We present an experimental method to perform dual-channel lock-in magnetometry of timedependent magnetic fields using a single spin associated with a nitrogen-vacancy (NV) color center in diamond. We incorporate multi-pulse quantum sensing sequences with phase estimation algorithms to achieve linearized field readout and constant, nearly decoherence-limited sensitivity over a wide dynamic range. Furthermore, we demonstrate unambiguous reconstruction of the amplitude and phase of the magnetic field. We show that our technique can be applied to measure random phase jumps in the magnetic field, as well as phase-sensitive readout of the frequency.PACS numbers: 07.55. Ge,85.75.Ss,76.30.Mi The coherent evolution of a quantum state interacting with its environment is the basis for understanding fundamental issues of open quantum systems 1 , as well as for applications in quantum information science and technology 2 . Traditionally in these fields, the extreme sensitivity of coherent quantum dynamics to external perturbations has been viewed as a barrier to be surmounted. By contrast, quantum sensors have emerged that instead take advantage of this sensitivity; recent examples include electrometers and magnetometers based on superconducting qubits 3 , quantum dots 4 , spins in diamond [5][6][7][8] and trapped ions 9 .The nitrogen-vacancy (NV) defect center in diamond ( Fig. 1(a)) shows great promise as an ultra-sensitive solid-state magnetometer and magnetic imager because it features potentially atomic-scale resolution 6 , wide temperature range operation from 4 K -700 K 10 , and long coherence times that allow for high magnetic field sensitivity 11 . Recent demonstrations include nanoscale magnetic imaging 7,12,13 , coupling to nano-mechanical oscillators [14][15][16] , detection of single proximal nuclear spins [17][18][19][20] and nanoscale volumes of external electron and nuclear spins [21][22][23][24] .Magnetometry with diamond spin sensors detects the frequency shift of the NV spin resonance caused by the magnetic field via the Zeeman effect. Highly sensitive quantum sensing techniques use multi-pulse dynamical decoupling (DD) sequences 3,6,9,[25][26][27] that are tuned to the frequency of a time-dependent field. The resulting fluctuating frequency shift is rectified and integrated by the pulse sequence to yield a detectable quantum phase, while effectively filtering out low frequency noise from the environment ( Fig. 1(b)). Another advantage of these DD sequences is that they make the magnetometer insensitive to instabilities such as drifts in temperature or applied bias magnetic field.However, these state of the art quantum sensing methods also have significant drawbacks: the dynamic range is limited by the quantum phase ambiguity 28,29 , the sensitivity is a highly nonlinear function of field amplitude requiring prior knowledge of a working point for accurate deconvolution, and the classical phase of the field has to be carefully controlled to obtain accurate field ampli-Illustration of experimental setup f...