To catch and reverse a quantum jump midflight
Abstract
In quantum physics, measurements can fundamentally yield discrete and random results. Emblematic of this feature is Bohr's 1913 proposal of quantum jumps between two discrete energy levels of an atom^{1}. Experimentally, quantum jumps were first observed in an atomic ion driven by a weak deterministic force while under strong continuous energy measurement^{24}. The times at which the discontinuous jump transitions occur are reputed to be fundamentally unpredictable. Despite the nondeterministic character of quantum physics, is it possible to know if a quantum jump is about to occur? Here we answer this question affirmatively: we experimentally demonstrate that the jump from the ground state to an excited state of a superconducting artificial threelevel 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 results demonstrate that the evolution of each completed jump is continuous, coherent and deterministic. We exploit these features, using realtime monitoring and feedback, to catch and reverse quantum jumps midflight—thus deterministically preventing their completion. Our findings, which agree with theoretical predictions essentially without adjustable parameters, support the modern quantum trajectory theory^{59} and should provide new ground for the exploration of realtime intervention techniques in the control of quantum systems, such as the early detection of error syndromes in quantum error correction.
 Publication:

Nature
 Pub Date:
 June 2019
 DOI:
 10.1038/s415860191287z
 arXiv:
 arXiv:1803.00545
 Bibcode:
 2019Natur.570..200M
 Keywords:

 Quantum Physics
 EPrint:
 Clarified and expanded the introduction and conclusion. Reorganized Methods and Supplementary Information. Revised and expanded citations. Corrected a few remaining minor typos