Description of the PhD project
Superconducting Single Photon Detector (SSPD) is a key technology for quantum communication or space-to-ground communications. They consist of superconducting nanowires biased near their critical current. A (visible or near IR) photon absorption creates a hotspot that is converted into a measurable voltage pulse. Although the microscopic picture of the particle-to-signal conversion is far from being understood, low-Tc SSPD are the best single photon detectors commercially available, in terms of quantum efficiency, jitter and dark counts. The main problem which prevents this technology to spread-out is the energy cost and complexity of 4K cryogenics. Making SSPD with High Temperature Superconducting materials (HTS SSPD) would allow working with currently available simplified energy efficient cryo-coolers working in the 60-80 K range. The aim of this PhD project is to fabricate YBCO based SSPD and to investigate, by mean of global and local (STM/AFM) measurements, the microscopic mechanism leading to hotspot formation, to optimize them. On this route, we will study NbN-based SSPDs as well, whose elaboration and characterization is fully controlled. We recently fabricated YBCO nano-meanders showing the highest aspect ratios ever reported so far with preserved superconducting properties, and measured highly hysteretic I-V characteristics that are mandatory to reach single photon detection. We will reduce further the nanowire cross-section to approach the standard parameters for low-Tc SSPDs (4nm-thick, 100nm wide, 100µm long4), through the fruitful collaboration with J. Briatico at UMR Thales-CNRS. The goal is to show the photo-response of YBCO nanowires at a photon level in the visible and near IR.
The core of the project is to address specific issues to improve SSPD performances: How the presence of strong supercurrents modifies the superconducting properties of the wire? Are there “preferential locations” where absorption takes place? How the film structure, inhomogeneities, wire edges, bends affect… the detector efficiency? Are there vortices, and do they influence the detection process? Thanks to the unique UHV low-temperature Scanning Tunneling Microscopy/Atomic Force Microscope (STM/AFM) equipment installed at ESPCI Paris, we will study conventional low-Tc SSPD and HTS nano-meanders under biased current, to answer these questions, combining global transport measurements and local electronic properties such as superconducting current distribution and local density of states. We will study the role of local topography and inhomogeneities on the detector efficiency and dark count rate. Using magnetic field, the presence and the distribution of vortices under current biasing will be studied. The tip will be used as pulse current source to inject high-energy quasi-particles to trigger the detection event.
Description of the research Unit/subunit
UMR7636 Physics & Materials
The project will take place in the LPEM (UMR8213) in both QuantumSpec (D. Roditchev, S. Pons, S. Vlaic) and Phasme Group (C. Feuillet-Palma, J. Lesueur and N. Bergeal) .
The QuantumSpec group is headed by D. Roditchev, who recently moved from INSP to the LPEM in ESPCI-Paris, an assistant professor S. Vlaic and a CNRS researcher S. Pons.
D. Roditchev is internationally recognized for his expertise in building and operating low temperature STM and has built the 300mK apparatus used in INSP. He is a worldwide expert recognized internationally in superconductivity.
The Phasme group has been studying superconductors at a meso and nanoscale for many years, from High-Tc cuprates to 2-DEG at oxide interfaces, from basic research to applications. The group made essential contributions to understand the fundamental properties of superconducting cuprates, and at the same time, developed a new technology to make HTc nanowires and Josephson junctions which are used in various functional circuits (SQUID, SQUIFS, THz mixers and single photon detectors).
The complementarity of the teams brings the full span of skills necessary for the successful completion of the project.