| Abstract: | Unobserved quantum systems evolve coherently in superposition, while
strong measurement forces definite outcomes. But what happens as we
gradually turn up the strength of observation?
Using a continuously monitored superconducting qubit [1], we show that
the crossover between these two regimes is not smooth, but unfolds
through three sharp transitions [2]. First, coherent Rabi oscillations
abruptly halt at an exceptional point. Next, trajectories freeze near
a stable measurement eigenstate. Finally, the system enters the
quantum Zeno regime, where stronger measurement paradoxically slows
relaxation [3]. Decoherence does not wash out this transition cascade.
Instead, it reorders the transitions and separates signatures that
coincide in the ideal model.
Time permitting, I will also present preliminary results on
topological transitions in observed quantum systems, where hidden
exceptional points organize quantum-jump behavior into distinct
topological phases, observable without postselection.
[1] Minev et al., Nature 570, 200–204 (2019).
[2] Guttel*, Gov*, et al., arXiv:2602.02672 (2026).
[3] Snizhko et al., Physical Review Research 2, 033512 (2020).
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