Costantino Agnesi

We propose Qubit4Sync, a synchronization method for Quantum Key Distribution (QKD) setups, based on the same qubits exchanged during the protocol and without requiring additional hardware other than the one necessary to prepare and measure the quantum states, in a similar fashion to the clock recovery used in classical communications. Our approach introduces a new cross-correlation algorithm achieving the lowest computational complexity, to our knowledge, for high channel losses. We tested the robustness of our scheme in a real QKD implementation, and we believe it may find application in other quantum communication protocols

Here we present a simple and robust polarization encoded QKD experiment where synchronization, polarization compensation and QKD are all performed with the same optical setup, without requiring any changes or any additional hardware, by exploiting only the transmission of quantum states. Furthermore, the developed polarization encoder exhibits high stability and the lowest intrinsic Quantum Bit Error Rate ever reported.

Space-based quantum key distribution would allow, in the near future, secure communications be- tween parties over continental distances, complementing short-range fiber-based quantum networks. However, further demonstrations of daylight operations over free-space channels and the full compatibility with the telecom-based fiber infrastructure are still necessary. Here we present the prototype for daylight QKD at 1550 nm we developed as a demonstrator for application of QKD both on ground and in Space. Our QKD source, exploiting integrated silicon photonics technology, allows to reach a QBER of 1% during the field-test performed over a 145 m link, and represents a promising resource to design quantum optical payloads to be implemented in future satellite missions.

The successful application of Quantum Key Distribution is dependent on the accurate generation and detection of quantum states, and a communication mechanism that can withstand disturbances caused in the channel. The selection of the optimal encoding strategy is complex and is influenced by external elements such as the characteristics of the quantum channel. Polarization encoding is acknowledged for its dependability and low error rate, rendering it ideal for free-space links, whereas time-bin encoding is robust to birefringence, thereby making it suitable for optical fiber networks. The strength of polarization-based protocols is based on the full characterization of the receiver, to reconstruct the information encoded in the shared qubits. This is typically achieved through tomographic analysis, which adds to the complexity of the final protocol. In this research, we introduce a unique cross-encoded method, where high precision quantum states are produced using a self-regulating, calibration-free polarization modulator and then conveyed through a polarization-to-time-bin converter. A hybrid receiver is used to carry out both time-of-arrival and polarization measurements for decoding the quantum states. Moreover, the suggested receiver is optimized to perform a self-characterization process, utilizing the same photons in which the information is encoded. The adaptability of our approach can lead to a significant advancement in the creation of hybrid networks that incorporate both optical fiber and free-space networks.

Entanglement is a unique and invaluable resource for quantum information processing because it highlights the non-locality property, which allows for device-independent (DI) quantum communication protocols, such as Quantum Key Distribution and Quantum Random Number Generation. However, the distribution of entanglement over long distances is challenging due to propagation losses and instability. Time-bin entanglement is a promising solution since it is robust in long-distance distribution over fiber optics and immune to the polarization distortion such a channel can introduce. Time-bin has also been demonstrated to be compatible with NV center-based quantum technologies, representing a crucial interface between the different devices in quantum networks. Nevertheless, its most common implementation suffers from a post-selection loophole (PSL), which invalidates Bell non-locality tests and renders it vulnerable to quantum hacking attacks, thus preventing its use for device-independent protocols. We present a scheme using optical switches to obtain only detection events displaying non-local interference, thereby closing the PSL. Our scheme works with 1550nm biphotons for entanglement distribution over existing fiber-based telecom networks. The switches show a high extinction ratio of up to 30dB and stability over extended periods. We also measure interferometric visibilities of over 94%, which corresponds to a CHSH S parameter of 2.65

We describe a QKD field trial running on urban fibers deployed in Padua, Italy. This is the first validation outside of the laboratory environment of a new low-error and calibration-free polarization encoder, called iPOGNAC, which we also present here. Our system is resource- and cost-effective, and can be installed quickly in an existing fiber network.