous nanotube-quantum dot studies investigated quantum dots formed by nanotube defects or by tunnel barriers in a metal/nanotube interface. With these methods, it was not possible to independently control device parameters, and there also were strict constraints on device design. The new technique gets around these restrictions because the quantum dots are defined by controlling gates.

The researchers began by growing carbon nanotubes on a silicon wafer. They selected single-walled nanotubes with a diameter of less than 3 nm and created 15-nm palladium contacts. These contacts butted up against each end of the nanotubes, leading to devices that ranged in length from 5 to 25 μm. They covered the palladium and nanotubes with 25 to 35 nm of either silicon dioxide or aluminum oxide and topped everything with a layer of chromium/gold. Finally, they patterned the metal, resulting in 150- to 300-nm-wide controlling gates that did not overlap the buried palladium contacts.

The simplest quantum dot device configuration they created consisted of three controlling gates. They cooled the device to a few degrees above absolute zero and charged the two outer gates to create barrier regions in the nanotube below. They used the middle gate to form a quantum dot, which appeared in the nanotube directly beneath the gate.

From various measurements, the researchers showed that the resulting quantum dot was roughly 2 μm long, and that it behaved electrically like other nanotube quantum dots. Using a five-gate arrangement, they created a double quantum dot. By adjusting the charge of the middle gate, they could merge the two dots into one. Again, this was verified by various electrical and other measurements.

Because the location and existence of these quantum dots are controlled by the placement and charge on the gates, quantum dots can be positioned anywhere along a nanotube. Such capabilities could be used to make coupled quantum dots for quantum computation devices. Moreover, carbon nanotubes are attractive for spin-based quantum information storage.

Nano Letters, ASAP Article, doi:10.1021/nl050364v.