No abstract

Robust implementation of quantum key distribution requires precise state generation and measurements, as well as the choice of an optimal encoding to minimize channel disturbance. Time‐bin encoding represent a good candidate for fiber links as birefingence does not perturb this kind of states whereas stable and low‐error encoders and decoders are available for polarization encoding. Here a cross‐encoded scheme where high accuracy quantum states are prepared through a self‐compensating, calibration‐free polarization modulator and transmitted using a polarization‐to‐time‐bin converter is presented. The receiver performs time‐of‐arrival measurements in the key‐generation basis and converts qubits back to polarization encoding for measurements in the control basis. Temporal synchronization between the transmitter and receiver is performed with a qubit‐based method that does not require additional hardware to share a clock reference. The system is tested in a 12‐h run and demonstrates good and stable performance in terms of key and quantum bit error rates. The flexibility of this approach represents an important step toward the development of hybrid networks with both fiber‐optic and free‐space links.

One of the most advanced technologies within the field of quantum mechanics is quantum key distribution (QKD), which allows the secure generation of secret keys among remote users. In order for QKD to be more widely adopted, it must be integrated into existing classical communication systems. However, this can be difficult due to the use of various technologies and channels in deployed networks. Recently, we developed a QKD network in the metropolitan area of Padova, which connects various nodes across the city through a combination of fiber and free-space links. By utilizing a modular design based on the iPOGNAC encoder and the Qubit4Sync method, we have realized portable and adaptable systems that operate in the C and O bands. This allowed us to deploy and test the compatibility of both research and commercial QKD systems by ThinkQuantum with classical communication over a variety of links, as well as their ability to switch between free-space and fiber connections. Finally, we developed and experimentally implemented complex network configurations such as star networks, where a fiber-based transmitter and free-space transmitter could operate with a single receiver.

Here we present cross-encoded Quantum Key Distribution where state encoding is performed with a self-compensating and calibration-free polarization modulator, while transmission is performed in time-bin encoding resistant to perturbances from the fiber channel.

Intermodal quantum key distribution (IM-QKD) enables the integration of fiber networks and free-space connections, which can be ground-to-ground links or involve satellite nodes in orbit. IM-QKD permits to extend the reach of free-space links without trusting any additional node, but this requires to efficiently couple the free-space signal into a single-mode fiber (SMF). We present the implementation of different IM-QKD networks realized in Padova and Vienna, exploiting km-long deployed fibers and free-space channels up to 620 meters. We show that such an intermodal scheme is compatible with both in-house QKD systems and commercially available solutions exploiting polarization encoding at 1550 nm. Remarkably, we realized different QKD tests in daylight and also in rainy conditions.