Spin Qubits

This lecture will introduce the rich physics of spin qubits, which are among the leading platforms for quantum technologies as they offer three main advantages compared to other physical systems: They are hot, dense and coherent.

In this context, hot means that they can operate at elevated temperatures. While other platforms require mK or even uK temperature, spins in solids can be initialized, controlled, and even entangled up to room temperature.

Dense means that spin qubits can be packed close together without affecting one another. This is important for the integration of a large number of qubits in a single device, such as a quantum computer that will require millions of qubits to achieve universal, fault-tolerant computations.

Finally, the coherence of spin qubits by far outperforms that of any other known system. While most quantum systems can preserve quantum states for less than a millisecond, spin qubits have demonstrated coherence times of more than 6 hours at cryogenic, and 40 minutes at room temperature. This opens the door towards long-term storage of quantum information.

This lecture will give a general introduction to the most prominent experimental platforms for spin qubits: quantum dots, donors, defects and rare-earth dopants. It will further introduce the different techniques to initialize, control, and read the state of spin qubits, as well as techniques to generate entanglement and perform universal quantum gates for quantum computation. Finally, it will explore the different applications in which spin qubits show unique promise: Nanoscale quantum sensors, scalable quantum computers, and quantum communication in global quantum networks.

After successful completion of the module the students are able to:

• Compare different techniques to initialize, control, and read the state of spin qubits
• Describe the main mechanisms to controllably couple spin qubits and generate entanglement between them
• Analyze different experimental platforms and in the context of quantum computing, quantum sensing and quantum communication
• Describe mechanisms that lead to decoherence, and explain how it can be avoided with spin qubits
• Understand and explain recent scientific publications on spin qubits and related topics

The lecture is targeted to students of the M.Sc. programs in Quantum Science and Technology, Condensed Matter Physics and Applied and Engineering Physics. The lecture and tutorial will be given in English.

The module consists of a lecture (2 SWS) and a tutorial (1 SWS).

In the thematically structured lecture the learning content is presented. With cross references between different topics, the universal concepts of spin qubits are explained. In scientific discussions the students are involved to stimulate their analytical skills.

In the exercise the learning content is deepened and exercised using problem examples involving both analytical and numerical solutions. Thus, the students are able to explain and apply the learned physics knowledge.

Blackboard/Tablet and PowerPoint presentation

Exercise sheets

S. Haroche & J.M. Raimond: Exploring the Quantum, ISBN 0198509146

More literature recommendations are given in the lecture.

There will be an oral exam of 25 minutes duration. Therein the achievement of the competencies given in section learning outcome is tested exemplarily at least to the given cognition level using comprehension questions and sample calculations.

For example an assignment in the exam might be:

• Make a sketch of a control sequence that generates a maximally entangled state between two spin qubits.
• Explain how the individual steps of this control sequence can be implemented with nitrogen-vacancy centers.
• Describe the main sources of decoherence for spin qubits in silicon, and how this decoherence can be reduced by control and materials engineering.

Participation in the exercise classes is strongly recommended since the exercises prepare for the problems of the exam and rehearse the specific competencies.