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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - SuSiPOD (Broadband Superconducting Nanowire Single Photon Detectors)

Teaser

Summary

Single-photon detection is an emerging technology, with applications ranging from medical imaging and LIDAR systems to space communication and fundamental quantum optics. Moreover, single-photon detectors are considered an enabling technology for the development of quantum information science, and the quantum computer.

Currently, single-photon detection is carried out using semiconductor-based avalanche photodiodes; however, this technology is limited by large timing jitter, unavoidable dark counts, afterpulsing, and limited detection efficiency. A recently proposed alternative relies on a superconducting nanowire biased just below its critical current, so that an impinging photon triggers a transition from the superconducting to the normal state, resulting in a voltage spike at the nanowire leads. Superconductivity is then recovered within a few nanoseconds.

The detection efficiency can be boosted close to 100% by coupling the superconducting nanowire to the evanescent field propagating in a waveguide. However, the fabrication of high quality, ultrathin superconducting layers is challenging (e.g., the critical temperature of superconducting NbN thin films on silicon is typically about 10 K, compared to 16 K for bulk NbN), and the operation wavelength of such devices is limited by the waveguide band gap.

We have identified GaN/AlN as the best suited waveguide material system, approximately lattice matched with NbN, and with a transparent band from 400 to 6000 nm. The target of the SuSiPOD project is the establishment of a technology platform for the fabrication of a new generation of broadband superconducting nanowire single-photon detectors built on III-nitride waveguides, in which photons are coupled laterally with the help of a tapered optical fibre. This new geometry should allow near-unity absorption probability in a wide spectral range, since the substrate is transparent to visible and infrared light. The project success will be proven by the realization of a working prototype which will greatly outperform state-of-the-art single-photon detectors.

Work performed

The first months of the project have been dedicated to the design and dimensioning of the detectors. Extensive simulations have been performed, and the performance achievable with the proposed approach has been quantified. The simulation results confirm that a waveguide-coupled device would allow highly efficient broadband photon detection, as predicted. In addition, the detector efficiency would be almost independent on the photon polarization, which is currently a major problem of normal-incidence detectors.

Furthermore, some innovative concepts to improve normal-incidence detectors have been tested by simulation, with very promising results. Relying on these calculation, the Partner Organization â€œSingle Quantum BVâ€ could demonstrate a device having a detection efficiency of 93%, which outperforms the current state of the art for NbTiN-based SNSPDs.

During four stays at the Partner Organization, the Marie Sklodowsca-Curie Fellow has learned much of the fabrication and characterization techniques of SNSPDs, and has transferred them to the Host Organization CEA Grenoble. Additional travel funding has been obtained by the Fellow through a French-Dutch grant (Partenariat Hubert Curien â€“ Van Gogh), to enhance two-way exchanges between Grenoble and Delft.

Concerning the experimental work, GaN/AlN waveguides have been fabricated on sapphire, and NbN films have been deposited and characterized by different means (AFM, XRR, XRD, EPMA, XPS). The technology for the fabrication of normal-incidence SNSPDs has been succesfully transferred from the Partner to the Host organization by the Fellow, and working devices have been demonstrated. A characterization setup for device characterization in liquid helium has been developed and installed. In spite of the sub-optimal characterization temperature (4.2 K, while state-of-the-art results are obtained 2.5K or below) and the absence of a cavity (which limits the achievable efficiency to about 40%) the realized devices have efficiencies in excess of 20%, a very promising result. The developement of a cavity structure to approach and go beyond the state of the art is being developed.

Meanwhile, cooperations with different scientific organizations are being developed, in order to fabricate SSPDs on innovative supercounducting films. One notable example is the fabrication of SSPDs on CVD-deposited NbTiN, which would be a first worldwide demonstration.

Final results

The scientific results have been presented in three international workshops, including two invited oral presentations (PICQUE summer school in Rome, Italy; Single Photon Workshop in Geneva, Switzerland; Heimbach international Workshop in Heldrungen, Germany (invited) ; Photonics West in San Francisco, CA, USA (invited) ). The Fellow has also given four seminars to promote the divulgation of the scientific results (twice at the CEA Grenoble, at the TU Delft, and at the CNRS â€“ institute NÃ©el).

Two peer-review articles have been published in an international journal (Web Of Science Impact Factor 2.7), with the Fellow as first author.

Some of the results obtained so far appear to be ripe for valorization on the market: filing of a patent is under consideration. Furthermore, the Fellow has taken part in a three-day training entitled â€œvalorisation de lâ€™innovation dans lâ€™entrepriseâ€ (French for: â€œvalorization of innovation in the private sectorâ€), in order to acquire the necessary competences to connect public research with the private sector. Considering the structure and goals of SuSiPOD, this kind of knowledge could be a valuable asset to enhance the societal and economic impact of this project.