The use of quantum technologies has risen drastically in the last decades, offering an enhancement in terms of the technological capabilities available nowadays. Found in this field of research, quantum communications based on the transmission of single photons offer new approaches to redefine the use of protocols and devices for specialised applications. As a final result, this leads to the accomplishment of long-range quantum networks with terrestrial and satellite-based communications set to establish a new scale of Telecommunication applications. Within these kinds of applications, this project focuses on the assembly of a single-photon detector using a photodiode of type Single-Photon Avalanche Diode (SPAD), essential device to define the level of performance of any quantum communication system. To carry this out, the system is assembled around its centre piece, an Excelitas APD operated in Geiger mode above its breakdown voltage. The electronic circuit that drives the photodiode is characterised to work with its adjusted bias voltage, its dedicated temperature management system and its active quenching circuit to detect pulses with certainty and precision. The final version of the project is adjusted to work in the 1300nm and 1500nm operating wavelengths as the photodiode used is formed by an InGaAs/InP semiconductor structure. Working on top of the original circuit provided initially, the experimental measurements obtained in the course of the project prove that the design modifications implemented have improved the functioning of the whole system. The main achievements accomplished encompass the generation of pulses for photon detection at 20kHz, the reduction of electromagnetic interferences by a factor 40, the enhancement of the manipulated signals in the circuit by a factor of tens, and the correct management of the SPAD temperature to maintain a fixed value of dark counts for a long time.
With the growing use of online communications in our modern society, information security is becoming a big concern. Along with that, the progress in quantum computers is posing severe threats to such communications. Once powerful quantum computers are available, most of today's encryption schemes, which are based on computationally hard problems, will be broken within a short period. Researchers are therefore making a great effort to establish quantum-safe encryption schemes. One such scheme is quantum key distribution (QKD), which utilizes the laws of quantum mechanics. These cryptography protocols offer unconditional security to the communication between two distant parties by providing a secure way of sharing encryption keys between them. While over the last few decades QKD has continuously progressed, it is limited to a distance of up to several hundred kilometers using terrestrial quantum links. Satellites are therefore being considered to extend the QKD range for global coverage, although implementations of the satellite-based QKD infrastructure are still in their early stage. There are many aspects of QKD that need further assessment and advancement for establishing long-term satellite-based quantum communication (QC). My thesis works were focused on developing advanced systems for single-photon detectors and quantum sources. Single-photon avalanche diodes (SPADs) are the most viable option for satellite-based quantum communications. They must travel to outer space either for receiving quantum-states in the ground-to-satellite QC or for characterizing the quantum-sources in the satellite-to-ground QC. However, while in space, SPADs exhibit damage caused by the space radiation that gradually increases their dark counts. Performing QKD is not possible when the dark counts exceed a specific threshold. Hence, methods of reducing the detectors' dark counts by mitigating the damages would help to extend the SPADs' in-space useful lifetime. Laser annealing is one such effective method, found in lab experiments, to heal the radiation-induced detector damages. We now aim to carry out this method in low Earth orbit (LEO) to verify its in-orbit effectiveness. On that goal, we are building an annealing payload (APL) for a cube satellite (CubeSat) in collaboration with the University of Illinois at Urbana Champaign (UIUC). We, the University of Waterloo team, built one of the two segments of the APL-- a space-qualified detector module containing two Excelitas C30902SH and Excelitas SLiK detectors. Our miniaturized and compact module integrates the facilities required for the detector operation and laser annealing, as well as an active detector temperature control system. The operation of the detector module is highly flexible and software controllable. Our detector module will work together with the control board containing the laser annealing system (built by the UIUC team). Once the satellite has been launched in 2020, the in-orbit experiment will enable us to study the in-space SPAD radiation damage and their healing using the integrated annealing system. During a second project, we designed and built a new simple readout circuit for the negative feedback avalanche diodes (NFADs), which are free-running single-photon detectors at telecom wavelengths. These detectors suffer from strong afterpulsing effects, which limits their overall performances. Therefore, our readout system incorporates features to suppress NFAD afterpulses. We also used this custom readout to characterize two NFADs (from Princeton Lightwave) and assessed the performance of the new electronics. Our analysis showed that even at higher detection efficiencies, a 20
mus hold-off time after each avalanche event is enough to extensively reduce the number of afterpulses and to keep the dark count rate below 100 Hz at 192 K temperature. Both the detectors showed timing jitter of less than 75 ps FWHM at their maximum efficiencies. The best figure of merit is found to be 1.6
times107, which is comparable to that of the high-performing superconducting nanowire single-photon detectors. This result demonstrates the suitability of our readout and the NFADs in various quantum optics applications, such as in long-distance quantum key distribution, where the detection rate is usually low. We then performed a blinding attack, which enables an Eavesdropper in QKD to gain information on the key, on these NFADs using bright illumination. These detectors are usually threshold detectors that generate a click when the optical power is above a certain threshold, otherwise they do not click. Blinding attack utilizes detectors' inability to resolve the photon numbers. During the experiment, we sent controlled optical pulses with a high time resolution to deterministically force detection at the detectors. The result demonstrated the NFADs' susceptibility to these attacks, which tells us to include countermeasures into the system to protect the communications. Finally, we built a quantum source to produce 785 nm polarized photons to implement decoy-state BB84 QKD. Our source utilized the sum-frequency generation scheme to generate 785 nm laser pulses. Its modulator system includes an intensity modulator and two-phase modulators in the Mach-Zehnder configuration to prepare polarized quantum states with different intensities. Our source provides a repetition rate of 500 MHz, which was successfully used in an airborne QKD demonstration with a moving receiver up to
10 km distance. To summarize, my research projects are a contribution to the development of advanced devices, particularly single-photon detectors for quantum communications.
The implementation and commercialization of quantum cryptography technologies have to face some challenges related to the development of single-photon detectors operating at 1550 nm. The main requirements are: i) high detection efficiency; ii) low noise; iii) high count rate; iv) low timing jitter. Different technologies are currently available for single-photon detection at 1550 nm, but semiconductor devices (like Single-Photon Avalanche Diode, SPAD) offer a photon detection efficiency that is inherently higher than any photocathode employed in vacuum tube detectors. Additionally InGaAs/InP SPADs can detect single photons at 1550 nm with low noise when moderately cooled by means of thermo-electric coolers. Consequently, InGaAs/InP SPAD can be the enabling technology of practical quantum key distribution (QKD) systems, provided that the maximum count rate is increased above 1 Mcps. The main limit is the afterpulsing effect that usually sets too long (< 10 μs) a hold-off time after each avalanche ignition. We present the developments achieved in InGaAs/InP SPAD device design, fabrication technology and front-end electronics, aimed at decreasing the afterpulsing effect, while not impairing photon detection efficiency and timing jitter. The new InGaAs/InP SPADs provide count rates higher than 1 Mcps and temporal response with 60 ps Full-Width at Half Maximum and very short (30 ps time constant) tail. The front-end electronics includes a wide-band pulse generator able to gate the SPAD up to 133 MHz repetition rate. Eventually, a fast avalanche-quenching scheme minimizes quenching time to less than 1 ns, thus effectively reducing afterpulsing by decreasing the total charge flowing through the junction.
Single-photon detectors (SPDs) are the most sensitive instruments for light detection. In the near-infrared range, SPDs based on III–V compound semiconductor avalanche photodiodes have been extensively used during the past two decades for diverse applications due to their advantages in practicality including small size, low cost and easy operation. In the past decade, the rapid developments and increasing demands in quantum information science have served as key drivers to improve the device performance of single-photon avalanche diodes and to invent new avalanche quenching techniques. This Review aims to introduce the technology advances of InGaAs/InP single-photon detector systems in the telecom wavelengths and the relevant quantum communication applications, and particularly to highlight recent emerging techniques such as high-frequency gating at GHz rates and free-running operation using negative-feedback avalanche diodes. Future perspectives of both the devices and quenching techniques are summarized. Recent progress in single-photon detectors for quantum communication based on III–V compound semiconductor avalanche photodiodes is reviewed. Specifically, Jun Zhang and Jian-Wei Pan at the University of Science and Technology of China and their colleagues in the USA and Switzerland introduce technological advances for InGaAs/InP single-photon detector systems in the telecommunication band along with their associated applications in quantum communication. III–V single-photon avalanche diodes are the most practical tools available for detecting ultraweak near-infrared light. The scientists overview important parameters for evaluating the performance of detector systems based on single-photon avalanche diodes and describe the experimental characterization of these parameters. They also consider emerging techniques, including high-frequency gating at gigahertz rates and free-running operation using negative-feedback avalanche diodes. Finally, the future prospects of these devices are considered.
In order to improve the sensitivity of single-photon detection systems,InGaAs/InP avalanche photodiode(InGaAs/InP APD) is used in the design of single photon detector of quantum communication.Single photon detector bias generation circuit in gate pulse mode,single-photon signal amplification circuit,single photon signal detection circuit and temperature control module were realized.By selecting high-precision OP37 preamplifier and precision comparator AD8561,quantum efficiency was increased to 18.3%,dark count was controlled within 4.1 × 10-6/ns.
In our experiment, two InGaAs/InP avalanche photodiodes (APD) were used in the quantum key distribution (QKD) system for single-photon detection at telecom communication wavelength (1310/1550 nm). By using properly designed passive-quenching circuit, the photo-detection efficiency of the APD could reach 10% at 1550 nm. In order to decrease the dark count rate efficiently, we designed a coincident counting circuits for single-photon detection. Before the APD signals were sent to the signal processing circuit, the avalanche signals were amplified and shaped into the TTL pulses. In order to obtain an accurate and efficient data acquisition and to transfer synchronically several signals such as the TTL signals and pulses from Random generators, we designed a signal processing circuit with the parallel port whose speed limitation is about 100Kb/s. In our experiments of single-photon interference and quantum cryptography, the electronic designs presented us satisfactory performance and results. To achieve much higher speed for the data acquisition, we also employed the Universal Serial Bus System (USB2.0) to interface our single-photon avalanche detectors and random generators used in our QKD system, which was believed to improve the performance of QKD such as the key distribution speed.
The application of quantum key distribution (QKD) has raised particular demands for single-photon detectors. One of the most promising candidates at the low-loss optical fibre communications windows is the planar geometry InGaAs/InP single-photon avalanche diode. These detectors have been modelled, fabricated and characterised at 1.55 µm wavelength. Their performance in terms of single-photon detection efficiency, dark count rate, timing jitter and afterpulsing behaviour are reported and compared with the best commercially available, linear multiplication avalanche photodiodes operated in Geiger-mode. Their use in the application of QKD is discussed.