In this paper, we numerically evaluated an optically switched multi-user Quantum Key Distribution (QKD) network for the end-to-end secure signal transmission with the metropolitan area network (MAN) applications. The multi-user network performance can be enhanced via MEMS (Micro-Electro-Mechanical Systems) technology to get the maximum secure key rates due to the less insertion loss and crosstalk for switching operation. We have analyzed a four-state, quadrature-phase shift keying (QPSK) transmitter to generate quantum keys, i.e. Alice, while a classical coherent receiver aided by the digital post-processing unit is being used as Bob. To route the secure QKD traffic to the end-users, 1×2, 1×4, 1×8 and 1×12 switching configurations have been analyzed for the transmission distance ranging from 0 to 60 km. We have achieved the secure key rates of 30 Mbit/s and 100 Mbit/s for the 1×12 and 1×2 switching configurations, respectively. We have also compared the performance of routed QKD traffic with a simple point-to-point QKD system and the classical DV-QKD protocol. The QKD networks need an effective low-loss fibre channel and a distribution system to enhance the transmission distance of the secure keys and applications. This paper provides multi-user used case as an important paradigm with the corresponding scheme and simulation performances.
In this paper, we numerically evaluated an optically switched multi-user Quantum Key Distribution (QKD) network for the end-to-end secure signal transmission with the metropolitan area network (MAN) applications. The multi-user network performance can be enhanced via MEMS (Micro-Electro-Mechanical Systems) technology to get the maximum secure key rates due to the less insertion loss and crosstalk for switching operation. We have analyzed a four-state, quadrature-phase shift keying (QPSK) transmitter to generate quantum keys, i.e. Alice, while a classical coherent receiver aided by the digital post-processing unit is being used as Bob. To route the secure QKD traffic to the end-users, 1×2, 1×4, 1×8 and 1×12 switching configurations have been analyzed for the transmission distance ranging from 0 to 60 km. We have achieved the secure key rates of 30 Mbit/s and 100 Mbit/s for the 1×12 and 1×2 switching configurations, respectively. We have also compared the performance of routed QKD traffic with a simple point-to-point QKD system and the classical DV-QKD protocol. The QKD networks need an effective low-loss fibre channel and a distribution system to enhance the transmission distance of the secure keys and applications. This paper provides multi-user used case as an important paradigm with the corresponding scheme and simulation performances.
Encrypted data transmission is becoming increasingly more important as information security is vital to modern communication networks. Quantum Key Distribution (QKD) is a promising method based on the quantum properties of light to generate and distribute unconditionally secure keys for use in classical data encryption. Significant progress has been achieved in the performance of QKD point-to-point transmission over a fibre link between two users. The transmission distance has exceeded several hundred kilometres of optical fibre in recent years, and the secure bit rate achievable has reached megabits per second, making QKD applicable for metro networks. To realize quantum encrypted data transmission over metro networks, quantum keys need to be regularly distributed and shared between multiple end users. Optical switching has been shown to be a promising technique for cost-effective QKD networking, enabling the dynamic reconfiguration of transmission paths with low insertion loss. In this thesis, the performance of optically switched multi-user QKD systems are studied using a mathematical model in terms of transmission distance and secure key rates. The crosstalk and loss limitations are first investigated theoretically and then experimentally. The experiment and simulation both show that negligible system penalties are observed with crosstalk of -20 dB or below. A practical quantum-safe metro network solution is then reported, integrating optically-switched QKD systems with high speed reconfigurability to protect classical network traffic. Quantum signals are routed by rapid optical switches between any two endpoints or network nodes via reconfigurable connections. Proof-of-concept experiments with commercial QKD systems are conducted. Secure keys are continuously shared between virtualised Alice-Bob pairs over effective transmission distances of 30 km, 31.7 km, 33.1 km and 44.6 km. The quantum bit error rates (QBER) for the four paths are proportional to the channel losses with values between 2.6% and 4.1%. Optimising the reconciliation and clock distribution architecture is predicted to result in an estimated maximum system reconfiguration time of 20 s, far shorter than previously demonstrated. In addition, Continuous Variable (CV) QKD has attracted much research interest in recent years, due to its compatibility with standard telecommunication techniques and relatively low cost in practical implementation. A wide band balanced homodyne detection system built from modified off-the-shelf components is experimentally demonstrated. Practical limits and benefits for high speed CVQKD key transmission are demonstrated based on an analysis of noise performance. The feasibility of an optically switched CV-QKD is also experimentally demonstrated using two virtualised Alice-Bob pairs for the first time. This work represents significant advances towards the deployment of CVQKD in a practical quantum-safe metro network. A method of using the classical equalization technique for Inter-symbol-interference mitigation in CVQKD detection is also presented and investigated. This will encourage further research to explore the applications of classical communication tools in quantum communications.
This paper reports a practical quantum-safe metro network, integrating optically-switched QKD systems with high speed reconfigurability to protect classical network traffic. Quantum signals are routed by millisecond optical switches and secure keys are shared between any two endpoints or network nodes via low-latency reconfigurable connections. Efficient quantum encryption topologies between different end-users are also presented. We show experimentally the feasibility of a rapidly reconfigured QKD transmission system between multiple users in the proposed network. Classical data and control signals coexist with the quantum signals in the same fibre. Proof-of-concept experiments are conducted over effective transmission distances of 30 km, 31.7 km, 33.1 km, and 44.6 km. Software controlled QKD transmission is established between four different transmitters (Alice) and one receiver (Bob) with a switching time of a few milliseconds. The quantum bit error rates for the four paths are proportional to the channel losses with values between 2.6% and 4.1%. Optimization of the reconciliation and clock distribution architecture is predicted to result in a maximum key generation delay of 20 s, far shorter than previously demonstrated.
Data encryption is gaining much attention these days from the research community and industry for transmitting secure information over access networks, i.e. `fiber-to-the-home (FTTH)' networks and data centers. It is important that the newly designed encrypted networks are fully functional, reconfigurable, compatible, flexible and scalable with the existing deployed optical fiber networks around the globe. The prime benefit of having FTTH networks is the optical end-to-end data encryption that can best be implemented by quantum-keydistribution (QKD) protocols using state-of-the-art telecommunication components, i.e. continuous-variable quantum key distribution (CV-QKD). In this paper, we numerically investigate the quadrature phase shift keying (QPSK) based CV-QKD network that is compatible with the next generation (NG) services such as point-to-point (P2P) transmission and multicast overlay (MCO) traffic for audio/video signalling. We have further investigated the performance of quantum signals on multi-user fibers by emulating 7-, 12- and 19-core multi-core fibers (MCF). 100 Mbits/s secure key rates (SKRs) can be generated for inter-core crosstalk (XT) values of <; -20 dB. These results and theoretical framework will allow the telecommunication industry to extend these scenarios not only to accommodate more complex trust constraints, but also to take into account the robustness and resiliency of a complex encrypted network.
For secure data transmission to the end users in a conventional fiber-to-the-home (FTTH) network, quantum cryptography (QC) is getting much consideration nowadays. QC or more specifically quantum key distribution (QKD) promises unconditionally secure protocol, the Holy Grail of communication and information security that is based on the fundamental laws of quantum physics. In this chapter, we discuss the design issues in a hybrid quantum-classical communication network, performance of the cost-effective off-the-shelf telecommunication equipment, our latest results on a four-state (Quadrature Phase Shift Keying, ‘QPSK’) RF sub-carrier assisted continuous-variable quantum key distribution (CV-QKD) multiuser network based on ultra-low loss quantum channel (pure silica core fiber, ‘PSCF’) and microelectromechanical systems (MEMS) based add/drop switch. The results are thoroughly compared with the commercially available high-cost encryption modules. It is expected that the discussed cost-effective and energy efficient QKD network can facilitate the practical applications of the CV-QKD protocol on the commercial scale in near future for smart access networks.
For imparting data security to the end-users in a archetypal fiber-to-the-home (FTTH) network, quantum cryptography (QC) is getting much attention now-a-days. QC or more specifically quantum key distribution (QKD) promises unconditionally secure protocol, the Holy Grail of communication and information security, that is based on the fundamental laws of quantum physics. In this talk, we will discuss our latest experiments on a four-state (Quadrature Phase Shift Keying `QPSK') RF sub-carrier assisted continuous-variable quantum key distribution (CV -QKD) multi-user network based on ultra low loss quantum channel (pure silica core fiber `PSCF') and micro-electromechanical systems (MEMS) based add/drop switch. A coherent receiver with local local oscillator (LLO) is implemented, which ideally could not be accessed by eavesdroppers (Eve), aided with digital signal processing (DSP) module for phase noise cancellation (PNC). With 10 Gbit/s QPSK classical WDM signals, we have recorded secure key rates of 8.65 Mbit/s over 20 km and upto 12 Mbit/s over lossless channel. The experimental setup is further extended to a optically switched multi-user network, i.e. multiple Bobs, for implementing add/drop operations to achieve key rates of 5.98 Mbit/s for a 2_2 MEMS switch. It is expected that the proposed cost-effective and energy efficient QKD network can facilitate the practical application of the CV-QKD protocol on commercial scale in near future for smart access networks.
Quantum Key Distribution (QKD) is currently receiving much attention as it provides a secure source of encryption keys. Discrete-Variable QKD (DV-QKD) is possible for single photon transmission in QKD to-coexist with and encode classical wavelength division multiplexed (WDM) data with appropriate system design. Nevertheless, previous QKD field trials adopted either or both of key relay via trusted nodes and transparent link via optical switching. The former requires guaranteed physical security of the relay nodes, but can expand key distribution distance arbitrarily. The latter can realize key establishment for more users with less complexity of key management over an untrusted network. To realise the adaption of the QKD system for future high speed and long distance metropolitan world exploitation at lower cost, there has to be investigations on existing fibre infrastructures. Prior to this work, previous researches over similar distances feature extremely low secure key rates. For example, the Swiss Quantum Network between three sites displayed secure bit rates of 2.5 kbps at a fibre length of 17km. Quantum Key distribution within the 25km Cambridge Quantum Network have demonstrated the highest long-term secure key rates yet demonstrated in a field trial of at least 2.5Mb/s which is the fastest and much higher than 0.8 kbps which was reached over the similar channel loss field trial up to date. Additional field trials have been performed on the UK Quantum Network using a 66km path having 16dB loss. Combined wavelength division multiplexed 2 x 100 Gb/s traffic encrypted using QKD co-existing on the same fibres has operated for several months, with a long-term key rate of 80kb/s that is also faster than any other similar long-term QKD trial systems. In addition to this advanced commercial QKD system, there have been secure key rate analysis comparisons between laboratory fibre coils and practical field trials more than field trials only conducted before.These comparisons help to identify factors that limit future QKD network scale in both quantity and quality aspects. Also, the limit for the highest secure key rate at longest fibre length QKD in the multiplexing environment is discussed and determined in this research thesis. Nevertheless, in this thesis, improvements have been made to minimise the corresponding negative effects by investigations on the dependence of temperature have been done in order to ensure system operation environment effects. It was found from the trial results that there exists a relationship between temperature and secure key rate and further study has been done to evaluate the system sensitivity to operating temperature. Although the conventional DV-QKD system, original BB84 coding scheme, was designed to exploit the quantum properties of single photon polarization states, the trial equipment operates based upon the phase coding schemes. These coding schemes are based on the properties of interferometers and the coding is implemented by changing the relative optical path lengths or phase between the internal arms of the interferometer, while in the real transmission environment, temperature or polarization variation happens unpredictably. The existing polarisation controllers operate at relative low speed align within the interferometer, which slows to operation environment such as a punch to fibre causing phase difference. Therefore, in this project, there has been an improvement in the QKD-WDM system performance by adding an external polarization controller to minimize the Raman noise and increase the secure key rate at the longest fibre length up to date. In Summary, transmitting quantum keys over a coil of fibre in the lab differs a lot from actually putting it in the ground. This work contrasts the world fastest QKD system at the longest distance in field trials with lab fibre reels and then characterises and identifies two of the key factors, temperature and polarizations, influencing performance in practical wavelength-multiplexed secure communication systems. This is a significant step towards the coexistence of the quantum and conventional data channels on the same fibre for metropolitan networks and paves a way for an information-secure communication infrastructure.