Category Publications

Free-space quantum key distribution by rotation-invariant twisted photons

“Twisted photons” are photons carrying a well-defined nonzero value of orbital angular momentum (OAM). The associated optical wave exhibits a helical shape of the wavefront (hence the name) and an optical vortex at the beam axis. The OAM of light is attracting a growing interest for its potential in photonic applications ranging from particle manipulation, microscopy, and nanotechnologies to fundamental tests of quantum mechanics, classical data multiplexing, and quantum communication. Hitherto, however, all results obtained with optical OAM were limited to laboratory scale. Here, we report the experimental demonstration of a link for free-space quantum communication with OAM operating over a distance of 210 m. Our method exploits OAM in combination with optical polarization to encode the information in rotation-invariant photonic states, so as to guarantee full independence of the communication from the local reference frames of the transmitting and receiving units. In particular, we implement quantum key distribution, a protocol exploiting the features of quantum mechanics to guarantee unconditional security in cryptographic communication, demonstrating error-rate performances that are fully compatible with real-world application requirements. Our results extend previous achievements of OAM-based quantum communication by over 2 orders of magnitude in the link scale, providing an important step forward in achieving the vision of a worldwide quantum network.

Experimental Satellite Quantum Communications

Quantum communication (QC), namely, the faithful transmission of generic quantum states, is a key ingredient of quantum information science. Here we demonstrate QC with polarization encoding from space to ground by exploiting satellite corner cube retroreflectors as quantum transmitters in orbit and the Matera Laser Ranging Observatory of the Italian Space Agency in Matera, Italy, as a quantum receiver. The quantum bit error ratio (QBER) has been kept steadily low to a level suitable for several quantum information protocols, as the violation of Bell inequalities or quantum key distribution (QKD). Indeed, by taking data from different satellites, we demonstrate an average value of QBER=4.6% for a total link duration of 85 s. The mean photon number per pulse μsat leaving the satellites was estimated to be of the order of one. In addition, we propose a fully operational satellite QKD system by exploiting our communication scheme with orbiting retroreflectors equipped with a modulator, a very compact payload. Our scheme paves the way toward the implementation of a QC worldwide network leveraging existing receivers.

Random bits, true and unbiased, from atmospheric turbulence

Random numbers represent a fundamental ingredient for secure communications and numerical simulation as well as to games and in general to Information Science. Physical processes with intrinsic unpredictability may be exploited to generate genuine random numbers. The optical propagation in strong atmospheric turbulence is here taken to this purpose, by observing a laser beam after a 143 km free-space path. In addition, we developed an algorithm to extract the randomness of the beam images at the receiver without post-processing. The numbers passed very selective randomness tests for qualification as genuine random numbers. The extracting algorithm can be easily generalized to random images generated by different physical processes.

Experimental Quantum Key Distribution with finite-key security analysis for noisy channels

Exchanging unconditionally secure cryptographic keys by means of a free-space quantum channel is possible even under realistic conditions, that is, in the presence of environmental noise and with the transmission of a limited number of photons, as for Satellite Quantum Communications. In their work, just published on Nature Communications, Davide Bacco, Matteo Canale, Nicola Laurenti, Giuseppe Vallone and Paolo Villoresi, all with the Department of Information Engineering at the University of Padova, have experimentally proved the feasibility of Quantum Key Distribution (QKD) in such conditions and by considering different attack models. This result opens perspectives for scenarios where the transmission window is limited by physical constraints, as for Satellite Communications, where the passage of one terminal over the other is restricted to a few minutes.

Optimization of Two-Photon Wavefunction in Parametric Down Conversion by Adaptive Optics Control of the Pump Radiation

We present an efficient method to control the spatial modes of entangled photons produced through SPDC process. Bi-photon beam propagation is controlled by a deformable mirror that shapes a 404nm CW diode laser pump interacting with a nonlinear BBO type-I crystal. Thanks to adaptive optical system, the propagation of 808nm SPDC light produced is optimized over a distance of 2m. The whole system optimization is carried out by a feedback between deformable mirror action and entangled photon coincidence counts.

Impact of Turbulence in Long Range Quantum and Classical Communications

The purpose of the work was to investigate the propagation of single photon beams over the inter-island link, and in particular between a transmitter on the roof of the Jacob Kapteyn Telescope at ORM, La Palma, and the ESA-Optical Ground Station at Izana, Tenerife as receiver. Indeed, the study of the free-space propagation of quantum correlations is necessary for any future application of quantum communication aiming to connect two remote locations. The problem related to the free-space propagation is represented by the atmospheric turbulence that acts as a temporal and spatial variation of the air refraction index. A turbulent channel acts as an increment of the losses on the transmitted photons due to beam wandering of the beam centroid or to scintillation, increasing the role of the noise.