Observation of Intensity Squeezing in Resonance Fluorescence from a Solid-State Device

Intensity squeezing—i.e., photon number fluctuations below the shot-noise limit—is a fundamental aspect of quantum optics and has wide applications in quantum metrology. It was predicted in 1979 that intensity squeezing could be observed in resonance fluorescence from a two-level quantum system. However, its experimental observation in solid states was hindered by inefficiencies in generating, collecting, and detecting resonance fluorescence. Here, we report the intensity squeezing in a single-mode fiber-coupled resonance fluorescence single-photon source based on a quantum dot–micropillar system. We detect pulsed single-photon streams with 22.6% system efficiency, which show sub-shot-noise intensity fluctuation with an intensity squeezing of 0.59 dB. We estimate a corrected squeezing of 3.29 dB at the first lens. The observed intensity squeezing provides the last piece of the fundamental picture of resonance fluorescence, which can be used as a new standard for optical radiation and in scalable quantum metrology with indistinguishable single photons.

Figure 1

(a) Single-photon counts with CW laser resonant excitation. The saturation count is $1.87\times {10}^9\mathrm{per}$ second. The data are fitted by Eq. (8) in the Supplemental Material. (b) Time-resolved measurement of the resonance fluorescence by a superconducting nanowire single-photon detector with a time resolution of $\sim 20\mathrm{ps}$. The fitted lifetime of the quantum dot is $T_1=58.60\pm 0.02\mathrm{ps}$. Comparing this to the quantum dot lifetime in the slab ($\sim 1.08\mathrm{ns}$), the Purcell factor is 18.4. (c) Rabi oscillation under pulsed resonant excitation. At $\pi$ pulse, $17.2\times {10}^6$ pure single photons are detected per second [22], by a superconducting nanowire single-photon detector with an efficiency of 0.86. (d) High-resolution spectrum of resonance fluorescence measured by a Fabry-Perot cavity with a frequency resolution of 220 MHz and a free spectral range of 37.4 GHz. The data is fitted by a Voigt function (red line), and the linewidth is $2.74\pm 0.02\mathrm{GHz}$.

Figure 2

(a) Real-time monitoring of resonance fluorescence single-photon counts at $\pi$ pulse with a time bin of $1.0\mu \mathrm{s}$. The average count per time bin is $17.5/\mu \mathrm{s}$. (b) The corresponding histogram, where the frequency of observed counts (i.e., the intensity fluctuation, shown as deep blue dots) is fitted by a binomial function. Comparing this to the shot-noise-limited source with the same intensity (green line), the quantum dot single-photon source shows a 12.68% reduction of histogram linewidth—that is, intensity fluctuation noise. The blue line displays the corrected intensity squeezing at the first lens (see the main text). (c) Intensity squeezing parameter as a function of the excitation laser amplitude. At $\pi$ pulse, the intensity squeezing reaches a minimal value of 87.32%. The black dotted line is the normalized shot-noise limit.