On the pathway to quantum key distribution on a global scale, will be the realization of metropolitan-sized Memory Assisted Measurement-Device-Independent Quantum Key Distribution (MA-MDI-QKD) systems. Here, we present a simplistic and intuitive stochastic model to predict key distribution rates in a MA-MDI-QKD scheme that addresses the real-world parameters inherent to free-space quantum communication channels. Specific to our algorithm, the memory-assisted based system allows us to leverage the advantage of asynchronously loaded quantum memory when predicting the distribution rates. Specifically, by focusing on metropolitan distances, we perform simulations tailored toward a system based on free-space links and field-deployable quantum memory. We show the capabilities of our model to predict key rate distributions over ranges of 10-50 km for a set of atmospheric-based parameters and selection of QM efficiencies and coherence times. This tool provides impactful insights into the deployment and optimization of practical MA-MDI-QKD networks in urban environments. Our streamlined approach is a valuable addition to existing quantum network simulators for the smooth integration of quantum networking into the field of communications engineering.
On the pathway to quantum key distribution on a global scale, will be the realization of metropolitan-sized Memory Assisted Measurement-Device-Independent Quantum Key Distribution (MA-MDI-QKD) systems. Here, we present a simplistic and intuitive stochastic model to predict key distribution rates in a MA-MDI-QKD scheme that addresses the real-world parameters inherent to free-space quantum communication channels. Specific to our algorithm, the memory-assisted based system allows us to leverage the advantage of asynchronously loaded quantum memory when predicting the distribution rates. Specifically, by focusing on metropolitan distances, we perform simulations tailored toward a system based on free-space links and field-deployable quantum memory. We show the capabilities of our model to predict key rate distributions over ranges of 10-50 km for a set of atmospheric-based parameters and selection of QM efficiencies and coherence times. This tool provides impactful insights into the deployment and optimization of practical MA-MDI-QKD networks in urban environments. Our streamlined approach is a valuable addition to existing quantum network simulators for the smooth integration of quantum networking into the field of communications engineering.
Memory-assisted measurement-device-independent quantum key distribution (MA-MDI-QKD) is a promising scheme that aims to improve the rate-versus-distance behavior of a QKD system by using the state-of-the-art devices. It can be seen as a bridge between current QKD links to quantum repeater based networks. While, similar to quantum repeaters, MA-MDI-QKD relies on quantum memory (QM) units, the requirements for such QMs are less demanding than that of probabilistic quantum repeaters. Here we present a variant of MA-MDI-QKD structure that relies on only a single physical QM: a nitrogen-vacancy center embedded into a cavity where its electronic spin interacts with photons and its nuclear spin is used for storage. This enables us to propose a simple but efficient MA-MDI-QKD scheme resilient to memory errors and capable of beating, in terms of rate and reach, existing QKD demonstrations. We also show how we can extend this setup to a quantum repeater system, reaching, thus, larger distances.
Quantum key distribution (QKD) is one of the most prominent methods for secure exchange of cryptographic keys between two users. The laws of physics provide it with an immense tool \ntowards secure communications. Although QKD has been proven to reach distances on the order of a few hundreds of kilometers, the transmission rate of the key significantly drops when we go \nto further distances. One possible solution to this is to build a network of trusted nodes. The trust requirement will however narrow its scope of deployability. In this thesis, we focus on improving the key rate performance of secure communications by introducing imperfect quantum memories (QMs) in a measurement-device-independent (MDI) QKD system. \n \nIn this thesis, a protocol with the potential of beating the existing distance records for conventional QKD systems is proposed. It borrows ideas from quantum repeaters by using memories in the middle of the link, and that of MDI-QKD, which only requires optical source equipment at the user’s end. For certain fast memories, our scheme allows a higher repetition rate than that of quantum repeaters, thereby requiring lower coherence times. By accounting for various sources of nonideality, such as memory decoherence, dark counts, misalignment errors, and background noise, as well as timing issues with memories, we develop a mathematical framework within which we can compare QKD systems with and without memories. In particular, we show that with the state-of-the-art technology for quantum memories, it is possible to devise memory-assisted QKD systems that, at certain distances of practical interest, outperform current QKD implementations. \n \nTo extend this work, we consider a suitable candidate that fullfils the requirements we have set for the QMs, i.e., the ensemble-based QMs. This type of memories, nevertheless, suffers from multiple-excitation effects, which can deteriorate the performance of the memory-assisted MDI-QKD system. As a solution we propose an alternative approach to the memory-assisted MDI-QKD by employing entangled-photon sources. We fully analyse this system by including modulation errors during the state-preparation at a single-photon source. We identify under which regimes of operation this system outperforms present QKD implementations. Overall we obtain a realistic account of what can be done with current technologies in order to improve the performance, in terms of rate versus distance, of QKD systems. Our findings can guide us toward implementing larger quantum networks.
Measurement-device-independent quantum key distribution (MDI-QKD) protocol can effectively resist all possible attacks targeting the measurement devices in a quantum key distribution (QKD) system, thus exhibiting high security. However, due to the protocol’s high sensitivity to channel attenuation, its key generation rate and transmission distance are significantly limited in practical applications. To improve the performance of MDI-QKD, researchers have proposed quantum-memory (QM)-assisted MDI-QKD protocol, which has enhanced the protocol's performance to a certain extent. Nevertheless, under finite-size conditions where the total number of transmitted pulses is limited, accurately estimating the relevant statistical parameters is still a challenge. As a result, existing QM-assisted MDI-QKD schemes still encounters issues such as low key rates and limited secure transmission distances. To solve these problems, this work proposes a novel improved finite-size QM-assisted MDI-QKD protocol. By utilizing quantum memories to temporarily store early-arriving pulses and release them synchronously, the protocol effectively reduces the influence caused by channel asymmetry. Additionally, the protocol introduces a four-intensity decoy-state method to improve the estimation accuracy of single-photon components. Meanwhile, to mitigate the influence of finite-length effects on QM schemes, the proposed protocol combines a collective constraint model and a double-scanning algorithm to jointly estimate scanning error counts and vacuum-related counts. This approach enhances the estimation accuracy of the single-photon detection rate and phase error rate under finite-size conditions, thereby significantly improving the secure key rate of the MDI-QKD system. Simulation results show that under the same experimental conditions, compared with the existing QM-assisted three-intensity decoy-state MDI-QKD protocol and the four-intensity decoy-state MDI-QKD protocol based on Heralded Single-photon Source (HSPS), the proposed protocol extends the secure transmission distance by more than 30 km and 100 km, respectively. This proves that under the same parameter settings, the proposed scheme exhibits significant advantages in both key rate and secure transmission distance. Therefore, this research provides important theoretical references and valuable benchmarks for developing long-distance, high-security quantum communication networks.
Measurement-device-independent quantum key distribution (MDI-QKD), based on two-photon interference, is immune to all attacks against the detection system and allows a QKD network with untrusted relays. Since the MDI-QKD protocol was proposed, fiber-based implementations aimed at longer distance, higher key rates, and network verification have been rapidly developed. However, owing to the effect of atmospheric turbulence, MDI-QKD over a free-space channel remains experimentally challenging. Herein, by developing a robust adaptive optics system, high-precision time synchronization and frequency locking between independent photon sources located far apart, we realized the first free-space MDI-QKD over a 19.2-km urban atmospheric channel, which well exceeds the effective atmospheric thickness. Our experiment takes the first step toward satellite-based MDI-QKD. Moreover, the technology developed herein opens the way to quantum experiments in free space involving long-distance interference of independent single photons.
Memory-assisted measurement-device-independent quantum key distribution (MA-MDI-QKD) has recently been proposed as a technique to improve the rate-versus-distance behavior of QKD systems by using existing, or nearly-achievable, quantum technologies.The promise is that MA-MDI-QKD would require less demanding quantum memories than the ones needed for probabilistic quantum repeaters.Nevertheless, early investigations suggest that, in order to beat the conventional memoryless QKD schemes, the quantum memories used in the MA-MDI-QKD protocols must have high bandwidth-storage products and short interaction times.Among different types of quantum memories, ensemble-based memories offer some of the required specifications, but they typically suffer from multiple excitation effects.To avoid the latter issue, in this paper, we propose two new variants of MA-MDI-QKD both relying on single-photon sources for entangling purposes.One is based on known techniques for entanglement distribution in quantum repeaters.This scheme turns out to offer no advantage even if one uses ideal single-photon sources.By finding the root cause of the problem, we then propose another setup, which can outperform single memory-less setups even if we allow for some imperfections in our single-photon sources.For such a scheme, we compare the key rate for different types of ensemble-based memories and show that certain classes of atomic ensembles can improve the rate-versus-distance behavior.
Memory-assisted measurement-device-independent quantum key distribution (MA-MDI-QKD) has recently been proposed as a technique to improve the rate-versus-distance behavior of QKD systems by using existing, or nearly-achievable, quantum technologies. The promise is that MA-MDI-QKD would require less demanding quantum memories than the ones needed for probabilistic quantum repeaters. Nevertheless, early investigations suggest that, in order to beat the conventional memory-less QKD schemes, the quantum memories used in the MA-MDI-QKD protocols must have high bandwidth-storage products and short interaction times. Among different types of quantum memories, ensemble-based memories offer some of the required specifications, but they typically suffer from multiple excitation effects. To avoid the latter issue, in this paper, we propose two new variants of MA-MDI-QKD both relying on single-photon sources for entangling purposes. One is based on known techniques for entanglement distribution in quantum repeaters. This scheme turns out to offer no advantage even if one uses ideal single-photon sources. By finding the root cause of the problem, we then propose another setup, which can outperform single memory-less setups even if we allow for some imperfections in our single-photon sources. For such a scheme, we compare the key rate for different types of ensemble-based memories and show that certain classes of atomic ensembles can improve the rate-versus-distance behavior.