Raman quantum memory based on an ensemble of nitrogen-vacancy centers coupled to a microcavity

We propose a scheme to realize optical quantum memories in an ensemble of nitrogen-vacancy centers in diamond that are coupled to a microcavity. The scheme is based on off-resonant Raman coupling, which allows one to circumvent optical inhomogeneous broadening and store optical photons in the electronic spin coherence. This approach promises a storage time of order 1 s and a time-bandwidth product of order ${10}^7$. We include all possible optical transitions in a nine-level configuration, numerically evaluate the efficiencies, and discuss the requirements for achieving high efficiency and fidelity.

Figure 1

Raman quantum memory scheme. The figure shows the nine-level configuration of the electronic ground and excited states for a NV${}^{-}$ center. Initially, a microwave $\pi$ pulse transfers the population from $|0\rangle$ to $|+\rangle$. A $\widehat{x}$-polarized signal is coupled to a cavity to be stored. For storage, a collective spin excitation is generated through off-resonant coupling to an $\widehat{x}$-polarized cavity field and a $\widehat{y}$-polarized control field. For retrieval, a similar control field pulse is applied to read out the stored excitation. $\Delta$ is the cavity and control field detuning from the excited state.

Figure 2

Simulation results of storage and retrieval of input pulse (black dashed line). The blue solid line presents the cavity output field. Simulation parameters: pulse bandwidth $\Delta \omega =110$ MHz, excited-state inhomogeneous broadening ${\gamma }_e=1$ GHz, and spin inhomogeneous broadening ${\gamma }_s=200$ kHz. The detuning from the excited state, $\Delta =0.8$ GHz (other parameters are provided in the main text). This results in 91% absorption efficiency and 81% total efficiency. The red dash-dotted line is associated with the noise intensity (multiplied by 10 for clarity), corresponding to 1% total noise probability. This gives 99% conditional fidelity (see text for more details).