Quantum Lock-In Detection via Successive Adiabatic Evolution

Traditional quantum lock-in detection typically relies on pulse-based control sequences, which generate square-wave modulation. This approach suffers from spectral leakage—where unwanted frequency components interfere with the measurement—and complicates the extraction of signal characteristics when the initial phase is unknown. The researchers sought to develop a more robust quantum sensing protocol that mimics the precision of classical lock-in amplifiers.

Instead of using discrete pulses, the authors propose a protocol based on successive quantum adiabatic evolution. By adiabatically controlling the time evolution of a two-level quantum probe (such as a nitrogen-vacancy center in diamond), the system can implement triangular modulation functions. This continuous control allows the probe to act as a filter that effectively isolates the target signal from background noise. The protocol uses specific sequences of adiabatic evolution to measure both the in-phase and quadrature components of the signal, allowing for the full reconstruction of the signal's amplitude, frequency, and phase.

The study demonstrates that adiabatic quantum lock-in detection fundamentally solves the spectral leakage problem inherent in pulse-based methods. Because the adiabatic evolution is inherently robust against parameter variations and timing errors, the proposed protocol is highly resilient to experimental imperfections. The authors provide a practical implementation scheme using nitrogen-vacancy centers in diamond and show that the protocol can also determine the carrier frequency of a signal even when it is initially unknown, without requiring complex post-processing.