Cryogenic technologies for scalable trapped ion quantum computing

Grounds, Anton (2021) Cryogenic technologies for scalable trapped ion quantum computing. Doctoral thesis (PhD), University of Sussex.

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There has been a great deal of effort and progress made towards building a fully scalable quantum computer but there are still significant engineering challenges to overcome. Trapped ions currently represent the best fundamental technology towards achieving this goal and so they form the basis for the work in this thesis.

In this thesis I describe the work towards creating a closed cycle cryogenic vacuum system driven by a Gifford McMahon cryogenic cooler. I will show the equipment required to trap a Ytterbium 174/171 ion on a cryogenic surface ion trap. The primary goal is to create a fast turn-around system for ion trap research. Room temperature systems suffer from long baking times and as such ion traps are rarely changed, this means that there is little room for testing and comparing of trap designs and predicting optimum trapping parameters as each trap is usually not changed unless there is a fault. In the cryogenic system, every effort is made to make the system generic enough to allow for any trap design to be tested and to be replaced within 24 hours.

To create reliable trapping parameters we require a toolkit that can numerically simulate the required RF and DC voltages used to trap the ions above the surface of the ion trap. There are many numerical methods that can be used (FEM, FVM, BEM, FTDI, etc) and in this thesis I describe multiple methods and what their relative benefits are. Since we want to create a full toolkit to allow for chip design and optimisation, most commercial programs suffer from inadequate API interfaces or they dictate which programming languages you can interface with. This would limit our abilities to develop and optimise trapping parameters, especially in a cryogenic system where the goal is rapid deployment. To this end I created a new toolkit based on the Scuff BEM solver engine that allows us to go from AutoCAD layout to field solver and gives us accurate field potentials that match micromotion compensated voltages to within 1%.

I describe a novel cryogenic resonator built out of superconducting wire based on an auto-transformer topology, I compare its strengths and weaknesses compared to other technologies already in use for trapped ion research.

I also describe a novel DSUB design with integrated modular filtering which allows for 50 connections per DSUB. This DSUB is used to connect two sets of 50 wires to a PCB for wirebonding to the surface trap. This allows me to save space within the 4k shield where other PCB designs would not fit. Also it is modular in design which allows for different cutoff frequencies to be swapped out as desired.

I also describe different chip mounting technologies including epoxy based, physical clamping and indium foil based. After extensive testing the indium foil diebonded approach is deemed as being the most reliable and fastest non-destructive method. I also describe a novel cryogenic coil design for generating 12T m−1 field gradients for microwave based ion gates with Ytterbium 171. I then describe the creation of a combined RF and microwave antenna for delivering coherent RF and 12.6GHz microwave radiation inside the cryogenic environment. This design is much smaller than typical waveguide typologies used.

I finish by showing the first trapped ions in a cryogenic surface ion trap in the UK and heating rate tests performed on two different linear surface traps. The first trap was a sapphire based, 150µm ion height linear trap with a measured heating rate of Se(1MHz) = 5.93(10) ∗ 10−14(V /m)2. The second ion trap is a 50k Silicon based, 100µm ion height linear trap with a measured heating rate of Se(1MHz) = 2.45(10)∗10−13(V /m)2

Item Type: Thesis (Doctoral)
Schools and Departments: School of Mathematical and Physical Sciences > Physics and Astronomy
Subjects: Q Science > QA Mathematics > QA0075 Electronic computers. Computer science > QA076.889 Quantum computers
T Technology > TP Chemical technology > TP0480 Low temperature engineering. Cryogenic engineering. Refrigeration
Depositing User: Library Cataloguing
Date Deposited: 28 Jun 2021 12:49
Last Modified: 28 Jun 2021 12:59

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