The security of communication systems is based on key cryptography techniques. Communications on a protected channel impose the exchange of a key between Alice and Bob. Eve, a third actor in the scene tries to obtain this key without making her presence notorious to the others. The quantum security results from the impossibility to Eve to duplicate successfully signals or to extract a significant part of them without giving evidence of her intervention. Since her intervention introduces important changes in the error rate of the signals received by Bob. These errors result from incompatible observations of the same quantum object, as well as phase measurements of a single photon coded on two different basis. A weak rate of error guarantees confidentiality of the key. The protocol BB84 authorizes the elaboration and the exchange of key between Alice and Bob, it requires four quantum states forming two basis, called A1 and A2 containing two symbols, named 0 and 1, each. The A1 and A2 bases are conjugated. This thesis presents for consideration a research and an experimental realization of a quantum key distribution system using the protocol BB84 by coding in phase on a single photon at l = 1,5 µm. The generation of this single photon is assured by an ILM laser. The optical pulses are strongly attenuated. The QPSK modulation satisfies independent choices of base and symbol assured by dual-electrode Mach-Zehnder modulators. Three coherent detection schemes are proposed and compared. The successive modifications to our system take to propose a one optical way quantum key distribution scheme by DQPSK coding.
The security of communication systems is based on key cryptography techniques. Communications on a protected channel impose the exchange of a key between Alice and Bob. Eve, a third actor in the scene tries to obtain this key without making her presence notorious to the others. The quantum security results from the impossibility to Eve to duplicate successfully signals or to extract a significant part of them without giving evidence of her intervention. Since her intervention introduces important changes in the error rate of the signals received by Bob. These errors result from incompatible observations of the same quantum object, as well as phase measurements of a single photon coded on two different basis. A weak rate of error guarantees confidentiality of the key. The protocol BB84 authorizes the elaboration and the exchange of key between Alice and Bob, it requires four quantum states forming two basis, called A1 and A2 containing two symbols, named 0 and 1, each. The A1 and A2 bases are conjugated. This thesis presents for consideration a research and an experimental realization of a quantum key distribution system using the protocol BB84 by coding in phase on a single photon at l = 1,5 µm. The generation of this single photon is assured by an ILM laser. The optical pulses are strongly attenuated. The QPSK modulation satisfies independent choices of base and symbol assured by dual-electrode Mach-Zehnder modulators. Three coherent detection schemes are proposed and compared. The successive modifications to our system take to propose a one optical way quantum key distribution scheme by DQPSK coding.
Quantum key distribution (QKD) is a technology that allows two users to communicate with theoretically perfect security using standard optical fibres. This is possible by transmitting the key on single photons, meaning a measurement by an eavesdropper disturbs the system in a way observable to the legitimate parties. The technology has advanced since the first protocol proposed in 1984, to the stage where there are now many protocols that can be experimentally implemented. These protocols have allowed secure keys to be generated over distances greater than 400 km and with secure key rates over 10 Mbit/s. In a metropolitan QKD network, it would be desirable for as many users to be connected as possible. Unfortunately, each protocol comes with different requirements on the transmitter and receiver. Even within a single protocol, different clock rates can require individualised transmitter and receiver hardware. This prohibits users from communicating with all receivers, unless they have complex transmitters with hardware for many protocols. This thesis develops a transmitter for practical QKD that is able to adapt to a number of different protocols with no changes to the hardware. The transmitter works using optical injection locking, where a pulse preparation laser adopts the phase of a phase preparation laser. Controlling the phase and intensity of the pulses in this way removes the side channel ordinarily present with direct modulation, in that the phase, intensity and frequency simultaneously change in response to an applied current. The cavity-enhanced electro-optic effect allows for the first demonstration of sub-volt half-wave phase modulation at high clock rates. The transmitter successfully demonstrates phase encoding, intensity encoding and on-demand phase randomisation. This allows for the experimental realisation and direct comparison of different QKD protocols, including one that has never before been implemented due to experimental complexity. A stable intensity modulator is also developed, based on a Sagnac interferometer. This removes a side channel in QKD systems and integrates well with the directly-modulated quantum transmitter. This development also means that the transmitter can implement all current two-party QKD protocols based on weak coherent pulses. The transmitter has the potential to become the standard transmitter for future quantum communication networks due to its stability, versatility and power efficiency. The design could also be demonstrated on a photonic chip, making it compact enough to fit in small transmitter units.
We discuss the unconditional security of a quantum key distribution protocol in which bit values are encoded in the phase of a weak coherent-state pulse relative to a strong reference pulse, which is essentially the one proposed by Bennett in 1992 (the B92 scheme). In the BB84 protocol with a perfect single photon source, the key rate decreases linearly with the transmission η of the channel. If we simply replace this source with a weak coherent-state pulse, the key rate drops more rapidly (as O(η)) since the presence of multiple photons favors the eavesdropper, Eve. The B92 protocol, if modified to be encoded on the polarization of a single photon, also shows O(η) dependence. This comes from Eve’s option of sending the vacuum state, which is always registered as an inconclusive result and never causes bit errors. In the original B92 scheme with phase coding, Eve has no such option replacing the weak pulse by the vacuum cannot avoid errors completely, thanks to the strong reference pulse. This should make the scheme stronger against the loss in the channel even coherent-state pulses from conventional lasers are used. We have obtained an unconditional security proof allowing the presence of small bit errors, and confirmed the above observation in a rigorous way. The key rate is found to drop linearly with the transmission η, and it is lower than the BB84 with a perfect single-photon source only by a constant.
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.
Quantum Key Distribution (QKD) provides a secure way of generating shared cryptographic keys between a sender (Alice) and a receiver (Bob). The original BB84 protocol uses a two-dimensional polarization basis, limiting the information content of a single photon to 1 bit. Using the transverse position of single photons as one basis and the Fourier space as a second basis, one can construct a pair of mutually unbiased higher-dimensional bases. This improves not only the security of the protocol, but also the key generation rate. We present experimental results with a Spatial-Light-Modulator-based encoding scheme employing two nearly orthogonal alphabets with on the order of 103symbols each and an information content of about 10 bit.
We discuss the unconditional security of a quantum key distribution protocol in which bit values are encoded in the phase of a weak coherent-state pulse relative to a strong reference pulse, which is essentially the one proposed by Bennett in 1992 (the B92 scheme). In the BB84 protocol with a perfect single photon source, the key rate decreases linearly with the transmission T of the channel. If we simply replace this source with a weak coherent-state pulse, the key rate drops more rapidly (as O(η2)) since the presence of multiple photons favors the eavesdropper, Eve. The B92 protocol, if modified to be encoded on the polarization of a single photon, also shows O(η2) dependence. This comes from Eve's option of sending the vacuum state, which is always registered as an inconclusive result and never causes bit errors. In the original B92 scheme with phase coding, Eve has no such option--replacing the weak pulse by the vacuum cannot avoid errors completely, thanks to the strong reference pulse. This should make the scheme stronger against the loss in the channel even coherent-state pulses from conventional lasers are used. We have obtained an unconditional security proof allowing the presence of small bit errors, and confirmed the above observation in a rigorous way. The key rate is found to drop linearly with the transmission η, and it is lower than the BB84 with a perfect single-photon source only by a constant.