Estimation of the Laser Frequency Nosie Spectrum by Continuous Dynamical Decoupling.

Decoherence induced by the laser frequency noise is one of the most important obstacles in the quantum information processing. In order to suppress this decoherence, the noise power spectral density needs to be accurately characterized. Here, we experimentally obtain the spectrum of laser frequency noise based on the continuous dynamical decoupling technique. We first estimate the mixture-noise (including laser and magnetic noises) spectrum over a frequency range 0-$(2\pi)$530 kHz by monitoring the transverse relaxation from an initial state $\ket{+\sigma_x}$, followed by a gradient descent data process protocol. Then the contribution from the laser noise is extracted by enconding the qubits on different Zeeman sublevels. We also investigate two sufficiently strong noise components by making an analogy between these noises and driving lasers whose lineshape is assumed to be Lorentzian. This method is verified by the experimental data and finally helps to characterize the noise.