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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)

A Modular Source of Entangled Photon Pairs in Femtosecond-Laser Written Waveguide Circuits

Simone Atzeni, Roberto Osellame, Adil Syed Rab, Giacomo Corrielli, Emanuele Polino, Mauro Valeri, Paolo Mataloni, Nicolò Spagnolo, Andrea Crespi, Fabio Sciarrino

Published Jun 1, 2019• 0 citations
Publisher

Abstract

Sources of entangled photons are cornerstones of photonic quantum technologies, providing the basic resources for a wide range of quantum protocols for computing, simulation and sensing. A typical technique to generate entangled photon pairs exploits spontaneous parametric down conversion (SPDC) in non-linear crystals. In this context, an integrated-optics approach based on non-linear waveguides [1] can boost source brightness due to the enhanced mode confinement, and can take great advantage from the unique compactness and stability of on-chip devices. However, integrated devices are often specialized to produce a given quantum state.

DOI
10.1109/cleoe-eqec.2019.8872233

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Sources of entangled photons are cornerstones of photonic quantum technologies, providing the basic resources for a wide range of quantum protocols for computing, simulation and sensing. A typical technique to generate entangled photon pairs exploits spontaneous parametric down conversion (SPDC) in non-linear crystals. In this context, an integrated-optics approach based on non-linear waveguides [1] can boost source brightness due to the enhanced mode confinement, and can take great advantage from the unique compactness and stability of on-chip devices. However, integrated devices are often specialized to produce a given quantum state.
Integrated optical circuits enable stable and scalable realization of quantum logic devices, which can form a basis for the mass production of photonic chips for quantum communications and computations. An important challenge is the integration of single-photon sources, which should provide on-chip generation and preparation of quantum photon states. Spontaneous parametric down-conversion (SPDC) provides an attractive solution for experimental generation of correlated and entangled photon pairs [1,2]. However, internal losses in the waveguides may still be present, affecting nontrivially the emerging photon state [3].
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.
Photon entanglement is an important state of light that is at the basis of many protocols in photonic quantum technologies, from quantum computing, to simulation and sensing. The capability to generate entangled photons in integrated waveguide sources is particularly advantageous due to the enhanced stability and more efficient light-crystal interaction. Here we realize an integrated optical source of entangled degenerate photons at telecom wavelength, based on the hybrid interfacing of photonic circuits in different materials, all inscribed by femtosecond laser pulses. We show that our source, based on spontaneous parametric down-conversion, gives access to different classes of output states, allowing to switch from path-entangled to polarization-entangled states with net visibilities above 0.92 for all selected combinations of integrated devices.
Entanglement represents a fundamental resource for quantum technologies, including quantum communication tasks. In this context, photons are the ideal physical systems due to their capability of carrying information over long distances. Hence, it is fundamental to design reliable sources of entangled photons in the telecom wavelength regime (around 1.55 μm), where optical fiber losses are low. In this paper we review different platforms used to generate telecom entangled photon pairs, and we focus in particular on an integrated source realized by the femtosecond laser writing (FLW) technique, which allows to devise stable and compact optical circuits in glass. We show how this technique can be employed to inscribe waveguides in nonlinear crystals for generation of telecom entangled photon pairs on a chip.
The generation of entangled photons by spontaneous parametric down-conversion (SPDC) or spontaneous four-wave mixing (SFWM) attracted enormous interest in the field of quantum optics. Depending on applications, entangled photon sources prioritize either bandwidth (e.g. for quantum imaging) or brightness (e.g. for quantum key distribution). Layered materials offer unique advantages for both. They have already been used to realize thinnest SPDC sources [1], which, like other layered materials such as transition-metal dichalcogenides (TMDs) [2], offer nearly unlimited bandwidth thanks to relaxed phase-matching constraints. Further, their easy integration on photonic platforms is promising for bright on-chip entangled photon sources. In this context, SPDC-based solutions are limited by phase-matching, whereas SFWM would be an almost phase-matching-free process, as pump, idler, and signal photons can be generated at similar wavelengths, thus propagating at the same group velocity in integrated devices. Moreover, exploiting excitonic resonances could enhance FWM even more. However, to date, experiments with resonant FWM in TMDs have been limited to signals in the visible [3], which is unsuitable for integrated photonics and telecom systems due to reabsorption during propagation in the photonic device.