Prepare-and-measure quantum key distribution (QKD) schemes with polarization encoding require the minimization of polarization degradation, i.e., undesired variations in the state of polarization, in the transmitter and receiver designs. In particular, the polarization extinction ratio (PER) should be maintained as high as possible to reduce the quantum bit error rate (QBER). In this paper, we analyze the polarization degradation and its correction in the design of free-space QKD systems. The polarization of a wavefront can be degraded in two different ways: uniformly, or non-uniformly; the latter known as polarization aberrations. While uniform polarization degradation of the beam wavefront can be easily corrected with standard retardation plates, polarization aberrations are more complex and require a different correction approach. BB84 diagonal linearly-polarized states are especially affected by polarization degradation as they are reflected off certain optical elements, and its correction is essential to achieve a low QBER. We have experimentally characterized the polarization degradation in a QKD transmitter and its impact in the QBER and achieved an almost perfect correction of the diagonal linearly polarized states when choosing a specific optical design.
Prepare-and-measure quantum key distribution (QKD) schemes with polarization encoding require the minimization of polarization degradation, i.e., undesired variations in the state of polarization, in the transmitter and receiver designs. In particular, the polarization extinction ratio (PER) should be maintained as high as possible to reduce the quantum bit error rate (QBER). In this paper, we analyze the polarization degradation and its correction in the design of free-space QKD systems. The polarization of a wavefront can be degraded in two different ways: uniformly, or non-uniformly; the latter known as polarization aberrations. While uniform polarization degradation of the beam wavefront can be easily corrected with standard retardation plates, polarization aberrations are more complex and require a different correction approach. BB84 diagonal linearly-polarized states are especially affected by polarization degradation as they are reflected off certain optical elements, and its correction is essential to achieve a low QBER. We have experimentally characterized the polarization degradation in a QKD transmitter and its impact in the QBER and achieved an almost perfect correction of the diagonal linearly polarized states when choosing a specific optical design.
In polarization-encoding free-space Quantum Key Distribution (QKD), unwanted variations in the states of polarization may cause errors, leading to an increase in the quantum bit error rate (QBER), and a reduction of the Secure Key Rate (SKR). Optical elements that reflect or transmit the quantum signal at the transmitter and/or receiver are the main cause of polarization variations in the quantum states. Although most of these optical elements affect polarization in a constant manner, requiring relatively simple compensation techniques, mobile elements such as steering mirrors, cause time-varying polarization variations that are more complex to characterize and correct. In this work, we simulate the effect of steering mirrors’ typical values of angular rotation on the QBER. In particular, we show that, for most cases, an active correction of the polarization reference frame and a passive correction of the phase shift between the electric field components is sufficient to obtain QBER values due to the steering mirrors below 0.2%. This avoids the need of a fully active correction strategy, which increases the complexity, weight, and cost of the implementation. Furthermore, we present different design considerations to further reduce the steering mirrors’ contribution to the QBER (less than 0.05%).
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.
In polarization-encoded satellite-to-ground quantum communication, the on-board transmitter and a receiver at the optical ground station encode and decode linearly polarized quantum bits for execution of quantum key distribution (QKD) protocol. Dynamic polarization tracking and compensation is a key technique in realizing such a space based QKD system. In this work, we report the design and development of motorized waveplate based active polarization compensation system. Integrated testing and validation with Decoy-state enabled BB84 QKD testbed over ~5m free space channel and quantum link polarization extinction ratio (PER) ≥ 100:1 has been carried out successfully. Satellite motion was emulated as a continuous 1°/s polarization basis rotation at the Quantum Tx output. When active polarization compensation system was enabled, sazimuth (Az.) deviation was measured with ±1.5° accuracy. During this experiment, overall signal QBER was consistently maintained at <3.5% with secure key generation rate of ~1350kbps, proving the suitability of this scheme for realistic satellite-to-ground QKD scenarios.
Wavefront tilt correction is necessary for reducing beam deviations caused by atmospheric turbulence in the receiver of an optical system. This reduction allows decreasing the receiver's field of view, which in the case of free-space quantum key distribution (QKD), reduces the solar background noise that reaches the detectors, and with this, the quantum bit error rate (QBER). A wavefront tilt correcting system that stabilizes the beam in two different points of the receiver's optical axis has been developed and characterised, and it is capable of operating under realistic conditions of strong turbulence with almost perfect ideal correction. The reduced area of beam deviations at the focal plane of the receiver after correction enables a reduction of the QBER of more than 80%, which paves the way to free-space quantum key distribution in daylight under strong turbulence.
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.
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.