We propose and analyze a generic technique to engineer the two-photon quantum state generated by spontaneous parametric downconversion (SPDC) in nonlinear photonic nanowires using any suitable material system. Through dispersion engineering in nanowires, the group velocity of each photon involved in the SPDC process can be tuned such that pure heralded single photons or maximally polarization entangled photons can be directly generated on a chip. Implementations in III–V semiconductor and ferroelectric waveguides demonstrate minimal frequency correlations with Schmidt numbers of ∼1 for heralded single photons, and maximal entanglement with concurrences of ∼1 for polarization entangled photons.
We propose and analyze a generic technique to engineer the two-photon quantum state generated by spontaneous parametric downconversion (SPDC) in nonlinear photonic nanowires using any suitable material system. Through dispersion engineering in nanowires, the group velocity of each photon involved in the SPDC process can be tuned such that pure heralded single photons or maximally polarization entangled photons can be directly generated on a chip. Implementations in III–V semiconductor and ferroelectric waveguides demonstrate minimal frequency correlations with Schmidt numbers of ∼1 for heralded single photons, and maximal entanglement with concurrences of ∼1 for polarization entangled photons.
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.
The aim of this thesis is to explore nonlinear nanostructures – photonic systems with one or more dimension with nanoscale feature size – as new platforms for parametric photon-pair generation. This is motivated by the need for photon-pair sources in many technological applications of quantum optics, such as quantum communication, imaging and computing. The initial hypothesis is that nonlinear nanostructures are a promising platform to complement established photon-pair source schemes based on bulk crystals or photonic integrated circuits in terms of quantum state-engineering, footprint, functional density etc. However, nonlinear pair generation, e.g. via spontaneous parametric down-conversion (SPDC), is a new subfield in nanophotonics with a very limited number of studies existing. This constitutes the knowledge-gap addressed in this thesis. Therefore, the objective is to provide first experimental demonstrations of SPDC in subwavelength-scale and nonlinear nanostructures. We investigate the ability of such systems to control the properties of two-photon quantum states such as frequency and angular spectrum or polarization entanglement. To this end we analyze nanostructures with different degrees of complexity, starting with an unstructured thin crystal with one nanoscale dimension, leading up to a 3D nano-scale resonator with subwavelength-sized features in all dimensions. Furthermore, we test different types of nonlinear materials such as lithium niobate, group III-V semiconductors and emerging, highly nonlinear materials such as molybdenum disulfide. Additionally, we also advance the theoretical understanding by developing a comprehensive model of the quantum process of nonlinear parametric pair-generation in photonic nanostructures.
We present theoretical and experimental advances on photon-pair generation through spontaneous parametric down-conversion (SPDC) in nonlinear nanostructures and metasurfaces, underpinning quantum entanglement engineering at sub-wavelength scale for photon shaping with tailored polarization and spatial correlations. We derive explicit theoretical expressions for the measurable photon counts and quantum correlations, incorporating material dispersion and absorption characteristics [Phys. Rev. Lett. 117, 123901 (2016)], and demonstrate experimentally the generation of quantum heralded photon pairs from a single AlGaAs nanodisk [Optica 6, 1416 (2019)]. We further reveal the opportunities to enhance the generation rate through high-Q resonances including bound states in the continuum [doi:10.1117/12.2539888 (2019)]. We also outline the applications of metasurfaces for the manipulation and measurement of multi-photon quantum states for free-space quantum imaging and communications.
Tunable photonic quantum entanglement promises to advance information processing beyond classical approaches, facilitating applications in quantum communications, imaging and computing. A practical approach to preparing quantum entanglement at room temperature is to engineer photon-pair generation through spontaneous parametric down-conversion (SPDC). Nonlinear flat-optics devices, incorporating subwavelength-thick nonlinear materials, are emerging as new platforms for generating photon pairs at the nanoscale. In particular, nanostructured metasurfaces supporting optical resonances can enhance and tailor SPDC processes, allowing for the generation of spectral, polarization, and spatial entanglement [1]. However, the tunability of polarization entanglement is subject to the symmetry of nonlinear tensors of materials, where the pump photon polarization serves as the only tuning parameter [2].
Photonic entanglement and heralded single photons are vital for optical quantum information processing, and are realized by spontaneous parametric down-conversion (SPDC) in chiefly just two materials: LiNbO3 and KTiOPO4. The particular properties of these materials, however, strongly restrict advanced techniques. Thus the authors study four other nonlinear optical materials, plus KTiOPO4, and configurations in which they can be used to generate pure photon states and entanglement of polarization or frequency. The team finds a broad variety of promising SPDC setups that are impossible to implement using the traditional materials.
Photon pairs generated by spontaneous parametric downconversion are essential for optical quantum information processing, in which the quality of biphoton states is crucial for the performance. To engineer the biphoton wave function (BWF) on-chip, the pump envelope function and the phase matching function are commonly adjusted, while the modal field overlap has been considered as a constant in the frequency range of interest. In this work, by using modal coupling in a system of coupled waveguides, we explore the modal field overlap as a new degree of freedom for biphoton engineering. We provide design examples for on-chip generations of polarization entangled photons and heralded single photons. This strategy can be applied to waveguides of different materials and structures, offering new possibilities for photonic quantum state engineering.