So far we have seen how qubits can be realized in physical systems, and in what ways we can control them. We will now shift our focus to the kinds of errors that can occur and how to characterize them. In this video, Menno will explain relaxation and dephasing.

• T1 is the characteristic time over which a spin relaxes to its ground state. It is generally very long, in the order of seconds.
• The characteristic dephasing time is called the T2* time and can be measured with a Ramsey experiment.
• Coherence time can be increased by removing noise affecting the experiment. Different pulse sequences serve this purpose, the simplest one is called Hahn Echo and it consists of a single pi pulse between two pi/2 pulses.