Controlling coherence between waveguide-coupled quantum dots

We present a novel waveguide design that incorporates a split-diode structure, allowing independent electrical control of transition energies of multiple emitters over a wide range with minimal loss in waveguide coupling efficiency. We use this design to systematically map out the transition from superradiant to independent emission from two quantum dots. We perform both lifetime as well as Hanbury Brown-Twiss measurements on the device, observing anti-dips in the photon coincidences indicating collective emission while at the same time observing a drop in lifetime around zero detuning, indicating superradiant behaviour. Performing both measurement types allows us to investigate detuning regions which show both superradiant rate enhancement and inter-emitter coherence, as well as regions in which correlations persist in the absence of rate enhancement.

💡 Research Summary

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In this work the authors introduce a novel nanophotonic platform that enables independent electrical tuning of two spatially separated quantum dots (QDs) coupled to a single‑mode waveguide, and they use this platform to explore the transition between superradiant and independent emission regimes. The device is built around a split p‑i‑n diode structure in which a shallow etch electrically isolates two sections of the waveguide. Each section can be biased separately, allowing Stark tuning of the negatively charged trion transition of each QD over a range of tens of micro‑electronvolts while preserving the waveguide’s transmission (loss < 1 %) and maintaining a high β‑factor (~0.8). The two InAs QDs are positioned ≈20 µm apart (≈70 wavelengths in the waveguide), demonstrating that long‑range coherent coupling can be achieved without sacrificing individual addressability.

Experimentally, QD1 is excited with a femtosecond above‑band pulse, while QD2 is “optically gated” with a weak continuous‑wave laser that switches its charge state without populating the excited trion. This gating scheme lets the authors switch QD2 on and off rapidly, thereby probing the system both as a single emitter and as a coupled pair while keeping the excitation conditions identical. By sweeping the voltage on QD1 they vary the spectral detuning Δ between the two trions from well beyond the individual linewidths down to near zero.

Time‑resolved photoluminescence measurements reveal that when the two QDs are resonant and QD2 is gated on, the decay becomes noticeably faster: the characteristic decay time shortens by about 20 % compared with the single‑dot case. This acceleration is a clear signature of superradiant rate enhancement, although it falls short of the ideal factor‑of‑two speed‑up expected for perfectly identical emitters with β = 1. The authors attribute the reduced enhancement to a combination of non‑unity β, spectral wandering (σ ≈ 1.3 µeV), pure dephasing (γ_d ≈ 8 ns⁻¹), and residual non‑radiative channels. A simple metric τ(ε) – the time for the intensity to fall to a fraction ε of its peak – is used to quantify the decay without assuming a single exponential, and the experimental τ(ε) curves match well with a master‑equation model that includes waveguide loss, dephasing, and the non‑resonant excitation of QD1.

Second‑order correlation (Hanbury Brown‑Twiss) measurements are performed under continuous‑wave excitation of both dots. For large detunings the autocorrelation g²(τ) shows the expected dip to 0.5 at zero delay, indicating uncorrelated emission from two independent sources. As the detuning is reduced, an “anti‑dip” emerges: g²(0) rises toward unity, reflecting the preparation of a bright, delocalised state after the first photon is emitted. Remarkably, the anti‑dip persists even when the detuning exceeds several times the single‑dot linewidth, a regime where the lifetime measurements show no superradiant speed‑up. In this intermediate region the correlation function exhibits beating at a frequency equal to the spectral detuning, confirming coherent oscillations between the bright and dark collective states. Theoretical calculations that incorporate the measured detuning, the same σ and γ_d as above reproduce both the height and the beating frequency of the anti‑dip, validating the model.

The combined lifetime and HBT data thus provide a comprehensive picture: (i) at exact resonance the system displays both a modest superradiant rate enhancement and strong inter‑dot coherence (anti‑dip); (ii) as detuning grows, the rate enhancement vanishes while coherence survives over a broader range than the linewidth, manifested as beating in g²(τ); (iii) at large detuning the emitters behave independently. This demonstrates that rate enhancement and coherence are not strictly synonymous and that both observables are required to unambiguously identify superradiance in solid‑state platforms.

Beyond the fundamental physics, the work showcases a scalable, fast, and reversible tuning method that can be extended to many emitters. The split‑diode waveguide architecture preserves high coupling efficiency while allowing independent control of each emitter’s transition frequency, a crucial requirement for building larger quantum photonic circuits, quantum networks, or collective light‑matter devices such as super‑absorbers, quantum batteries, or sub‑radiant quantum memories. The ability to maintain coherent coupling over distances of tens of wavelengths also opens the door to distributed quantum processing architectures where emitters need not be placed in the near‑field of each other. In summary, the paper delivers both a novel device platform and a thorough experimental validation of superradiant physics, establishing a solid foundation for future multi‑emitter quantum photonic technologies.