Instead, free-running is exploited if SPADs need to detect photons in every instants. In gated-mode, the SPADs are sensitive to photons just in a defined nanoseconds time interval, synchronous with an external gate signal. The designed VLQC can operate in both gated-mode and free-running. The SPAD is driven by its anode and thick-oxide transistors are used to help the quenching action by raising this node from 0V to 5V. The SPAD breakdown voltage is about 25 V: 30 V are applied to its cathode so to operate at 5 V of excess bias. Such quenching circuitry is needed to promptly quench the avalanche right after a photon detection. Four different Variable Load Quenching Circuits (VLQC) drive the four SPADs independently and provide a digital pulse corresponding to the detected photon. This choice makes the array compatible with different fiber diameters. The pixel architecture is composed by four SPADs with different diameters: 5 μm, 10 μm, 20 μm, and 50 μm. The most critical step is the assembly between optical and electronic parts, requiring perfect matching between waveguides and SPADs. The latter, in particular, relies on a single photon source, an integrated optical circuit able to randomly route the single photon through multiple paths and, at the end of each path, a single photon detector that provide a digital signal if the photon gets detected. Single photons can be exploited in many ways for random number generation: e.g., by the number of counted photons, by inter-arrival time measurements, or by the different possible optical paths the photon propagated through. A reliable random generation of the basis needs to be provided, and Quantum Random Number Generators (QRNGs) are generally used for this task.
Quantum Key Distribution (QK) is one interesting application that ensure high security by trusting single photon quantum states the communication is encrypted using keys, distributed in random basis.
Quantum physics starts to become one of the most reliable way in sensitive data distribution. Multi Hit Mode, used to identify a coincidence of a certain number of photons(more than one, two, three or four) detected within a specified time window, thanks to a multi threshold coincidence detection logic. Two operation modes are implemented: Single-Hit Mode, needed to reveal the (5-bit) position of the pixel triggered by a single photon, representing a random number, in a time window synchronous with the laser emission. The linear array architecture consists of 32 pixels, pitched at 125 μm, each made by 4 SPADs with different diameter (5 μm, 10 μm, 20 μm, 50 μm). Such detector is a 32×1 linear array based on Single-Photon Avalanche-Diode (SPAD) detectors for the generation of a raw random number, by revealing the position on the array of the single photon impinging on it, realized in a BCD 0.16 μm technology. The Integrated Circuit (IC) designed contributes in the QRNG block of the system, tailored to communicate with the integrated non-linear optics circuit. This work takes part of the European project UNIQORN (Affordable Quantum Communication for Everyone: Revolutionizing the Ecosystem from Fabrication to Application) whose aim is to develop a Quantum System on Chip (QSoC) for telecom application. Quantum communication is a fast-growing field that takes advantage of the quantum physics laws to protect and secure sensitive data.
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