Quantum Cryptography

Quantum entanglement is the main resource to endow the field of quantum information processing with powers that exceed those of classical communication and computation. In view of applications such as quantum cryptography or quantum teleportation, extension of quantum-entanglement-based protocols to global distances is of considerable practical interest. Here we experimentally demonstrate entanglement-based quantum key distribution over 144 km. One photon is measured locally at the Canary Island of La Palma, whereas the other is sent over an optical free-space link to Tenerife, where the Optical Ground Station of the European Space Agency acts as the receiver. This exceeds previous free-space experiments by more than an order of magnitude in distance, and is an essential step towards future satellite-based quantum communication and experimental tests on quantum physics in space.

Figure showing the experimental setup for client-server rfiQKD. The server side holds a telecom wavelength (1550 nm) laser with a 1 MHz pulse generator (PG) and fixed polarizer, to send light pulses to the client through a polarization maintaining fiber (PMF). At the client side, an integrated polarization controller (PC) encodes qubits into the polarization of the attenuated (Att.) light. A fiber beam splitter (FBS) and photodetector (PD) continuously monitor power for malicious attacks. Qubits received back at the server side are measured with a similar PC, fiber polarizing beam splitter (FPBS), and superconducting single photon detectors (SSPDs), all controlled by an electronic board synchronization (Sync.), function programmable gate array (FPGA), and processor. The Bloch sphere illustrates the effects of an unstable environment on polarization.


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