Scientists create identical quantum dots and place them with pinpoint precision
The ambitious goal of creating quantum dots with digital fidelity by eliminating variations in their size, shape and arrangement has remained elusive. Such perfect reproducibility is important as it opens the door to quantum dot architectures free of uncontrolled variations, which is necessary for technologies ranging from nanophotonics to quantum information processing.
Now scientists from Paul-Drude-Institute for Solid-State Physics in Berlin, NTT Basic Research Laboratories, Japan; and the Naval Research Laboratory (NRL), USA have managed to create quantum dots with identical, deterministic sizes, according to a recent report in Nature Nanotechnology.
Quantum dots are often called artificial atoms because, like real atoms, they confine electrons to quantised states with discrete energies. But real atoms are identical, whereas most quantum dots comprise hundreds or thousands of atoms, with variations in size and shape and, consequently, unavoidable variability in their wavefunctions and energies.
Creating atomically precise quantum dots requires every atom of the quantum dot to be placed in a precisely specified location without error, and multiple dots to be arranged in exactly defined configurations without variation. The researchers achieved this goal using a scanning tunnelling microscope (STM) to manipulate the atoms and an atomically precise surface template to define a lattice of allowed atomic positions.
The template was the surface of an InAs crystal, which has a regular pattern of indium vacancies and a low concentration of native indium adatoms adsorbed above the vacancy sites. 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 N = 6 to 25 indium atoms.
Because the indium atoms are strictly confined to the regular lattice of vacancy sites, every quantum dot with N atoms is essentially identical, with no intrinsic variation in size, shape, or position. This means that quantum dot "molecules" consisting of several coupled chains will reflect the same invariance.
Steve Erwin, a physicist at NRL and the team's theorist, pointed out that "this greatly simplifies the task of creating, protecting, and controlling degenerate states in quantum dot molecules, which is an important prerequisite for many technologies." In quantum computing, for example, qubits with doubly degenerate ground states offer protection against environmental decoherence.
By combining the invariance of quantum dot molecules with the intrinsic symmetry of the InAs vacancy lattice, the team say they have created degenerate states that are surprisingly resistant to environmental perturbations by defects.
The reproducibility and high fidelity offered by these quantum dots makes them excellent candidates for studying fundamental physics. Looking forward, the team also anticipates that the elimination of uncontrolled variations in quantum dot architectures will offer many benefits to a broad range of future quantum dot technologies from nanophotonics to quantum information processing.
Figures a,b, c and d above show the quantized states of a digital quantum dot in which electrons are confined by a chain of ionized indium adatoms. Picture a, is a topographic STM image (0.1 nA, _0.3 V) of a chain of indium adatoms assembled on InAs(111)A. Twenty-two indium atoms were placed on adjacent indium-vacancy sites of the (2 x 2)-reconstructed surface. b, shows the atomic structure of the image section indicated in a. The surface consists of indium (green) and arsenic (orange) atoms, and the chain is formed by In adatoms (black circles) adsorbed above vacancy sites. c, shows the differential conductance (dI/dV) spectra (red and blue) recorded at the off-chain tip positions indicated in a, revealing quantized electron states with quantum numbers n = 1-7. The reference spectrum of pristine InAs(111)A (green) reveals that the Fermi level is pinned in the conduction band due to intrinsic electron accumulation at the surface. d, Spatial DOS maps D(x,y) obtained by constant-height dI/dV scanning at the bias voltages corresponding to the resonances in c. Quantized states for n = 1-6, each with n lobes and n- 1 nodes, are clearly revealed.
This work is described in Nature Nanotechnology 9, 505-508 (2014) 'Quantum dots with single-atom precision' by Stefan Folsch et al.
doi:10.1038/nnano.2014.129
