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10.1117/2.1200609.0403
Tunable quantum dots in carbon
nanotubes
Sami Sapmaz and Carola Meyer
Lithographically defined top gates on carbon nanotubes create quantum
dots isolated by tunable tunnel barriers that can serve as a building
block for quantum computers.
Quantum dots (QDs), also known as artificial atoms, are small
structures to which electrons can be added, one by one, into
discrete energy levels. Their study in the past decade has led
to a considerable understanding of atomic-like systems in the
solid state.1 Recently, the interest in QDs has shifted from their
mesoscopic physics to properties related to quantum information technology, since the electron spin in a QD can serve as an
elementary quantum bit, or qubit.2
To date, the material used for most QD studies has been GaAs.
Although much progress has been made, this material presents
intrinsic barriers to application for quantum information processing. For example, the strong spin-orbit interaction severely
limits the relaxation time of electron spins in these types of QDs,
and the Ga and As nuclear spins limit the decoherence time.3
Overcoming such limitations requires the investigation of alternative materials, with new properties.
Our group in Delft has chosen to explore carbon nanotubes
(CNTs) as a novel system for creating high quality QDs that
might be useful as elementary qubits. Since they are based on
carbon, CNTs have the advantages of having no nuclear spin and
very small spin-orbit interaction.
Initially, we studied QDs formed in CNT segments defined by
the deposition of top metal electrodes. We formed high quality
QDs, with a variety of coupling strengths, in both semiconducting and metallic CNTs. Besides the known effects of Coulomb
blockade and energy-level quantization, the peculiar band structure of CNTs enabled us to measure novel types of Kondo effect,
such as orbital and SU(4). A variety of materials can be used to
make electrical contact to CNTs, and the first superconducting
QD has been recently made by depositing superconducting electrodes on CNTs.
In all of these experiments, however, the coupling between the
QD and the leads is not controlled. We have therefore developed
Figure 1. (a) An atomic-force microscope image shows a carbonnanotube double-quantum-dot device, contacted with Pd leads and
with Al electrostatic top gates. Tunnel barriers can be induced and
tuned by voltages applied to these gates. In panels (b) and (c), the voltages on the top gates are tuned such that the carbon-nanotube double
quantum dot is in the intermediate and weakly tunnel-coupled regimes,
respectively. The current is plotted in color scale as a function of the
left- and right-side gate voltage.
a novel technique to define QDs, with tunable coupling strength,
at arbitrary locations in CNTs (see Figure 1). The method, based
on the evaporation of thin electrostatic top gates, has let us create
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SPIE Newsroom
fully tunable double quantum dots in CNTs, a necessary step for
more advanced microwave-based experiments to measure the
electron spin relaxation and decoherence times in CNTs.
Figure 1(a) shows an atomic-force-microscope image of a CNT
contacted by Pd source and drain electrodes, which make good
contact to the CNT. On top of the CNT, but separated from it
by a thin insulating layer, we have formed very narrow (about
25 – 50nm) electrostatic top gates (TGL , TGR , TGM ) of Al. The top
gates are formed by evaporating 2nm of Al, then oxidizing this
layer, and finally by continuing the evaporation with, typically,
50nm of Al.
By applying voltages to the Al top gates we can locally induce
an electrostatic barrier, thus forming QDs. Varying the top-gate
voltage allows us to tune the barrier and explore different experimental regimes, which have different degrees of coupling between the dot and the leads. Until quite recently, such tunable
barriers were only possible in semiconductor-heterostructure
QDs. Traditional side gates are used to change the potential of
the individual QDs.
Figure 1(b) shows the current (in color scale) as a function of
the left- and right-side gate (SGL , SGR ) for a fixed source-drain
bias voltage of 1mV. The characteristic honeycomb pattern indicates that we have formed a double dot in the CNT, which is
in the tunnel-coupled regime.4 By applying voltages on the top
gates, the barriers are increased and the double dot is brought
into the weakly-tunnel-coupled regime, as shown in Figure 1(c).
Furthermore, the excited states of the CNT double dot are observed (lines inside the triangular regions).
These results5 open up the road to much exciting research. The
ability to create a CNT double quantum dot with controllable
barriers, together with the expected long orbital and spin relaxation times make it an ideal candidate for a qubit, the elementary
building block of a quantum computer.
Author Information
Sami Sapmaz
TU Delft
Kavli institute of Nanoscience Delft
Delft, The Netherlands
Carola Meyer
Institut für Festkörperforschung
Forschungszentrum Jülich GmbH
Jülich, Germany
References
1. L. P. Kouwenhoven, D. G. Austing, and S. Tarucha, Few-electron quantum dots,
Reports on Prog. in Phys. 64 (701), 2001.
2. D. Loss and D. P. DiVincenzo, Quantum computation with quantum dots, Phys. Rev.
A 57 (120), 1998.
3. J. R. Petta, A. C. Johnson, J. M. Taylor, et al., Coherent manipulation of coupled electron spins in semiconductor quantum dots, Sci. 309 (2180), 2005.
4. W. G. van der Wiel, S. De Franceschi, J. M. Elzerman, et al., Electron transport
through double quantum dots, Rev. Mod. Phys. 75 (1), 2003.
5. S. Sapmaz, C. Meyer, P. Beliczynski, et al., Excited State Spectroscopy in Carbon
Nanotube Double Quantum Dots, Nano Lett. 6 (1350), 2006.
c 2006 SPIE—The International Society for Optical Engineering