Design of Quantum Dot-Nanowire Single-Photon Sources that are Immune to Thermomechanical Decoherence

Nanowire antennas embedding a single quantum dot (QD) have recently emerged as versatile platforms to realize bright sources of quantum light. In this theoretical work, we show that the thermally driven, low-frequency vibrations of the nanowire have a major impact on the QD light emission spectrum. Even at liquid helium temperatures, these prevent the emission of indistinguishable photons. To overcome this intrinsic limitation, we propose three designs that restore photon indistinguishability thanks to a specific engineering of the mechanical properties of the nanowire. We anticipate that such a mechanical optimization will also play a key role in the development of other high-performance light-matter interfaces based on nanostructures.

Figure 1

Nanowire antennas with a “needle” (a) and “trumpet” (b) top taper. The thermal excitation of nanowire vibration modes generates a fluctuating stress which modulates the quantum dot (QD) band gap energy and thus broadens the spectrum $S(\omega )$ of QD photons. (c) Flexural ($F_1$), longitudinal ($L_1$) and torsional ($T_1$) fundamental vibration modes for the needle antenna shown in (a). The color codes the amplitude of the ${\sigma }_{\mathrm{zz}}$ stress component.

Figure 2

Impact of the thermal vibrations of a needle antenna at $T=4\mathrm{K}$. (a) ${\theta }_m^2$ and ${\eta }_m^2$ for the first 15 vibration modes. Inset: rms values of the thermal top facet displacement $⟨u_m^2{⟩}^{1/2}$ and of the longitudinal stress $⟨{\sigma }_{\mathrm{zz},m}^2{⟩}^{1/2}$ for a few modes. For $F$ modes, we plot the maximum, on-sidewall values. (b) Line shape $S_m(\omega )$ for ${\theta }_m^2\gg 1$ and for ${\theta }_m^2\ll 1$ (in both cases ${\eta }_m^2\ll 1$). The arrows represent Dirac $\delta$ peaks. (c) Solid line: Calculated QD emission profile, $S(\omega )$. Dotted line: Calculated QD emission profile without the 7 first mechanical modes (6 $F$ modes and 1 $L$ mode). Filled curve: Lorentzian, radiatively limited emission line, $S_{\mathrm{rad}}(\omega )$. Top panel: on-axis QD; Bottom panel: on-sidewall QD.

Figure 3

Photon indistinguishability $I$ as a function of the operation temperature $T$ for a typical needle (a) and trumpet (b) nanowire antenna. We consider four transverse QD positions: on axis, 20 nm of axis, 30 nm of axis, and on sidewall. The vertical dashed lines indicate the typical base temperature in a dilution fridge (20 mK), a pumped ${}^3\mathrm{He}$ cryostat (300 mK) and a liquid ${}^4\mathrm{He}$ cryostat (4 K).

Figure 4

Photon indistinguishability $I$ as a function of the operation temperature $T$ for three mechanically engineered antennas. (a) Needlelike antenna covered with a conformal, low-index dielectric shell (thickness of 500 nm). (b) Suspended antenna. (c) Mechanically decoupled top taper. In (b) and (c) the suspension arms are connected to large pillars (not shown).