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arXiv:1710.09618 · Optica

Integrated sources of entangled photons at the telecom wavelength in femtosecond-laser-written circuits

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

Published Mar 19, 2018• 90 citations
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Abstract

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.

DOI
10.1364/optica.5.000311

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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.
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.
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.
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.
Photonics is a powerful framework for testing in experiments quantum information ideas, which promise significant advantages in computation, cryptography, measurement and simulation tasks. Linear optics is in principle sufficient to achieve universal quantum computation, but stability requirements become severe when experiments have to be implemented with bulk components. Integrated photonic circuits, on the contrary, due to their compact monolithic structure, easily overcome stability and size limitations of bench-top setups. Anyway, for quantum information applications, they have been operated so far only with fixed polarization states of the photons. On the other hand, many important quantum information processes and sources of entangled photon states are based on the polarization degree of freedom. In our work we demonstrate femtosecond laser fabrication of novel integrated components which are able to support and manipulate polarization entangled photons. The low birefringence and the unique possibility of engineering three-dimensional circuit layouts, allow femtosecond laser written waveguides to be eminently suited for quantum optics applications. In fact, this technology enables to realize polarization insensitive circuits which have been employed for entangled Bell state filtration and implementation of discrete quantum walk of entangled photons. Polarization sensitive devices can also be fabricated, such as partially polarizing directional couplers, which have enabled on-chip integration of quantum logic gates reaching high fidelity operation.
Integrated quantum photonics is a key tool towards large scale quantum technologies. In this work we present an AlGaAs-based photonic circuit for on-chip generation and manipulation of broadband polarization-entangled photon pairs. The quantum state is generated by Type-II spontaneous parametric down conversion in AlGaAs Bragg reflection waveguides at telecom wavelengths and room temperature. Polarization entangled photons are manipulated over a frequency band of around 50 nm by an integrated polarizing beam splitter. We demonstrate Hong-Ou-Mandel visibility of 80% for a quantum device where the photon pairs generation and manipulation are implemented in a single chip.
The demand for integrated photonic chips combining the generation and manipulation of quantum states of light is steadily increasing, driven by the need for compact and scalable platforms for quantum information technologies. While photonic circuits with diverse functionalities are being developed in different single material platforms, it has become crucial to realize hybrid photonic circuits that harness the advantages of multiple materials while mitigating their respective weaknesses, resulting in enhanced capabilities. Here, we demonstrate a hybrid III-V/Silicon quantum photonic device combining the strong second-order nonlinearity and direct bandgap of the III-V semiconductor platform with the high maturity and CMOS compatibility of the silicon photonic platform. Our device embeds the spontaneous parametric down-conversion (SPDC) of photon pairs into an AlGaAs source and their vertical routing to an adhesively-bonded silicon-on-insulator circuitry, within an evanescent coupling scheme managing both polarization states. This enables the on-chip generation of broadband (> 40 nm) telecom photons by type 0 and type 2 SPDC from the hybrid device, at room temperature and with internal pair generation rates exceeding 10510^5105 s−1s^{-1}s−1 for both types, while the pump beam is strongly rejected. Two-photon interference with 92% visibility (and up to 99% upon 5 nm spectral filtering) proves the high energy-time entanglement quality of the produced quantum state, thereby enabling a wide range of quantum information applications on-chip, within an hybrid architecture compliant with electrical pumping and merging the assets of two mature and highly complementary platforms in view of out-of-the-lab deployment of quantum technologies.