We analyze the impact of fiber random birefringence in realistic quantum communication systems in which information is encoded in single-photon polarization qubits. We discuss an algorithm to dynamically compensate the polarization random rotations. We assess the algorithm performance considering both the polarization drift dynamics and the QBER threshold. We apply the polarization drift control algorithm to a realistic quantum key distribution system based on the BB84 and we assess its performance in terms of key generation rate for different values of polarization linewidth.
We analyze the impact of fiber random birefringence in realistic quantum communication systems in which information is encoded in single-photon polarization qubits. We discuss an algorithm to dynamically compensate the polarization random rotations. We assess the algorithm performance considering both the polarization drift dynamics and the QBER threshold. We apply the polarization drift control algorithm to a realistic quantum key distribution system based on the BB84 and we assess its performance in terms of key generation rate for different values of polarization linewidth.
Long-term quantum key distribution (QKD) using polarization encoding requires a random drift compensation method. We propose a method to compensate any state of polarization based on the quantum bit error rate (QBER) of two states from two non-orthogonal mutually unbiased bases. The proposed method does not require dedicated equipment, and through a simple but highly efficient feedback loop it compensates the polarization random drift suffered by photons while transmitted over the optical fiber quantum channel. A QBER lower than 2% was observed even considering imperfect single photon detectors. Besides, we verify a 82% secret key rate generation improvement in a finite-key size BB84 implementation for a 40 km fiber-optics quantum channel.
Polarization drift due to propagation medium birefringence is a primary cause of increased quantum bit error ratio (QBER) in polarization-encoded quantum key distribution (QKD). The random nature of polarization drift and environmental instabilities cause it to vary over time, necessitating algorithms for effective polarization drift compensation in polarization-encoded optical fiber QKD links. I propose an algorithm based on processing time-multiplexed control pulses by unscented Kalman filter that minimizes the number of additional measurements that interrupt key exchange. I evaluated the robustness of the algorithm under various conditions, including random polarization drift velocities, and additional noise due to depolarization for weak-coherent BB84-like protocols, to determine the applicability limits of the proposed algorithm and its overhead.
An analysis of the most critical issues in quantum key distribution systems with polarization encoded photons is presented. Effects like the fiber birefringence, fiber losses and some technical aspects can difficult the state of polarization control in such kind of systems, leading to an undesired quantum bit-error-rate increment. It is shown that a system with a long fiber, presenting a typical PMD value, only assures a strong correlation between two Stokes vectors for narrow wavelength separations. In terms of time correlation, it is shown that the typical drift times are much longer than delays between reference and data signals. The problematic of the reference pulse leakage to the data detector is also analyzed; a good agreement between experimental and theoretical data is observed.
Polarization qubits offer a direct route to fiber-based quantum communication, yet the fiber that carries them also scrambles their reference frame through uncontrolled birefringence. Active compensation commonly relies on monitoring and feedback, raising a natural question: can the link itself suppress coherent polarization drift before it reaches the receiver? We show that it can within a regime of sufficiently correlated unitary drift. Our central idea is to embed a fixed cyclic sequence of Pauli rotations along the fiber, turning propagation distance into a spatial toggling frame. Rather than estimating and inverting the unknown transformation, the sequence repeatedly reverses its leading action. We establish exact refocusing for constant generators compatible with a two-segment echo, show that a four-frame Pauli cell cancels the leading contribution of any traceless quasi-static generator, and bound the residual error for smoothly varying birefringence. We then connect these guarantees to operational BB84 quantities, including measured QBERs, the resulting secret fraction, and insertion loss. In simulations of a 50 km fiber with spatially correlated birefringence, a representative design reduces the mean QBER from 6.11% to 0.224% at a device spacing of 1.25 km. With an assumed per-device transmission of t = 0.997, this raises the asymptotic key rate per launched pulse by a factor of approximately 2.5. For this parameter set, the best design in the tested scan is not the densest one: error suppression and optical loss create a finite operating window. These results provide a loss-aware design principle for passive polarization stabilization and suggest a low-overhead complement to active tracking in polarization-encoded QKD networks.
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.
A quantum bit error rate (QBER) based algorithm for polarization random drift compensation is proposed. For a transmission window of 8 ms, for instance in aerial fiber installations, the algorithm overhead is below 1%. In an extreme turbulent situation, where the transmission window is as shorter as 0.8 ms, the overhead is still below 10%. Besides being able to operate smoothly, even in a very extreme situation, the algorithm overhead is also insensitive to the length of the communication system. It is upper layer protocol agnostic, and it is based on the mapping of the QBER on the Poincaré sphere. The algorithm finds the polarization reversal operator, which results in much lower overhead contrary to the blind methods currently used. The algorithm reverts the polarization random drift performing two QBER estimations and applying three rotations, at most. The uncertainty on the two QBER estimations defines an area over the sphere surface that is related with upper-layer protocols QBER threshold.