Superconducting Single Photon Detector (SSPD) is a key technology for quantum communication or space-to-ground communications1. 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 pulse2. 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 properties3, 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-CNRS5. 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, inhomogeneities3, 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 states6. 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. [1] Nature Photon. (2009) [2] SUST (2012) [3]. APL (1999) [4] SUST (2018) [5] Begon-Lours, thesis (2017) [6] Jalabert et al Private Comm.
Keywords
Scanning Tunneling Miscroscope, Superconducting Single Photon detector, Local electronic properties, superconducting to insulating transition
Research unit
UMR7636 Physics & Materials
Description of the research Unit/subunit
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.
Name of the supervisor
Dimitri Roditchev (Dimitri.Roditchev@espci.fr)
Name of the co-supervisor
Cheryl Feuillet-Palma (cheryl.palma@espci.fr)
3i Aspects of the proposal
Intersectoriality
This project aims at studying the fundamental issues related to the study of the parameters affecting the global and local phase coherence of disordered superconducting thin films and nanowires to give clues to the microscopic mechanism below the formation of the hotspot.
Moreover it has a strong industrial impact, indeed SSPDs overcome in many ways their semiconductors based counterparts and are commercially available. Some tremendous advantages have been reported recently such as high quantum efficiency ( 93% at l 1.54μm ), high operating frequency ( 1 GHz), low intrinsic dark count rate ( 0.1cps ), low jitter ( 18ns full width at half maximum) and broad spectral range (from visible to mid-infrared). Being able to identify the microscopic mechanism responsible for the detection of the photon, will allow improving this available low Tc technology and give better understanding of superconducting properties in High Tc nanowire to make the first high Tc SSPDs working upon 40K
Then this project is at the very early stage of thinking regarding potential applications for society and will not for instance drive the development of a new industry in France in near future.
Interdisciplinarity
The project will involve several foreign collaborators mostly in European institutions. On one hand, the QuantumSpec group has a long-standing collaboration with IMS-KIT (Institute of Micro- and Nanoelectronic Systems, Karlsruhe Institute of Technology) in particular they supply high quality NbN ultra-thin films. This collaboration has been strengthened by the ANR SUPERTRIPES grants fot both collaborators. On the other hand the PHASME group has established a long-standing collaboration with a group of theoreticians at La Sapienza University in Roma (Italy). Around ten co-authored scientific articles have been produced over the past years through this fruitful collaboration.
International mobility
The project will involve several foreign collaborators mostly in European institutions. On one hand, the QuantumSpec group has a long-standing collaboration with IMS-KIT (Institute of Micro- and Nanoelectronic Systems, Karlsruhe Institute of Technology) in particular they supply high quality NbN ultra-thin films. This collaboration has been strengthened by the ANR SUPERTRIPES grants for both collaborators. On the other hand the PHASME group has established a long-standing collaboration with a group of theoreticians at La Sapienza University in Roma (Italy). Around ten co-authored scientific articles have been produced over the past years through this fruitful collaboration.
Expected Profile of the candidate
We are looking for motivated applicants with an excellent academic background in particular in quantum physics and solid state physics. The Phd applicant should be motivated by experimental physic and able to work independently. He/She should have good communication skills and be proficient in spoken and written English.
Although not mandatory, previous experiences in the following fields will be positively considered :
- scanning tunneling microscope
- atomic force microscope
- ultra-high vacuum
- electronic transport measurements
- cryogenic systems operation
- micro and nanofabrication
Important dates
Call for applications : from February 1st to March 31st 2019
Eligibility check results : Mid April
3i Committee evaluation results : Mid May
Interviews from the shortlisted candidates with the Selection Committee : Late June-Early July
Final results : Mid July