The 'lock-in' technique is a valuable tool in experimental physics, used to increase measurement sensitivity while reducing susceptibility to noise. A team from the Weizmann Institute in Rehovot, Israel, reports the implementation of a quantum analogue to the classical lock-in amplifier, using a single trapped strontium ion. The system achieves magnetic-field sensitivity two orders of magnitude greater than that seen with established single-spin probe technologies. The quantum lock-in technique is generic and can potentially enhance the sensitivity of any quantum sensor.