In recent years, many applications have been proposed that require detection of light signals in the near-infrared range with single-photon sensitivity and time resolution down to few hundreds of picoseconds. InGaAs/InP singlephoton avalanche diodes (SPADs) are a viable choice for these tasks thanks to their compactness and ease-of-use. Unfortunately, their performance is traditionally limited by high dark count rates (DCRs) and afterpulsing effects. However, a recent demonstration of negative feedback avalanche diodes (NFADs), operating in the free-running regime, achieved a DCR down to 1 cps at 10 % photon detection efficiency (PDE) at telecom wavelengths. Here we present our recent results on the characterization of NFAD detectors for temperatures down to approximately 150 K. A FPGA controlled test-bench facilitates the acquisition of all the parameters of interest like PDE, DCR, afterpulsing probability etc. We also demonstrate the performance of the detector in different applications: In particular, with low-temperature NFADs, we achieved high secret key rates with quantum key distribution over fiber links between 100-300 km. But low noise InGaAs/InP SPADs will certainly find applications in yet unexplored fields like photodynamic therapy, near infrared diffuse optical spectroscopy and many more. For example with a large area detector, we made time-resolved measurements of singlet-oxygen luminescence from a standard Rose Bengal dye in aqueous solution.
Single-photon detectors are the best option for applications where low noise measurements and/or high timing
resolution are required. At wavelengths between 900 nm and 1700 nm, however, low noise detectors have typically
been based on cryogenic superconducting technology, precluding their extended use in industrial or clinical
applications. Here we present a practical (i.e. compact, reliable and affordable) detector, based on a negative
feedback InGaAs/InP avalanche photodiode and exhibiting dark counts < 1 count-per-second at 10% efficiency, and
with efficiencies of up to 27%. We show how this detector enables novel applications such as singlet-oxygen
luminescence detection for Photo Dynamic Therapy (PDT) but can be an enabling technology also for a diverse set
of applications in both quantum communication (e.g. long-distance quantum key distribution) and biomedical
Free-running single photon detectors at telecom wavelengths are attractive for many tasks in quantum optics. However, until recently, the convenient and compact InGaAs/InP avalanche photodiodes did not operate with satisfactory performance in this regime due to high dark count rates and afterpulsing effects. Recent development of negative feedback avalanche diodes (NFADs) enabled very fast passive quenching of the avalanche current, effectively reducing the afterpulse probability and subsequently allowing free-running operation. Here, we present analysis of NFAD operation at low temperatures, down to 163 K, which reveals a significant reduction of the dark count rate. We succeeded in developing a compact single photon detection system with a dark count rate of ~1 cps at 10% detection efficiency. To ensure that the NFAD is in a well-defined initial condition during the characterization of the detection efficiency and afterpulsing, we use a recently developed FPGA based test procedure suitable for free-running detectors. To demonstrate the performance of the detector in a real-world application we integrate it into a 1.25 GHz clocked quantum key distribution system. An optimization of the detector temperature allowed secret key distribution in the presence of more than 30 dB of loss in the quantum channel.
We present the results of a Swiss project dedicated to the development of high speed quantum key distribution and data encryption. The QKD engine features fully automated key exchange, hardware key distillation based on finite key security analysis, efficient authentication and wavelength division multiplexing of the quantum and the classical channel and one-time pas encryption. The encryption device allows authenticated symmetric key encryption (e.g AES) at rates of up to 100 Gb/s. A new quantum key can uploaded up to 1000 times second from the QKD engine.