11.4 Decoherence

There are three measurements that quantum computing researchers frequently use to measure coherence time: T₁, T₂, and its cousin T₂^*. They are single physical qubit measurements, 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. 19 Bloch, Felix

Let’s begin with T₁.

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 energy loss 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...