In recent years, there has been a spurring research interest growing on photonics integrated circuits as the way to scale up quantum communications. In particular, building a reliable integrated source of entangled photons is one of the main challenges remaining for commercial exploitation in quantum sensing and metrology. Therefore, advances in design of integrated sources of entangled photons is a necessary and crucial task to be performed for miniaturization and cost-effectiveness. Microring resonators are one of the most promising candidates to generate entangled photons via nonlinear interactions, especially Spontaneous Four-Wave Mixing (SFWM). But fabrication limitations in reproducibility of high-quality resonators mandate the use of active devices. Here we present the results of preparing the excitation pulse with different spectral, intensity and dispersion parameters to optimize the quantum efficiency in passive devices after fabrication. Different numerical methods such as Finite Difference Time Domain (FDTD) and Split-Step Fourier Method (SSFM) are used to reproduce the nonlinear propagation, interplay with dispersion and losses. Exploiting the degree of freedom of the excitation pulse can bring more energy efficient and passive devices as quantum entangled pair sources in an integrated chip. Considerations on different degrees of entanglement are also discussed for applications in quantum sensing and quantum metrology.
In recent years, there has been a spurring research interest growing on photonics integrated circuits as the way to scale up quantum communications. In particular, building a reliable integrated source of entangled photons is one of the main challenges remaining for commercial exploitation in quantum sensing and metrology. Therefore, advances in design of integrated sources of entangled photons is a necessary and crucial task to be performed for miniaturization and cost-effectiveness. Microring resonators are one of the most promising candidates to generate entangled photons via nonlinear interactions, especially Spontaneous Four-Wave Mixing (SFWM). But fabrication limitations in reproducibility of high-quality resonators mandate the use of active devices. Here we present the results of preparing the excitation pulse with different spectral, intensity and dispersion parameters to optimize the quantum efficiency in passive devices after fabrication. Different numerical methods such as Finite Difference Time Domain (FDTD) and Split-Step Fourier Method (SSFM) are used to reproduce the nonlinear propagation, interplay with dispersion and losses. Exploiting the degree of freedom of the excitation pulse can bring more energy efficient and passive devices as quantum entangled pair sources in an integrated chip. Considerations on different degrees of entanglement are also discussed for applications in quantum sensing and quantum metrology.
The intrinsic third-order nonlinearity in silicon has proven it to be quite useful in the field of quantum optics. Silicon is suitable for producing time-correlated photon pairs that are sources of heralded single-photon states for quantum integrated circuits. A quantum signal source in the form of single photons is an inherent requirement for the principles of quantum key distribution technology for secure communications. Here, we present numerical simulations of a silicon ring with a 6 μ m radius side-coupled with a bus waveguide as the source for the generation of single photons. The photon pairs are generated by exploring the process of degenerate spontaneous four-wave mixing (SFWM). The free spectral range (FSR) of the ring is quite large, simplifying the extraction of the signal/idler pairs. The phase-matching condition is considered by studying relevant parameters like the dispersion and nonlinearity. We optimize the ring for a high quality factor by varying the gap between the bus and the ring waveguide. This is the smallest ring studied for photon pair generation with a quality factor in the order of 10 5 . The width of the waveguides is chosen such that the phase-matching condition is satisfied, allowing for the propagation of fundamental modes only. The bus waveguide is pumped at one of the ring resonances with the minimum dispersion (1543.5 nm in our case) to satisfy the principle of energy conservation. The photon pair generation rate achieved is comparable to the state of the art. The photon pair sources exploiting nonlinear frequency conversion/generation processes is a promising alternative to atom-like single-photon emitters in the field of integrated photonics. Such miniaturized structures will benefit future on-chip architectures where multiple single-photon source devices are required on the same chip.
Photonic integrated circuits (PICs) based on silicon-on-insulator (SOI) substrates provide a lightweight, compact platform for applications in quantum optics. We show the evaluation and characterization of an SOI-bascd PIC design for an entangled photon pair source, generating photon pairs by means of the nonlinear process of spontaneous four-wave mixing in microring resonators. Experimental results following from the chip fabrication fed back into the simulated parameter tuning effects, culminating in photon pair generation with measured correlations exceeding classically predicted limits.
Quantum entanglement has been an extremely active field of research, both theoretically and experimentally. Studies on entanglement are fundamental to improving our basic understanding of quantum mechanics. As our understanding of the phenomenon grows, so does interest in using it to advance various technological fields: communications, computing, and sensing, to name just a few. It is now known that entangled photon pairs can be generated through a process known as Spontaneous Four-Wave Mixing (SFWM). In this paper, we report the generation of entangled photon pairs in time intervals by spontaneous four-wave mixing in a nonlinear fiber optic ring resonator. The requirements for efficient SFWM are quite similar to those for classical FWM, but they require even greater precision, especially for quantum applications. To achieve a measurable pair generation rate, a high-intensity pump laser is essential. The nonlinear medium must have high nonlinearity and confinement. This technique is compatible with contemporary fiber, enabling compact, low-cost, and scalable implementations such as quantum memories.
One of the key building blocks of quantum photonic systems is a source of indistinguishable photons. Silicon ring resonators can be used as photon pair sources by taking advantage of silicon's large third order nonlinearity with a process known as spontaneous four wave mixing. These sources are capable of producing pairs of indistinguishable photons but typically suffer from an effective 50% loss. By slightly decoupling the input waveguide from the ring, the desired photons generated in the ring can be directed to the drop port. Thus, the ratio between the coincidences from the drop port and the total number of coincidences from all ports (coincidence efficiency) can be significantly increased, with the trade-off being that the pump is less efficiently coupled into the ring. This asymmetric coupling scheme was used to demonstrate a dual-bus microring source with a coincidence efficiency of ~96%. Unfortunately, the penalty to pump power was quite large, with a required ~10x increase in the pump power to maintain the same pair generation rate.\nTo further improve upon this result, a microring source was designed with multipoint evanescent couplers to induce a strong spectral dependence in the coupling coefficients. This coupler design can be used on both sides of the ring resonator so that resonances supported by one of the couplers are suppressed by the other. This is the ideal configuration for a ring-based photon-pair source as it can only support the pump photons at the input side while only allowing the generated photons to leave through the output side. Asymmetric coupling was also used in this configuration but in a different way than in the previous source. In this case, the input side of the ring was configured to be critically coupled for maximal resonance enhancement while the output side was configured to be over coupled to minimize photon-pair loss within the ring. This device design was used to demonstrate a photon-pair source with coincidence efficiencies of 0.9981+-6.425x10^-4 and 0.9979+-1.665x10^-3 for degenerate and non-degenerate pump configuration respectively.\nIn addition to the source development, work was done to move some of the pump rejection filtering onto the photonic chip. While the fabricated device only exhibited an extinction of 50 dB, the root cause of the limited extinction was explored and a new filter design was proposed to overcome these limitations. With the combination of the microring source with its greatly improved efficiency and the movement toward reducing the required off-chip filtering, this work represents a significant advancement in the scalability of integrated quantum photonic systems which can enable high performance quantum computing and communication systems.
Nowadays, Quantum Photonics finds a number of significant applications, such as Quantum Computing and Quantum Communication. Silicon on Insulator (SOI) technology has proven its potential for the generation of correlated photon pairs through the spontaneous four wave mixing (SFWM) effect, but not in the visible wavelength range. In this invited paper we present the physical investigation and design guidelines of AlGaN microring resonators for generating photon entanglement, as induced by SFWM in the visible. The work theoretically demonstrates the possibility of realizing the on-chip generation of correlated pairs of photons within integrated microring resonators based on the AlGaN/AlN technological platform, due to its wide bandgap, crystalline structure and low losses. Furthermore, general physical features are presented by means of a parametric analysis of the biphoton flux and cross correlation as a function of the device parameters, i.e. ring radius, resonator cross section and sizes, pulley coupling. Moreover, the spectral features of the generated signal and idler photons have been analyzed as a function of the pump resonance mismatch for several Kerr coefficients and input visible “blue” pump powers. Cross correlation values around 2.3 ×104at a biphoton flux of about 9×104pairs/s and 2×103at about 2.9×106pairs/s for ring radius of 200 μm and 30 μm have been achieved, respectively, assuming a input pump power of 25 mW, while the signal is generated at wavelength of 369.5 nm and idler at 535.72 nm, for ring radius of 200 μm, and 369.5 and 457.45 nm for ring radius of 30 μm, respectively. These devices can be important for many applications, going from UV-visible spectroscopy to quantum sensing to image processing to nonlinear optics.
Integrated photonics is emerging as a key technology for use in quantum information and communication. Driven by classical communication, which already has a swathe of scalable, efficient, and robust components, integrated photonics promises to transfer quantum science from a mere lab curiosity into a commercial reality. Chip-based quantum light sources are increasingly being recognized as a practical and high-performance alternative to bulky and power-hungry, table-top photon-pair sources. In particular, photon-pair sources based on spontaneous four-wave mixing in microring resonators (MRR) have emerged as a viable integrated photonic solution for quantum information and communication. This thesis relies on silicon nitride (Si3N4) MRR to realize narrowband, telecom photonpair sources. High quality factor MRRs are enabled by the low propagation losses in the Si3N4 platform; in this thesis, they are exploited to realize bright photon-pair sources with heralded spectral purity up to 0.98. Low quality factor MRRs are exploited for a proof-of-concept high-rate sequential time-bin entanglement with net visibilities up to 99.96 ± 0.03 %. For the goal of practical, real-world deployment of integrated photon-pair sources, it is not enough to demonstrate the performance of a single source. One also needs to demonstrate their high-performance operation in complex quantum networking protocols, such as entanglement swapping between genuinely independent sources. Here, for the first time, we demonstrate entanglement swapping between two independent, spatially separated, and asynchronously-pumped MRR photon-pair sources. Our sources operate in the continuous-wave regime, and time-resolved detection results in high Hong-Ou-Mandel (93.2 ±1.6 %) and entanglement swapping (91.2 ± 3.4 %) visibilities. Finally, to further improve our rates while maintaining high visibilities in multi-source interference scenarios, the origin of an on-chip-generated noise must be identified and mitigated. Here, we present our first efforts in the quest for tackling this notorious noise problem.