The decoy-state BB84 protocol for quantum key distribution (QKD) is used on Micius, the world’s first satellite for quantum communications. The method of decoy states can detect photon-number-splitting eavesdropping and thus enables, in theory, secure QKD using weak coherent pulses over long distances with high channel loss inherent in satellite communication systems. However, it is widely known that realistic QKD devices can be vulnerable to various types of side-channel attacks that rely on flaws in experimental implementation. In most free-space QKD systems, including that on board Micius, multiple semiconductor lasers with passive optics are utilized to randomly generate polarization states. Optical pulses from independent laser diodes can to some extent differ in their temporal, spectral, and/or spatial distribution, and the quantum states can thus be distinguishable. Such distinguishability of photons in additional non-operational degrees of freedom compromises unconditional security of QKD since an eavesdropper can exploit this loophole to improve their attack strategy. The author carried out a thorough analysis of the experimental data obtained during multiple communication sessions between Micius and one of the ground stations designed specifically for it. Relative time delays between all the eight laser diodes on board have been found. The typical desynchronization between the lasers exceeds 100 ps, which is comparable with their pulse duration of 200 ps. The largest time delay was observed between signal and decoy states for vertically polarized photons and is about 300 ps. With such mismatch in timing, a potential attacker using as perfect equipment as possible unless it violates the laws of physics was shown to be capable of distinguishing decoy states from signal ones in at least 98.7% of cases. It offers great potential for hacking since one of the assumptions that the security of the decoy-state protocol is based on is that intensity levels selected for transmission of qubits are not known in advance. Based on a prior theoretical study, where a photon number splitting attack strategy that leverages the benefits of distinguishable decoy states is proposed, the author shows that distribution of quantum keys from Micius was insecure.
The decoy-state BB84 protocol for quantum key distribution (QKD) is used on Micius, the world’s first satellite for quantum communications. The method of decoy states can detect photon-number-splitting eavesdropping and thus enables, in theory, secure QKD using weak coherent pulses over long distances with high channel loss inherent in satellite communication systems. However, it is widely known that realistic QKD devices can be vulnerable to various types of side-channel attacks that rely on flaws in experimental implementation. In most free-space QKD systems, including that on board Micius, multiple semiconductor lasers with passive optics are utilized to randomly generate polarization states. Optical pulses from independent laser diodes can to some extent differ in their temporal, spectral, and/or spatial distribution, and the quantum states can thus be distinguishable. Such distinguishability of photons in additional non-operational degrees of freedom compromises unconditional security of QKD since an eavesdropper can exploit this loophole to improve their attack strategy. The author carried out a thorough analysis of the experimental data obtained during multiple communication sessions between Micius and one of the ground stations designed specifically for it. Relative time delays between all the eight laser diodes on board have been found. The typical desynchronization between the lasers exceeds 100 ps, which is comparable with their pulse duration of 200 ps. The largest time delay was observed between signal and decoy states for vertically polarized photons and is about 300 ps. With such mismatch in timing, a potential attacker using as perfect equipment as possible unless it violates the laws of physics was shown to be capable of distinguishing decoy states from signal ones in at least 98.7% of cases. It offers great potential for hacking since one of the assumptions that the security of the decoy-state protocol is based on is that intensity levels selected for transmission of qubits are not known in advance. Based on a prior theoretical study, where a photon number splitting attack strategy that leverages the benefits of distinguishable decoy states is proposed, the author shows that distribution of quantum keys from Micius was insecure.
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.
With the potential of quantum computers to compromise current public-key cryptosystems, Quantum Key Distribution (QKD), which is based on the physical laws of quantum mechanics, has garnered significant attention. However, while QKD security proofs typically assume ideal single-photon sources, realistic implementations rely on Weak Coherent Pulses (WCP) due to the difficulties associated with commercializing true single-photon sources. Unfortunately, the multi-photon components inherent in WCPs are vulnerable to Photon Number Splitting (PNS) attacks, which limits the secure transmission distance to a few tens of kilometers. This article introduces the Decoy State method, which effectively resolves this issue. Proposed by W.-Y. Hwang in 2003 and further advanced by H.-K. Lo and X.-B. Wang, this technique utilizes the random interleaving of pulses with different mean photon numbers during transmission. By doing so, it enables the accurate estimation of single-photon yield and error rates, thereby effectively detecting PNS attacks. Because it can be implemented using standard commercial lasers and intensity modulators, the Decoy State method has become the standard technology in most modern QKD systems. It serves as a core component for long-distance quantum communication, enabling major milestones reported recently. This article reviews the principles, mathematical analysis, and latest applications of the Decoy State method.
Problem statement: Quantum key distribution provides unconditional security guaranteed by the fundamental laws of quantum physics. Unfortunately, for real-life experimental set-ups, which mainly based on faint laser pulses, the occasional production of multi-photons and channel loss make it possible for sophisticated eavesdroppers to launch various subtle eavesdropping attacks including the Photon Number Splitting (PNS) attack. The decoy state protocols recently proposed to beat PNS attack and to improve dramatically distance and secure key generation rate of Quantum Key Distribution (QKD). Approach: Objective of this study was experimental implementation of weak decoy + vacuum states QKD for increasing the performance of QKD system. To show conceptually how simple it was to apply the weak decoy + vacuum state idea to a commercial QKD system, we chosen ID-3000 commercial quantum key distribution system manufactured by id quantique. To implement the weak decoy + vacuum state protocol, we had to add some new optical and electronics components to id quantique and to attenuate each signal to the intensity of either signal state or weak decoy or vacuum state randomly. Results: In our implementation, the attenuation will be done by placing a VOA (variable optical attenuator) in Alice’s side. Specifically, our QKD system required the polarizations of 2 pulses from the same signal to be orthogonal. Therefore the VOA must be polarization independent so as to attenuate the two pulses equally. The VOA utilized in experiment to attenuate signals dynamically was Intensity Modulator (IM). We had implemented weak + vacuum protocol on a modified commercial QKD system over a 25 km of telecom fibers with an unconditionally secure key rate of 6.2931×10-4 per pulse. Conclusion: By making simple modifications to a commercial quantum key distribution system, we could achieve much better performance with substantially higher key generation rate and longer distance than QKD system without decoy state.
The Micius satellite is the pioneering initiative to demonstrate quantum teleportation, entanglement distribution, quantum key distribution (QKD), and quantum-secured communications experiments at the global scale. In this work, we report on the results of the 600-mm-aperture ground station design which has enabled the establishment of a quantum-secured link between the Zvenigorod and Nanshan ground stations using the Micius satellite. As a result of a quantum communications session, an overall sifted key of 2.5 Mbits and a total final key length of 310 kbits have been obtained. We present an extension of the security analysis of the realization of satellite-based QKD decoy-state protocol by taking into account the effect of the detection-efficiency mismatch for four detectors. We also simulate the QKD protocol for the satellite passage and by that validate our semi-empirical model for a realistic receiver, which is in good agreement with the experimental data. Our results pave the way to the considerations of realistic imperfection of the QKD systems, which are important in the context of their practical security.
In the age of information and globalisation, secure communication as well as the protection of sensitive data against unauthorised access are of utmost importance. Quantum cryptography currently provides the only way to exchange a cryptographic key between two parties in an unconditionally secure fashion. Owing to losses and noise of today's optical fibre and detector technology, at present quantum cryptography is limited to distances below a few 100 km. In principle, larger distances could be subdivided into shorter segments, but the required quantum repeaters are still beyond current technology. An alternative approach for bridging larger distances is a satellite-based system, that would enable secret key exchange between two arbitrary points on the globe using free-space optical communication. The aim of the presented experiment was to investigate the feasibility of satellite-based global quantum key distribution. In this context, a free-space quantum key distribution experiment over a real distance of 144 km was performed. The transmitter and the receiver were situated in 2500 m altitude on the Canary Islands of La Palma and Tenerife, respectively. The small and compact transmitter unit generated attenuated laser pulses, that were sent to the receiver via a 15-cm optical telescope. The receiver unit for polarisation analysis and detection of the sent pulses was integrated into an existing mirror telescope designed for classical optical satellite communications. To ensure the required stability and efficiency of the optical link in the presence of atmospheric turbulence, the two telescopes were equipped with a bi-directional automatic tracking system. Still, due to stray light and high optical attenuation, secure key exchange would not be possible using attenuated pulses in connection with the standard BB84 protocol. The photon number statistics of attenuated pulses follows a Poissonian distribution. Hence, by removing a photon from all pulses containing two or more photons, an eavesdropper could measure its polarisation without disturbing the polarisation state of the remaining pulse. In this way, he can gain information about the key without introducing detectable errors. To protect against such attacks, the presented experiment employed the recently developed method of using additional "decoy" states, i.e., the the intensity of the pulses created by the transmitter were varied in a random manner. By analysing the detection probabilities of the different pulses individually, a photon-number-splitting attack can be detected. Thanks to the decoy-state analysis, the secrecy of the resulting quantum key could be ensured despite the Poissonian nature of the emitted pulses. For a channel attenuation as high as 35 dB, a secret key rate of up to 250 bit/s was achieved. Our outdoor experiment was carried out under real atmospheric conditions and with a channel attenuation comparable to an optical link from ground to a satellite in low earth orbit. Hence, it definitely shows the feasibility of satellite-based quantum key distribution using a technologically comparatively simple system.
Quantum key distribution (QKD) can in principle offer unconditional security by making use of the fundamental laws of quantum mechanics. In practice, this is typically achieved by preparing individual photons in quantum superposition states and sending them to a remote receiver. To date, most commercially available QKD systems rely on the transmission of the photons via optical fibers, which, due to channel loss and detector noise, limits the distance over which QKD is feasible to a few hundred kilometers. Alternatively, satellite-based QKD facilitates low photon loss and negligible signal disturbance and offers a viable solution for establishing a global scale quantum network. Recently, a quantum science mission of the Chinese Academy of Sciences (CAS) in collaboration with the Austrian Academy of Sciences (AAS) and the University of Vienna aimed at a satellite-based intercontinental quantum-key relay. Using the Chinese Quantum Science Satellite “Micius” as a trusted relay, a quantum network consisting of three optical ground stations located in China (Xinglong, Nanshan) and Austria (Graz-Lustbühel) has been demonstrated successfully. Here we report on the development of a quantum receiving module, installed at the Satellite Laser Ranging Station in Graz (Austria) capable of implementing the so-called decoy-state QKD protocol in a downlink scenario from the LEO satellite “Micius”. Furthermore, we will present the experimental results obtained during several downlinks from the Chinese satellite focusing on the performance of the Austrian receiving station.