The decoy-state method serves as a cornerstone for practical quantum key distribution (QKD) with inherent immunity to photon-number-splitting attacks. Leveraging optical injection locking, we propose a directly modulated light source with a scheme that simultaneously generates intensity- and phase-modulated signals without external modulators. Removing external modulators eliminates modulator-related side-channel vulnerabilities, fundamentally enhancing QKD security. Integrated into a high-speed phase-encoding BB84 system with optimized one-decoy protocol and finite-size analysis, our system achieves positive key rates up to 20 dB channel loss (100 km fiber) with commercial InGaAs single-photon detectors, reducing transmitter complexity while ensuring security for practical QKD applications.
The decoy-state method serves as a cornerstone for practical quantum key distribution (QKD) with inherent immunity to photon-number-splitting attacks. Leveraging optical injection locking, we propose a directly modulated light source with a scheme that simultaneously generates intensity- and phase-modulated signals without external modulators. Removing external modulators eliminates modulator-related side-channel vulnerabilities, fundamentally enhancing QKD security. Integrated into a high-speed phase-encoding BB84 system with optimized one-decoy protocol and finite-size analysis, our system achieves positive key rates up to 20 dB channel loss (100 km fiber) with commercial InGaAs single-photon detectors, reducing transmitter complexity while ensuring security for practical QKD applications.
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.
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.
The decoy-state protocol has been considered to be one of the most important methods to protect the security of quantum key distribution (QKD) with a weak coherent source. Here we test two experimental approaches to generating the decoy states with different intensities: modulation of the pump current of a semiconductor laser diode, and external modulation by an optical intensity modulator. The former approach shows a side channel in the time domain that allows an attacker to distinguish s signal state from a decoy state, breaking a basic assumption in the protocol. We model a photon-number-splitting attack based on our experimental data, and show that it compromises the system's security. Then, based on the work of Tamaki et al. [New J. Phys. 18, 065008 (2016)], we obtain two analytical formulas to estimate the yield and error rate of single-photon pulses when the signal and decoy states are distinguishable. The distinguishability reduces the secure key rate below that of a perfect decoy-state protocol. To mitigate this reduction, we propose to calibrate the transmittance of the receiver (Bob's) unit. We apply our method to three QKD systems and estimate their secure key rates.
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.
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.
Many security loopholes exist in quantum key distribution (QKD) due to the imperfections of realistic devices. On the source side, the decoy-state method can defend against the most severe photon-number-splitting (PNS) attack and improve the performance of QKD. Measurement-device-independent QKD and twin-field QKD have further closed all loopholes on the detection side. In this paper, we propose a valuable optical injection-locking loophole for the case when the internal isolator inside the laser is removed. We first introduce the effects of the injection locking with different injection intensities on the source frequency. Through the successive responses of adjacent pulses from frequency ports of the dense wavelength division multiplexer, the decoy state and the signal state can be partially distinguished. The specific injection-locking-loophole analysis of the isolatorless decoy phase-encoding Bennet-Brassard 1984 QKD and decoy phase-matching QKD protocols with external optical injection has been deeply studied. Simulation results show that, if we maintain the same observed gain statistics as normal after an external optical injection locking, the loophole cannot be exploited for a PNS attack when the QKD distance is short. As the QKD distance increases, a PNS attack at a medium distance using the optical injection-locking loophole does not threaten the security; the security is still valid. With the further increase of the QKD distance, the lower-bound secure key rate is higher than the upper-bound secure key rate given by a PNS attack at a long distance, some of the keys must be insecure, and information will be leaked.