Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)
Nonlinear waveguides for integrated quantum light source
Renato R. Domeneguetti, Hauke Conradi, Moritz Kleinert, Christian Kießler, Michael Stefszky, Harald Herrmann, Christine Silberhorn, Ulrik L. Andersen, Jonas Schou Neergaard-Nielsen, Tobias Gehring
Squeezed quantum states of light are essential tools for sensing, quantum computing and secure communication tasks, such as quantum key distribution [1] - [4] . For practical applications, integrated devices combining quantum light sources with additional active and passive components are attractive since they reduce bulky tabletop optical experiments into a single and highly compact chip, eventually leading to lower fabrication costs and the potential for large scale production with improved reproducibility and stability.
Squeezed quantum states of light are essential tools for sensing, quantum computing and secure communication tasks, such as quantum key distribution [1] - [4] . For practical applications, integrated devices combining quantum light sources with additional active and passive components are attractive since they reduce bulky tabletop optical experiments into a single and highly compact chip, eventually leading to lower fabrication costs and the potential for large scale production with improved reproducibility and stability.
Nonclassical states of light are an essential resource for many emerging quantum technologies and applications ranging from information processing, encrypted communications, and networking to sensing, metrology, and imaging. Nonlinear optical processes in solid-state materials are widely used for generating quantum light, including single photons, entangled-photon pairs, and quadrature-squeezed states. Recent advances in nonlinear photonics have enabled the functionality of benchtop nonlinear instruments to be scaled down to a single chip without sacrificing efficiency or degrading the key performance metrics. The dramatic improvement in the size, weight, power, cost, and stability enabled by photonic integrated circuits has been essential for enabling the chip-scale generation, manipulation, and detection of quantum light at a steadily increasing degree of complexity and scale. Within the last decade, the authors have seen the progression from few-component photonic circuits operating on two photons to arrays of 18 identical heralded single-photon sources and reconfigurable devices operating with more than 650 components for multidimensional entanglement and arbitrary two-photon quantum gates. In this review, the authors summarize the history and recent key technological developments of chip-scale nonlinear quantum light generation based on integrated nonlinear photonics, recent advances in heterogeneous integrated methods, and approaches for system-level integration and demonstrated applications.
Optical chips for quantum photonics are cutting-edge technology, merging photonics and quantum mechanics to manipulate light at the quantum level. These chips are crucial for advancing quantum computing, secure communication, and precision sensing by integrating photonic components such as waveguides, beam splitters, and detectors to manipulate single photons, the fundamental carriers of quantum information. Key advancements in optical chips include low-loss waveguides, efficient single-photon sources, and high-fidelity quantum gates, all essential for scalable quantum circuits. Integrating these circuits on a chip offers significant advantages in miniaturization, stability, and reproducibility over traditional bulk optics setups. Recent breakthroughs in materials science and nanofabrication have propelled the field forward, enabling the production of chips with higher precision and lower defect rates. Silicon photonics, in particular, has become a prominent platform due to its compatibility with existing semiconductor manufacturing processes, facilitating the integration of quantum photonic circuits with classical electronic systems. Here, we share our vision of the future of optical chips for quantum photonics, which hold promise for various applications. In quantum computing, they enable the development of compact and scalable quantum processors. In communication, they provide the foundation for ultra-secure quantum networks through quantum key distribution. In sensing, they allow for high-precision measurements that surpass classical limits. As research progresses, optical chips are expected to play a critical role in realizing the full potential of quantum technologies.
``Squeezed'' states of light are the fundamental building blocks for continuous-variable quantum optics, so a quantum network for optical communication would seem to require a compact, efficient source of such states. Combining clever design with recent advances in waveguide materials engineering, the authors produce a device that yields very high levels of squeezing, compared to other integrated architectures. Their innovative approach allows for rarely seen continuous-wave squeezing, paving the way for more advanced on-chip functions.
High-dimensional degrees of photons – such as orbital angular momentum, spatial modes of frequency – provide a powerful mean to increase the density and security of quantum communication, and to enhance flexibility in quantum computing. In particular, the spatial degree of freedom is particularly suited to on-chip integration. Rapid progress has been made in recent years to develop integrated circuits achieving on-chip quantum interference, entanglement and gate operations on spatially encoded states, culminating in the demonstration of the Boson Sampling task on-chip [1]. While many of these demonstrations have relied on external sources to generate quantum states of light, which were then fed into a passive circuitry, a next step has been taken recently with the demonstration of active chips combining the generation and manipulation of spatially encoded states [2].
Integrated quantum photonics can realize and process optical entangled quantum states in highly compact, robust, and scalable chips thereby enabling chip scale implementations of long-distance quantum-secured communication, quantum-accelerated information processing, and non-classical metrology [1]. Notably, all previous on-chip entangled quantum photonic sources have relied on an external laser to excite nonlinear parametric processes, thereby making these systems overall non-reproducible, bulky, impractical, and thus unsuitable for out-of-lab use as well as production at large scale [2]. To date, the major challenge inhibiting a fully on-chip quantum light system is to integrate a stable, tunable laser together with a high rejection filter that eliminates laser sideband noise [3], [4] and a nonlinear parametric source of entangled photons. Fig. 1 A) laser-integrated photonic quantum light source of frequency-bin entangled photon states: an electrically pumped inp gain-section, i.e. A reflective semiconductor optical amplifier (rsoa) providing optical gain, is coupled to a si3n4 chip containing a three microring resonator (r1, r2, and r3) vernier filter and a mach-zehnder interferometer (mzi) with a sagnac loop mirror. B) measurements utilizing single photon detectors allowed to determine an smsr≈ 112 db, and an ase suppression of> 56 db. C) car for respective frequency pairs for an excitation power of 3.9 mw. D) density matrices of two-dimensional (qubit) and three-dimensional (qutrit) frequency-bin entangled states reconstructed via quantum state tomography (qst).