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Fundamentals

01 /Quantum computers with magical advantage? 02 /Bra-ket notation 03 /Born rule 04 /Bloch sphere 05 /Measurement operators 06 /Experimental and theoretical measurements 07 /How to measure in any basis 08 /State decomposition 09 /Why every maximally entangled state is secretly a Bell state 10 /Coherence 11 /Relaxation 12 /Quantum materials: the environment of a qubit 13 /Introduction to quantum error correction (QEC) 14 /Multiqubit systems 15 /Multiqubit gates 16 /Entanglement 17 /Measuring multiqubit systems 18 /Superdense coding 19 /Teleportation 20 /Entanglement swapping

Coherence

Quantum coherence is a desired property for a qubit. Its coherence time – the duration of the qubit coherence – is used to make quality comparisons between qubits. Coherence tells us something about how long a qubit retains its information, and thus dictates some sort of lifetime. In this video, Ph.D. student Jonas Helsen (QuTech) introduces this concept and its meaning. 

Prerequisite knowledge


 

Further thinking

What do you think can improve the coherence of a qubit? Can you think of what decoheres a qubit?

Solution

Further reading

For an extensive overview of quantum coherence and how a qubit loses its information to the environment, we recommend taking a look at the Wikipedia page on quantum decoherence. That page does an excellent job of explaining all the low-level details.

For those looking for an expert insight in quantum coherence, take a look at this insights issue of Nature on Quantum coherence (Vol. 453, No. 7198 pp 1003–1049)