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Germanium Quantum Technology

01 /What can CMOS technology do for quantum computing? 02 /Semiconductor devices and scaling with industrial approach 03 /Semiconductor foundry facilities: the good and the bad for qubits 04 /IMEC's latest qubit developments 05 /Summary and outlook 06 /Semiconductors for spin qubits 07 /Germanium quantum wells on silicon 08 /Germanium quantum wells on silicon-germanium 09 /Growth methods 10 /Characterization techniques 11 /Electrical characterization of germanium quantum wells 12 /Fabrication of H-FET 13 /Semiconductor qubits: vision and challenges 14 /Quantum materials 15 /Quantum dot qubits and their operation 16 /Types of quantum dot qubits 17 /Qubit Readout 18 /Qubit Control 19 /Qubit Coherence and Fidelity 20 /Multi-qubit control and quantum algorithms 21 /Quantum links and scaling 22 /Physics of holes 23 /Hole spin qubits 24 /Introduction to electronics for quantum computing 25 /Room temperature electronics 26 /Cryogenic qubit chip carriers 27 /Contribution to IGNITE 28 /Quantum Control Stack for Spin Qubits 29 /Auto-tuning a quantum computer 30 /Tuning and operation of arrays 31 /Finding operation points example 32 /What is auto-tuning? 33 /Neural network tuning 34 /Navigating charge stability diagrams 35 /Experimental implementation 36 /Towards universal quantum algorithms 37 /Classical error correction 38 /Quantum error correction 39 /Progress and challenges 40 /Introduction to quantum algorithms 41 /The first algorithms 42 /Quantum annealing 43 /Quantum Machine Learning

Semiconductors for spin qubits

The Ignite project is based around germanium, the semiconductor with the right balance of material properties which make it the best material in which to fabricate spin-based qubits. 

Spin qubits in germanium are based on holes rather than electrons. The energy difference between spin states for holes in germanium is larger than for electrons in either silicon or gallium arsenide, so that weaker magnetic fields can be used, and indeed this can even be tuned by the geometry of the quantum dot. Strong spin-orbit coupling means that spin can be manipulated with electric fields without needing a nearby micromagnet. The effective mass for holes is smaller than for electrons in silicon, so that qubits can be made larger (50-100 nm). The dominant isotopes of germanium have no nuclear spin, unlike both gallium and arsenic, which greatly extends the qubit coherence lifetime. 

Prerequisite knowledge

  • Some background solid state physics is useful e.g. the electronic band structure of typical semiconductors, including the concept of effective mass 
  • Zeeman splitting 
  • Spin-orbit coupling 

Main takeaways

  • Electron qubits in silicon have a long coherence lifetime but need to be small and are relatively slow. 
  • Electron qubits in gallium arsenide have a short coherence lifetime (due to nuclear spins). 
  • Hole qubits in germanium have long coherence lifetimes, don’t need to be so small, and can be operated quickly. 

Further thinking

True or False: A small effective mass means that a quantum dot has to be smaller in order to demonstrate quantum confinement.

Solution

Further reading

A review on “Semiconductor Spin Qubits” by Guido Burkard et al. https://arxiv.org/abs/2112.08863