Two-photon interference between mutually-detuned resonance fluorescence signals scattered off a semiconductor quantum dot

Abstract

The radiative linewidth of a two-level emitter (TLE) fundamentally limits the bandwidth available for quantum information processing. Despite its importance, no prior experiment has systematically examined how driving detuning affects the indistinguishability of photons scattered from a TLE - a parameter critical for photonic quantum computing. Here, we perform post-selective two-photon interference measurements between mutually detuned resonance fluorescence signals from an InAs quantum dot embedded in a micropillar cavity. At small mutual laser detunings (<=0.5GHz), the results are accurately described by the pure-state model [Nat. Commun. 16, 6453 (2025)], which treats all resonance-fluorescence photons as spontaneous emission. At larger detunings, we uncover an anomalous feature in the two-photon interference, where the normalised second-order correlation function under orthogonal polarisations yields g2_vert(0) < 0.5.

Figures (11)

• Figure 1: Dual-color two-photon interference (TPI) setup. The RF signals are collected in the same polarization as the excitation lasers, and no spectral filtering is applied to them in any of the TPI experiments. a, High-resolution RF spectra measured with a Fabry-Pèrot interferometer (FPI); The dashed line represents the QD-micropillar reflectance spectrum measured under $\bar{n} = 0.05$. b, The auto-correlation function $g^{(2)}(0)$ values measured under $\bar{n} = 0.05$ using a Hanbury Brown-Twiss setup (HBT). The red dashed line shows the cavity reflectivity under a strong excitation of 465 nW, corresponding to $\bar{n} = 139$. BS: beam splitter; SPD: single-photon detector.
• Figure 2: Dual-color HOM interference results under two different mutual detunings: $\Delta =0$ GHz (left column) and $\Delta =0.5$ GHz (right column).a, b, HOM interference traces for the parallel (blue) and orthogonal (red) polarization settings; c, d, Magnified view of the data shown in panels a and b; e, f, TPI visibility. In all panels, experimental data are shown as dots while theoretical simulations are displayed as solid lines. In this set of measurements, the excitation flux is fixed at $\bar{n} = 0.05$.
• Figure 3: Visualization of two-photon coincidence contributions.a, Two mutually-detuned RF beams, with global phases of $\phi_a$ and $\phi_b$, respectively, meet at a 50/50 beam splitter, and their interference is recorded by two single-photon detectors; b, Four possible cases involving two input photons may contribute to a coincidence event with the two detectors clicking respectively at time $t_1$ and $t_2 = t_1 + \tau$. Cases (ii) and (iii), in which each input arm contributes one photon, share the common global phase ($\phi_a + \phi_b$) and therefore their quantum probability amplitudes interfere. For simplicity, cases involving three or more input photons are omitted from the diagram, but they are fully accounted for in Eq. \ref{['eq:HOM_all']}.
• Figure 4: Excitation flux dependence of the HOM correlation under the parallel polarization setting. The excitation detuning is fixed at $\Delta = 0.5$ GHz. a, HOM correlation traces. Symbols (solid lines) denote experimental data (theoretical fits). b, The oscillation amplitudes (solid dots) and the zero-delay HOM dip values (open squares), extracted from the data shown in panel a, along with the fit (dashed line) to the former using Eq. \ref{['eq:p0']}.
• Figure 5: Excitation detuning dependencies of the 0-delay TPI visibility and the HOM correlations. For all measurements, the excitation flux is fixed at $\bar{n} = 0.05$.