In prepare-and-measure quantum key distribution systems, careful preparation of quantum states within the transmitter device is a significant driver of both complexity and cost. Moreover, the security guarantees of such systems rest on the correct operation of high speed quantum random number generators (QRNGs) and the high-fidelity modulation of weak optical signals by high-speed optoelectronic devices, all of which must be hardened against a variety of known side-channel attacks. A fully passive state preparation approach elegantly resolves these problems by combining state preparation and QRNG stages into a single optical instrument. By using pairs of optical pulses from a gain-switched laser diode as ready-to-use qubits, the QKD transmitter can be radically simplified, eventually comprising a single laser and local phase tomography stage. We demonstrate our simplified transmitter by establishing a QKD link over a 10 km fiber, generating a secret key rate 110 bits/s, sufficient for practical deployment in "last mile" urban quantum networks. Our results show promise in making QKD simpler and more accessible, closing a critical technology gap in building a secure quantum communication infrastructure.
In prepare-and-measure quantum key distribution systems, careful preparation of quantum states within the transmitter device is a significant driver of both complexity and cost. Moreover, the security guarantees of such systems rest on the correct operation of high speed quantum random number generators (QRNGs) and the high-fidelity modulation of weak optical signals by high-speed optoelectronic devices, all of which must be hardened against a variety of known side-channel attacks. A fully passive state preparation approach elegantly resolves these problems by combining state preparation and QRNG stages into a single optical instrument. By using pairs of optical pulses from a gain-switched laser diode as ready-to-use qubits, the QKD transmitter can be radically simplified, eventually comprising a single laser and local phase tomography stage. We demonstrate our simplified transmitter by establishing a QKD link over a 10 km fiber, generating a secret key rate 110 bits/s, sufficient for practical deployment in "last mile" urban quantum networks. Our results show promise in making QKD simpler and more accessible, closing a critical technology gap in building a secure quantum communication infrastructure.
Quantum Key Distribution (QKD) technologies are on the way to becoming a practical solution for quantum-secured communication networks. One of the major issues pointed out by governmental agencies [1], [2] is the high cost of equipment needed to build large scale connected quantum networks. To overcome distance limitations, one can use satellites as a trusted node or backbone quantum networks depending on distance and fiber availability. Nevertheless, both solutions are not well-suited for connecting end users to a quantum network. In this work, we use passive state preparation to make the QKD transmitter as simple as possible. Combined with an optical switch, the receiver can share time among multiple transmitters, allowing us to build optimal quantum networks. An additional advantage of the passive state approach is the reduced number of critical elements affecting engineering vulnerabilities since active elements are harder to test.
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.
Most experimental realizations of quantum key distribution are based on the Bennett-Brassard 1984 (the so-called BB84) protocol. In a typical optical implementation of this scheme, the sender uses an active source to produce the required BB84 signal states. While active state preparation of BB84 signals is a simple and elegant solution in principle, in practice passive state preparation might be desirable in some scenarios, for instance, in those experimental setups operating at high transmission rates. Passive schemes might also be more robust against side-channel attacks than active sources. Typical passive devices involve parametric down-conversion. In this paper, we show that both coherent light and practical single-photon sources are also suitable for passive generation of BB84 signal states. Our method does not require any externally driven element, but only linear optical components and photodetectors. In the case of coherent light, the resulting key rate is similar to the one delivered by an active source. When the sender uses practical single-photon sources, however, the distance covered by a passive transmitter might be longer than that of an active configuration.
Quantum key distribution (QKD) allows two users to communicate with theoretically provable secrecy [1]. This is vitally important to secure the confidential data of governments, businesses and individuals. As the technology is adopted by a wider audience, a quantum network will become necessary for multi-party communication, as in the classical communication networks in use today. Unfortunately, a number of phase-encoded QKD protocols based on weak coherent pulses have been developed. Whilst the first protocol, proposed by Bennett and Brassard in 1984 (BB84), is still commonly used, other protocols such as differential phase shift [2] or coherent one way QKD [3] are also adopted. Each protocol has its benefits; however all would require a different transmitter and receiver, increasing the complexity and cost of quantum networks. In this work we demonstrate a multi-protocol transmitter [4-6] that also has the benefits of small footprint, low power consumption and low complexity. We use this transmitter to give the first experimental demonstration of an improved version of the BB84 protocol, known as the differential quadrature phase shift protocol. We have achieved megabit per second secure key rates at short distances, and have shown secure key rates that are, on average, 2.71 times higher than the standard BB84 protocol. This enhanced performance over such a commonly adopted protocol, at no expense to experimental complexity, could lead to a widespread migration to the new protocol. The security of the BB84 protocol relies on each signal and reference pulse pair being globally phase randomised with respect to all other pulse pairs. In the DQPS protocol, blocks with a length L ≥ 2 are used and each block has a globally random phase with respect to all other blocks. Implementing this protocol would ordinarily require a high-speed random number generator and a phase modulator. As well as increasing device complexity, it would also require an unrealistic continuous source of electrical modulation signals for complete security. The transmitter we use injects light from a master laser diode into a 2 GHz gain-switched slave laser diode. The principal of optical injection locking means that the slave laser inherits the phase of the master laser. We apply modulations to the master laser current within a block to control the phase of the slave laser output pulses, and then drive the master laser below threshold for a short period of time when phase randomisation is required. This ensures the lasing comes from below threshold, thus the phase adopted by the slave laser pulse is completely random. We perform an autocorrelation measurement on the blocks to show their randomness. [1] N. Gisin et al. Rev. Mod. Phys. 74, 145 (2002). [2] K. Inoue et al. Phys. Rev. Lett. 89, 037902 (2002). [3] D. Stucki et al. Appl. Phys. Lett. 87 194108 (2005). [4] Z. Yuan et al. Phys. Rev. X. 6, 031044 (2016). [5] G. L. Roberts et al. Laser Phot. Rev. 11, 1700067 (2017). [6] G. L. Roberts et al. arXiv:1709.04214 [quant-ph] (2017).
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.