Abstract
Single photons are ideal for carrying quantum information in the form of qubits because they exhibit very low decoherence and are therefore well-suited for quantum communications. For this same reason, they constitute a promising path for quantum computing exploiting the MBQC (Measurement-Based Quantum Computing) paradigm. The miniaturization of functions for generating, encoding, manipulating, and detecting single photons is essential for the future industrialization of quantum applications. Integrated photonics, combined with standard CMOS technologies, offers a privileged path.
Discrete Superconducting Nanowire Single-Photon Detectors (SNSPDs) exhibit near-100% efficiency and good temporal performance, but their on-chip integration remains a challenge. The object of this thesis is the development of SNSPDs monolithically integrated by evanescent coupling on Si or SiN waveguides on 200 mm diameter silicon substrates in order to eliminate coupling losses with other components. Si waveguides are well-suited for telecom wavelengths used for quantum communications, while SiN waveguides allow for an extended range of wavelengths, notably at 925 nm, which is the emission wavelength of single-photon sources with III-V quantum dots, currently the most efficient for photonic quantum computing. SiN waveguides are also interesting at 1550 nm because they exhibit very low propagation losses. Four nanowires configurations of different lengths were designed at 1550 and 925 nm, from the simplest, where the nanowire is in a straight line centered above the waveguide, to the most compact and least sensitive geometries to lithographic misalignment in U, S, and W shapes.
The manufacturing technology was developed using a fully CMOS-compatible process on 200 mm diameter substrates, including the planarization of Si or SiN waveguides, the deposition of a high-quality thin layer of NbN, the structuring of 100 nm wide nanowires, and their metallization. The crystalline quality of the superconducting NbN material used directly influences its critical temperature (Tc). Continuing previous studies on the optimization of ultrathin NbN superconducting films deposited on crystalline silicon, we demonstrate that the introduction of a 10 nm AlN buffer layer can also increase the critical temperature of NbN by more than 2 K on an amorphous SiN layer. In both cases, the AlN is sufficiently textured to also orient the NbN layer through an epitaxial relationship between AlN and NbN. This slight increase in Tc is important because it ensures the operation of the detectors in compact closed-cycle cryostats at a temperature of 2.3 K. The room temperature resistivity measurements of the nanowires show excellent manufacturing uniformity of over 98%.
A robust optical and electrical packaging technique has been developed to test the detectors made at 2.3K. These exhibit efficiencies between 70 and 80% at 925 and 1550 nm, with limited noise between 10 to 100 Hz, recovery times not exceeding 5 ns, maximum counting rates of over 100 MHz, and timing jitters of less than 100 ps. These very promising results pave the way for the integration of SNSDPs into reception circuits for quantum communication protocols as well as into photonic quantum computing circuits.