Heralded Emission Detection in InAs/ZnSe Quantum Dot Solids Using Time-Correlated Photons

TL;DR

This work demonstrates heralded emission detection (HED) under entangled-photon excitation in solid-state InAs/ZnSe quantum dot films at cryogenic temperatures, establishing a solid-state platform for quantum-light spectroscopy. By constructing a cw SPDC-based source with high photon indistinguishability and utilizing both SNSPDs and large-area APDs, the authors achieve high time resolution and substantial count rates to extract exciton lifetimes and bright–dark state dynamics via second-order photon correlations. The study shows that NIR colloidal QDs are bright, tunable model systems compatible with optical cavities, and provides a detailed analysis of detector tradeoffs, downconversion efficiency, and temperature-dependent decay pathways. While current coincidence rates remain modest, the results pave the way for phase-sensitive measurements and on-chip quantum photonic integrations that could extend entangled-photon spectroscopy to solid-state materials and devices.

Abstract

Harnessing quantum correlations between photons is an emerging frontier in optical spectroscopy, yet experimental demonstrations have largely remained limited to molecular systems at room temperature. Here, we investigate heralded emission detection (HED) under continuous-wave entangled-photon excitation of near-infrared (NIR)-emitting colloidal III-V quantum dot (QD) solids at low temperatures. We demonstrate the advantages of superconducting nanowire single-photon detectors (SNSPDs) for high time resolution ($\sim$72 ps) and large-area NIR avalanche photodiodes (APDs) for high emission count rates ($\sim$2000 cps). Second-order photon-correlation analysis reveals exciton lifetimes and fine-structure energy splittings. These results establish NIR colloidal QDs as a bright, tunable model system for quantum-light spectroscopy and highlight their compatibility with optical cavities as a further experimental control parameter.

Figures (14)

• Figure 1: Experimental setup, source characteristics, and properties of InAs/ZnSe nanocrystals. (a) Experimental setup for heralded emission detection and preparation of InAs/ZnSe quantum dot (QD) films. (b) Normalized spectra of the entangled photons from Type-II SPDC used in the heralding and sample arms. (c) The impulse response function (IRF), obtained by replacing the sample with a cover slip coated with a thin gold film and collecting the reflected photons. (d) Ensemble absorption and emission spectra of InAs/ZnSe QDs dispersed in toluene at room temperature. The shaded region marks the twin-pair excitation wavelength in HED. For clarity, the excitation line is drawn with exaggerated width and does not represent the actual narrow bandwidth. Inset: representative transmission electron microscopy (TEM) images of InAs/ZnSe QDs. Scale bar: 30 nm.
• Figure 2: Data analyses in heralded emission detection. (a) In heralded emission detection (HED), photon pairs are split into a heralding arm (blue) and a sample arm (orange). Pairs can be classified as coincidence pairs, which are time-correlated and produce bunching in the $g^{(2)}(\tau)$, or non-coincidence pairs, which contribute uncorrelated background. The sample emission therefore possesses a similar statistics with emission photon from both twins (red) and accidental pairs (light red). (b) Intensity traces of the heralding arm and the emission signal during a 20h measurement at 4.7 K, demonstrating stable down-conversion over a ten-hour period. (c) Normalized cross-correlation, $g^{(2)}(\tau)$, between the heralding arm and the emission at 4.7 K. The orange-shaded region corresponds to contributions from correlated heralding–emitted photon pairs (entangled two-photon down-conversion), while the purple region represents uncorrelated background with $g^{(2)}(\tau)=1$. (d) Schematic for the $g^{(2)}$ correlation analysis of time-tagged HED data. Delaying one sequence relative to the other only shifts the bunching peak without changing its shape. (e) Schematic of the effect of adopting a 'start–stop' alternative from time-correlated single-photon counting (TCSPC) to HED, introducing distortions.
• Figure 3: Temperature-dependent HED with SNSPD and APD for sample emission detection. (a) Normalized cross-correlation, $g^{(2)}(\tau)$, at various temperatures, measured by tagging the sample emission with an SNSPD. The bunching peak reflects the time correlations determined by the photoluminescence lifetime of the InAs/ZnSe quantum dots (QDs). The inset shows a zoomed-in view on a logarithmic scale, with $g^{(2)}(\tau)$ normalized so that all peak values are equal, facilitating comparison of the decay dynamics. (b) $g^{(2)}(\tau)$ of a second QD sample measured with an avalanche photodiode (APD). (c) Data from (b) plotted as $G^{(2)}(\tau)$ normalized on a logarithmic scale. (d) Extracted lifetimes as a function of temperature, along with bright–dark model fits. The inset illustrates the model, where G, B, and D denote the ground, bright, and dark state, respectively. (e) Brightness and bunching peak area extracted from $g^{(2)}(\tau)$ data as a function of temperature. The bunching area is fitted with a Bose–Einstein model. (f) External quantum efficiency (EQE) of the twin photon downconversion by the sample at different temperatures, following the trend in brightness.
• Figure S1: Characterization of the setups. (a) Hong–Ou–Mandel (HOM) dip of the entangled photon pairs, demonstrating their indistinguishability, with a visibility of approximately 86%. (b) Impulse response function (IRF) of the HED system with an APD detector in the emission arm. (c) IRF of the TCSPC system with an SNSPD detector in the emission arm.
• Figure S2: Characterization of Sample 1 in solution. (a) Absorption spectrum of the InAs cores. (b) TEM image of InAs cores. (c) Size distribution of InAs cores extracted from TEM images. (d) Absorption and photoluminescence spectra of InAs/ZnSe QDs. (e) TEM image of InAs/ZnSe QDs. (f) Size distribution of InAs/ZnSe QDs extracted from TEM images. (g) Elliott formula fitting of the absorption spectrum shown in (d).