Loss is inevitable for the optical system due to absorption of materials, scattering caused by the defects and surface roughness. In quantum optical circuits, the loss can not only reduce the intensity of signal, but also affect the performance of quantum operations. In this work, we divide losses into unbalanced linear loss and shared common loss, and provide a detailed analysis on how loss affects the integrated linear optical quantum gates. It is found that the orthogonality of eigenmodes and the unitary phase relation of the coupled waveguide modes are destroyed by the loss. As a result, the fidelity of single- and two-qubit operations decrease significant as the shared loss becomes comparable to the coupling strength. Our results are important for the investigation of large-scale photonic integrated quantum information process.
Loss is inevitable for the optical system due to absorption of materials, scattering caused by the defects and surface roughness. In quantum optical circuits, the loss can not only reduce the intensity of signal, but also affect the performance of quantum operations. In this work, we divide losses into unbalanced linear loss and shared common loss, and provide a detailed analysis on how loss affects the integrated linear optical quantum gates. It is found that the orthogonality of eigenmodes and the unitary phase relation of the coupled waveguide modes are destroyed by the loss. As a result, the fidelity of single- and two-qubit operations decrease significant as the shared loss becomes comparable to the coupling strength. Our results are important for the investigation of large-scale photonic integrated quantum information process.
Loss was typically considered a nemesis to optics, which should always be avoided or compensated. On the other hand, gain would be a valuable asset, allowing for mitigating losses and increasing the signal power. However, research has blurred those boundaries, demonstrating that their role can not be simply categorized this way. For instance, loss gives access to nonlinear eects in linear devices, and linear gain introduce additional noise to a quantum system. Undeniably, their role in quantum optics can not be ignored, and this is the path this Ph.D. dissertation follows. First, a general method to compute the input-output relations in any lossy linear N-port device is presented. The method, based on the singular value decomposition (SVD), allows for the analysis of quantum interference in the device and, at the same time, provides an intuitive picture of its classical operation principle. The method's applicability is demonstrated by using dierent reciprocal and nonreciprocal devices. In particular, using this SVD-based procedure to analyze quantum interference in a Wilkinson power divider (WPD) reveals interesting physics in the device. It is theoretically demonstrated that WPDs grant access to coherent perfect absorption (CPA) quantum state transformations in CPA networks with a smaller footprint and a reduced number of elements, which might contribute to reducing the size and complexity of optical networks. Additionally, numerical simulations suggest that WPDs can be designed in a pure silicon-on-insulator platform by taking advantage of radiative losses. Balanced gain-loss optical systems are also investigated. In particular, a method that identies the photonic quantum eigenmodes of the spatial evolution in coupled gain-loss waveguides is proposed, indicating that the associated statistics could be measured using standard detection techniques. Therefore, the noise generation in dierent coupled waveguides is studied, emphasizing its relation with the linear or nonlinear nature of the gain/loss mechanism. The existence of phase transition points in the eigenvalue/eigenvector structure, the possibility of gain and loss mechanisms of different nature to compensate for each other, and the nontrivial properties of the system's uctuations are some of the outcomes of the presented study, thus contributing to a better understanding of non-Hermitian quantum systems. Last but not least, relevant results on scattering theory are presented. The generalized optical theorem is used to derive upper bounds on the cross-polarized backscattering cross-section of systems integrated with a ground plane. Moreover, viable strategies to design these backscatterers based on highly-directive scattering systems are identified.
Quantum photonic integrated circuits, composed of linear-optical elements, offer an efficient way for encoding and processing quantum information on-chip. At their core, these circuits rely on reconfigurable phase shifters, typically constructed from classical components such as thermo- or electro-optical materials, while quantum solid-state emitters such as quantum dots are limited to acting as single-photon sources. Here, we demonstrate the potential of quantum dots as reconfigurable phase shifters. We use numerical models based on established literature parameters to show that circuits utilizing these emitters enable high-fidelity operation and are scalable. Despite the inherent imperfections associated with quantum dots, such as imperfect coupling, dephasing, or spectral diffusion, we show that circuits based on these emitters may be optimized such that these do not significantly impact the unitary infidelity. Specifically, they do not increase the infidelity by more than 0.001 in circuits with up to 10 modes, compared to those affected only by standard nanophotonic losses and routing errors. For example, we achieve fidelities of 0.9998 in quantum-dot-based circuits enacting controlled-phase and - not gates without any redundancies. These findings demonstrate the feasibility of quantum emitter-driven quantum information processing and pave the way for cryogenically-compatible, fast, and low-loss reconfigurable quantum photonic circuits.
Integrated optical waveguide circuits are becoming increasingly important in quantum information processing. They provide a platform that is scaleable to multiple qubits, offers stability over the timescales required for multiple photon coincidence based measurements, and by utilizing telecommunications sources and hardware, allows low-loss and efficient operation. This work will present recent results and indicate research directions that are leading towards the demonstration of the required range of functions on chip, particularly on chip, sources, system tomography, phase control operations and detection.
Optical chips for quantum photonics are cutting-edge technology, merging photonics and quantum mechanics to manipulate light at the quantum level. These chips are crucial for advancing quantum computing, secure communication, and precision sensing by integrating photonic components such as waveguides, beam splitters, and detectors to manipulate single photons, the fundamental carriers of quantum information. Key advancements in optical chips include low-loss waveguides, efficient single-photon sources, and high-fidelity quantum gates, all essential for scalable quantum circuits. Integrating these circuits on a chip offers significant advantages in miniaturization, stability, and reproducibility over traditional bulk optics setups. Recent breakthroughs in materials science and nanofabrication have propelled the field forward, enabling the production of chips with higher precision and lower defect rates. Silicon photonics, in particular, has become a prominent platform due to its compatibility with existing semiconductor manufacturing processes, facilitating the integration of quantum photonic circuits with classical electronic systems. Here, we share our vision of the future of optical chips for quantum photonics, which hold promise for various applications. In quantum computing, they enable the development of compact and scalable quantum processors. In communication, they provide the foundation for ultra-secure quantum networks through quantum key distribution. In sensing, they allow for high-precision measurements that surpass classical limits. As research progresses, optical chips are expected to play a critical role in realizing the full potential of quantum technologies.
High-fidelity quantum information processing with cascaded photonic integrated circuits demands precise phase control and minimal optical loss at every stage. Low-loss phase modulators therefore constitute a critical component, directly governing quantum coherence, interference visibility, and operational fidelity. In this perspective, we examine key strategies for achieving low-loss phase modulation in integrated photonic platforms. Recent advances are systematically categorized into thermo-optic, mechanical-optic, magneto-optic, and electro-optic approaches. We discuss the underlying physical mechanisms and performance trade-offs, and the advantages and limitations of each strategy in the context of quantum photonic applications. Finally, we outline future directions and emerging approaches for developing optical modulators tailored for large-scale, programmable quantum photonic systems.
Abstract An on‐chip quantum light source based on spontaneous four‐wave mixing is an essential element for developing quantum photonic integrated circuit technology, which has the advantage of no connection loss owing to the integration of the source into photonic circuits. The waveguide‐based quantum light source inevitably causes propagation loss owing to imperfections in the fabrication process, but the propagation loss effects on photon‐pair generation have not been extensively studied. In this study, propagation loss effects were examined using theoretical and experimental methods. In theory, the performance of quantum light sources, such as brightness, heralding efficiency, and coincidence‐to‐accidental ratio, strongly depend on propagation loss. We fabricate several waveguides with a moderate propagation loss of 2.2 dB/cm to investigate the loss dependence and ascertain that the brightness, heralding efficiency, and coincidence‐to‐accident ratio strongly correlate with the length of the optical waveguide. The maximum coincidence‐count brightness occurred at an optimization length of 1/ α , where α is the absorption coefficient. In contrast, the single‐count brightness shows slightly different waveguide length dependence owing to loss‐induced one‐photon states. We expect that the results obtained in this study will greatly assist in determining the proper waveguide length for photon‐pair generation according to the source’s application fields. The results will be helpful in the development of a quantum light source suitable for practical and quantum optical integrated circuits and will lead to the development of high‐fidelity quantum technologies.