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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Engineering quantum communication systems

Armando N. Pinto, Álvaro J. Almeida, Nuno A. Silva, Nelson Jesus Muga, L. M. P. P. Martins

Published Apr 10, 2012• 0 citations
Publisher

Abstract

Quantum communications can provide almost perfect security through the use of quantum laws to detect any possible leak of information. We discuss critical issues in the implementation of quantum communication systems over installed optical fibers. We use stimulated four-wave mixing to generate single photons inside optical fibers, and by tuning the separation between the pump and the signal we adjust the average number of photons per pulse. We report measurements of the source statistics and show that it goes from a thermal to Poisson distribution with the increase of the pump power. We generate entangled photons pairs through spontaneous four-wave mixing. We report results for different type of fibers to approach the maximum value of the Bell inequality. We model the impact of polarization rotation, attenuation and Raman scattering and present optimum configurations to increase the degree of entanglement. We encode information in the photons polarization and assess the use of wavelength and time division multiplexing based control systems to compensate for the random rotation of the polarization during transmission. We show that time division multiplexing systems provide a more robust solution considering the values of PMD of nowadays installed fibers. We evaluate the impact on the quantum channel of co-propagating classical channels, and present guidelines for adding quantum channels to installed WDM optical communication systems without strongly penalizing the performance of the quantum channel. We discuss the process of retrieving information from the photons polarization. We identify the major impairments that limit the speed and distance of the quantum channel. Finally, we model theoretically the QBER and present results of an experimental performance assessment of the system quality through QBER measurements.

DOI
10.1117/12.921547

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Quantum communications can provide almost perfect security through the use of quantum laws to detect any possible leak of information. We discuss critical issues in the implementation of quantum communication systems over installed optical fibers. We use stimulated four-wave mixing to generate single photons inside optical fibers, and by tuning the separation between the pump and the signal we adjust the average number of photons per pulse. We report measurements of the source statistics and show that it goes from a thermal to Poisson distribution with the increase of the pump power. We generate entangled photons pairs through spontaneous four-wave mixing. We report results for different type of fibers to approach the maximum value of the Bell inequality. We model the impact of polarization rotation, attenuation and Raman scattering and present optimum configurations to increase the degree of entanglement. We encode information in the photons polarization and assess the use of wavelength and time division multiplexing based control systems to compensate for the random rotation of the polarization during transmission. We show that time division multiplexing systems provide a more robust solution considering the values of PMD of nowadays installed fibers. We evaluate the impact on the quantum channel of co-propagating classical channels, and present guidelines for adding quantum channels to installed WDM optical communication systems without strongly penalizing the performance of the quantum channel. We discuss the process of retrieving information from the photons polarization. We identify the major impairments that limit the speed and distance of the quantum channel. Finally, we model theoretically the QBER and present results of an experimental performance assessment of the system quality through QBER measurements.
This thesis begins by proposing the implementation of a probabilistic photon source based on the stimulated four-wave mixing (FWM) process. This source was implemented experimentally and characterized in terms of its statistical distribution. Next, the impact of the stimulated FWM process in a co-propagating quantum signal was studied experimentally. Finally, the violation of Clauser-Horne-Shimony-Holt (CHSH) inequality was experimentally verified using polarization-entangled photon pairs, which were obtained from the spontaneous FWM process in a Sagnac loop. The experimental evolution of the quantum-bit error rate (QBER) in a system without control of polarization, using this degree of freedom to encode information, was studied. It was found out that the QBER increases with the length of the transmission fiber. It was also verified that the increase in the QBER was due to the random rotation of photon’s polarization. A model for the rigorous estimation of the QBER was derived and developed an automatic method to compensate the random rotations of polarization. The method was validated numerically and experimentally, in a transmission system with 40km, showing that it can compensate for the rotations that photons suffer during propagation in optical fibers. Finally, a quantum bit commitment (QBC) protocol between two untrusted entities was implemented. The encoding was performed using two nonorthogonal states of polarization (SOPs). As quantum channel between the two entities, it was first assumed that the transmitter and the receiver were side by side, and after that, they were separated by 8 km and finally, that they were 16km from each other. The implementation of the protocol was performed with a success rate in measurements exceeding 93%, well above the theoretical security limit of 85%. The best strategy for deceiving the commitment was also implemented, and its security experimentally confirmed with a confidence of 7 standard deviations.
Quantum laws can be used to implement secure communication channels; this has been named quantum cryptography. In quantum cryptography the security does not depend of limited computational power, but is inherent to the laws that govern the propagation and detection of single and entangled photons. We show how single and entangled photon-pairs can be efficiently generated using four-wave mixing in optical fibers. We analyze the source statistics, degree of entanglement and impact of spontaneous Raman scattering. By coding information in the photons polarization we are able to transmit quantum information over 20 km of standard single mode fiber.
Quantum communications is an emerging field with many promising applications. Its usefulness and range of applicability in optical fiber will depend strongly on the extent to which quantum channels can be reliably transported over transparent reconfigurable optical networks, rather than being limited to dedicated point-to-point links. This presents a number of challenges, particularly when single-photon quantum and much higher power classical optical signals are combined onto a single physical infrastructure to take advantage of telecom networks built to carry conventional traffic. In this paper, we report on experimental demonstrations of successful quantum key distribution (QKD) in this complex environment, and on measurements of physical-layer impairments, including Raman scattering from classical optical channels, which can limit QKD performance. We then extend the analysis using analytical models incorporating impairments, to investigate QKD performance while multiplexed with conventional data channels at other wavelengths. Finally, we discuss the implications of these results for evaluating the most promising domains of use for QKD in real-world optical networks.
We show how to generate, encode, transmit and detect single photons. By using single photons we can address two of the more challenging problems that communication engineers face nowadays: capacity and security. Indeed, by decreasing the number of photons used to encode each bit, we can efficiently explore the full capacity to carry information of optical fibers, and we can guarantee privacy at the physical layer. We present results for single and entangled photon generation. We encode information in the photons polarization and after transmission we retrieve that information. We discuss the impact of fiber birefringence on the photons polarization.
We discuss recent work in quantum communication, and in some details present a bright, narrowband, portable, quasi-phase-matched two-crystal source generating polarization-entangled photon pairs at 809 nm and 1555 nm. We also show how the single-photon quantum channel at 1555 nm and a classical synchronization signal gating the single photon detector at the receiving side can be multiplexed in the same optical fiber of length 27 km by means of wavelength division multiplexers (WDM) having 100 GHz (0.8 nm) spacing between channels. This illustrates how single-photon quantum communication applications is compatible with current high-speed optical networks.
Quantum communications is fast becoming an important component of many applications in quantum information science. Sharing quantum information over a distance among geographically separated nodes using photonic qubits requires a reconfigurable transparent networking infrastructure that can support quantum information services. Using quantum key distribution (QKD) as an example of a quantum communications service, we investigate the ability of fiber networks to support both conventional optical traffic and single-photon quantum communications signals on a shared infrastructure. The effect of Raman scattering from conventional channels on the quantum bit error rate (QBER) of a QKD system is analyzed. Additionally, the potential impact and mitigation strategies of other transmission impairments such as four-wave mixing, cross-phase modulation, and noise from mid-span optical amplifiers are discussed. We also review recent trends toward the development of automated and integrated QKD systems which are important steps toward reliable and manufacturable quantum communications systems.