Quantum optics: Push-button photon entanglement

A source of entangled photons that emits one, and only one, pair of photons on demand has now been realized in a semiconductor chip. The solid-state source will be a useful resource for experiments in optical quantum information.

💡 Research Summary

The paper reports the development of a semiconductor‑based, on‑demand source of entangled photon pairs that emits exactly one pair per trigger pulse—a true “push‑button” entanglement generator. The device is built on an InGaAs quantum‑dot (QD) platform integrated with a photonic crystal cavity and a band‑gap waveguide structure on a III‑V wafer. By resonantly exciting the QD with sub‑3 ps laser pulses, a single electron‑hole pair is created and recombines to emit two photons simultaneously. The cavity enforces strong polarization selection, ensuring that the emitted photons are in orthogonal linear polarizations (H and V). A built‑in wavelength‑division multiplexer separates the photons into the telecom bands around 1.3 µm and 1.55 µm, while on‑chip phase‑shifters fine‑tune the relative phase to zero, thereby generating the Bell state |Φ⁺⟩ = (|HH⟩ + |VV⟩)/√2 with high fidelity.

Key performance metrics demonstrate the source’s superiority over traditional bulk‑crystal spontaneous parametric down‑conversion (SPDC) systems. The second‑order correlation measurement yields g^(2)(0) ≈ 0.01, confirming that multi‑pair emission is suppressed to below 1 % per pulse. Quantum state tomography reveals an entanglement fidelity of F = 0.98 ± 0.01 and a CHSH Bell parameter of S = 2.71 ± 0.04, exceeding the classical bound by more than 17 σ. The device operates at an 80 MHz repetition rate, synchronized with an electronic trigger, and maintains a pair‑generation probability of >95 % per pulse, demonstrating both high speed and reliability.

The authors discuss several technical innovations that enable these results. First, resonant pulsed excitation minimizes carrier re‑excitation and reduces background fluorescence. Second, the photonic crystal cavity provides a Purcell enhancement that shortens the radiative lifetime, allowing rapid emission while preserving indistinguishability. Third, the on‑chip polarization control and wavelength routing eliminate the need for bulky external optics, paving the way for fully integrated quantum photonic circuits.

Beyond the immediate experimental achievements, the paper outlines a roadmap for scaling the technology. Long‑term stability will be improved through active temperature regulation and feedback‑controlled electric tuning of the QD transition. Arrays of quantum dots can be fabricated to produce multiple entangled pairs in parallel, enabling the generation of cluster states required for measurement‑based quantum computing. Finally, hybrid integration with silicon photonics is proposed to combine the excellent light‑matter interaction of III‑V quantum dots with the mature CMOS manufacturing ecosystem, facilitating large‑scale quantum networks.

In summary, this work delivers the first semiconductor chip that can generate a single, high‑quality entangled photon pair on demand, with near‑deterministic efficiency, low multi‑pair probability, and telecom‑compatible wavelengths. Its compact, electrically controllable architecture makes it a practical resource for quantum key distribution, quantum repeaters, and photonic quantum information processing, marking a significant step toward scalable, real‑world quantum technologies.