Subnatural Linewidth Single Photons from a Quantum Dot

The observation of quantum-dot resonance fluorescence enabled a new solid-state approach to generating single photons with a bandwidth approaching the natural linewidth of a quantum-dot transition. Here, we operate in the small Rabi frequency limit of resonance fluorescence—the Heitler regime—to generate subnatural linewidth and high-coherence quantum light from a single quantum dot. The measured single-photon coherence is 30 times longer than the lifetime of the quantum-dot transition, and the single photons exhibit a linewidth which is inherited from the excitation laser. In contrast, intensity-correlation measurements reveal that this photon source maintains a high degree of antibunching behavior on the order of the transition lifetime with vanishing two-photon scattering probability. Generating decoherence-free phase-locked single photons from multiple quantum systems will be feasible with our approach.

Figure 1

Collection efficiency and background analysis of RF. (a) Integrated photon counts for RF at the saturation point as a function of laser detuning. The curve is a Lorentzian fit to the data (open circles). Inset: Total off-resonance (detuned by 35 GHz) background counts (dark) compared to the APD dark counts alone (light). (b) Integrated photon detection events when the laser is on-resonance with the QD transition and (c) off-resonance. (d) RF SBR. At saturation ($\Omega \sim \Gamma /\sqrt{2}$) a value of 1050 is achieved. The excitation power for (b)-(d) is scaled in units of the saturation power ${\Omega }^2={\Gamma }^2/2$.

Figure 2

Spectrum and intensity-correlation measurements on QD RF. (a) A narrowband single-mode laser is fixed on-resonance with the QD transition at $\Omega \sim 1.5\Gamma$ and the RF spectrum is recorded by an APD as the transmission through a scanning Fabry-Perot cavity (FWHM $\sim 29\mathrm{MHz}$), shown as filled circles. The spectrum (fitted with the solid curve) comprises the onset of the Mollow triplet superimposed by the coherently scattered component. The open circles display the residual laser background under the same conditions, but when the QD transition is gate-detuned. (b) Same measurement as panel (a) performed when the laser power is around saturation ($\Omega \sim 0.6\Gamma$) and (c) when the laser power is below saturation ($\Omega \sim 0.2\Gamma$). The spectrum collapses to a single Lorentzian of width indistinguishable from the resolution of the scanning cavity. The offset is caused by detector dark counts and ambient light. (d)–(f) Intensity-correlation measurements of RF for the excitation power regimes of the spectra presented in panels (a)–(c), respectively. In all the plots the data are superimposed by the theoretical prediction (continuous curves) convolved with the system response function [32], while the dotted black lines indicate how our measurements would appear for an ideal system response. All plots display the ubiquitous antibunching behavior at zero time delay.

Figure 3

First-order correlation measurement of QD RF. (a) The visibility of the interference fringes observed for RF for the laser excitation power corresponding to $\Omega \sim 0.22\Gamma$ (squares) and $\Omega \sim 0.17\Gamma$ (diamonds). The dotted line is the measured mean fringe visibility for the laser and marks the highest attainable coherence of the scattered photons. The first-order correlation measurement for incoherently generated photons (above GaAs band gap excitation) (circles) shows a coherence time of $\sim 540-\mathrm{ps}$, while the RF displays a coherence time of $\sim 22\mathrm{ns}$ in the Heitler limit, i.e., $\sim 15$ times longer than the antibunching time scale. The detailed description of the theoretical fits to the data (solid curves) can be found in the Supplemental Material [32]. (b) A close-up of the raw interference fringes for $\Omega \sim 0.22\Gamma$ for a relative time delay of 330 ps. The fringe visibility here is 0.90. (c) Same as panel (b) for a relative time delay of 2.68 ns and visibility of 0.75.