Bad Qubits — Learn Quantum by Doing

Multiple choice

A qubit has relaxation time T1 and dephasing (coherence) time T2, related by 1/T2 = 1/(2*T1) + 1/Tphi, where Tphi is the pure dephasing time. What constraint does this place on T2?

T2 is unbounded and can be arbitrarily larger than T1
T2 >= 2*T1 always, because dephasing is slower than relaxation
T2 <= 2*T1, with equality only when pure dephasing is absent
T2 = T1 exactly, since both describe the same decay process