We demonstrate a chip-integrated semiconductor source that combines polarization and frequency entanglement, allowing the generation of entangled biphoton states in a hybrid degree of freedom without post-manipulation. Our AlGaAs device is based on type-II spontaneous parametric downconversion in a counterpropagating phase-matching scheme in which the modal birefringence lifts the degeneracy between the two possible nonlinear interactions. This allows the direct generation of polarization–frequency entangled photons at room temperature and telecom wavelength, and in two distinct spatial modes, offering enhanced flexibility for quantum information protocols. The state entanglement is quantified by a combined measurement of the joint spectrum and Hong–Ou–Mandel interference (raw visibility 70.1 % ± 1.1 % ) of the biphotons, allowing to reconstruct a restricted density matrix in the hybrid polarization–frequency space.
We demonstrate a chip-integrated semiconductor source that combines polarization and frequency entanglement, allowing the generation of entangled biphoton states in a hybrid degree of freedom without post-manipulation. Our AlGaAs device is based on type-II spontaneous parametric downconversion in a counterpropagating phase-matching scheme in which the modal birefringence lifts the degeneracy between the two possible nonlinear interactions. This allows the direct generation of polarization–frequency entangled photons at room temperature and telecom wavelength, and in two distinct spatial modes, offering enhanced flexibility for quantum information protocols. The state entanglement is quantified by a combined measurement of the joint spectrum and Hong–Ou–Mandel interference (raw visibility 70.1 % ± 1.1 % ) of the biphotons, allowing to reconstruct a restricted density matrix in the hybrid polarization–frequency space.
High-dimensional entangled states of light provide novel capabilities for quantum information, from fundamental tests of quantum mechanics to enhanced computation and communication protocols. In this context, the frequency degree of freedom is attracting a growing interest due to its robustness to propagation in optical fibers and its capability to convey large scale of quantum information into a single spatial mode. This provides a strong incentive for the development of efficient and scalable methods for the generation and manipulation of frequency-encoded quantum states. Nonlinear parametric processes are powerful tools to generate such states, but up to now the manipulation of the generated frequency states has been carried out mostly by post-manipulation, which demands complex and bulk-like experimental setups. Direct production of on-demand frequency-states at the generation stage, and preferably using a chip-based source, is crucial in view of practical and scalable applications for quantum information technologies. Here we employ parametric down-conversion in an integrated AlGaAs chip to engineer the wavefunction and exchange statistics of frequency-entangled photon pairs directly at the generation stage, without post-manipulation [1]. Tuning the pump spatial intensity allows to produce frequency-anticorrelated, correlated and separable states, while tuning the spatial phase allows to tailor the symmetry of the spectral wavefunction. As revealed by Hong-Ou-Mandel interferometry, this allows to simulate either bosons, fermions, or anyons, i.e. particles displaying a fractional exchange statistics intermediate between bosons and fermions [2]. Finally, we show that the frequency entanglement can be combined with the polarization entanglement of our source to produce hybrid frequency-polarization entangled states [3], leading to quantum beating in two-photon interference. These results, obtained at room temperature and telecom wavelength, open promising perspectives for implementing quantum simulation tasks with tailored wavefunction and particle statistics in a chip-integrated platform. [1] S. Francesconi et al, Optica 7, 316 (2020). [2] S. Francesconi et al., ACS Photonics 8, 2764 (2021). [3] S. Francesconi et al., in preparation.
We demonstrate the direct generation of polarization-entangled photon pairs at room temperature and telecom wavelength in an AlGaAs semiconductor waveguide. The source is based on spontaneous parametric down-conversion with a counterpropagating phase-matching scheme. The quality of the two-photon state is assessed by the reconstruction of the density matrix giving a raw fidelity to a Bell state of 0.83; a theoretical model, taking into account the experimental parameters, provides ways to understand and control the amount of entanglement. Its compatibility with electrical injection, together with the high versatility of the generated two-photon state, make this source an attractive candidate for completely integrated quantum photonics devices.
Quantum states of photons such as entangled and single photons are indispensable components for applications in optical quantum information processing. Being able to generate and manipulate quantum states of photons on a chip is a crucial step towards realizing real-world quantum information processing systems such as an optical quantum computer. In this thesis, theoretical and experimental research was carried out for on-chip generation of quantum states of photons via the nonlinear process of spontaneous parametric down-conversion (SPDC), including polarization entangled photons, hyperentangled photons and heralded pure single photons. A majority of the study was based on III-V semiconductor AlGaAs, which could lead to monolithic integration of pump lasers and enable electrically pump sources operating at room temperature. The generation of cross-polarized entangled photons via a type-II SPDC in an AlGaAs Bragg reflection waveguide (BRW) was first investigated. The lack of material birefringence enables the direct generation of wavelength-multiplexed polarization entangled photons with a concurrence of at least 0.77 in a bandwidth of 95 nm. Second, a technique using concurrent type-0 and type-I processes on an AlGaAs chip to generate polarization entangled photon was developed. This technique allows the generation of both co-polarized and cross-polarized entangled photons on the same chip without the need of off-chip compensation, interferometer and bandpass filter. Third, a technique of generating hyperentangled photons on an AlGaAs chip was developed by utilizing spatial modes of different guiding mechanisms. High quality entanglement could be achieved with bandpass filtering of a few nanometers without off-chip phase compensation. Four, a generic technique using nonlinear photonic nanowires to engineer two-photon quantum states was proposed. Via dispersion engineering which controls the group velocity of each mode, maximally entangled polarization entangled photons and pure heralded single photons can be generated on-chip. Design examples were applied to III-V semiconductor and ferroelectric materials. Last, a strategy to enhance the bandwidths of wavelength conversion and SPDC spectrum was developed by tapering the waveguides. Combined with further integration of pump lasers, photonic circuits and detectors, the techniques developed in this thesis provides a viable route to achieve fully integrated generation and manipulations of photonic quantum qubits.
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 105s−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.
Summary form only given. In these last years, a great deal of effort has been devoted to the miniaturization of quantum information technology on semiconductor chips. In the context of photon pair sources, the bi-exciton cascade of a quantum dot and the four-wave mixing in Silicon waveguides have been used to demonstrate the generation of entangled states. Spontaneous parametric down-conversion in III-V semiconductor waveguides combines the advantages of room temperature and telecom wavelength operation, while keeping open the possibility of electrically pumping of the device. Here we present a source consisting of a multilayer AlGaAs waveguide grown on a GaAs substrate and then chemically etched to achieve lateral confinement in a ridge. The structure design is such that a pump beam (around 775 nm), impinging on the surface of the waveguide with an incidence angle θ, generates two counterpropagating orthogonally polarized beams (around 1550 nm). The waveguide core is surrounded by distributed Bragg reflectors to enhance the pump field. We demonstrate the direct emission of polarization entangled photons by pumping the device with two symmetric angles of incidence corresponding to frequency degeneracy and performing a quantum tomography measurement. Most common entanglement witnesses are satisfied and a raw fidelity of 0.8 to the Bell state ( Hν +eiφ vx ) is obtained. A theoretical model, taking into account the experimental parameters, provides ways to understand and control the amount of entanglement.These results open the route to the demonstration of other interesting features of our device such as the generation of hyper-entangled states via the control of the frequency correlation degree through the spatial and spectral pump beam profile, leading to a new generation of completely integrated devices for quantum information.