The performance of Quantum Key Distribution (QKD) protocols that encodeinformation in the polarization state of light can be affected by polarizationtransformations induced by the optical fiber channel. This thesis investigates whethera real, deployed fiber link can support polarization-based QKD, building on apolarization stabilization system developed in an earlier project course to activelycorrect the fiber’s polarization drift. To answer this, the channel-induced quantum biterror rate (QBER) is decomposed into a rotation-induced misalignment term,correctable by active polarization stabilization, and a depolarization term set by thefiber’s differential group delay (DGD) relative to the coherence time of thetransmitted photons, a term no stabilization system can correct. The stabilizationsystem’s performance and the fiber’s Polarization Mode Dispersion (PMD) vector,from which the DGD is extracted via a wavelength-swept Jones matrix reconstructionmethod, are both characterized experimentally on two deployed metropolitan fiberlinks in the Stockholm area. The results show the rotation term is fully within reachof the stabilization system, and reduce the remaining question — whether thedepolarization term is also acceptable — to a single, precisely defined externalparameter, the coherence time of the photon source eventually used for the quantumsignal, which this thesis shows exactly how to evaluate.
The performance of Quantum Key Distribution (QKD) protocols that encodeinformation in the polarization state of light can be affected by polarizationtransformations induced by the optical fiber channel. This thesis investigates whethera real, deployed fiber link can support polarization-based QKD, building on apolarization stabilization system developed in an earlier project course to activelycorrect the fiber’s polarization drift. To answer this, the channel-induced quantum biterror rate (QBER) is decomposed into a rotation-induced misalignment term,correctable by active polarization stabilization, and a depolarization term set by thefiber’s differential group delay (DGD) relative to the coherence time of thetransmitted photons, a term no stabilization system can correct. The stabilizationsystem’s performance and the fiber’s Polarization Mode Dispersion (PMD) vector,from which the DGD is extracted via a wavelength-swept Jones matrix reconstructionmethod, are both characterized experimentally on two deployed metropolitan fiberlinks in the Stockholm area. The results show the rotation term is fully within reachof the stabilization system, and reduce the remaining question — whether thedepolarization term is also acceptable — to a single, precisely defined externalparameter, the coherence time of the photon source eventually used for the quantumsignal, which this thesis shows exactly how to evaluate.
The polarization mode dispersion (PMD) in optical fibers poses a major challenge for maintaining the fidelity of quantum states for quantum communications. In this work, a comprehensive model linking the probability of quantum measurement errors (infidelity) to PMD is developed and validated by experimental measurements of differential group delay and quantum bit error rate (QBER). Our research proposes effective methods to mitigate PMD effects for broadband entangled photons and evaluates the impact of higher-order PMD effects. The model provides an experimentally verified framework for the optimization of commercial quantum key distribution systems in deployed fiber-optic lines.
We study Polarization Mode Dispersion (PMD) in fiber-based Quantum Key Distribution (QKD) using a broadspectrum polarization-entangled photon source. We analyze wavelength-dependent polarization transformations over 10 km deployed fiber channels, with measurements spanning one year of evolution. By examining the rotation of polarization states on the Poincare sphere and projecting them along a PMD trajectory onto a predefined state within a measurement basis, we derived a simple new formula that accurately links the measurement error probability to finite signal bandwidth and the observed differential group delay (DGD). Finally, we propose strategies to mitigate the PMD effect for entangled-based QKD by optimizing the orientation of the measurement bases and by performing the optimal spectral filtering of the source.
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.
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.
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.
Polarization variations in the installed fibers are complex and volatile, and would severely affect the performances of polarization-sensitive quantum key distribution (QKD) systems. Based on the recorded data about polarization variations of different installed fibers, we establish an analytical methodology to quantitatively evaluate the influence of polarization variations on polarization-sensitive QKD systems. Using the increased quantum bit error rate induced by polarization variations as a key criteria, we propose two parameters - polarization drift time and required tracking speed - to characterize polarization variations. For field buried and aerial fibers with different length, we quantitatively evaluate the influence of polarization variations, and also provide requirements and suggestions for polarization basis alignment modules of QKD systems deployed in different kind of fibers.