When quantum information is encoded in optical polarization, practical protocols must contend with the fact that preparation and measurement reference frames may be misaligned, owing to physical orientations and birefringence effects. For long-distance transmissions, a common solution involves multiplexing strong classical reference signals, providing a basis for correction. Here we numerically investigate the performance of a polarization-frame alignment protocol utilizing single-photon-level signals and measurements, in the context of quantum key distribution (QKD) over channels with detector noise and imperfect sources. To the extent possible, the same states and measurement bases are used as for the QKD protocol we intend to support. Our results show that a small fraction of resources from the overall signal---a few hundred photon detections, in total---are required for good performance, restoring the state to better than 99% of its original quality, and we illustrate the protocol's successful operation and adaptability through experimental demonstrations in which it was used in the laboratory and the field.
When quantum information is encoded in optical polarization, practical protocols must contend with the fact that preparation and measurement reference frames may be misaligned, owing to physical orientations and birefringence effects. For long-distance transmissions, a common solution involves multiplexing strong classical reference signals, providing a basis for correction. Here we numerically investigate the performance of a polarization-frame alignment protocol utilizing single-photon-level signals and measurements, in the context of quantum key distribution (QKD) over channels with detector noise and imperfect sources. To the extent possible, the same states and measurement bases are used as for the QKD protocol we intend to support. Our results show that a small fraction of resources from the overall signal---a few hundred photon detections, in total---are required for good performance, restoring the state to better than 99% of its original quality, and we illustrate the protocol's successful operation and adaptability through experimental demonstrations in which it was used in the laboratory and the field.
Abstract Owing to physical orientations and birefringence effects, practical quantum information protocols utilizing optical polarization need to handle misalignment between preparation and measurement reference frames. For any such capable system, an important question is how many resources – for example, measured single photons – are needed to reliably achieve alignment precision sufficient for the desired quantum protocol. Here, we study the performance of a polarization-frame alignment scheme used in prior laboratory and field quantum key distribution (QKD) experiments by performing Monte Carlo numerical simulations. The scheme utilizes, to the extent possible, the same single-photon-level signals and measurements as for the QKD protocol being supported. Even with detector noise and imperfect sources, our analysis shows that only a small fraction of resources from the overall signal – a few hundred photon detections, in total – are required for good performance, restoring the state to better than 99% of its original quality.
Measurement-device-independent quantum key distribution (MDI-QKD) is proved to be able to eliminate all potential detector side channel attacks. Combining with the reference frame independent (RFI) scheme, the complexity of practical system can be reduced because of the unnecessary alignment for reference frame. Here, based on polarization multiplexing, we propose a time-bin encoding structure, and experimentally demonstrate the RFI-MDI-QKD protocol. Thanks to this, two of the four Bell states can be distinguished, whereas only one is used to generate the secure key in previous RFI-MDI-QKD experiments. As far as we know, this is the first demonstration for RFI-MDI-QKD protocol with clock rate of 50 MHz and distance of more than hundred kilometers between legitimate parties Alice and Bob. In asymptotic case, we experimentally compare RFI-MDI-QKD protocol with the original MDI-QKD protocol at the transmission distance of 160 km, when the different misalignments of the reference frame are deployed. By considering observables and statistical fluctuations jointly, four-intensity decoy-state RFI-MDI-QKD protocol with biased bases is experimentally achieved at the transmission distance of 100km and 120km. The results show the robustness of our scheme, and the key rate of RFI-MDI-QKD can be improved obviously under a large misalignment of the reference frame.
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.
Quantum key distribution (QKD) is a secure key-transmitting method based on quantum mechanics to deliver private keys for encryption. Satellite QKD is a promising technique that overcomes the limited transmission distance in optical-fibre-based systems. To enable polarisation encoded QKD, polarisation reference-frame mismatches between the satellite and a receiving optical ground station (OGS) have to be compensated. We developed and implemented a compensation method for a BB84 protocol designed QKD receiver by an additional set of three waveplates (HWP, QWP, QWP). In simulations and laboratory demonstrations, we show the robustness under conditions that we expect on our future satellite downlink mission. As a result, we expect good compensation quality for a wide range of signal to noise ratios and satellite rotation frequencies by requiring only a few signal photons of 2000 of two non-orthogonal polarisation states, respectively, to measure the reference-frame mismatch used for one compensation sequence.
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%.
Standard quantum key distribution (QKD) protocols typically assume that the distant parties share a common reference frame. In practice, however, establishing and maintaining a good alignment between distant observers is rarely a trivial issue, which may significantly restrain the implementation of long-distance quantum communication protocols. Here we propose simple QKD protocols that do not require the parties to share any reference frame, and study their security and feasibility in both the usual device-dependent (DD) case—in which the two parties use well characterized measurement devices—as well as in the device-independent (DI) case—in which the measurement devices can be untrusted, and the security relies on the violation of a Bell inequality. To illustrate the practical relevance of these ideas, we present a proof-of-principle demonstration of our protocols using polarization entangled photons distributed over a coiled 10-km long optical fiber. We consider two situations, in which either the fiber spool's polarization transformation freely drifts, or randomly chosen polarization transformations are applied. The correlations obtained from measurements allow, with high probability, to generate positive asymptotic secret key rates in both the DD and DI scenarios (under the fair-sampling assumption for the latter case).