Memory-assisted measurement-device-independent quantum key distribution (MA-MDI-QKD) has recently been proposed as a possible intermediate step towards the realization of quantum repeaters. Despite its relaxing some of the requirements on quantum memories, the choice of memory in relation to the layout of the setup and the protocol has a stark effect on our ability to beat existing no-memory systems. Here, we investigate the suitability of nitrogen vacancy (NV) centers, as quantum memories, in MA-MDI-QKD. We particularly show that moderate cavity enhancement is required for NV centers if we want to outperform no-memory QKD systems. Using system parameters mostly achievable by today's state of the art, we then anticipate some total key rate advantage in the distance range between 300 and 500 km for cavity-enhanced NV centers. Our analysis accounts for major sources of error including the dark current, the channel loss, and the decoherence of the quantum memories.
Memory-assisted measurement-device-independent quantum key distribution (MA-MDI-QKD) has recently been proposed as a possible intermediate step towards the realization of quantum repeaters. Despite its relaxing some of the requirements on quantum memories, the choice of memory in relation to the layout of the setup and the protocol has a stark effect on our ability to beat existing no-memory systems. Here, we investigate the suitability of nitrogen vacancy (NV) centers, as quantum memories, in MA-MDI-QKD. We particularly show that moderate cavity enhancement is required for NV centers if we want to outperform no-memory QKD systems. Using system parameters mostly achievable by today's state of the art, we then anticipate some total key rate advantage in the distance range between 300 and 500 km for cavity-enhanced NV centers. Our analysis accounts for major sources of error including the dark current, the channel loss, and the decoherence of the quantum memories.
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.
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.
A protocol with the potential of beating the existing distance records for conventional quantum key distribution (QKD) systems is proposed. It borrows ideas from quantum repeaters by using memories in the middle of the link, and that of measurement-device-independent QKD, which only requires optical source equipment at the userʼs end. For certain memories with short access times, our scheme allows a higher repetition rate than that of quantum repeaters with single-mode memories, 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 potentially possible to devise memory-assisted QKD systems that, at certain distances of practical interest, outperform current QKD implementations.
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.
A protocol with the potential of beating the existing distance records for conventional quantum key distribution (QKD) systems is proposed. It borrows ideas from quantum repeaters by using memories in the middle of the link, and that of measurement-device-independent QKD, which only requires optical source equipment at the users end. For certain memories with short access times, our scheme allows a higher repetition rate than that of quantum repeaters with single-mode memories, 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 potentially possible to devise memory-assisted QKD systems that, at certain distances of practical interest, outperform current QKD implementations.