Single photon detector(SPD) problems arise in most quantum tasks, especially for measuring states going through high-lost channels. They are particularly prominent in quantum key distribution(QKD), which could be the most significant application in quantum information theory. In recent years, QKD distance has been improved dramatically but is still restricted because the bit error rate(QBER) caused by SPD dark counts will be out of control as the distance increases. If this problem can be solved, QKD can be implemented over arbitrarily long distances. However, previous solutions often result in impractical requirements such as superconductors while they can only reduce the dark count rate to finite low levels. In this paper, we solve SPD problems with today's technologies only. Although it is the no-cloning theorem that prevents a state from being measured multiple times to obtain a more reliable result, we propose a scheme circumventing the no-cloning theorem in certain tasks to allow a single state to be employed several times. The scheme demonstrates that imperfect detectors can provide nearly perfect results, namely, the QBER caused by dark counts can be reduced to arbitrarily low while in the meantime, detective efficiency can be improved to arbitrarily high. Consequently, QKD distance is not limited by the imperfect SPD anymore and can be improved from hundreds of kilometers to thousands without high-technology detectors. Furthermore, similar schemes can be applied for reducing measurement errors or improving the performance of sources. Finally, it is worth noting that although the paper is mainly discussed in the context of QKD, our scheme is an independent scheme that could be employed in other protocols wherever SPD are employed.
Single photon detector(SPD) problems arise in most quantum tasks, especially for measuring states going through high-lost channels. They are particularly prominent in quantum key distribution(QKD), which could be the most significant application in quantum information theory. In recent years, QKD distance has been improved dramatically but is still restricted because the bit error rate(QBER) caused by SPD dark counts will be out of control as the distance increases. If this problem can be solved, QKD can be implemented over arbitrarily long distances. However, previous solutions often result in impractical requirements such as superconductors while they can only reduce the dark count rate to finite low levels. In this paper, we solve SPD problems with today's technologies only. Although it is the no-cloning theorem that prevents a state from being measured multiple times to obtain a more reliable result, we propose a scheme circumventing the no-cloning theorem in certain tasks to allow a single state to be employed several times. The scheme demonstrates that imperfect detectors can provide nearly perfect results, namely, the QBER caused by dark counts can be reduced to arbitrarily low while in the meantime, detective efficiency can be improved to arbitrarily high. Consequently, QKD distance is not limited by the imperfect SPD anymore and can be improved from hundreds of kilometers to thousands without high-technology detectors. Furthermore, similar schemes can be applied for reducing measurement errors or improving the performance of sources. Finally, it is worth noting that although the paper is mainly discussed in the context of QKD, our scheme is an independent scheme that could be employed in other protocols wherever SPD are employed.
Discrete-variable quantum key distribution (QKD) requires the use of highly sensitive single-photon detectors (SPD) to detect decoded quantum states. These can form a significant portion of the cost and the physical layer vulnerabilities in a QKD system. Previously, a single-detector architecture which reduced the cost of simple, polarization-encoded fiber QKD systems was proposed, which decoded polarization states into varying fiber delays before reaching a single SPD. We describe the parameter space available for a single detector receiver and, using simulation, optimise performance for a given example. It is demonstrated that the order of state delays can be changed to reduce quantum bit error rate (QBER), and that delay change between states need only exceed some small minimum value in the ns range, which saves on fiber length, weight and cost, as well as loss. We also explore time gating and efficient (asymmetric basis selection) BB84 to reduce QBER. This work improves the understanding and performance of single-SPD-based QKD receivers, allowing simpler and more robust receivers with fewer physical loopholes and a lower cost, reducing the barrier to entry of fiber QKD.
Discrete-variable quantum key distribution (QKD) requires the use of highly sensitive single-photon detectors (SPD) to detect decoded quantum states. These can form a significant portion of the cost and the physical layer vulnerabilities in a QKD system. Previously, a single-detector architecture which reduced the cost of simple, polarization-encoded fiber QKD systems was proposed, which decoded polarization states into varying fiber delays before reaching a single SPD. We describe the parameter space available for a single detector receiver and, using simulation, optimise performance for a given example. It is demonstrated that the order of state delays can be changed to reduce quantum bit error rate (QBER), and that delay change between states need only exceed some small minimum value in the ns range, which saves on fiber length, weight and cost, as well as loss. We also explore time gating and efficient (asymmetric basis selection) BB84 to reduce QBER. This work improves the understanding and performance of single-SPD-based QKD receivers, allowing simpler and more robust receivers with fewer physical loopholes and a lower cost, reducing the barrier to entry of fiber QKD.
Discrete-variable quantum key distribution (QKD) requires the use of highly sensitive single-photon detectors (SPD) to detect decoded quantum states. These can form a significant portion of the cost and the physical layer vulnerabilities in a QKD system. Previously, a single-detector architecture which reduced the cost of simple, polarization-encoded fiber QKD systems was proposed, which decoded polarization states into varying fiber delays before reaching a single SPD. We describe the parameter space available for a single detector receiver and, using simulation, optimise performance for a given example. It is demonstrated that the order of state delays can be changed to reduce quantum bit error rate (QBER), and that delay change between states need only exceed some small minimum value in the ns range, which saves on fiber length, weight and cost, as well as loss. We also explore time gating and efficient (asymmetric basis selection) BB84 to reduce QBER. This work improves the understanding and performance of single-SPD-based QKD receivers, allowing simpler and more robust receivers with fewer physical loopholes and a lower cost, reducing the barrier to entry of fiber QKD.
Quantum communications and quantum cryptography are developing rapidly during the last decades caused partly by a fast progress in quantum computing. Quantum cryptography provides an unconditionally secure way of communicating whereas traditional classical cryptographic protocols are likely to be broken by super powerful quantum computers. In the past few years distances covered by quantum communications have increased by an order of magnitude. To provide a global coverage for the quantum networks, a satellite based quantum communications is the most promising solution. \n \nAs an emerging field, QKD systems are still under evolution process. Despite outstanding security proven theoretically, it has loopholes caused by their implementations. To test QKD, find the possible loopholes and suggest ways to fix them, is a job of many scientific groups. \nIn this thesis I start with presenting my work for a securing test of a commercial QKD system Clavis2. A Trojan-horse attack on Bob's apparatus was prepared by testing reflections and transmissions of all optical components in Bob's scheme. The attack was implemented and found to be unsuccessful at the tested wavelengths due to afterpulsing effect in Bob's single-photon detectors reacting to the bright light attack pulses. \n \nThree chapters of the thesis are dedicated to custom built single-photon detectors (SPDs) based on commercial Silicon avalanche photodiodes (APDs). Those detectors demonstrate parameters that altogether are not possible to find in commercially available SPDs, especially if combined with a very compact size. One of the in-lab-built SPDs was implemented in 143 km teleportation experiment, where a low dark count rate was crucial for the success of the experiment. The next generation SPD is already built, characterized and ready to be implemented. \n \nAnother 4-channel SPD was built as a prototype for a quantum satellite SPD. It has light weight, low electrical power consumption, low dark count rate and decent other parameters. It was used in the airborne demonstration of QKD receiver payload experiment, when a secret key was successfully generated between a moving aircraft and a ground station. \n \nSPDs installed on a satellite have to be able to work in the harsh space environment during a mission life time. Space radiation dramatically increases dark count rate of APDs. The last project presented in the thesis committed to a radiation test of three types of APDs and one type of photo multiplier tube. The experiment included characterization of all SPDs before and after irradiation by four levels of proton radiation, equivalent to 3 months - 2 years duration in a 600 km low Earth orbit. Three methods for mitigating radiation damage were tested and found to be successful with perspective to use some of them on a quantum satellite to extent life time of SPDs. \n \nTo summarize, this work makes a contribution to the development of SPDs for global quantum communications.
Quantum key distribution represents one of the most mature and well-studied advances in quantum technologies. Many protocols have been implemented, breaking record distances both employing satellites [1] and fiber-based systems [2]. However, there still are open challenges in achieving long-distance links without implementing quantum repeaters. One of the main limiting factors comes from the non-ideality of detection systems, in particular from intrinsic dark counts. As the covered distances increase, the signal decreases in intensity and the intrinsic noise in the detector prevails, hindering the extraction of a secret key. In this work, we propose the use of high-performance state-of-the-art superconducting nanowire single-photon detectors (SNSPD) showing ultra-low dark count rates to reduce the quantum bit error rate (QBER) in the detection system. Overall we achieved a link distance of 55 dB equivalent to 275 km in ultra low-loss fiber.
A measurement-device-independent quantum key distribution (MDI-QKD) protocol is immune to all detection side-channel attacks and guarantees the information-theoretical security even with uncharacterized single photon detectors. A weak coherent source is used in the current MDI-QKD experiments, it inevitably contains a certain percentage of vacuum and multi-photon pulses. The security issues introduced by these source imperfections can be avoided by applying the decoy state method. Here, through modeling experimental devices, and taking into account the weak coherent source and the threshold detectors, we have evaluated the gain, the probability to get successful Bell measurement and incorrect Bell measurement, and the quantum bit error rate (QBER), given a practical setup. In our simulation, we show how QBER varies with different transmission distances in the cases when the average photon numbers per pulse from Alice and Bob are symmetric and asymmetric. Result shows that the multi-photon pulses do not cause error in the Z basis of polarization encoding scheme, but produce a large QBER in phase encoding scheme and in the X basis of polarization encoding scheme. QBER is affected by the dark count rate and the system optical error associated with the multi-photon pulses. For different encoding schemes, QBER caused by each kind of average photon numbers from Alice and Bob increases to different degrees with the transmission distance, and finally is close to 50%. With the increase of the transmission distance, the average photon number per pulse decreases and the fraction of the dark count rate causing QBER gradually increases. Under the same effect of the dark count rate, the smaller the average photon number per pulse, the bigger the QBER. After a certain transmission and at the same transmission distance, the QBER is largest when average photon numbers used by Alice and Bob are both smallest. For the short distance transmission of phase encoding scheme and the X basis, we find that QBER is larger when average photon numbers from the two arms are asymmetric, as compared to the symmetric case. For the Z basis, the QBER caused by the system optical error and the dark count rate is very small.