A positive economic effect from the implementation of quantum key distribution (QKD) technology can be achieved only with significant scaling, which involves the deployment of branched metropolitan area networks. The creation of QKD systems suitable for such networks is an important task for the coming years. This paper considers a method for preparing quantum states using pulsed optical injection, which can be used as a basis for a compact modulator-free transmitter ideally suited for QKD at typical distances within a city. Considering the relative proximity between nodes of a MAN, we suggest to abandon the decoy states, which, together with the proposed method of quantum state preparation, allows making the transmitter extremely simple. We report here the results of an experiment confirming the operating principle and provide a security analysis of the three-state decoy-free QKD protocol that can be implemented using such a device.
A positive economic effect from the implementation of quantum key distribution (QKD) technology can be achieved only with significant scaling, which involves the deployment of branched metropolitan area networks. The creation of QKD systems suitable for such networks is an important task for the coming years. This paper considers a method for preparing quantum states using pulsed optical injection, which can be used as a basis for a compact modulator-free transmitter ideally suited for QKD at typical distances within a city. Considering the relative proximity between nodes of a MAN, we suggest to abandon the decoy states, which, together with the proposed method of quantum state preparation, allows making the transmitter extremely simple. We report here the results of an experiment confirming the operating principle and provide a security analysis of the three-state decoy-free QKD protocol that can be implemented using such a device.
Quantum key distribution (QKD) allows secret key exchange between two users with unconditional security. For QKD to be widely deployed, low cost and compactness are crucial requirements alongside high performance. Currently, the majority of QKD systems demonstrated rely on bulk intensity and phase modulators to generate optical pulses with precisely defined amplitude and relative phase difference—i.e., to encode information as signal states and decoy states. However, these modulators are expensive and bulky, thereby limiting the compactness of QKD systems. Here, we present and experimentally demonstrate a novel optical transmitter design to overcome this disadvantage by generating intensity- and phase-tunable pulses at GHz clock speeds. Our design removes the need for bulk modulators by employing directly modulated lasers in combination with optical injection locking and coherent interference. This scheme is, therefore, well suited to miniaturization and photonic integration, and we implement a proof-of-principle QKD demonstration to highlight potential applications.
Quantum key distribution (QKD) is a technology that allows two users to communicate with theoretically perfect security using standard optical fibres. This is possible by transmitting the key on single photons, meaning a measurement by an eavesdropper disturbs the system in a way observable to the legitimate parties. The technology has advanced since the first protocol proposed in 1984, to the stage where there are now many protocols that can be experimentally implemented. These protocols have allowed secure keys to be generated over distances greater than 400 km and with secure key rates over 10 Mbit/s. In a metropolitan QKD network, it would be desirable for as many users to be connected as possible. Unfortunately, each protocol comes with different requirements on the transmitter and receiver. Even within a single protocol, different clock rates can require individualised transmitter and receiver hardware. This prohibits users from communicating with all receivers, unless they have complex transmitters with hardware for many protocols. This thesis develops a transmitter for practical QKD that is able to adapt to a number of different protocols with no changes to the hardware. The transmitter works using optical injection locking, where a pulse preparation laser adopts the phase of a phase preparation laser. Controlling the phase and intensity of the pulses in this way removes the side channel ordinarily present with direct modulation, in that the phase, intensity and frequency simultaneously change in response to an applied current. The cavity-enhanced electro-optic effect allows for the first demonstration of sub-volt half-wave phase modulation at high clock rates. The transmitter successfully demonstrates phase encoding, intensity encoding and on-demand phase randomisation. This allows for the experimental realisation and direct comparison of different QKD protocols, including one that has never before been implemented due to experimental complexity. A stable intensity modulator is also developed, based on a Sagnac interferometer. This removes a side channel in QKD systems and integrates well with the directly-modulated quantum transmitter. This development also means that the transmitter can implement all current two-party QKD protocols based on weak coherent pulses. The transmitter has the potential to become the standard transmitter for future quantum communication networks due to its stability, versatility and power efficiency. The design could also be demonstrated on a photonic chip, making it compact enough to fit in small transmitter units.
Abstract Quantum key distribution (QKD) networks represent a foundational step toward the future quantum internet and have become a focus of intensive research. By analogy with the architecture of classical optical networks, we can observe that the high-capacity backbone typically relies on complex external modulation, while the cost-sensitive access layer prefers simpler, more compact, and more economical direct modulation. As the cornerstone of quantum networks, today’s QKD networks share this backbone-access hierarchy, yet their implementations remain heavily reliant on external modulators, leaving a clear ecological niche unfilled for directly modulated QKD in the access layer. Here, we propose and experimentally demonstrate the first fully modulator-free, full-degree-of-freedom direct-modulation QKD architecture that fills this niche. The transmitter combines optical injection locking with coherent optical interference to achieve simultaneous phase and intensity control without external modulators, enabling arbitrary quantum-state preparation while preserving inherent compatibility with standard time-bin-encoding receivers. Built with relatively inexpensive laser diodes and passive optical components, our experimental implementation achieves a secure key rate of 4.78 kbps at 30 dB channel loss, matching the performance of conventional modulator-based systems. By filling the long-standing direct-modulation niche in the QKD ecosystem, this work delivers a practical solution to the access-layer deployment challenge and opens a viable new direction for compact, scalable QKD implementation.
Abstract Quantum key distribution (QKD) has convincingly been proven compatible with real life applications. Its wide-scale deployment in optical networks will benefit from an optical platform that allows miniature devices capable of encoding the necessarily complex signals at high rates and with low power consumption. While photonic integration is the ideal route toward miniaturisation, an efficient route to high-speed encoding of the quantum phase states on chip is still missing. Consequently, current devices rely on bulky and high power demanding phase modulation elements which hinder the sought-after scalability and energy efficiency. Here we exploit a novel approach to high-speed phase encoding and demonstrate a compact, scalable and power efficient integrated quantum transmitter. We encode cryptographic keys on-demand in high repetition rate pulse streams using injection-locking with deterministic phase control at the seed laser. We demonstrate record secure-key-rates under multi-protocol operation. Our modulator-free transmitters enable the development of high-bit rate quantum communications devices, which will be essential for the practical integration of quantum key distribution in high connectivity networks.
One of the most advanced technologies within the field of quantum mechanics is quantum key distribution (QKD), which allows the secure generation of secret keys among remote users. In order for QKD to be more widely adopted, it must be integrated into existing classical communication systems. However, this can be difficult due to the use of various technologies and channels in deployed networks. Recently, we developed a QKD network in the metropolitan area of Padova, which connects various nodes across the city through a combination of fiber and free-space links. By utilizing a modular design based on the iPOGNAC encoder and the Qubit4Sync method, we have realized portable and adaptable systems that operate in the C and O bands. This allowed us to deploy and test the compatibility of both research and commercial QKD systems by ThinkQuantum with classical communication over a variety of links, as well as their ability to switch between free-space and fiber connections. Finally, we developed and experimentally implemented complex network configurations such as star networks, where a fiber-based transmitter and free-space transmitter could operate with a single receiver.
Quantum Key Distribution (QKD) is recognized for its unconditional security through the principles of quantum mechanics. The decoy-state method is widely used in QKD for its resilience to photon-number-splitting attacks [1]. In this method, Alice prepares decoy states alongside the standard signal state, which differ only in intensity. This approach enabled the BB84 system to transmit over distances of up to 421 km and achieve key rates as high as 115.8 Mbps. While high-speed intensity modulators are essential for these systems, directly modulated lasers offer an alternative by simplifying the transmitter and reducing system costs.