Manipulating photon coherence to enhance the security of distributed phase reference quantum key distribution

Abstract

Quantum key distribution (QKD) allows two users to communicate with theoretically provable secrecy by encoding information on photonic qubits. Current encoders are complex, however, which reduces their appeal for practical use and introduces potential vulnerabilities to quantum attacks. Distributed-phase-reference (DPR) systems were introduced as a simpler alternative, but have not yet been proven practically secure against all classes of attack. Here we demonstrate the first DPR QKD system with information-theoretic security. Using a novel light source, where the coherence between pulses can be controlled on a pulse-by-pulse basis, we implement a secure DPR system based on the differential quadrature phase shift protocol. The system is modulator-free, does not require active stabilization or a complex receiver, and also offers megabit per second key rates, almost three times higher than the standard Bennett-Brassard 1984 (BB84) protocol. This enhanced performance and security highlights the potential for DPR protocols to be adopted for real-world applications.Quantum key distribution (QKD) has developed strongly since the proposal of the first protocol in 1984 1-3 . The future could see widespread quantum networks similar to those in Tokyo 4 and Vienna 5 and global secure communication enabled by QKD over satellites 6 . These advances depend on the development of simple, cost-effective and high performance implementations. Innovations in both protocols and system hardware are required to achieve this.Nearly two decades after the inception of BennettBrassard 1984 (BB84) 1 , distributed phase reference (DPR) QKD was proposed, allowing for much simpler experimental implementations. The class includes the differential phase shift 7,8 and coherent-one-way 9,10 protocols. One advantage is that the transmitters needed to realize these DPR protocols can be made using off-the-shelf telecom lasers and modulators. However the benefit of their simpler implementation is outweighed by a seriously degraded performance when full security is taken into account 3,11,12 . To plug the security gap, two further DPR protocols were proposed: round-robin differential phase shift and differential quadrature phase shift (DQPS). The former simplifies the estimation of Eve's information, but requires an overly complicated QKD receiver setup 13-16 , making it impractical. The latter separates the signal from the differential phase shift protocol into blocks, each having a global phase that varies randomly, ensuring the protocol is immune against coherent attacks 17,18 . It does, however, stray from the main goal of DPR protocols to provide simpler QKD implementations, due to the phase randomization requirement that would ordinarily require extra system components. In this work we show it is possible to produce phase coherent and phase randomized pulses from a single device. This device is based on optical injection of one laser diode into another, removing the need for a phaserandomization component in DQPS by relying on the randomness pr...

Experimental quantum key distribution beyond the repeaterless secret key capacity

“…This produced the histogram in Fig. 5, which closely resembles the one expected from phase-randomised laser fields [38,39,48]. We then confirmed the phase coordination between the users by interfering pulse pairs emitted by the users' lasers at the same clock and obtained a first-order interference visibility of 98.7%, showing that active discrete phase randomisation and passive continuous phase randomisation generate a similar visibility.…”

“…For the global random phase, figure. Also shown is the simulation line that accounts for experimental imperfections (solid black line). The agreement between the experimental results and simulation indicates that the pulses have random phase [48]. An analogous measurement on Bob's slave laser gives similar results.…”

Advanced Laser Technology for Quantum Communications (Tutorial Review)

“…[93] In fact, by directly modulating the phase of the primary laser it is possible to encode the qubits in the pulses emitted by the secondary laser, without the need of the interferometer and the phase modulator. The versatility of this so-called phase-seeding approach is remarkable since it makes possible to implement multiple QKD protocols by simply changing the current modulation format of the primary laser, [94][95][96] as we will illustrate in next section.…”

Performance evaluation and security analysis of ground‐to‐satellite FSO system with CV‐QKD protocol

“…Furthermore, a straightforward and cost-effective system can be satisfied using non-coherent detection techniques known as intensity modulation/direct detection [4]. Unlike the previous two schemes, the DPR coding scheme resorts to the phase difference between two successive signal pulses or the photon arrival times to encode the critical information [5]. EB-QKD protocol generates pairs of photons (entangled-photons), each entangled photon-pair distributes between Alice and Bob, who independently measure the photon distributed and jointly form a secret key based on a series of measurements [6].…”