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arXiv:1809.01645 · Physical Review Letters

Cascaded Cavities Boost the Indistinguishability of Imperfect Quantum Emitters

Hyeongrak Choi, Di Zhu, Yoseob Yoon, Dirk Englund

Published May 10, 2019• 47 citations
arXivHTMLPDF

Abstract

Recently, Grange et al. [Phys. Rev. Lett. 114, 193601 (2015)PRLTAO0031-900710.1103/PhysRevLett.114.193601] showed the possibility of single-photon generation with a high indistinguishability from a quantum emitter despite strong pure dephasing, by "funneling" emission into a photonic cavity. Here, we show that a 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 an ultrasmall mode volume and does not require ultrafast excitation of the emitter. These results were obtained by numerical and closed-form analytical models with strong emitter dephasing, representing room-temperature quantum emitters.

DOI
10.1103/physrevlett.122.183602

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Recently, Grange et al. [Phys. Rev. Lett. 114, 193601 (2015)PRLTAO0031-900710.1103/PhysRevLett.114.193601] showed the possibility of single-photon generation with a high indistinguishability from a quantum emitter despite strong pure dephasing, by "funneling" emission into a photonic cavity. Here, we show that a 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 an ultrasmall mode volume and does not require ultrafast excitation of the emitter. These results were obtained by numerical and closed-form analytical models with strong emitter dephasing, representing room-temperature quantum emitters.
We investigate theoretically the generation of indistinguishable single photons from a strongly dissipative quantum system placed inside an optical cavity. The degree of indistinguishability of photons emitted by the cavity is calculated as a function of the emitter-cavity coupling strength and the cavity linewidth. For a quantum emitter subject to strong pure dephasing, our calculations reveal that an unconventional regime of high indistinguishability can be reached for moderate emitter-cavity coupling strengths and high-quality factor cavities. In this regime, the broad spectrum of the dissipative quantum system is funneled into the narrow line shape of the cavity. The associated efficiency is found to greatly surpass spectral filtering effects. Our findings open the path towards on-chip scalable indistinguishable-photon-emitting devices operating at room temperature.
Highly efficient sources of indistinguishable single photons that can operate at room temperature would be very beneficial for many applications in quantum technology. We show that the implementation of such sources is a realistic goal using solid-state emitters and ultrasmall mode volume cavities. We derive and analyze an expression for photon indistinguishability that accounts for relevant detrimental effects, such as plasmon-induced quenching and pure dephasing. We then provide the general cavity and emitter conditions required to achieve efficient indistinguishable photon emission and also discuss constraints due to phonon sideband emission. Using these conditions, we propose that a nanodiamond negatively charged silicon-vacancy center combined with a plasmonic-Fabry-P\'erot hybrid cavity is an excellent candidate system.
The concept of cavity funneling has emerged recently as a promising route towards creating indistinguishable photons from highly dephased emitters. So far, all suggested solutions are solely based on dielectric cavities that require extremely high quality factors that are difficult to reach at visible wavelengths. Here we suggest a hybrid funneling architecture where a dephased emitter is coupled to a plasmonic nanoresonator that is enclosed by an outer dielectric cavity. The estimated lower limit of the outer cavity quality factor is found to be ∼2\sim2∼2 orders of magnitude lower compared to a cascaded cavity system. Furthermore, the surrounding topology of our approach allows for a partial direct coupling between the emitter and the outer cavity which in turn can increase the overall system extraction efficiency (β)\left(β\right)(β) by a factor of 12, boosting the probability of photon collection.
Resonant laser excitation of a two-level system with subsequent single-photon emission can be used to generate single photons with high indistinguishability or Hong-Ou-Mandel (HOM) visibility. However, spectral overlap between excitation laser and emitted photons generally poses significant challenges. Furthermore, emitter re-excitation intrinsically limits achievable single-photon purity. Established solutions mitigate these issues at significant cost to source efficiency and with increased source complexity. This motivates the use of few-level systems with spectral separation of excitation and emission pathways. One option is a three-level cascade. However, without targeted lifetime engineering of emitting states, the cascade naturally limits achievable photon indistinguishability. Here we study a semiconductor quantum dot with resonant and selective cavity-enhancement of biexciton-to-exciton transition. Following resonant two-photon excitation of the biexciton state, we collect the emitted single photon with the cavity. This approach circumvents emitter re-excitation and naturally introduces spectral separation of excitation laser and emitted single photon. Supported by first experimental results, we demonstrate theoretically that with selective Purcell enhancement, the observed quality quantifiers of single-photon emission (purity, equivalently g(2)(0)g^{(2)}(0)g(2)(0), and HOM visibility V\mathcal{V}V, equivalently indistinguishability) are competitive with respect to high-quality deterministic quantum-dot single-photon sources. This is already achieved without systematic optimization or targeted system engineering, which firmly places the reported approach as a viable route to the next generation of highest-quality quantum-dot based deterministic single-photon sources.
Motivated by recent advances in the development of single photon emitters for quantum information sciences, here we design and formulate a quantum cascade model that describes cascade emission by a quantum dot (QD) in a cavity structure while preserving entanglement that stores information needed for single photon emission. The theoretical approach is based on a photonic structure that consists of two orthogonal cavities in which resonance with either the first or second of the two emitted photons is possible, leading to amplification and rerouting of the entangled light. The cavity-QD scheme uses a four-level cascade emitter that involves three levels for each polarization, leading to two spatially entangled photons for each polarization. By solving the Schrodinger equation, we identify the characteristic properties of the system, which can be used in conjunction with optimization techniques to achieve the "best" design relative to a set of prioritized criteria or constraints in our optical system. The theoretical investigations include an analysis of emission spectra in addition to the joint spectral density profile, and the results demonstrate the ability of the cavities to act as frequency filters for the photons that make up the entanglements and to modify entanglement properties. The results provide new opportunities for the experimental design and engineering of on-demand single photon sources.