The goal of quantum key distribution is to safely transfer secret data between two legitimate users through an unreliable network. This is done so by exploiting the properties of quantum mechanics. The security proofs of standard quantum key distribution protocols rely heavily on the characterization of the measurements and prepared quantum states. These assumptions, however, prove to be difficult to meet in real-life implementations. The obvious solution would come as device-independent (DI) security proofs. However, this type of implementation remains a challenge to this day. The alternative to DI found was a semi-device independent approach. Here the devices are non-characterized, and the only assumption made is the inner product information of the sent coherent states. As it is currently one of the most well-established quantum-information technologies, I shall provide a brief introduction and state-of-the-art of quantum key distribution. In this dissertation, I will expound on the implementation of a semi-device independent quantum key distribution protocol. Firstly, state preparation is discussed. The accuracy of the state preparation as well as the measurement operation will have a great impact on the performance of the protocol based on polarization states encoded on weak coherent light pulses. To ensure these are correctly implemented, a full characterization of the polarization controllers used to encode the states is made. After that, the estimation of the parameters needed to prepare the desired polarization states and their respective optimization is explained. In this chapter, the building of the systems needed to control the polarization is also discussed. In the second part of the dissertation, the experimental implementation of the semi-device independent protocol is examined in more depth. Here, the components used shall be specified and their choice is explained. The full control of the experimental set-up will also be discussed. This includes an analysis of the alignment procedures and a characterization of the weak coherent pulses. Lastly, we shall discuss the experimental realization of the protocol and the discussion of the obtained results.
The goal of quantum key distribution is to safely transfer secret data between two legitimate users through an unreliable network. This is done so by exploiting the properties of quantum mechanics. The security proofs of standard quantum key distribution protocols rely heavily on the characterization of the measurements and prepared quantum states. These assumptions, however, prove to be difficult to meet in real-life implementations. The obvious solution would come as device-independent (DI) security proofs. However, this type of implementation remains a challenge to this day. The alternative to DI found was a semi-device independent approach. Here the devices are non-characterized, and the only assumption made is the inner product information of the sent coherent states. As it is currently one of the most well-established quantum-information technologies, I shall provide a brief introduction and state-of-the-art of quantum key distribution. In this dissertation, I will expound on the implementation of a semi-device independent quantum key distribution protocol. Firstly, state preparation is discussed. The accuracy of the state preparation as well as the measurement operation will have a great impact on the performance of the protocol based on polarization states encoded on weak coherent light pulses. To ensure these are correctly implemented, a full characterization of the polarization controllers used to encode the states is made. After that, the estimation of the parameters needed to prepare the desired polarization states and their respective optimization is explained. In this chapter, the building of the systems needed to control the polarization is also discussed. In the second part of the dissertation, the experimental implementation of the semi-device independent protocol is examined in more depth. Here, the components used shall be specified and their choice is explained. The full control of the experimental set-up will also be discussed. This includes an analysis of the alignment procedures and a characterization of the weak coherent pulses. Lastly, we shall discuss the experimental realization of the protocol and the discussion of the obtained results.
Quantum key distribution (QKD) allows two remote parties to generate secret keys for cryptographic purposes. Its security has been proven with some assumptions. However, practical realizations may not comply with all the assumptions, leading to various attacks. Founded on the observation that almost all attacks are on the detection part, measurement-device-independent QKD (MDI-QKD) was proposed to remove all such attacks. This thesis presents an implementation of the protocol. In our implementation, key bit information was encoded in the polarization states of weak coherent pulses at 1542 nm wavelength in optical fibers, and decoy state techniques were employed. We ensured stable polarization preparation and alignment and developed a QKD system over 10 km of standard Telecom fibers at 500 KHz repetition rate. Our work demonstrates the practicality of MDI-QKD protocol of removing all attacks, existing and yet to be discovered, on the detection part of a QKD system.
There is a large gap between theory and practice in quantum key distribution (QKD) because real devices do not satisfy the assumptions required by the security proofs. Here, we close this gap by introducing a simple and practical measurement-device-independent-QKD type of protocol, based on the transmission of coherent light, for which we prove its security against any possible imperfection and/or side channel from the quantum communication part of the QKD devices. Our approach only requires to experimentally characterize an upper bound of one single parameter for each of the pulses sent, which describes the quality of the source. Moreover, unlike device-independent (DI) QKD, it can accommodate information leakage from the users' laboratories, which is essential to guarantee the security of QKD implementations. In this sense, its security goes beyond that provided by DI QKD, yet it delivers a secret key rate that is various orders of magnitude greater than that of DI QKD.
We demonstrate the first implementation of polarization encoding measurement-device-independent quantum key distribution (MDI-QKD), which is immune to all detector side-channel attacks. Active phase randomization of each individual pulse is implemented to protect against attacks on imperfect sources. By optimizing the parameters in the decoy state protocol, we show that it is feasible to implement polarization encoding MDI-QKD with commercial off-the-shelf devices. A rigorous finite key analysis is applied to estimate the secure key rate. Our work paves the way for the realization of a MDI-QKD network, in which the users only need compact and low-cost state-preparation devices and can share complicated and expensive detectors provided by an untrusted network server.
A key aspect in quantum information is to understand the advantage offered by quantum systems over classical ones in communication tasks. In recent years, a fundamental approach to this problem has been developed, focusing on quantum correlations in prepare-and-measure scenarios. Inspired by the developments in Bell nonlocality and device-independent information processing, this line of research aims to characterize the possibilities and limits of quantum systems for communication, in particular to precisely capture the advantage they offer over classical systems. In addition to fundamental insights, these ideas also underpin the concept of semi-device-independent quantum information processing. Exploring trade-offs between security, performance and ease-of-implementation, this approach opens promising directions for novel quantum information processing technologies and devices. A number of protocols and proof-of-principle demonstrations have been reported in recent years, in particular for quantum randomness certification and key distribution. Here, we provide a comprehensive introduction to quantum prepare-and-measure correlations and semi-device independent applications.
Quantum key distribution (QKD) enables two legitimate parties to share a secret key even in the presence of an eavesdropper. The unconditional security of QKD is based on the fundamental laws of quantum physics. Original security proofs of QKD are based on a few assumptions, e.g., perfect single photon sources and perfect single-photon detectors. However, practical implementations of QKD systems do not fully comply with such assumptions due to technical limitations. The gap between theory and implementations leads to security loopholes in most QKD systems, and several attacks have been launched on sophisticated QKD systems. Particularly, the detectors have been found to be the most vulnerable part of QKD. Much effort has been put to build side-channel-free QKD systems. Solutions such as security patches and device-independent QKD have been proposed. However, the former are normally ad-hoc, and cannot close unidentified loopholes. The latter, while having the advantages of removing all assumptions on devices, is impractical to implement today. Measurement-device-independent QKD (MDI-QKD) turns out to be a promising solution to the security problem of QKD. In MDI-QKD, all security loopholes, including those yet-to-be discovered, have been removed from the detectors, the most critical part in QKD. In this thesis, we investigate issues related to the practical implementation and security of MDI-QKD. We first present a demonstration of polarization-encoding MDI-QKD. Taking finite key effect into account, we achieve a secret key rate of 0.005 bit per second (bps) over 10 km spooled telecom fiber, and a 1600-bit key is distributed. This work, together with other demonstrations, shows the practicality of MDI-QKD. Next we investigate a critical assumption of MDI-QKD: perfect state preparation. We apply the loss-tolerant QKD protocol and adapt it to MDI-QKD to quantify information leakage due to imperfect state preparation. We then present an experimental demonstration of MDI-QKD over 10 km and 40 km of spooled fiber, which for the first time considers the impact of inaccurate polarization state preparation on the secret key rate. This would not have been possible under previous security proofs, given the same amount of state preparation flaws.
In this dissertation it is realized a study about quantum key distribution (QKD) in optical networks. Initially, a review of the theory of quantum key distribution and some of its implementations with strongly attenuated coherent states, as well a review of an optical system for physical encryptation using mesoscopic coherent states are realized. Following, it is analyzed the use of an error correction scheme in the physical encryptation system, and two new schemes for quantum key distribution are proposed. The first is a possible implementation of a hybrid system, that is, using weak and mesoscopic coherent states, for QKD and quantum authentication of classical messages. The second is an implementation of a QKD system based on relative phase modulation of sidebands frequency, using two RF carriers and an amplitude modulator in Alice and a phase modulator in Bob. After, an experimental realization of a simple QKD setup using polarization states of strongly attenuated coherent states for B92 protocol is presented. At last, it is realized an experimental characterization of an optical receiver for optical communication applications.