To catch and reverse a quantum jump mid-flight

TL;DR

This paper demonstrates that quantum jumps in a superconducting atom can be tracked in real-time, allowing for their reversal mid-flight, challenging the notion of their fundamental unpredictability and opening new avenues for quantum control.

Contribution

It provides the first experimental evidence that quantum jumps can be monitored continuously and reversed mid-flight, supporting quantum trajectory theory with real-time feedback.

Findings

Quantum jumps can be tracked during their evolution.

Real-time feedback allows reversing quantum jumps mid-flight.

Results align with theoretical predictions without adjustable parameters.

Abstract

Quantum physics was invented to account for two fundamental features of measurement results -- their discreetness and randomness. Emblematic of these features is Bohr's idea 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 while under strong continuous energy measurement. The times at which the discontinuous jump transitions occur are reputed to be fundamentally unpredictable. Can there be, despite the indeterminism of quantum physics, a possibility to know if a quantum jump is about to occur or not? Here, we answer this question affirmatively by experimentally demonstrating that the jump from the ground 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 results demonstrate that the jump evolution when completed is continuous, coherent, and deterministic. Furthermore, exploiting these features and using real-time monitoring and feedback, we catch and reverse a quantum jump mid-flight, thus deterministically preventing its completion. Our results, which agree with theoretical predictions essentially without adjustable parameters, support the modern quantum trajectory theory and provide new ground for the exploration of real-time intervention techniques in the control of quantum systems, such as early detection of error syndromes.