We present the results of simplified polarimetric methods for detecting disturbances on a fibre-optic link in a quantum key distribution system. The proposed methods use a polarisation coupler and a single polariser as a polarisation-sensitive detection system. The effectiveness of the disturbance detection was considered in a plug-and-play quantum key distribution system with phase encoding. This paper compares the performance of two simplified methods for determining changes in the state of polarisation in response to three types of disturbances, including touching, bending, and inserting a clip-on coupler. Also, the results of an experiment involving the entire procedure of introducing eavesdropping into a telecommunications fibre-optic cable are presented. The results of the measurements indicate the subsequent steps when attacking a fibre-optic cable. This article shows that a simplified polarisation-sensitive sensor can be implemented into a quantum key distribution system to detect disturbances and eavesdropping attempts, as well as enhance the security of the system.
We present the results of simplified polarimetric methods for detecting disturbances on a fibre-optic link in a quantum key distribution system. The proposed methods use a polarisation coupler and a single polariser as a polarisation-sensitive detection system. The effectiveness of the disturbance detection was considered in a plug-and-play quantum key distribution system with phase encoding. This paper compares the performance of two simplified methods for determining changes in the state of polarisation in response to three types of disturbances, including touching, bending, and inserting a clip-on coupler. Also, the results of an experiment involving the entire procedure of introducing eavesdropping into a telecommunications fibre-optic cable are presented. The results of the measurements indicate the subsequent steps when attacking a fibre-optic cable. This article shows that a simplified polarisation-sensitive sensor can be implemented into a quantum key distribution system to detect disturbances and eavesdropping attempts, as well as enhance the security of the system.
The most mature quantum key distribution systems available on the market is the class of prepare-and-measure systems which are vulnerable to eavesdropping attacks. Therefore, monitoring optical properties of such a system should be enhanced. This paper presents a measurement methodology, characterisation, and testing of a four-channel Stokes polarimeter implemented into the send-and-return quantum key distribution system with faint pulses. The developed polarimetric setup is a complete amplitude-division Stokes polarimeter, the main objective of which is to define changes in the polarization state of light pulses. The measurement method is based on intensity measurements of four selected polarization components. The four-channel Stokes polarimeter was characterised and its performance was compared with a commercial instrument. At this stage of the polarimeter development, different signal processing paths were proposed. The influence of fibre infringement was observed as a rapidly changing polarisation state and, as a result, intrusion threshold levels were determined for the analysed signals for the quantum key distribution fibre-optic link tested.
Quantum Key Distribution (QKD) employs the laws of quantum mechanics for the purpose of cryptography. Two parties, commonly called Alice and Bob, are able to share a random key which is used to encrypt a message. Any eavesdropper trying to intercept their key will have to make measurements, thereby disturbing the system. This can be detected by Alice and Bob and they will then discard their key. Polarisation encoded QKD protocols use the polarisation of single photons as qubits to generate a cryptographic key. This can be implemented using a fibre optic link between Alice and Bob but the polarisation of light is altered when passed through a fibre due to birefringence caused by asymmetries in the fibre. This causes refractive differences for orthogonal components of the state of polarisation of light, so the polarisation is rotated as the photon is transmitted through the fibre. If the fibre is fixed, the change of polarisation will be unique and constant. This can be compensated by rotating each photon appropriately to its original state. Under typical environmental conditions, such as temperature changes and vibrations, the birefringence effects vary and should be compensated in real time. Therefore, an active polarisation controller is needed in order to maintain the state of polarisation of each qubit. An investigation was done to first track how the state of polarisation changes over time in a natural environment. Both wavelength-division multiplexing and time-division multiplexing were investigated as testing methods for the compensation system. A timedivision multiplexed system was developed to compensate the changes in polarisation. Since QKD protocols such as BB84 and B92 utilise two non-orthogonal bases, two polarisation controllers are usually used for compensation. However, by using a search algorithm, one polarisation controller was able to isolate the plane on the Poincare sphere that passes through both bases, thus compensating non-orthogonal states with one device.
Robust, simple, and flexible quantum key distribution (QKD) is vital for realising practical applications of this technique. Contrary to typical phase-coded QKD schemes, the plug-and-play QKD design requires only one arm-length-insensitive interferometer without active feedback, in which the noise is automatically compensated by the two-way structure. However, there are certain possible loopholes in the typical plug-and-play designs, which require consideration and strict monitoring. This study proposes a new design of plug-and-play QKD based on two-way protocol with enhanced security and presents an experimental demonstration of said scheme. The security is analysed under a collective attack scenario assisted by the decoy state method. The scheme was implemented in a 50.4 km commercial fibre without active feedback. The system showed highly robust performance with an ultra-low error rate and maintained an ultra-high visibility of 0.9947+/-0.0002 through significant environmental changes over 24 hours. We analysis the performance of the system and give a brief discussion of the possible loopholes in practical applications.
In 1946, C. E. Shannon proved that One-Time Pad is truly unbreakable [1].However, stringent conditions pose difficulties such as the key distribution problem which limited its practicability.Fortunately, public key cryptosystem widely used today was developed to solve the key distribution problem.Its security is based on the assumption that an adversary has limited computational power to factor a number with large prime factors.With the increase in computational power and technological advancement, this security may one day be compromised.In contrast, quantum key distribution (QKD) offers a platform for secure key distribution and everlasting secrecy.Unlike conventional cryptography, quantum key distribution's security is governed by the laws of quantum mechanics.The basic principle of quantum key distribution is to encode classical binary bit information onto the properties of quantum states such as the polarisation of a photon.Because of the quantum no-cloning theorem, an eavesdropper is unable to simply duplicate these photons.Moreover, by intercepting these photons, the eavesdropper will leave detectable trace which will reveal its presence.In 1989, the first experimental demonstration of QKD using the polarisations of single photons based on the BB84 protocol occurred through 32cm of air [2].Since then, QKD over hundreds of kilometres of optical fibre Abstract iv and free space have been reported [3][4][5].Key generation rate of a few megahertz have also been demonstrated [6][7][8].However, photons travelling through optical fibres are subjected to random polarisation drifts.Moreover, clock drifts often causes inaccuracy during the detection of these photons.If these performance issues were not addressed, the reliability and availability of QKD systems and their cryptographic keys will be affected.Therefore, polarisation recovery schemes have been implemented to mitigated polarisation drifts [9][10][11][12][13][14].However, these schemes often limit the transmission distance, slow down or even disrupt the key generation process for polarisation recovery.These limitations lead to the proposed development of a polarisation-encoded QKD system based on an adaptive polarisation state monitoring and recovery scheme that adapts the system to the existing polarisation drift condition in the transmission link to enhance its reliability and availability.On the other hand, current high-speed single-photon detection schemes are often designed to work with idealised parameters such as fixed gating rate and operating temperature.Therefore, such schemes are unable to accommodate changes in gating frequency induced by clock drifts which results in the reduction of detection efficiency.Hence, a proposed robust high-speed singlephoton avalanche diode with tunable sinusoidal gate frequency was developed to mitigate the effect caused by clock drift in order to maintain the detection efficiency over varying operating conditions.
Abstract Robust, simple, and flexible quantum key distribution (QKD) is vital for realizing practical applications of this technique. Contrary to typical phase-coded QKD schemes, the plug-and-play QKD design requires only one arm-length-insensitive interferometer without active feedback, in which the noise is automatically compensated by the two-way structure. However, there are certain possible loopholes in the typical plug-and-play designs, which require consideration and strict monitoring. This study proposes a new design of theoretically loophole-free plug-and-play QKD scheme with two-way protocol and presents an experimental demonstration of said scheme. The security is analyzed under a collective attack scenario assisted by the decoy state method. The scheme was implemented in a 50.4 km commercial fibre without active feedback. The system showed highly robust performance with an ultra-low error rate and maintained an ultra-high visibility of 0.9947 ± 0.0002 through significant environmental changes over 24 hours.
Abstract Quantum key distribution (QKD) based on entangled photon pairs holds the potential for repeater-based quantum networks connecting clients over long distance. We demonstrate long-distance entanglement distribution by means of polarisation-entangled photon pairs through two successive deployed 96 km-long telecommunications fibres in the same submarine cable. One photon of each pair was detected directly after the source, while the other travelled the fibre cable in both directions for a total distance of 192 km and attenuation of 48 dB. The observed two-photon Bell state exhibited a fidelity 85 ± 2% and was stable over several hours. We employed neither active stabilisation of the quantum state nor chromatic dispersion compensation for the fibre.