On the dot: Quantum reproducibility

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  • Published: Jul 15, 2014
  • Author: David Bradley
  • Channels: Atomic
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Deterministically quantum

An international collaboration has developed a technique for creating quantum dots with identical, deterministic sizes as revealed by scanning tunnelling microscopy. The research could allow quantum dot architectures to be constructed with complete control facilitating novel technologies from nanophotonics to quantum information processing as well as for fundamental scientific studies. Credit: Fölsch et al

An international collaboration has developed a technique for creating quantum dots with identical, deterministic sizes as revealed by scanning tunnelling microscopy. The research could allow quantum dot architectures to be constructed with complete control allowing technologists to develop new nanophotonics and quantum information processing systems as well as enabling various fundamental scientific studies.

Physicists from the Paul Drude Institute for Solid State Electronics (PDI) in Berlin, Germany, Nippon Telegraph and Telephone Corporation (NTT) Basic Research Laboratories (NTT-BRL) in Atsugi, Japan, and the US Naval Research Laboratory (NRL) in Washington DC have used scanning tunnelling microscopy to help them create quantum dots with identical, deterministic sizes. The researchers explain in the July 2014 issue of the journal Nature Nanotechnology that perfect reproducibility of these sub-microscopic devices might lead the way to new technologies for information processing and computation.

Atomic allowance

Quantum dots are in some senses "artificial" atoms. They are a system that confines electrons to quantized energy states just as occurs in the conventional model of the atom. However, whereas all atoms of a given isotope are identical, quantum dots are much more complex, themselves comprising hundreds or thousands of individual atoms, and so variations in the size and shape between individual, although purportedly identical quantum dots are inevitable. Researchers have previously demonstrated that they can use external electrostatic gates to reduce these variations. However, a much more ambitious goal would be to devise a way to fabricate quantum dots that are as near identical as possible, precluding statistical variations between them.

Creating atomically precise quantum dots requires every atom to be placed in a precisely specified location with no mistakes. The team has now assembled the dots atom-by-atom, using an STM, and relied on an atomically precise surface template to define a lattice of allowed atom positions. The template was the surface of a crystal of indium arsenide, InAs. This material has a regular pattern of indium vacancies and a the concentration of native indium adatoms adsorbed above the vacancy sites is low. NTT's Kiyoshi Kanisawa, an expert in crystal growth, used molecular beam epitaxy to fabricate this InAs surface. The adatoms are ionized +1 donors and can be moved with the STM tip by vertical atom manipulation. The team assembled quantum dots consisting of linear chains of 6 to 25 indium atoms.

PDI physicist Stefan Fölsch, who led the team, explains that, "The ionized indium adatoms form a quantum dot by creating an electrostatic well that confines electrons normally associated with a surface state of the InAs crystal. The quantized states can then be probed and mapped by STM measurements of the differential conductance." The resulting spectra show a series of resonances while spatial maps reveal the wave functions of the quantized states, which are exactly as expected for a quantum-mechanical electron in a box.

Hi-fi quantum dots

The team adds that because the indium atoms are strictly confined to the regular lattice of vacancy sites, every quantum dot with a given number of atoms is essentially identical, with no intrinsic variation in size, shape, or position. This means that quantum dot "molecules" that are made up of numerous coupled chains will also show the same invariance. NRL's physicist Steven Erwin, the theorist of the team, points out that, "This simplifies considerably the task of creating, protecting, and controlling degenerate states in quantum dot molecules, another important prerequisite for many technologies."

These high fidelity quantum dots might make the perfect candidates for studying fundamental physics that is usually rendered opaque by stochastic variations in size, shape, or position of the chains. The team suggests that eradicating uncontrolled variations in quantum dot structures will have many benefits in a wide range of applications for these novel systems.

Related Links

Nature Nanotech, 2014, 9, 505-508: "Quantum dots with single-atom precision"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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