Experimental polarization encoded quantum key distribution over optical fibres with real-time continuous birefringence compensation

In this paper we demonstrate an active polarization drift compensation scheme for optical fibres employed in a quantum key distribution experiment with polarization encoded qubits. The quantum signals are wavelength multiplexed in one fibre along with two classical optical side channels that provide the control information for the polarization compensation scheme. This set-up allows us to continuously track any polarization change without the need to interrupt the key exchange. The results obtained show that fast polarization rotations of the order of 40*pi rad/s are effectively compensated for. We demonstrate that our set-up allows continuous quantum key distribution even in a fibre stressed by random polarization fluctuations. Our results pave the way for Bell-state measurements using only linear optics with parties separated by long-distance optical fibres

Review on Channel Estimation in OFDM

OFDM is a wireless connectivity technique that sends multiple data streams over a particular channel while efficiently handling inter-symbol interference and boosting the frequency and bandwidth available.  Since the antenna is used for signal transmission, predicting the noise present  in a noisy channel is essential. In noisy channels, the evaluation method for estimating channel can be used to explore the impact of noise on transmitted signal. Orthogonal frequency division multiplexing (OFDM) is important in wireless communication for its elevated transmission rate. Thus this paper is based on the analysis of Orthogonal frequency division multiplexing and modulation techniques in multiple input multiple output (MIMO) user

Simulation of 1 x 2 OTDM router employing symmetric Mach-Zehnder switches

In high-speed all-optical time division multiplexed (OTDM) routers it is desirable to carry out data routing, switching, clock recovery and synchronisation in the optical domain in order to avoid the bottleneck due to optoelectronics conversion. The authors propose an optical switch based on all-optical symmetric Mach–Zehnder (SMZ) switching and investigate its characteristics. The proposed switch is to be used as a building block for a simple 1x2 OTDM router for asynchronous OTDM packet routing, where clock recovery, address recognition and payload routing are all carried out in the optical domain. Simulation and numerical results demonstrate that clock recovery, address recognition and payload routing are possible with small amounts of crosstalk. Also presented are simulation results for bit error rate (BER) performance for the 1x2 router. For a BER of 10e-9 the receiver sensitivity is -26 dB compared with baseline detection without a router of -38 dB. The proposed router displays great potential for use in ultrahigh- speed OTDM networks

Single and entangled photon manipulation for photonic quantum technologies

Photonic quantum technologies that harness the fundamental laws of quantum physics open the possibility of developing quantum computing and communication that could show unprecedented computational power on specific problems and unconditional information security, respectively. However, the lack of high-efficiency single-photon sources and integrated photonic circuits that can generate, manipulate and analyse entanglement states are the major hurdles to demonstrate the quantum advantages. The potential solutions are clearly explained in this thesis. Chapter 1 provides a brief overview that explains the theme of each chapter. Chapter 2 emphasises the importance of a high-efficiency single-photon source and an integrated time-bin entanglement chip, after explaining the advantages of photonic quantum computing and communication over their classical counterparts. In Chapter 3, three different temporal multiplexing schemes are experimentally demonstrated as the potential solutions to build a high-efficiency single-photon source. Chapter 3 also identifies the potential limitations of temporal multiplexing with high repetition rate. In Chapter 4, the linear processing circuits and nonlinear photon source are separately demonstrated in a low-loss double-stripe silicon nitride waveguide. In the final section of Chapter 4, an integrated silicon nitride time-bin entanglement chip that combines linear processing circuits and nonlinear photon sources is demonstrated as a potential solution to build a robust, scalable and cost-efficient quantum network in the real world. After a succinct summarisation, the final chapter briefly discusses the promising strategies and platforms to build an integrated high-efficiency single-photon source and an integrated quantum node with broad bandwidth and long storage time