It is shown that the transition process to BEC phase of excitons can be manifested as non‐monotonic kinetics of photoluminescence decay. This decay looks as a series of subsequent peaks resulting from superradiance phenomenon. We show that decay of PL of certain InAs‐GaSb coupled quantum wells having, for instance, in average 5 ground state excitons per island may demonstrate up to 5 consecutive maximums followed by exponential decay. This dependence may be considered as a manifestation of transition to the Bose condensate phase. The radiation kinetics of such a system is a generalization of the known two‐level Dicke superradiance to an ensemble of multilevel systems.
Transmon qubits are a cornerstone of modern superconducting quantum computing platforms. Temporal fluctuations of energy relaxation in these qubits are widely attributed to microscopic two-level systems (TLSs) in device dielectrics and interfaces, yet isolating individual defects typically relies on tuning the qubit or the TLS into resonance. We demonstrate a novel spectroscopy method for fixed-frequency transmons based on multilevel relaxation: repeated preparation of the second excited state and simultaneous
T1 extraction of the first and second excited states reveals characteristic correlations in the decay rates of adjacent transitions. From these correlations we identify one or more dominant TLSs and reconstruct their frequency drift over time. Remarkably, we find that TLSs detuned by
≳100MHz from the qubit transition can still significantly influence relaxation. The proposed method provides a powerful tool for TLS spectroscopy without the need to tune the transmon frequency, either via a flux-tunable inductor or AC-Stark shifts.