## Hyeongrak Choi

*Massachusetts Institute of Technology*

Hyeongrak "Chuck" Choi was born in Seoul, South Korea. He attended the Seoul National University, where he obtained a B.S. majoring in Electrical Engineering. His undergraduate research work in the laboratory of Prof. Nam-Gyu Park focused on the metamaterial. In 2014, he joined the laboratory of Prof. Dirk Englund as a graduate student at MIT. His current research focuses on quantum optics and quantum computing.

Recently, Grange et al. (Phys. Rev. Lett. 114, 193601 (2015)) showed generating single photons with high indistinguishability from a quantum emitter, despite strong pure dephasing, by `funneling' emission into a photonic... [ view full abstract ]

Recently, Grange et al. (Phys. Rev. Lett. 114, 193601 (2015)) showed generating single photons with high indistinguishability from a quantum emitter, despite strong pure dephasing, by `funneling' emission into a photonic cavity. Here, we show that cascaded two-cavity system can further improve the photon characteristics and greatly reduce the *Q*-factor requirement to levels achievable with present-day technology. Our approach leverages recent advances in nanocavities with ultrasmall mode volume and does not require ultrafast excitation of the emitter.

The cascaded two-cavity system considered in this abstract is illustrated in Fig. 1(c). The emitter is assumed to be dipole-coupled with the first cavity (*C*_{1}). This cavity can have a relatively low *Q-*factor < 10^{5}, as long as it has a small *V*_{eff} to efficiently collect the emitter fluorescence. However, the indistinguishability of the emission from *C*_{1} would be low. A high *I* can then be achieved by coupling to a second cavity (*C*_{2}), which provides additional degrees of freedom to optimize the single photon emission from the two-cavity-emitter system.

To investigate the dynamics quantitatively, we assume a strong pure dephasing, *γ** ^{}= 10^{4}, normalized to spontaneous emission rate, *γ* = 1. For simplicity, we first ignore spectral diffusion (*Δδ*=0). We applied the master equation approach to calculate indistinguishability (*I*) and collection efficiency (*η*) as a function of cavity-cavity coupling rate (*g*_{2}) and second cavity decay rate (*κ*_{2}). The results in Fig. 2 show two regimes of interest. In `Reg. 1' of *g*_{2}, *κ*_{2} < *κ*_{1}, we find high *I* and small *η*. `Reg. 2' of *g*_{2}, *κ*_{2} > *κ*_{1} leads to moderate *I* and large *η*.

In Reg. 1, the emitter and *C*_{1} serves as a `composite emitter' with decoherence rate *R*_{1} = 4g_{1}^{2}/(*Γ+κ*_{1}). Fig. 3 (a) plots *η *and *I* as a function of *κ*_{2}. Our analytical model (dashed lines) show excellent agreement with the numerical simulations with the master equation.

Next, we investigate Reg. 2, for which large *η* and moderate *I* are possible. Fig. 4(a) plots *η* and *I* of the photon emitted by *C*_{2} as a function of *κ*_{2}. Notably, *I* for a *κ*_{2} = 300 exceeds the maximum achievable *I* for a single-cavity system with *g* = *g*_{1} (green dashed line), corresponding to the same *V*_{eff}_{}. Though the improvement of *η-**I product* is less significant because of a reduced efficiency (Fig.4(b)), *I* is still higher than the single-cavity system allows.

Fundamental science for quantum technologies , Quantum optics and non-classical light sources , Solid states and hybrid systems