In this paper we propose and validate an algorithm for the automatic and deterministic state of polarization (SOP) generation, with direct application in polarization-encoded quantum key distribution (QKD) systems. The proposed SOP generation algorithm uses an electronic polarization controller (EPC), whose active elements entail the concatenation of several fiber-squeezing-based waveplates. Initially, we carry out a characterization of the waveplates. After that, we present in detail the working principle of the proposed algorithm. Finally, the effectiveness of the algorithm is experimentally assessed through the calculation of the quantum bit error rate (QBER), where an Arduino Due is used to control the different waveplates of the EPC. The QBER was measured for periods of 4 hours and 10 hours when generating two or four SOPs, respectively, with an overall average QBER value of 2%. Results show the potential of the proposed method regarding the generation of automatic and deterministic SOPs, thus demonstrating viability for its practical implementation in polarization-encoding QKD systems.
In this paper we propose and validate an algorithm for the automatic and deterministic state of polarization (SOP) generation, with direct application in polarization-encoded quantum key distribution (QKD) systems. The proposed SOP generation algorithm uses an electronic polarization controller (EPC), whose active elements entail the concatenation of several fiber-squeezing-based waveplates. Initially, we carry out a characterization of the waveplates. After that, we present in detail the working principle of the proposed algorithm. Finally, the effectiveness of the algorithm is experimentally assessed through the calculation of the quantum bit error rate (QBER), where an Arduino Due is used to control the different waveplates of the EPC. The QBER was measured for periods of 4 hours and 10 hours when generating two or four SOPs, respectively, with an overall average QBER value of 2%. Results show the potential of the proposed method regarding the generation of automatic and deterministic SOPs, thus demonstrating viability for its practical implementation in polarization-encoding QKD systems.
Abstract We present the design and implementation of a state-of-polarization (SOP) management technique and two efficient synchronizing methods for quantum key distribution (QKD) systems. This is achieved following a wavelength-division multiplexing approach, where the classical synchronization signal and the quantum states are propagated in the same optical fiber. The employed frame synchronization method is based on the monitoring of the quantum bit error ratio (QBER) of the quantum channel, thus avoiding additional hardware and high computational resources. We evaluate the operation of SOP generation method through the assessment of the individual response of the waveplates that comprise the employed electronic polarization controller. Finally, the performance was assessed by computing the overall QBER and the QBER contributions of each of the four polarization states associated with the different qubits. The measurements, obtained during six hours, show a slight variation of the QBER values associated with the individual contributions, reaching an overall QBER of 0.75%. This demonstrates the capability of the presented methods to operate, stably, with very low QBER values, making its application in practical QKD systems reliable.
This work introduces a theoretical model for electronic polarization controllers (EPCs) based on fiber squeezing. First, a method to calculate the EPC configuration in order to transform between any two states of polarization (SOP) is presented. In a second stage, a technique to deterministically generate four SOPs in polarization-encoded quantum communication systems is proposed. Finally, the effectiveness of the proposed approach is experimentally assessed through the generation of two pairs of orthogonal SOPs. Results show that this polarization generation process is intrinsically stable, which evidences its potential for practical implementations of polarization-encoding quantum protocols, e.g. BB84 or B92 quantum key distribution protocols.
In this work, a fully functional polarization-encoding quantum key distribution system (QKD) is demonstrated. At the transmitter side, the polarization states are prepared using an algorithm to determine the voltages to be applied on the electronic polarization controller (EPC), to generate up to four polarization states. At the receiver, a second EPC is used to perform the basis alignment. The symbol synchronization is achieved using a reference optical signal that is wavelength division multiplexed with the quantum signal which carries the polarization-encoded qubit. Frame synchronization is achieved at a protocol level. The presented QKD system works at a repetition rate of 500 Hz, with an average quantum bit error rate of 1.35%.
This study presents a theoretical model for polarisation manipulation using electronic polarisation controllers (EPCs) based on fibre squeezing. A method to calculate the EPC configuration in order to transform between two arbitrary states of polarisation (SOP) is presented. After, a technique to deterministically generate four SOPs for use in polarisation‐encoded quantum communication systems is proposed. Moreover, the effectiveness of the proposed technique is experimentally assessed through the generation of two pairs of orthogonal SOPs. The experimental implementation used an field programmable gate array (FPGA) board to electrically control the four waveplates of the EPC, reaching a rate of 500 qubit/s. Results show that this polarisation generation process is intrinsically stable, demonstrating its potential for practical implementations of polarisation‐encoding quantum key distribution systems using the BB84 and B92 protocols.
Due to the fast-paced advancements of the quantum computing field, current public key cryptographic protocols need to be rethought. One promising solution is Quantum Key Distribution (QKD), whose security is robust to any computational advancement. The security of QKD relies on the laws of quantum mechanics, ensuring that any eavesdropper attempting to access the data will be detected. In particular, polarization-encoding QKD systems allow to generate secret keys in two remote locations to encode/decode information, using the State of Polarization (SOP) of single photons. Although its security has been theoretically proven, its practical implementation poses some challenges, part of which are the main focus of this PhD thesis. One relevant process in QKD systems is the synchronization between the transmitter and receiver, namely, the frame synchronization. This way, in this thesis, a frame synchronization method is developed and implemented, showing its potential for the use in prepare-and-measure QKD systems. The developed method has been shown to be effective, even with a low number of photons reaching the receiver. Furthermore, its architecture allows it to be adjusted for different attenuation regimes of the QKD system. The ability to encode data on the physical properties of single photons, in this case, polarization, highly depends on the capacity to accurately generate the required polarization states on demand. This way, an automatic SOP generation method, comprising an optical polarization monitoring system and a dedicated algorithm was developed and implemented, capable of generating four SOPs. This method reached an average Quantum Bit Error Rate (QBER) of 0.75%, which is well below the security threshold. After the SOP generation, and their transmission through the quantum channel, polarization alignment algorithms are required to compensate for the random rotations induced by birefringence in the optical fiber. In this thesis, a non-iterative polarization compensation method was implemented. This method requires only three QBER estimations to re-align the polarization, reaching average QBERs of 1.8%. Additionally, multiple iterative methods were implemented for comparison purposes, with the results clearly showing a better efficiency of the deterministic method. Beyond fiber-based transmissions, free-space transmissions were analysed due to their ability to reach sites without installed fiber and due to its potential for satellite-based quantum communications. This thesis demonstrates the feasibility of polarization-encoding QKD over free-space channels, by subjecting quantum signals to atmospheric turbulence to study its impact. In that sense, polarization variations over a 1.8 km outdoor free-space channel were monitored, to understand the effect of atmospheric turbulence on polarization. These results have shown that SOP variations in free-space links are slow, allowing their compensation. Furthermore, it was identified that, under conditions tested, the Secret Key Rate (SKR) could reach values up to 8.7×10ˉ⁴ bits per second, allowing QKD over free space for distances up to 2.31 km.
We present a method for optimizing the waveplate’s voltage range of an electro-optic based electrical polarization controller for qubit encoding in discrete-variable quantum key distribution (QKD). By increasing the number of used waveplates, the maximum voltage range to generate each of the states-of-polarization required by a QKD protocol can be reduced. The proposed machine learning algorithm, particle swarm optimization, was validated through numerical and experimental results. Results show that transitions between six states of polarization used in QKD systems can be generated using a maximum voltage range of 10V in each waveplate.