Andrew Shields

Current quantum key distribution (QKD) networks focus almost exclusively on transporting secret keys with the highest possible rate. Consequently, they are built as mostly fixed, ad hoc, logically, and physically isolated infrastructures designed to avoid any penalty to the quantum channel. This architecture is neither scalable nor cost-effective and future, real-world deployments will differ considerably. The structure of the MadQCI QKD network presented here is based on disaggregated components and modern paradigms especially designed for flexibility, upgradability, and facilitating the integration of QKD in the security and telecommunications-networks ecosystem. These underlying ideas have been tested by deploying many QKD systems from several manufacturers in a real-world, multi-tenant telecommunications network, installed in production facilities and sharing the infrastructure with commercial traffic. Different technologies have been used in different links to address the variety of situations and needs that arise in real networks, exploring a wide range of possibilities. Finally, a set of realistic use cases have been implemented to demonstrate the validity and performance of the network. The testing took place during a period close to three years, where most of the nodes were continuously active.

Random numbers play a crucial role in information technology, particularly as digital communication capacity continues to expand. Consequently, the need for secure and high-rate random number generation has become increasingly urgent. While integrated photonics technology holds promise for mass-producing optoelectronic quantum random number generators (QRNGs), there remains a challenge in developing fast, robust, and scalable solutions suitable for industrial deployment. Addressing this challenge, we present a fast QRNG solution in this study, leveraging a photonic integrated circuit (PIC) directly embedded onto a versatile electronic platform. Designed to withstand real-world applications, our PIC is packaged to align with industrial electronic assembly lines. To rigorously assess scalability and stability, these generators underwent week-long periods of continuous GHz operation. Furthermore, a QRNG was integrated into a quantum key distribution system, where despite operating in an uncontrolled environment, minimal variations in physical randomness were observed over 38 days, as measured from 2.9 million histograms. Finally, we implemented a security model for the QRNGs, enabling rate adjustment to match the actual randomness content and demonstrating secure generation at 2 Gbit/s.

Quantum Communications (QC) harness quantum mechanical phenomena such as superposition and entanglement to enhance information transfer between remote nodes. Coherent quantum communications refer to QC schemes relying on maintaining optical coherence between nodes for successful execution. These schemes typically involve single photon interference between optical fields generated by distant parties and represent a cornerstone of a promising architecture of the quantum internet. Despite their significant potential, scientific and technical hurdles - including optical coherence maintenance, integrating high-performance single-photon detectors, and precise stabilisation and synchronisation - have prevented the implementation of coherent QC over existing telecommunication infrastructure. Here we present the first realisation of a coherent QC fully integrated into standard telecommunication infrastructure over a link connecting the German cities of Frankfurt and Kehl. The implemented scheme is the Twin Field Quantum Key Distribution (QKD) protocol, enabling the distribution of a shared secret key for encryption at a rate of 110 bit/s over a highly asymmetric 254 km link. This result, obtained with a system featuring measurement-device-independent properties, marks the longest installed QKD implementation utilising non-cryogenic cooled detectors and was enabled by the QC system architecture we developed and by our approach to phase stabilisation, which involves active out-of-band phase stabilisation and avalanche photodiodes for single photon detection. This achievement, not only represents a milestone for practical quantum communications but also validates the compatibility of coherent QC with current telecommunication infrastructure, supporting the feasibility of a phase-based architecture for the quantum internet.

Hybrid integration has the potential to overcome various limitations of integrated photonic material platforms. Here, we present the results of applying edge-couple hybrid integration to produce high performance quantum key distribution chips. We show low quantum bit error rate operation (< 1%) and positive secure key rates over 250 km of fibre spool.

Integrated photonics presents an opportunity for low-cost and highly-reproducible quantum cryptographic systems. However, due to numerous challenges such as packaging, power consumption and interfacing multiple chips in real, a standalone deployable photonic integrated system is still missing. Here we address all these challenges to present a real-time quantum communication system using integrated photonics. The system operated without intervention over multiple days and is capable of secure key rates of > 470 kbps over 10 km of fiber

We demonstrate the first experimental overcoming of the repeaterless secret key capacity (PLOB) bound through the implementation of the Twin Field Quantum Key Distribution (TF-QKD) protocol. We distribute secret keys at record channel losses (> 90 dB). We assess the prospects for real-world implementation of TF-QKD.

As the volume of data and connections exchanged across telecom/datacom networks continues to increase, there is a growing need for technologies that deploy quantum key distribution (QKD) on a large scale in a practical and sustainable manner. To realize high-speed, real-time communication of large-volume data using one-time pad cryptography with QKD modules, it will be important to multiplex QKD modules in the future. Furthermore, it is necessary to consider the physical size of the device for the practical application of multiplexed QKD modules. In this study, we focused on miniaturizing the key distillation process required at the back end of the QKD chip. To reduce the size of the device, it is necessary to estimate as accurately as possible the minimum computing power required to run the key distillation process for the target secret key rate (SKR). However, the performance of the key distillation process requires computing power and involves the exchange of messages via classical channels. Therefore, we evaluate the performance by a network simulator before performing evaluations on the actual equipment. In this paper, we focus on the behavior of classical communication paths in the multiplexed QKD system, which is a problem in studying the key distillation process, and we evaluate it with the simulator. Specifically, we clarify the relationship between the required performance of the key distillation process (i.e., throughput) and the target SKR, which is necessary to realize a part of the key distillation process in hardware.

Quantum key distribution (QKD) ensures the sharing of secret cryptographic keys between distant entities (typically called Alice and Bob), whose intrinsic security is guaranteed by the laws of nature [1–3]. Besides pioneering experiments involving satellite transmission [4], the challenge is the integration of this technology in telecommunication fiber networks, in particular in long haul segments [5–11]. The longest achievable communication distance is limited by the channel loss which increases exponentially with the fiber length and noise in the deployed single photon detector. The secure QKD key rate decreases exponentially with the channel fiber length. Although the communication distance could be extended using quantum repeaters, the related research is still at a proof-of-principle level [12]. Presently the widely adopted solution is the exploitation of trusted nodes, whose security represents however a significant technical issue. An innovative approach that overcomes, at least partially, the need for trusted node is represented by the recently proposed QKD protocol dubbed twin-field QKD (TF-QKD) [13]. In TF-QKD, the information is encoded on dim laser pulses generated at distant Alice and Bob terminals and sent through optical fiber over half of the entire communication distance to the central node, Charlie, where they interfere. For this reason, the TF-QKD has weaker dependence on channel losses, essentially doubling the communication distance with respect to the conventional prepare-and-measure QKD solution. TF-QKDhas been proved secure against general attacks (see e.g. [14–18]), but its implementation is challenging as the optical pulses sent by Alice and Bob are required to be phase-coherent and preserve coherence when reaching Charlie after travelling the long fiber paths. While phase coherence can be achieved by phase-locking the two QKD lasers in Alice and Bob to a common reference laser transmitted through a service channel, uncorrelated phase changes due to the length and refractive index fluctuations in the long optical fibers still remain and will reduce the visibility of the interference measurement. In the TF-QKD proof-of-principle experiments [19–26], this effect was mitigated by interleaving the QKD frames with classical transmission frames that were used to periodically realign the phases of interfering pulses. Here we present an alternative solution derived from the metrological research community, more precisely from atomic clocks comparison technology. Specifically, transmission of coherent laser radiation over thousand-kilometer-long fibers is exploited for the comparison of distant atomic clocks at the highest accuracy [27–32]. In this case phase fluctuations in long fiber also need to be addressed, othewise they would substantially degrade the comparison results. Precise comparison among these atomic clocks are made possible by the use of ultra-stable lasers and the active cancellation of the noise introduced by connecting fibers. Here we demonstrate that this technique can be successfully adapted into a TF-QKD setup. More specifically, we designed and developed an apparatus suitable for actively cancelling phase fluctuations of both the lasers and of the connecting fibers in a TF-QKD setup. This is achieved by transmitting an additional sensing laser light at a slightly different wavelength in the same fiber as the QKD dim pulses. In Charlie, this sensing laser is used for the fiber optical length stabilisation. We show that this multiplexed solution can be properly tuned in order to avoid a sizeable impact on the number of background photons observed by the single-photon detectors in the QKD channels, allowing simultaneous key streaming and channels stabilization, ensuring longer duty-cycles of the QKD process and a tighter control of the optical phase on long-haul deployed fibers. Furthermore, we tested our solution in a real-world network where the net losses between Alice and Bob are as high as 65 dB, resulting here in a distance of 206 km, or equivalent at 325 km on a fiber haul at common nominal losses of 0.2 dB/km [33]. References [1] Bennett, C. H. & Brassard, G. Quantum cryptography: public key distribution and coin tossing. Theor. Comput. Sci. 560, 7–11 (2014). [2] Scarani, V. et al. The security of practical quantum key distribution. Rev. Mod. Phys. 81, 1301 (2009). [3] Kwong Lo, H., Curty, M. & Tamaki, K. Secure quantum key distribution. Nature Photonics 8, 595-604 (2014). [4] Liao, S-K., Cai, W-Q., Pan, J-W. Satellite-to-ground quantum key distribution, Nature 549, 43-47 (2017) [5] Peev, M. et al. The SECOQC quantum key distribution network in Vienna, New J. Phys. 11, 075001 (2009). [6] Sasaki, M. et al. Field test of quantum key distribution in the Tokyo QKD Network. Opt. Expr. 19, 10387 (2011). [7] Dynes, J. F. et al. Cambridge quantum network. npj Quantum Inf. 5, 101 (2019). [8] Shimizu K., et al. Performance of long-distance quantum key distribution over 90-km optical links installed in a field environment of Tokyo metropolitan area. J. Lightwave Technol. 32,, 141-51 (2014). [9] Bacco, D. et al. Field trial of a three-state quantum key distribution scheme in the Florence metropolitan area. EPJ Quantum Technol.6, 5 (2019). [10] Choi, I. et al. Field trial of a quantum secured 10 Gb/s DWDM transmission system over a single installed fiber. Opt. Expr 22, 23121-23128 (2014). [11] Dixon, A. R. et al. Quantum key distribution with hacking countermeasures and long term field trial, Sci. Rep. 7, 7583 (2017). [12] Xu, F., Ma, X., Zhang, Q., Lo, H-K. & Pan, J-W. Secure quantum key distribution with realistic devices. Rev. Mod. Phys. 92, 025002 (2020) [13] Lucamarini, M., Yuan, Z. L., Dynes, J. F., Shields, A. J. Overcoming the rate-distance limit of quantum key distribution without quantum repeaters. Nature 557, 400-403 (2018). [14] Ma, X. Zeng, P., & Zhou, H. Phase-Matching Quantum Key Distribution. Phys. Rev. X 8, 031043 (2018). [15] Wang, X-B., Yu, Z-W. & Hu, X-L. Twin-field quantum key distribution with large misalignment error. Phys. Rev. A 98, 062323 (2018). [16] Lin J. & Lutkenhaus, N. Simple security analysis of phase-matching measurement-device-independent quantum key distribution. Phys. Rev. A 98, 042332 (2018); [17] Curty, M., Azuma, K. & Lo, H.-K. 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We demonstrate a simple and compact MDI-QKD system design based on optical injection locking and gain-switching techniques, capable of directly encoding phase-modulated time-bin bits. Our results improve upon the state-of-the-art key rates by an order of magnitude.

Random number generators underpin the security of current and future cryptographic systems and are therefore a likely target for attackers. Quantum random number generators have been hailed as the ultimate sources of randomness. However, as shown in this work, the susceptibility of the sensitive electronics required to implement such devices poses a serious threat to their security. We present the first out-of-band electromagnetic injection attack on a quantum random number generator through which an adversary can gain full control of the output. In our first experiment, the adversary forces the binary output of the generator to become an alternating string of 1s and 0s, with near 100% success. This attack may be spotted by a vigilant user performing statistical tests on their output strings. We therefore envisage a second more subtle attack in which the adversary forces the output to be a random pattern known to them, thus rendering any protection based on statistical tests ineffective.

We developed a system for real-time transmission of genome sequence data using quantum cryptography and have succeeded in the quantum cryptography transmission of genome sequence data with data volumes exceeding several hundred gigabytes. This demonstrated that quantum cryptography can transmit large amounts of data and has practical applications in the fields of genomic research and genomic medicine.

Running single photon detectors (such as single photon avalanche photodiodes or superconducting nanowire detectors) at high speed is challenging and requires innovative driving and signal processing electronics. Self-differencing has been shown to be a very successful method to achieve detection rates in excess of 1 GHz using APDs while keeping low error rates from dark counts and afterpulses. Here, we present first results of high bit-rate QKD using a novel compact self-differencing setup which is integrated completely on a single printed circuit board (PCB). We achieve bit rates higher than 1 Mbit/s for a fibre distance of 50 km.