To catch and reverse a quantum jump mid-flight

A quantum system driven by a weak deterministic force while under strong continuous observation exhibits quantum jumps between its energy levels. Employing a three-level superconducting artificial atom of the V-type involved in the original observation of quantum jumps, we show that quantum jumps can be caught and even reversed mid-flight. The three required levels are: G (for Ground), B (for Bright), and D (for Dark). The D level is engineered to be decoupled from both any dissipative environment and any measurement apparatus. Quantum jumps between G and D are monitored indirectly by the combination of a Rabi drive between the G and B levels, together with the monitoring of the occupation of B, itself tracked by a dispersively-coupled readout cavity. Using digital low-latency feedback electronics, we demonstrate the catch of a quantum jump mid-flight, i.e. a coherent superposition of D (corresponding to having jumped) and G (corresponding to having not jumped). The fidelity of the mid-flight state is above 70%, in agreement with quantum trajectory theory simulations. Our monitoring scheme can be useful for other quantum information tasks, such as the continuous monitoring of error syndromes.