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Speaker: Michel Devoret (Applied Physics, Yale University)

Measurements in quantum physics, unlike their classical physics counterparts, can fundamentally yield discrete and random results. Emblematic of this feature is Bohr’s hypothesis of quantum jumps between two discrete energy levels of an atom. Experimentally, quantum jumps were first observed in an atomic ion driven by a weak deterministic force under strong continuous energy measurement. The times at which the discontinuous jump transitions occur are reputed to be fundamentally unpredictable. Despite the non-deterministic character of quantum physics, is it possible to know if a quantum jump is about to occur? Here we provide a positive answer to this question: we experimentally show that the jump from the ground state to an excited state of a superconducting artificial three-level atom can be tracked as it follows a predictable “flight” by monitoring the population of an auxiliary energy level coupled to the ground state. The experimental results¹ demonstrate that the evolution of the jump — once completed — is continuous, coherent, and deterministic. Based on these insights and aided by real-time monitoring and feedback, we then pinpoint and reverse one such quantum jump “mid-flight”, thus deterministically preventing its completion. Our findings, which agree with theoretical predictions essentially without adjustable parameters, lend support to the modern formulation of quantum trajectory theory; most important, they may provide new ground for the exploration of real-time intervention techniques in the control of quantum systems, such as the early detection of error syndromes.

¹Z. Minev et al., arXiv:1803.00545 (to appear in Nature)