The design, fabrication, and detailed calibration of essential building blocks towards fully integrated linear-optics quantum computation are discussed. Photonic devices are made from silicon nitride rib waveguides, where measurements on ring resonators show small propagation losses. Directional couplers are designed to be insensitive to fabrication variations. Their offset and coupling lengths are measured, as well as the phase difference between the transmitted and reflected light. With careful calibrations, the insertion loss of the directional couplers is found to be small. Finally, an integrated controlled-NOT circuit is characterized by measuring the transmission through different combinations of inputs and outputs. The gate fidelity for the CNOT operation with this circuit is estimated to be 99.81% after post selection. This high fidelity is due to our robust design, good fabrication reproducibility, and extensive characterizations.
The design, fabrication, and detailed calibration of essential building blocks towards fully integrated linear-optics quantum computation are discussed. Photonic devices are made from silicon nitride rib waveguides, where measurements on ring resonators show small propagation losses. Directional couplers are designed to be insensitive to fabrication variations. Their offset and coupling lengths are measured, as well as the phase difference between the transmitted and reflected light. With careful calibrations, the insertion loss of the directional couplers is found to be small. Finally, an integrated controlled-NOT circuit is characterized by measuring the transmission through different combinations of inputs and outputs. The gate fidelity for the CNOT operation with this circuit is estimated to be 99.81% after post selection. This high fidelity is due to our robust design, good fabrication reproducibility, and extensive characterizations.
Programmable waveguide circuits are crucial building blocks for integrated spectrometric applications and quantum photonic information processing. Amongst the dielectric material platforms in integrated photonics, silicon nitride stands out with highly attractive properties such as a large bandgap energy and a moderately high index contrast. This allows low propagation losses in a wide spectral range while simultaneously allowing a dense packing of components. The aforementioned properties, together with the inherent phase stability and phase programmability achievable in silicon nitride, enable the creation of complex photonic circuits. In this thesis we describe and demonstrate densely integrated programmable photonic circuits based on silicon nitride waveguides for wavelength metrology and quantum information processing. We concentrate on reconfigurable photonic integrated circuits based on silicon nitride waveguides with low-loss propagation, to explore interference in the spectral and temporal domain for advanced applications. We investigated two types of integrated interferometric devices featuring low loss in combination with programmability for classical and quantum photonic processing. The first is simple tunable microring resonator circuits in combination with neural network data processing for the analysis of classical light in the spectral domain as wavelength meter. The second is a complex tunable network of waveguide interferometers for controlling quantum correlations (coincidences) between single photons. Exploiting the long-term interferometric stability, low propagation loss and tight optical confinement of integrated silicon nitride waveguides, we have shown complex reconfigurable optical circuits both for classical and quantum photonic processing. For future development of integrated programmable photonic processors, various challenges need to be addressed such as compact and low-power phase shifters, a further increase of the component density and lower the propagation losses.
Directional couplers are fundamental building blocks for integrated quantum photonic circuits but are highly sensitive to fabrication imperfections. We demonstrate that composite segmented directional couplers (CSDCs) enhance gate fidelity without active tuning. We design and fabricate fully integrated photonic controlled-NOT (CNOT) gates using both uniform and composite coupler variants and compare their performance via simulation, classical characterization, and quantum two-photon interference. The composite design reduces the average computational-basis error probability by nearly a factor of two and variability by a factor of five, with residual error primarily limited by photon indistinguishability. These results establish CSDCs as passive, foundry-compatible building blocks for scalable quantum photonic circuits.
Silicon-on-insulator (SOI) wafers have been developed for electronic integrated circuits, but are also one of the most promising platforms in photonic integrated circuits due to its low loss, high refractive index, ability to fast phase modulation, and well-developed fabrication technologies. With advantages to handle light with a small volume and high modulation speed, silicon photonics has been intensively investigated as the next generation platform of signal processing, interconnection, and communication devices. Recently, silicon photonics has been attracted great interests in quantum optics communities as optical quantum systems for linear optical quantum computing are achievable using silicon photonic components including beam splitters, phase modulators, and filters and the small footprint of a silicon photonic quantum system has unprecedented performances of high stability and scalability. Here we show our recent results on the fabrication of silicon photonic elements for quantum applications. We fabricated low-loss optical waveguides, efficient fiber-to-waveguide grating couplers, resonator-based optical filters, and tunable beam splitters and tested their performances. A micro-sized heater is attached on a Mach-Zehnder interferometer, showing stable operation as a slow phase modulator. In addition, phase modulators using plasma dispersion effects are under investigation. Finally, the quantum characteristics of photon pairs generated by spontaneous four-wave mixing in silicon waveguides are investigated. These results will be an essential cornerstone for the realization of quantum optical computing and quantum communication devices.
We demonstrate controlled-NOT (CNOT) operation of a silicon nitride photonic integrated circuit using path-entangled photon pairs from a silicon-photonic integrated circuit as logical inputs. The CNOT gate is constructed by combining several silicon-nitride Mach–Zehnder interferometers for a linear-optical quantum gate. We package each of the two photonic circuits with electrical wire bonding and optical fiber bonding for stable measurement. The logical-basis fidelity of CNOT operation is measured better than 81% for the fully packaged photonic circuits.
Linear Optics Quantum Computing can be achieved by implementing a complete Silicon Photonic Integrated Circuit. This involves optimizing the layout and fabrication of photonic circuits tailored for quantum computing applications using linear optics principles. A prototype of this kind of circuit is the universal two-qubit Controlled-NOT gate. This scheme makes use of linear, coincidence basis gate that performs all the operations of a controlled-NOT gate and requires only single photons at the input. The key components in the photonic quantum circuit using the CNOT gate scheme are designed and simulated. Subsequently, the layouts for the chips incorporating the optimal parameters identified in simulations of all the components are designed.
In the past decades quantum optics has been at the forefront of quantum innovative technologies. For practical applications, scalable platforms for implementation of quantum optical circuits are vital. This thesis presents two new platforms for scalable implementation of quantum optical circuits, namely, modular approach and monolithic integration. Here, we take the first steps towards the integration of three main elements of every quantum optics circuits: Single-photon emitters, single-photon detectors, and quantum logics. Until now, most quantum optical circuits used separate platforms for single-photon generation and detection. The main challenge in the integration of these technologies, which have different requirements, has slowed down the research in the field. Here, we integrate sources and detectors by first fabricating the devices on their own platform and then transferring and combining them together. Plasma enhanced chemical vapor deposition of silicon nitride followed by etching optical waveguides connect these elements. Removing the Poissonian optical excitation field from the quantum circuit is necessary for integration. Classical optical excitation can be avoided if the sources are electrically pumped. However, fabrication of high-quality electrically pumped sources, suitable for integration, has been limited. The experiments described in chapter 4 are our first step towards addressing the mentioned problem. Defect-free nanowires are grown on <100> direction and their optoelectronic performance are characterized. Nanowire quantum dots, thanks to their waveguiding, purity, coherence and their potentials for deterministic integration with other optical circuits, are promising single-photon sources for on-chip quantum optics. However, precise control of the emission energy of the quantum dots by growth has not become possible yet. Chapter 5 describes a method for on-chip tuning of emission energy of nanowire quantum dots using strain fields. We show the emission energy of independent nanowire quantum dots can be brought into degeneracy without affecting their single-photon emission properties. The quantum optical components have to be routed and connected together to form functional circuits. On a chip, this is usually carried out using optical waveguides. Moreover, manipulation of single photons has to be done in a scalable fashion. Again optical waveguides and ring resonators are very good candidates for this task. Therefore, understanding the behavior of these circuits such as their losses, polarization dependence, and temperature behavior is important. The experiment described in chapter 6 studies the behavior of plasma enhanced silicon nitride waveguides in cryogenic temperatures. We concluded in this chapter that due to weak thermo-optic sensitivity of silicon nitride at cryogenic temperatures, the available thermal budget on the system should be carefully considered. An important step in achieving a scalable platform for quantum optical circuits is deterministic and efficient integration of single-photon sources. In chapter 7, we demonstrate successful integration of III-V nanowire quantum dots with silicon nitride waveguides. The nanowires are deterministically selected and transferred from the original growth chip to the new substrate where they are integrated with low-loss silicon nitride waveguides. Our measurements show that the integrated sources preserve their high quality emission properties. In chapter 8, we describe an alternative approach: amodular method for scalable quantum optics. The proposed technique is based on coupling the single-photon from sources into optical fibers where the photons can be processed and then fed into the single-photon detectors. This approach has high flexibility and is easier to implement but as described in the chapter, at the moment, losses in the interfaces between optical fibers and single-photon sources are a major limiting factor. We conclude the thesis with some possible future directions and exciting new results on integration of single-photon detectors with sources and waveguides. Finally, primary results on on-chip single-photon filtering and removal of the optical excitation field are demonstrated.