Tools – University of Copenhagen

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Center for Quantum Devices > Research > Tools

Tools

Specially designed PC boards

Specially designed PC boards that serve as sample holders for solid-state qubits offer easy mounting of the qubit chip in its center. It provides on-board filtering of tuning voltages, microwave transmission lines for fast qubit gates, RF tank circuits for high-fidelity qubit readout, and an array of gold pads for wire bonding. The board needs to survive repeated thermal cycling to below 100mK while maintaining electrical integrity and good thermal conductivity for cooling, with each component performing at milliKelvin temperatures and often high magnetic fields. As such, the development of PC boards for multi-qubit architectures is an active research field of QDev and its collaborators [Ref:Cryogenic High-Frequency Readout and Control Platform for Spin Qubits, J. I. Colless & D. J. Reilly, arXiv:1111.6440 (2011)].

 

After fabrication of devices on a chip is completed, a wirebonder connects the bonding pads on the chip with gold pads on the sample holder (PC board). The thin gold or aluminum wire (0.001 inches diameter) is bonded by application of an ultra-sonic pulse that travels down the steel needle of the wirebonder.

 

A wirebonder is used to electrically connect bonding pads of the sample holder to bonding pads on a chip. The thin gold or aluminum wire (0.001 inches diameter) is bonded by application of an ultra-sonic pulse that travels down the steel needle of the wirebonder.

 

Still, cold plate, mixing chamber plate, and cold finger of a cryofree dilution refrigerator that cools samples down to 10mK.

 

Mixing chamber plate of a cryofree dilution refrigerator. A series of heat sinks and electrical filtering is needed to reduce electrical noise and thermalize all electrical lines (i.e. to achieve a low electron temperature in addition to a low phonon temperature).

 

Close-up view of the still (top right) and a copper box with sapphire heat sinks mounted on the cold plate (bottom left).

 

Three-axis superconducting vector magnet lowered to give access to the coldfinger with sample holder. Once it is raised and conduction-cooled from above by a two-stage pulse tube cooler it allows application of magnetic fields up to 6 Tesla. The ability to point the magnetic field in any direction is particularly important for nanowire and nanotube experiments in which spin properties such as g factors are highly anisotropic [see: Gate-dependent spin-orbit coupling in multielectron carbon nanotubes, T. S. Jespersen, K. Grove-Rasmussen, J. Paaske, K. Muraki, T. Fujisawa, J. Nygård, and K. Flensberg, Nature Physics 5, 1 (2011)].

 

Chemical vapor deposition (CVD) allows synthesis of high quality single-walled carbon nanotubes (SWNTs). Gaseous carbon compounds are decomposed in a table-top furnace at temperatures around 1000 °C to nucleate carbon nanotubes from nanometer scale catalyst particles (move mouse over photo). Using isotopically purified methane or ethylene gas allows us to grow carbon nanotubes with nuclear spins (13C) or without nuclear spins (12C). This modifies their phonon spectrum as well as their spin-electronic properties [see: Carbon nanotubes for coherent spintronic devices, F. Kuemmeth, H. O. H. Churchill, P. K. Herring, C. M. Marcus, Materials Today 13, 18 (2010)].

One of the most important tools to measure quantum devices (apart from the cryostat that provides cooling power) are carefully designed electrical filters that thermalize electrons and reject noise while passing the signals of interest.

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