Abstract The possibility for quantum and classical communication to coexist on the same fiber is important for deployment and widespread adoption of quantum key distribution (QKD) and, more generally, a future quantum internet. While coexistence has been demonstrated for different QKD implementations, a comprehensive investigation for measurement-device independent (MDI) QKD—a recently proposed QKD protocol that cannot be broken by quantum hacking that targets vulnerabilities of single-photon detectors—is still missing. Here we experimentally demonstrate that MDI-QKD can operate simultaneously with at least five 10 Gbps bidirectional classical communication channels operating at around 1550 nm wavelength and over 40 km of spooled fiber, and we project communication rates in excess of 10 THz when moving the quantum channel from the third to the second telecommunication window. The similarity of MDI-QKD with quantum repeaters suggests that classical and generalized quantum networks can co-exist on the same fiber infrastructure.
Abstract The possibility for quantum and classical communication to coexist on the same fiber is important for deployment and widespread adoption of quantum key distribution (QKD) and, more generally, a future quantum internet. While coexistence has been demonstrated for different QKD implementations, a comprehensive investigation for measurement-device independent (MDI) QKD—a recently proposed QKD protocol that cannot be broken by quantum hacking that targets vulnerabilities of single-photon detectors—is still missing. Here we experimentally demonstrate that MDI-QKD can operate simultaneously with at least five 10 Gbps bidirectional classical communication channels operating at around 1550 nm wavelength and over 40 km of spooled fiber, and we project communication rates in excess of 10 THz when moving the quantum channel from the third to the second telecommunication window. The similarity of MDI-QKD with quantum repeaters suggests that classical and generalized quantum networks can co-exist on the same fiber infrastructure.
Abstract The forthcoming quantum Internet is poised to allow new applications not possible with the conventional Internet. The ability for both quantum and conventional networking equipment to coexist on the same fiber network would facilitate the deployment and adoption of coming quantum technology. Most quantum networking tasks, like quantum repeaters and the connection of quantum processors, require nodes for multi-qubit quantum measurements (often Bell-State measurements), and their real-world coexistence with the conventional Internet has yet to be shown. Here we field deploy a Measurement-Device Independent Quantum Key Distribution (MDI-QKD) system, containing a Bell-State measurement node, over the same fiber connection as multiple standard Internet Protocol (IP) data networks, between three nearby cities in the Netherlands. We demonstrate over 10 Gb/s classical data communication rates simultaneously with our next-generation QKD system, and estimate 200 GB/s of classical data transmission would be easily achievable without significantly affecting QKD performance. Moreover, as the system ran autonomously for two weeks, this shows an important step towards the coexistence and integration of quantum networking into the existing telecommunication infrastructure.
We assess the overall performance of our quantum key distribution (QKD) system implementing the measurement-device-independent (MDI) protocol using components with varying capabilities such as different single-photon detectors and qubit preparation hardware. We experimentally show that superconducting nanowire single-photon detectors allow QKD over a channel featuring 60 dB loss, and QKD with more than 600 bits of secret key per second (not considering finite key effects) over a 16 dB loss channel. This corresponds to 300 and 80 km of standard telecommunication fiber, respectively. We also demonstrate that the integration of our QKD system into FPGA-based hardware (instead of state-of-the-art arbitrary waveform generators) does not impact on its performance. Our investigation allows us to acquire an improved understanding of the trade-offs between complexity, cost and system performance, which is required for future customization of MDI-QKD. Given that our system can be operated outside the laboratory over deployed fiber, we conclude that MDI-QKD is a promising approach to information-theoretic secure key distribution.
Quantum key distribution (QKD) enables two legitimate parties to share a secret key even in the presence of an eavesdropper. The unconditional security of QKD is based on the fundamental laws of quantum physics. Original security proofs of QKD are based on a few assumptions, e.g., perfect single photon sources and perfect single-photon detectors. However, practical implementations of QKD systems do not fully comply with such assumptions due to technical limitations. The gap between theory and implementations leads to security loopholes in most QKD systems, and several attacks have been launched on sophisticated QKD systems. Particularly, the detectors have been found to be the most vulnerable part of QKD. Much effort has been put to build side-channel-free QKD systems. Solutions such as security patches and device-independent QKD have been proposed. However, the former are normally ad-hoc, and cannot close unidentified loopholes. The latter, while having the advantages of removing all assumptions on devices, is impractical to implement today. Measurement-device-independent QKD (MDI-QKD) turns out to be a promising solution to the security problem of QKD. In MDI-QKD, all security loopholes, including those yet-to-be discovered, have been removed from the detectors, the most critical part in QKD. In this thesis, we investigate issues related to the practical implementation and security of MDI-QKD. We first present a demonstration of polarization-encoding MDI-QKD. Taking finite key effect into account, we achieve a secret key rate of 0.005 bit per second (bps) over 10 km spooled telecom fiber, and a 1600-bit key is distributed. This work, together with other demonstrations, shows the practicality of MDI-QKD. Next we investigate a critical assumption of MDI-QKD: perfect state preparation. We apply the loss-tolerant QKD protocol and adapt it to MDI-QKD to quantify information leakage due to imperfect state preparation. We then present an experimental demonstration of MDI-QKD over 10 km and 40 km of spooled fiber, which for the first time considers the impact of inaccurate polarization state preparation on the secret key rate. This would not have been possible under previous security proofs, given the same amount of state preparation flaws.
Since its initial proposal in 2012, measurement-device-independent quantum key distribution (MDIQKD) has inspired rapid experimental progress as it is invulnerable to all detector side-channel attacks and ideally suitable for quantum key distribution (QKD) networks with star-topology, such as metropolitan networks. The main goal of this thesis is to develop MDIQKD systems for building a cost-effective metropolitan quantum network. Towards this end, we experimentally demonstrate the coexistence of MDIQKD with classical communication on the same fibre. This eliminates the use of dark fibre for quantum communication and minimises implementation costs by utilising the existing fibre infrastructure. Additionally, we move the quantum channel from the third telecommunication window (1530-1565 nm) to second (1260-1360 nm) to ensure MDIQKD can co-exist with classical communication rates of over 10-terabits per second. Furthermore, we enhance the performance of the MDIQKD systems by increasing the repetition rate from 20MHz to 200MHz and improve flexibility and reliability to facilitate the deployment of a metropolitan quantum network.
Quantum key distribution (QKD) can serve large numbers of users with unconditional secure communication, in which measurement-device-independent (MDI) QKD protocol can conveniently enhance realistic security by eliminating all detector side channels. In multi-user scenarios, MDI-QKD networks should meet the requirements of reliability and robustness. Around the issues, we complete a series of work on performance improvement, low-cost deployment, and robust networking. First is to answer how to change the trusted node to the untrusted node with low cost, here we propose and demonstrate an MDIQKD which is compatible with phase encoding BB84 networks, allowing for economical deployment of MDI-QKD networks and reconciliation between high key rate and high realistic security in the same network. Then we make the MDI networks more robust. We design an MDI-QKD networking scheme for multi-user access where more than two users can generate keys synchronously regardless of aligning reference frames and compensating channel disturbance on polarization. Third, we find a general method to distil secure keys from quantum discord, which is different to the previous entanglement-based frameworks of MDI-QKD, allowing imperfect preparation and measurement of quantum states. The series of work can help the practical application of MDI-QKD networks, and promote the development of quantum networks.
Abstract Measurement-device-independent quantum key distribution (MDI QKD) provides inherent immunity against attacks targeting practical measurement devices. Existing MDI QKD protocols all rely on Bell-state measurements (BSM) to establish correlations between the users. However, the success probability of linear-optics BSM is limited to 50%, which severely restricts the achievable key rate of MDI QKD. We propose a high-capacity MDI QKD protocol with entanglement-assisted linear-optical BSM. This protocol has three advantages. First, compared with the original MDI QKD, at least a 25% increase in the key rate can be achieved. Second, simulation results show that this protocol can effectively increase the maximum photon transmission distance of MDI QKD from 262 km to 272 km. Third, this protocol is feasible in a fully linear-optical system under current experimental conditions. Our protocol provides a potential approach for improving the performance of future quantum communication networks.