Nanowires and Nanotubes – University of Copenhagen

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Center for Quantum Devices > Research > Nanowires and Nanotubes

Nanowires and Nanotubes

QDev has a particular interest in materials where the spin environment or spin-orbit coupling can be controlled. Several options exist within nanotubes and nanowires that in addition exhibit one-dimensional transport and allows for integration in hybrid devices. Quantum dots and qubits can readily be made with these materials. The center houses facilities for growth of carbon nanotubes and III-V nanowires.

Chip with spin qubits based on Ge-Si core-shell
nanowires mounted in a dilution refrigerator. On-
board copper inductors serve as tank circuits for
fast qubit readout and capacitors provide electrical
filtering for various tuning voltages. Connectors for
high-bandwidth qubit manipulation and readout are
located on the back of the board.

Spin-based qubits is a central theme at the Center (see section on Solid-State Qubits). The coupling of the solid-state environment to the spin of an electron is much weaker than its coupling to charge, enabling long coherence times for spin-based quantum states.  Control of individual spins can be realized by applying an oscillating magnetic field but for devices a more fruitful approach takes advantage of spin-orbit coupling, which allows an oscillating electric field, applied by gates, to rotate individual spins through electric dipole spin resonance. However, spin-orbit coupling also allows a noisy electrical environment to couple to the spin. A careful consideration of spin-orbit coupling in several different materials, including heterostructures, wires, nanotubes, and quantum dots, is therefore an important activity within the Center.

Scanning electron micrograph (with false colour)
of a nanotube double dot with integrated charge
sensor. The carbon nanotube (not visible) runs
horizontally under the four Pd contacts (red). Top-
gates (blue) create voltage-tunable tunnel
barriers enabling the formation of a single or
double quantum dot. Plunger gates (green) control
the occupancy of the double dot. A separate single
dot contacted (right) is controlled with gate plunger
gate (grey) and is capacitively coupled to the
double dot by a coupling wire (orange). (H.O.H.
Churchill et al, Nature Physics 5, 312 (2009))

A range of materials with unique properties are employed for quantum devices. III-V nanowires (InAs, InSb) posses a strong spin-orbit coupling that can be exploited for spin manipulation and topological systems. On the other hand, interactions with nuclear spins can be a source of decoherence. Si and Ge/Si are predominantly zero-nuclear-spin materials and can also be grown as heterostructured wires for devices. In carbon nanotubes, the spin-orbit coupling is tunable and the effective g factor is directional, so that a bend in the nanotube produces an effective gradiant in the g factor. Isotopically pure nanotubes can be grown in order to tailor the nuclear spin environment.

While some materials are provided by external collaborators, materials growth within the Center focus on III-V nanowires grown by Molecular Beam Epitaxy, including fault-free wires as well as core-shell and segmented heterostructures. Our equipment also allows for in-situ deposition of metallic shells for contacting.  Carbon nanotubes are grown by Chemical Vapor Deposition, specializing in clean nanotubes and materials with a controlled ratio of 12C to 13C, which affects the nuclear spin environment. We employ electron microscopy, Raman spectroscopy and synchrotron x-ray scattering for structural characterization of wires and tubes in collaboration with other groups. Electrodes and gates defined by electron beam lithography turn the wires into devices.

Scanning and transmission electron micrographs of InAs nanowires grown in a period array (upper left panel) by Molecular Beam Epitaxy. High resolution microscopy reveals clearly the atomic planes of the perfect single-crystal wire and even allows for identification of the rows of different elements (lower left). (P. Krogstrup et al, Nano Lett. 9, 3689 (2009))

 

The ready-made one-dimensional materials enable simple schemes for fabrication of hybrid devices such as quantum dots and quantum wires with superconducting leads. For example, the combination of strong spin-orbit coupling and induced superconductivity in nanowires is of particular interest for construction of topological systems and the investigations of Majorana fermions (See section on Topological Quantum Systems).

A challenge of using 1D materials – nanowires and nanotubes – is how to move beyond the single-file constraint of the substrate by coherent coupling quantum states in different 1D systems.  We will investigate coherent transfer of quantum states across the intersections of wires.