“This work demonstrates for the first time that different sections of a single molecule can be controlled independently,” researcher Nadya Mason told nanotechweb.org. “It is especially significant that we were able to independently control the quantum properties of the molecular system - our measurements showed control over the positions of the energy levels of each dot, as well as the ability to tune the amount of electron hybridization between the two dots. This shows that it will be possible to fabricate more complex coherent electronic devices for applications such as quantum computing.”

Mason and colleagues made the device from a 2 nm diameter nanotube roughly 1.5 microns long. They positioned the nanotube between metal contacts and added three top gates and a doped silicon backgate. The resulting device showed a double periodicity of Coulomb charging phenomena, which the scientists say is consistent with a nanotube quantum dot, defined by tunnel barriers to the leads, that has been split into two dots of roughly equal size by a defect.

“A typical nanotube quantum device before our work consisted of one dot created by tunnel barriers between the tube and the source/drain contacts - in this case, all electronic properties were controlled by a global backgate,” explained Mason. “We decided to try to achieve greater control over nanotube electronic parameters by locally gating different sections of the tube. We hoped that local electrostatic gates would allow us to define and control multiple dots along a nanotube.”

The researchers tried various methods of creating multiple gates near nanotubes, including growing tubes across predefined metallic gates (“under gates”), lithographically defining lateral gates next to tubes (“side gates”), and aligning gates on top of tubes. According to Mason, the top-gate method, which used a vertically integrated geometry largely developed by researchers at IBM, was the most effective and reliable.

“Nanotube-based quantum dots are excellent candidates for schemes aimed at realizing solid-state qubits by controlling and exploiting the spin degree of freedom in quantum-dot systems,” said Mason. “Double quantum dots, in particular, provide a natural basis for realizing the two-qubit XOR gate, potentially allowing universal quantum computation.”

Mason says that because nanotubes have weak spin-orbit coupling and zero nuclear spin, spin lifetimes in nanotubes will be much longer than in GaAs-based quantum dots. What’s more, since nanotubes are so small, quantum mechanical effects are visible at much higher temperatures than in comparable semiconductor heterostructure devices. For example, for a 20 nm nanotube quantum dot – a length scale now achievable using electron-beam lithography – quantum effects would be accessible at room temperature. “Nanotubes are therefore a favourable medium for spin-based quantum computing,” added Mason. “This makes the development of controlled nanotube double dots a key milestone along the path toward solid-state quantum information.”

So far, the team has based its double quantum dots around naturally occurring defects in the nanotube. Now the researchers are aiming to induce defects controllably along a nanotube and form “arrays of quantum dots, each with a well-defined size, with independent control over each tunnel barrier as well as the energy levels of each quantum dot”. The ultimate plan is to create and perform simple quantum computations on a nanotube quantum dot-based quantum computer.

The researchers reported their work in Science.