Tools – University of Copenhagen

Tools

4K measurement station

To rapidly test and iterate device and material design parameters we have a Lakeshore Cryotronics probe station that has been modified, in-house, to be compatible with the board loading system used in our dilution refrigerators.

Samples are cooled to 4K by a cryo-free pulse tube in approximately 3 hours and material parameters such as field effect mobility can be tested in conditions that are close to the milli-Kelvin temperatures that are needed for our final device operation. If required these devices can then be directly loaded to a dilution refrigerator to access sub-Kelvin temperatures and to apply large magnetic fields.

Specially designed modular PCB sample holders

Fast operation of the quantum devices requires usage of specially designed sample holders that guarantee small cross-talk between wide-bandwidth lines, low parasitic capacitances and possibility of placing small electrical components in the nearest vicinity of the sample. In our lab we use modular PCB sample holders designed in a QDev-spin-off company QDevil. They use a set of components that work reliably in sub-kelvin temperatures and that survive repeated thermal cycling, and their layout is optimized to minimize pain of bonding the devices with tens on electrodes.

Spin-qubit device bonded to the module of a sample holder, surrounded by electrical components forming resonant circuits for reflectometry measurements.

Specially designed PCBs

Specially designed multi-layer microwave and low-frequency PCBs for cryogenic and room temperature applications: wire-bondable sample boards with additional SMT components, mainboards with integrated RF bias tees and low-pass filtering of the DC lines, supplementary filter boards etc. 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

Bonding

After fabrication of a device is complete, a wirebonder connects the bonding pads on the chip with bond pads on the sample holder (PCB board). The thin gold or aluminium wire (0.025 mm diameter) is bonded by application of an ultra-sonic pulse that travels down the steel needle of the wirebonder.

We have both manual KS bonders and an FS Bondtec 5630 Bonder which after aligning can automatically place all the preprogramed bonds.

Cryo-free dilution refigerators

The study of quantum devices at QDev is performed at sub-kelvin temperatures, for which purpose the center has 12 cryo-free dilution refrigerators, all with base temperature below 20 mK. The refrigerators are equipped with a bottom loading system, which enables to thermocycle samples between base and room temperature within 16 hours, providing efficient feedback in a cycle of design, fabrication and characterization of the quantum devices. Internally, each fridge is optimized for different kind of experiments. Some of the customizations are:

  • a superconducting vector magnet that can generate magnetic field up to 6 T vs. aluminum shielding for low magnetic noise operation of superconducting qubits
  • up to 42 high frequency coaxial lines for sub nanosecond voltage manipulation of the devices vs. heavy-filtered DC-only setup for low-temperature and low-noise material study
  • low-noise conventional and parametric cryogenic amplifiers for optimal RF reflectometry and transmition measurements

Fully automatic bottom-loading system for Blue Fors dilution refrigerator.

Wide range of low- and high-frequency electronics

Study of the novel devices requires a wide arsenal of manipulation techniques that explore bandwidth extending from DC to tens of gigahertz. The arsenal of equipment avaliable at QDev starts from conventional DC low-noise voltage and current sources and amplifiers, extends through microwave electronics that can be composed into custom RF circuits and ends at nanosecond-resolution arbitrary waveform generators and acquisition cards as well as super-gigagertz vector signal generators.

Rack with a variety of low- and high-frequency electronics, and the Oxford dilution refrigerator with the vacuum can and radiation shields removed

Custom built microwave device

Specially designed multi-channel bias tee with integrated band-stop filter for operation at 20mK temperature. This device intendent to decrease a number of cables for qubit control/read-out.

 

AJA sputtering & Evaporation

AJA 10kV electron beam evaporators allow depositing a variety of materials [Al, Au, Ge, Pt, Cr, Pd, V, Ti, SiOx, Cr]. It is also possible to sputter materials which do not evaporate well such as W or NbTiN. The evaporators also allow insitu Kaufman and RF milling.