Lecturer : Zhang Gengli
Abstract :
We present a physically clear and easy to implement solution to the decoherence problem of central spin for a general class of systems. The critical feature of this class of decoherence problems is that the renormalized energy for the central spin has a dressed form in the dressed basis, which is also fluctuating together with the fluctuation of the Overhauser field when nuclear spins flip or flop in the bath. In the renormalized basis for the bath, hyperfine mediated long range interactions are absorbed as fluctuation of the Overhauser field and contribute as single spin correlations in the total decoherence, so the central spin decoherence caused by the nuclear spin bath can be systemically solved by the cluster correlation expansion method.
Then we propose a new dynamical nuclear polarization (DNP) scheme that can auto-lock the Overhauser field to the sweet point of the nitrogen vacancy (NV) center. The DNP channel is established by the combining the bath state dependent microwave (MW) control, the conditional flipping of the nuclear spins and the optical pumping of the NV center electronic spin. The mechanism can also be understood as sequential weak measurements of the nuclear spins followed by entropy dumping through initialization of the electron spin. By locking to the sweet point, the coherence of the electron spin is well protected, which paves the way for further applications in QIP and quantum sensing, such as high-precision DC magnetic field nano-sensing.