Photon entanglement has a range of applications from secure communication to the tests of quantum mechanics. Utilizing optical nonlinearity for the generation of entangled photons remains the most widely used approach due to its quality and simplicity. The on-chip integration of entangled light sources has enabled the increase of complexity and enhancement of stability compared to bulk optical implementations. Entanglement over different optical paths is uniquely suited for photonic chips, since waveguides are typically optimized for particular wavelength and polarization, making polarization- and frequency-entanglement less practical. In this review we focus on the latest developments in the field of on-chip nonlinear path-entangled photon sources. We provide a review of recent implementations and compare various approaches to tunability, including thermo-optical, electro-optical and all-optical tuning. We also discuss a range of important technical issues, in particular the on-chip separation of the pump and generated entangled photons. Finally, we review different quality control methods, including on-chip quantum tomography and recently discovered classical-quantum analogy that allows to characterize entangled photon sources by performing simple nonlinear measurements in the classical regime.
Photon entanglement has a range of applications from secure communication to the tests of quantum mechanics. Utilizing optical nonlinearity for the generation of entangled photons remains the most widely used approach due to its quality and simplicity. The on-chip integration of entangled light sources has enabled the increase of complexity and enhancement of stability compared to bulk optical implementations. Entanglement over different optical paths is uniquely suited for photonic chips, since waveguides are typically optimized for particular wavelength and polarization, making polarization- and frequency-entanglement less practical. In this review we focus on the latest developments in the field of on-chip nonlinear path-entangled photon sources. We provide a review of recent implementations and compare various approaches to tunability, including thermo-optical, electro-optical and all-optical tuning. We also discuss a range of important technical issues, in particular the on-chip separation of the pump and generated entangled photons. Finally, we review different quality control methods, including on-chip quantum tomography and recently discovered classical-quantum analogy that allows to characterize entangled photon sources by performing simple nonlinear measurements in the classical regime.
Abstract High-dimensional quantum entanglement is an important resource for emerging quantum technologies such as quantum communication and quantum computation. The scalability of metres-long experimental setups limits high-dimensional entanglement in bulk optics. Advancements in quantum technology hinge on reproducible, and reconfigurable quantum devices—including photon sources, which are challenging to achieve in a scalable manner using bulk optics. Advances in nanotechnology and CMOS-compatible integration techniques have enabled the generation of entangled photons on millimeter-scale chips, significantly enhancing scalability, stability, replicability, and miniaturization for real-world quantum applications. In recent years we have seen several chip-scale demonstrations with different discrete degrees of freedom including path, frequency-bin, time-bin, and transverse modes, on many material platforms. A complete quantum photonic integrated circuit requires the generation, manipulation, and detection of quantum states, involving various active and passive quantum photonic components which further increase the degree of complexity. Here, we focus on the high-dimensional versions of qubits—qudits—and review the nonlinear optical processes that facilitate on-chip high-dimensional entangled photon sources, and the currently used material platforms. We discuss a range of current implementations of on-chip high-dimensional entangled photon sources and demonstrated applications. We comment on the current challenges due to the limitations of individual material platforms and present future opportunities in hybrid and heterogeneous integration strategies for the next generation of integrated quantum photonic chips.
Integrated optical platforms enable the realization of complex quantum photonic circuits for a variety of applications including quantum simulations, computations, and communications. The development of on-chip integrated photon sources, providing photon quantum states with on-demand tunability, is currently an important \nresearch area. A flexible approach for on-chip generation of entangled photons is based on spontaneous nonlinear \nfrequency conversion, with possibilities to integrate several photon-pair sources [1] and realize subsequent post \nprocessing using thermo-optically or electro-optically controlled interference [2, 3]. However, deterministic postprocessing can only provide a limited set of output states, whereas quantum gates with probabilistic operation are \nneeded to generate arbitrary two-photon states [4].
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.
The ability to generate complex optical photon states involving entanglement between multiple optical modes is not only critical to advancing our understanding of quantum mechanics but will play a key role in generating many applications in quantum technologies. These include quantum communications, computation, imaging, microscopy and many other novel technologies that are constantly being proposed. However, approaches to generating parallel multiple, customisable bi- and multi-entangled quantum bits (qubits) on a chip are still in the early stages of development. Here, we review recent advances in the realisation of integrated sources of photonic quantum states, focusing on approaches based on nonlinear optics that are compatible with contemporary optical fibre telecommunications and quantum memory platforms as well as with chip-scale semiconductor technology. These new and exciting platforms hold the promise of compact, low-cost, scalable and practical implementations of sources for the generation and manipulation of complex quantum optical states on a chip, which will play a major role in bringing quantum technologies out of the laboratory and into the real world.
In quantum technology, entanglement is the key resource to perform experiments in quantum computing, quantum communication, quantum sensing and metrology. Generation, characterization and distribution of entanglement deterministically is still an active research area. In contemporary, probabilistic entanglement sources based on parametric down-conversion process remain the workhorse for experimental demonstrations and practical applications, despite their inherent limitations of on-demand entangled photon generation. Over the years, three main technological architectures have evolved for generating probabilistic entangled-photon source; bulk-optics, waveguides and integrated photonic chips, each comprising several distinct architectural approaches tailored to different applications. This review article talks about the state of the art in probabilistic entangled photon source, examining the advancement from free space bulk non-linear optics to fiber based waveguides and scalable photonic chips with their current limitations, presenting a road map for researchers to choose the most appropriate platform for their quantum photonic applications.
Entangled photons generation is an interesting field of research, since progress in this area will directly affect the development of photonic quantum technologies, including quantum computing, simulation and sensing. Several methods have been sifted to increase the performances of entangled photon sources and the integrated optics approach represents a promising strategy. In particular, integrated waveguide sources represent a robust tool, thanks to their stability and the enhancement of nonlinear light-crystal interaction provided by waveguide field confinement. Here, we show the versatility of a hybrid approach, realizing an integrated optical source for the generation of entangled photon-pairs at telecom wavelength. The nonlinear active medium used is lithium niobate, while the routing and manipulation of the generated signal is performed in aluminum-borosilicate glass photonic circuits. The system is composed of three cascaded devices. First, a balanced directional coupler at the fundamental wavelength equally splits the pump in the lithium niobate waveguides, which generate single-photon pairs through type 0 spontaneous parametric down-conversion process. A third chip, encompassing directional couplers and waveplates, closes the interferometer and recombines the generated photons, thus giving access to different quantum states of light: path-entangled or polarization-entangled states. A thermal phase shifter, which controls the relative phase between the interferometer arms, gives an additional degree of freedom for engineering the output state of the presented photon pairs source. All these components are entirely fabricated by femtosecond laser micromachining, a direct and very versatile technique that allows to process different kind of materials and realize high quality optical circuits.