In this work, we propose and experimentally validate a polarization basis alignment method based on a hybrid approach, using the Particle Swarm Optimization (PSO) and a gradient search algorithm. This method uses the Quantum Bit Error Ratio (QBER) as metric to be minimized. At first, the PSO runs one iteration to perform a coarse basis alignment, followed by the gradient search algorithm which performs a fine basis alignment, until the QBER is below the threshold. The polarization basis alignment method here proposed is implemented and tested at the receiver of a Quantum Key Distribution (QKD) system testbed. With this proposed hybrid basis alignment approach, a QBER below 1% can be achieved after, on average, nineteen QBER estimations, when using five particles for the PSO. These nineteen QBER estimations represent a 24% improvement regarding the method mostly used in the state of the art which.
In this work, we propose and experimentally validate a polarization basis alignment method based on a hybrid approach, using the Particle Swarm Optimization (PSO) and a gradient search algorithm. This method uses the Quantum Bit Error Ratio (QBER) as metric to be minimized. At first, the PSO runs one iteration to perform a coarse basis alignment, followed by the gradient search algorithm which performs a fine basis alignment, until the QBER is below the threshold. The polarization basis alignment method here proposed is implemented and tested at the receiver of a Quantum Key Distribution (QKD) system testbed. With this proposed hybrid basis alignment approach, a QBER below 1% can be achieved after, on average, nineteen QBER estimations, when using five particles for the PSO. These nineteen QBER estimations represent a 24% improvement regarding the method mostly used in the state of the art which.
The ability to efficiently align the transmitter and receiver of a polarization-based Quantum Key Distribution (QKD) system at initialization and during the exchange of qubits is critical for its correct operation, otherwise resulting in a reduction of the secret key rate. We address this issue by implementing a deterministic polarization compensation method based on the reversal operator of polarization variations. The working principle of this reversal operator is based on Quantum Bit Error Rate (QBER) measurements and their mapping on the Poincaré sphere, enabling fast tracking and compensation of polarization misalignments. Our experimental implementation demonstrates that this method allows for an accurate alignment of the polarization coding and measurement basis using only three QBER measurements. The results confirm that the reversal operator method is exceptionally time-efficient when compared to the widely used compensation methods, such as gradient-based algorithms, requiring at least 5× fewer QBER measurements.
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.
The calibration of the polarization basis between the transmitter and receiver is an important task in quantum key distribution. A continuously working polarization-basis tracking scheme (PBTS) will effectively promote the efficiency of the system and reduce the potential security risk when switching between the transmission and calibration modes. Here, we proposed a single-photon level continuously working PBTS using only sifted key bits revealed during an error correction procedure, without introducing additional reference light or interrupting the transmission of quantum signals. We applied the scheme to a polarization-encoding BB84 QKD system in a 50 km fiber channel, and obtained an average quantum bit error rate (QBER) of 2.32% and a standard derivation of 0.87% during 24 h of continuous operation. The stable and relatively low QBER validates the effectiveness of the scheme.
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%.
The hybrid modulation quantum key distribution(QKD) via single-photon two-qubit states scheme is analyzed,and an improvement scheme is proposed.The input-output model of polarization is established.And the influence of the interferometers on the polarization coding is analyzed.Quantum bit error rate(QBER) of polarization coding caused by the phasic error of fast and slow axes and the alignment error between the basis vectors of polarization and polarization axis is simulated.Furthermore,the stability conditions of polarization are derived on this basis.The QBER of polarization coding can be influenced deeply by the fiber M-Z interferometers.The theoretic analysis reveals that the jitter of polarization state is not only derived from the birefringency,but also derived from coordinate system difference between incident light and fast and slow axes in fiber interferometer.Some necessary conditoins for the QKD via single-photon two-qubit states are given.The simulation show that when ρ and θ are both below 0.2 rad,the polarization QBER is less than 0.015.When ρ and θ are both below 0.4 rad,the polarization QBER is less than 0.06.The polarization QBER of 2%-5% in two-qubit QKD scheme could be accepted as far as the safety upper limit of QBER(11%) is concerned.
In this work, a reliable and low computational õ complexity frame synchronization method for quantum key distribution (QKD) systems is proposed. The proposed method is based on the computation of the quantum bit error rate (QBER) and requires no more hardware than the one used for qubit coding and decoding. In a initial stage, pre-shared and received sequences are shifted until a minimum QBER value is reached. In a second stage, to confirm that the frames are correctly positioned, and to ensure that during key transmission a correct association of each received bit is made with the corresponding bit sent by the transmitter, a pre-determined number of inverted frames is sent, rising the QBER value to maximum values. As the number of inverted frames is a pre-agreed number, after detecting the first inverted frame it is possible to determine exactly when the secret key transmission will start. We use a polarization-encoding QKD testbed to assess the overall performance of the method. With the proposed frame synchronization method, the synchronization is achieved after roughly 104clock cycles, despite the concerns imposed by the low probability detection inherent to the QKD systems.