We describe progress in developing a free-space QKD system that encodes information in both the radial and azimuthal degrees of freedom. Such a system makes optimum use of the sizes of sending and receiving telescopes.
In this paper we are looking at the milestones that were achieved in free−space quantum key distribution as well as the current state of this technology. First a brief overview introduces the technical prerequisites that will help to better understand the rest of the paper. After looking into the first successful demonstrations of short range free space QKD both indoor and outdoor, we are examining the longer range terrestrial QKD experiments. In the next step we look at some experiments that were aiming to take free space QKD to the next level by placing the sender or the receiver on moving vehicles. After the terrestrial demonstrations we focus on satellite based experiments. Finally, we explore hyper-dimensional QKD, utilising energy−time, polarization and orbital angular momentum (OAM) degrees of freedom.
It is established that the manipulation of quantum information is bound only by the laws of physics and thus information can be characterised, quantified and processed as a physical entity using the basic properties of quantum mechanics.The most advanced quantum information related technology at present is Quantum Key Distribution (QKD).QKD encodes information into a quantum data carrier, in particular, a single photon that is transported through a quantum channel to produce secure key.To date QKD has been demonstrated across two types of channels which is generally fibre or free space.Free space QKD is beneficial since it provides various mediums for the encoding.When considering the atmosphere as a medium for free space QKD, one of the challenges is the effects of turbulence.Apart from the turbulence effects, one of the other challenges faced with implementing a QKD system, is the use of an appropriate source of single photons for the encoding.A good implementation of a source of single photons would be to make use of entanglement.This thesis addresses the above two aspects of QKD Development of Encoding and Entanglement for Free Space Quantum Key Distribution DECLARATION 1 -PLAGIARISM I, Yaseera Ismail, declare that 1.The research reported in this thesis, except where otherwise indicated, is my original research.2. This thesis has not been submitted for any degree or examination at any other university.3. This thesis does not contain other persons' data, pictures, graphs or other information, unless specifically acknowledged as being sourced from other persons.4. This thesis does not contain other persons writing, unless specifically acknowledged as being sourced from other researchers.Where other written sources have been quoted, then:(a) Their words have been re-written but the general information attributed to them has been referenced (b) Where their exact words have been used, then their writing has been placed in italics and inside quotation marks, and referenced.5. This thesis does not contain text, graphics or tables copied and pasted from the Internet, unless specifically acknowledged, and the source being detailed in the thesis and in the References sections.
We report on the progress of our real-time entanglement based free-space quantum key distribution (QKD) system which uses polarization entangled photon pairs sent over a variety of free-space optical telescope links to distribute the key. An experiment with one photon from each pair sent over a 1,325 m long free-space link and the other photon detected locally next to the source is described. The system performs the complete QKD protocol including all error correction and privacy amplification algorithms. Over the course of 6.5 hours of communication at night an average raw key rate of 1,398 bits/s with an average quantum bit error rate (QBER) of 4.58% was observed producing an average final key rate of 244 bits/s. We also performed a Bell inequality experiment over two free-space links of 435 m and 1,325 m respectively, producing a total separation of 1,575 m between the two detectors in the experiment. During the Bell inequality experiment we observed an average Bell parameter of 2.51 ± 0.11.
It all begun by the early 1970s through, the work of Stephen Wiesner and Gilles Brassard, who introduced the concept of quantum conjugate coding, and it has now stretched to a level where today's agile communication systems constitute a substantial challenge for data security and safety. A number of research has been performed to achieve secure communication among legitimate parties in which QKD (Quantum Key Distribution) is prominent. Quantum Key Distribution (QKD), also called Quantum Cryptography, enables two communicating parties to share secret cryptographic keys with security that is theoretically guaranteed by the laws of quantum mechanics, assuming ideal conditions. However, practical implementations may be vulnerable to certain attacks, and the security of QKD can be compromised by imperfections in hardware, side-channel attacks, or limitations in real-world setups. In a practical QKD setup, two stations send and receive encoded quantum particles and some classical information. The most rudimentary and well-known QKD protocol is BB84, developed by Bennet and Brassard in 1984. We have developed the simulation and achieved the secret shared key with and without an intruder.
With the emergence of the field of quantum communications, the appropriate choice of photonic degrees of freedom used for encoding information is of paramount importance. Highly precise techniques for measuring the polarisation, frequency, and arrival time of a photon have been developed. However, the transverse spatial degree of freedom still lacks a measurement scheme that allows the reconstruction of its full transverse structure with a simple implementation and a high level of accuracy. Here we show a method to measure the azimuthal and radial modes of Laguerre-Gaussian beams with a greater than 99 % accuracy, using a single phase screen. We compare our technique with previous commonly used methods and demonstrate the significant improvements it presents for quantum key distribution and state tomography of high-dimensional quantum states of light. Moreover, our technique can be readily extended to any arbitrary family of spatial modes, such as mutually unbiased bases, Hermite-Gauss, and Ince-Gauss. Our scheme will significantly enhance existing quantum and classical communication protocols that use the spatial structure of light, as well as enable fundamental experiments on spatial-mode entanglement to reach their full potential.