11.4 Decoherence
There are three important measurements that quantum computing researchers use to measure coherence time: T1, T2, and its cousin T2*. Let’s begin with T1. They are single qubit measurements and so we can use the Bloch sphere to discuss them. Their use goes back to Felix Bloch’s work on nuclear magnetic resonance (NMR) in the 1940s. [1]
11.4.1 T1
T1 goes under several names, all of them connected to the physics of various underlying quantum processes:
- relaxation time,
- thermal relaxation,
- longitudinal relaxation,
- spontaneous emission time,
- amplitude damping, and
- longitudinal coherence time.
It is related to the loss of energy as the quantum state decays from the higher energy |1⟩ state to the |0⟩ ground state. This energy is transmitted to, or leaked into, the environment and lost from the qubit. T1 is measured in seconds or some fraction thereof such as microseconds. A microsecond...
