Distributed-phase-reference (DPR) systems were introduced as a method of decreasing the complexity of quantum key distribution systems for practical use. However, their information-theoretic security has only been proven when the added requirement of block-wise phase randomisation is met. Realisation of this with a conventional approach would result in a cumbersome transmitter, removing any practical advantage held by DPR systems. Here, we solve this problem using a light source that allows the coherence between pulses to be controlled on a pulse-by-pulse basis without the need for additional bulky components. The system is modulator-free, does not require a complex receiver, and features an excellent stability without an active stabilisation mechanism. We achieve megabit per second key rates that are almost three times higher than those obtained with the standard Bennet-Brassard 1984 protocol.
Distributed-phase-reference (DPR) systems were introduced as a method of decreasing the complexity of quantum key distribution systems for practical use. However, their information-theoretic security has only been proven when the added requirement of block-wise phase randomisation is met. Realisation of this with a conventional approach would result in a cumbersome transmitter, removing any practical advantage held by DPR systems. Here, we solve this problem using a light source that allows the coherence between pulses to be controlled on a pulse-by-pulse basis without the need for additional bulky components. The system is modulator-free, does not require a complex receiver, and features an excellent stability without an active stabilisation mechanism. We achieve megabit per second key rates that are almost three times higher than those obtained with the standard Bennet-Brassard 1984 protocol.
Quantum key distribution (QKD) and quantum communication enable the secure exchange of information between remote parties. Currently, the distributed-phase-reference (DPR) protocols, which are based on weak coherent pulses, are among the most practical solutions for long-range QKD. During the last 10 years, long-distance fiber-based DPR systems have been successfully demonstrated, although fundamental obstacles such as intrinsic channel losses limit their performance. Here, we introduce the first two-dimensional DPR-QKD protocol in which information is encoded in the time and phase of weak coherent pulses. The ability of extracting two bits of information per detection event, enables a higher secret key rate in specific realistic network scenarios. Moreover, despite the use of more dimensions, the proposed protocol remains simple, practical, and fully integrable.
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.
We prove the security of the Bennett-Brassard (BB84) quantum key distribution protocol in the case where the key information is encoded in the relative phase of a coherent-state reference pulse and a weak coherent-state signal pulse, as in some practical implementations of the protocol. In contrast to previous work, our proof applies even if the eavesdropper knows the phase of the reference pulse, provided that this phase is not modulated by the source, and even if the reference pulse is bright. The proof also applies to the case where the key is encoded in the photon polarization of a weak coherent-state pulse with a known phase, but only if the phases of the four BB84 signal states are judiciously chosen. The achievable key generation rate scales quadratically with the transmission in the channel, just as for BB84 with phase-randomized weak coherent-state signals (when decoy states are not used). For the case where the phase of the reference pulse is strongly modulated by the source, we exhibit an explicit attack that allows the eavesdropper to learn every key bit in a parameter regime where a protocol using phase-randomized signals is provably secure.
Abstract To guarantee the security of quantum key distribution (QKD), security proofs of QKD protocols have assumptions on the devices. Commonly used assumptions are, for example, each random bit information chosen by a sender to be precisely encoded on an optical emitted pulse and the photon-number probability distribution of the pulse to be exactly known. These typical assumptions imposed on light sources such as the above two are rather strong and would be hard to verify in practical QKD systems. The goal of the paper is to replace those strong assumptions on the light sources with weaker ones. In this paper, we adopt the differential-phase-shift (DPS) QKD protocol and drastically mitigate the requirements on light sources, while for the measurement unit, trusted and photon-number-resolving detectors are assumed. Specifically, we only assume the independence among emitted pulses, the independence of the vacuum emission probability from a chosen bit, and upper bounds on the tail distribution function of the total photon number in a single block of pulses for single, two and three photons. Remarkably, no other detailed characterizations, such as the amount of phase modulation, are required. Our security proof significantly relaxes demands for light sources, which paves a route to guarantee implementation security with simple verification of the devices.
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.