[Enlarge image]Left: Photograph of laser-integrated turnkey quantum light source, of a size comparable to a one-euro coin. Right: Hybrid source consists of an indium phosphide gain medium extended by a silicon nitride feedback circuit. Rings (R1, R2 and R3) form a Vernier filter, connected to a Mach-Zehnder interferometer (MZI). The circuit forms a lasing cavity made of RSOA and Sagnac mirror. R3 generates a quantum frequency comb (QFC) with a free-spectral range of ~200 GHz through SFWM.
Photonics is a compelling approach for quantum technologies, due to photons’ robustness to decoherence, sophisticated information-processing tools operable at room temperature, existing fiber optic networks and mature fabrication technologies. State-of-the-art nanofabrication facilities can already make compact, robust on-chip photonic devices capable of generating and controlling entangled photons—crucial resources for quantum information processing.1 A range of nonclassical functionalities has been realized in the lab on photonic chips.
Yet all previously demonstrated quantum photonic sources, on-chip quantum functionalities and integrated quantum photonic processors have relied on external, bulky excitation lasers. These lasers often occupy a large space and are tenuously coupled to the photonic chip, resulting in poor stability over time and losses at the interfaces. The overall system thus becomes non-reproducible, inefficient, impractical and unsuitable for out-of-laboratory use and scalable production. To address these challenges, we recently demonstrated a laser-integrated, fully on-chip quantum light source of entangled qubits and qudits2 that can provide the required stability and scalability.3
Previously, the major technical challenge inhibiting a turnkey system has been the combined integration of a stable, tunable laser with a high rejection filter (to eliminate noise) and an entangled-photon source (creating signal–idler pairs through nonlinear spontaneous parametric effects). Further, the absence of a unique material platform that supports all quantum photonic functionalities—low-loss guiding, filtering, efficient generation of entangled photons, active manipulation—while also providing laser gain has impeded monolithic integration.
Recently, through novel chip design and by leveraging hybrid integration, we realized a laser-integrated quantum light source. The source directly generates frequency-bin entangled photons—that is, a high-dimensional quantum frequency comb2,4—through spontaneous four-wave mixing (SFWM). The source consists of an electrically pumped reflective semiconductor optical amplifier (RSOA) based on an InP gain section, extended by a Si3N4 low-loss feedback circuit. Three microrings form a Vernier filter that provides a laser side-mode suppression ratio of 112 dB. A Sagnac mirror and reflecting coating at the RSOA form a lasing cavity. Metal microheaters align the rings’ resonances at 193.4 THz.
The high noise suppression of the Vernier filter facilitated a high coincidental-to-accidental ratio of ~80 with a remarkable pair-detection rate of greater than 600, over four resonance pairs in the telecom C-band. Quantum interference measurements with visibilities of more than 96%, and quantum state tomography with fidelities greater than 0.98 for 2D Bell states and ~0.85 for 3D Bell states, affirmed the generation of excellent-quality entangled qubits and qutrits.
Our work2 is the first demonstration of an on-chip, robust, lightweight quantum light source. We believe it will facilitate necessary technological advancements for building quantum processors, quantum internet devices and quantum satellite systems in a fully integrated, stable, scalable and field-deployable format, meeting industry requirements.
Raktim Haldar, Hatam Mahmudlu, Robert Johanning, Anahita Khodadad Kashi and Michael Kues, Leibniz University Hannover, Hannover, Germany
Albert van Rees and Klaus-J. Boller, University of Twente, Enschede, Netherlands
Jörn P. Epping, QuiX Quantum and Lionix International, Enschede, Netherlands
1. J. Wang et al. Nat. Photon. 14, 273 (2020).
2. H. Mahmudlu et al. Nat. Photon. 17, 518 (2023).
3. A.W. Elshaari et al. Nat. Photon. 14, 285 (2020).
4. M. Kues et al. Nature 546, 622 (2017).