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