From Isolated to Open: the Noise Dial
Same circuit, same qubit — but now the state is a density matrix, and you can dial in how much it leaks into its environment.
Circuit (unitary part)
q0 : |0⟩
+
System type
Isolated
Open
Noise channel
What the dial does. Applied once at the end of the circuit, as a channel acting on the density matrix — the same Kraus-operator idea behind depolarizing/amplitude-damping/phase-damping noise in Qiskit, just exposed as one slider here.
Density matrix ρ
ρ = [[1,0],[0,0]]
Purity Tr(ρ²)1.000
Von Neumann entropy S(ρ)0.000
Reading these two numbers together. Purity = 1 and entropy = 0 mean a pure state — full knowledge, maximal coherence. As noise increases, purity falls toward 0.5 and entropy rises toward 1 bit: the qubit is drifting toward the maximally mixed state, where a measurement result becomes a coin flip no matter what basis you choose.
Bloch sphere
drag to rotate · point moves inside the sphere as it mixes
T1 / T2 decay — illustrative
Excited-state population (T1)
Coherence |ρ01| (T2)
T1 vs T2. T1 tracks how fast a qubit prepared in |1⟩ decays toward |0⟩. T2 tracks how fast the off-diagonal coherence — the thing that makes interference possible — washes out. Physically T2 ≤ 2·T1, since losing energy necessarily randomizes phase too; try setting T2 above that and notice it's not physically realizable.