Synchronization of quantum communications over an optical classical communications channel.

Precise synchronization between a transmitter and receiver is crucial for quantum communications protocols such as quantum key distribution (QKD) to efficiently correlate the transmitted and received signals and increase the signal-to-noise ratio. In this work, we introduce a synchronization technique that exploits a co-propagating classical optical communications link and tests its performance in a free-space QKD system. Previously, existing techniques required additional laser beams or relied on the capability to retrieve the synchronization from the quantum signal itself; this approach, however, is not applicable in high channel loss scenarios.

Stable, low-error, and calibration-free polarization encoder for free-space quantum communication.

Polarization-encoded free-space quantum communication requires a quantum state source featuring fast modulation, long-term stability, and a low intrinsic error rate. Here we present a polarization encoder that, contrary to previous solutions, generates predetermined polarization states with a fixed reference frame in free-space. The proposed device does not require calibration either at the transmitter or at the receiver and achieves long-term stability.

All-fiber self-compensating polarization encoder for quantum key distribution.

Quantum key distribution (QKD) allows distant parties to exchange cryptographic keys with unconditional security by encoding information on the degrees of freedom of photons. Polarization encoding has been extensively used for QKD along free-space, optical fiber, and satellite links. However, the polarization encoders used in such implementations are unstable, expensive, and complex and can even exhibit side channels that undermine the security of the protocol.

Exploring the boundaries of quantum mechanics: advances in satellite quantum communications.

Recent interest in quantum communications has stimulated great technological progress in satellite quantum technologies. These advances have rendered the aforesaid technologies mature enough to support the realization of experiments that test the foundations of quantum theory at unprecedented scales and in the unexplored space environment. Such experiments, in fact, could explore the boundaries of quantum theory and may provide new insights to investigate phenomena where gravity affects quantum objects.

Extending Wheeler's delayed-choice experiment to space.

Gedankenexperiments have consistently played a major role in the development of quantum theory. A paradigmatic example is Wheeler's delayed-choice experiment, a wave-particle duality test that cannot be fully understood using only classical concepts. We implement Wheeler's idea along a satellite-ground interferometer that extends for thousands of kilometers in space.