Many applications and experiments consists of the ensemble of the two-level systems (TLS) and the electro-magnetic (EM) field to control the quantum state of TLS. In such situations, the ability to precisely control the population and phase of TLS plays crucial role in many applications like nuclear magnetic resonance spectroscopy and imaging (NMR), quantum computer, atomic clocks, and magnetometer. Dephasing degrades the quality of the control mentioned above. For example, in Ramsey spectroscopy, making the free precession time to be too long leads to degrading the quality of the Ramsey fringes, due to the dephasing.

In the past, open-loop (no feedback) approach, known as spin echo has been developed in NMR to suppress the dephasing error. In essence, the spin echo uses the phase of the control EM field (laser) as a reference and average out the phase error of each TLS. The similar technique can also be applied to isolate the non- classical effect like entanglement from the noise environment as well and called "dynamical decoupling''. Spin echo is very useful and easy to implement, but it cannot be used when the phase of the TLS should not be touched, such as atomic clock and magnetometer.

In this colloquium I will discuss a closed-loop control method to match a phase of laser to the phase of TLS, and we call this scheme "atom phase lock." The heart of this atom phase lock scheme is to monitor the phase difference and yet least affect the coherence of the spin by use of the non-destructive measurement. Atom phase lock is suited for atomic clock because spin phase lock views the atom (TLS) phase as a reference, in contrast to spin echo, which views the laser phase as a reference.

I will cover the basics of the atomic clock, present status and it's limit first. Then, I will introduce the idea of the atom phase lock and discuss how this scheme might help to significantly improve the stability of the atomic clock. Lastly, I will talk about the experimental implementations and the current status of our experiment.