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Experimental Satellite Quantum Communications
Giuseppe Vallone, Davide Bacco, Daniele Dequal, Simone Gaiarin, Vincenza Luceri, Giuseppe Bianco, Paolo Villoresi
TL;DR
The experiment examines satellite optical-link operation under pointing and timing constraints. It combines a pulsed laser transmitter, polarization-sensitive receiver, time-division shutters, and polarization compensation; detection duty cycles range from 0 to 15%, while modulating the retroreflector rotation enables two-way QKD.
Problem
Satellite optical-link operation is constrained by pointing errors and round-trip timing, which produce intermittent detections and limited effective transmission periods.
Method
The experiment uses a pulsed laser transmitter, a polarization-basis receiver, time-division optical shutters, and polarization compensation for a retroreflector-based two-way protocol.
Results
0 to 15%: the effective duty cycle for a LEO satellite varies across round-trip times, while uplink rotation is compensated by the downlink transformation.
Takeaways & Limitations
Modulating the Faraday-rotator angle φ rotates the received horizontal state by 2φ, enabling the two-way QKD protocol.
Abstract
from arXiv · showhide
Quantum Communications on planetary scale require complementary channels including ground and satellite links. The former have progressed up to commercial stage using fiber-cables, while for satellite links, the absence of terminals in orbit has impaired theirs development. However, the demonstration of the feasibility of such links is crucial for designing space payloads and to eventually enable the realization of protocols such as quantum-key-distribution (QKD) and quantum teleportation along satellite-to-ground or intersatellite links. We demonstrated the faithful transmission of qubits from space to ground by exploiting satellite corner cube retroreflectors acting as transmitter in orbit, obtaining a low error rate suitable for QKD. We also propose a two-way QKD protocol exploiting modulated retroreflectors that necessitates a minimal payload on satellite, thus facilitating the expansion of Space Quantum Communications.
SETUP
The experiment sends quantum pulses from the Matera observatory to satellite corner-cube retroreflectors and detects the reflected polarization qubits during synchronized time windows.
- SETUP: The setup uses a mode-locking laser synchronized to an atomic clock to generate 100 MHz qubit pulses and a 10 Hz satellite laser-ranging signal.The laser produces 100 ps pulses at 1064 nm; the ranging pulse is converted to 532 nm with 100 mJ energy.
- SETUP: A telescope beam path combines outgoing ranging and qubit signals and receives the reflected light from satellite retroreflectors.The received beams return through the Coudé path and are separated for quantum detection.
- SETUP: The receiver analyzes polarization with a rotating waveplate, polarizing beam splitter, and two single-photon photomultipliers.The detectors are connected to a time tagger with 81 ps resolution, and the waveplate selects between two polarization bases.
- SETUP: Time division protects the receiver from transmitter scattering by alternating uplink transmission and downlink detection within each ranging interval.The transmitter is open during the first half of the slot, while the reception shutter is closed; the roles reverse during detection.
- SETUP: A 4.5 ms shutter overhead reduces the effective duty cycle for LEO satellites to 0–15%, with the minimum approached near maximum elevation.The usable period is also bounded by the satellite round trip time, which varies from 5 to 20 ms.
RADAR EQUATION
The radar-equation analysis estimates the transmitter gain from measurements across several satellite passages, then uses it to predict received photon rates and compare them with observations.
- RADAR EQUATION: The full radar equation is used to extrapolate the transmitter gain as an independent confirmation of the measured satellite photon number.The resulting gain is inserted into the link-budget calculation.
- RADAR EQUATION: The analysis treats θt as the upgoing-beam divergence angle and θ as the pointing error, including turbulence broadening in θt.Because the two parameters cannot be measured separately, the gain is estimated by comparing data from different LEO-satellite passages.
- RADAR EQUATION: 1.1 × 10^9 is the effective transmitter gain obtained by averaging the most stable data from Ajisai, Jason, and Starlette.The gain is then used to estimate the received photon frequency and compare the fit with collected data.
POLARIZATION COMPENSATION IN THE DOWNLINK
The downlink analysis shows how the telescope and retroreflector transformations affect polarization. The resulting compensation supports a two-way protocol in which a modulated retroreflector rotates the received state.
- POLARIZATION COMPENSATION IN THE DOWNLINK: The MLRO Coudé path applies a unitary polarization transformation through its sequence of mirrors.The telescope mirrors, including the primary and secondary mirrors, contribute to the uplink transformation.
- POLARIZATION COMPENSATION IN THE DOWNLINK: The telescope rotation is described using the azimuth and elevation angles and a reference-frame rotation R(θ).These angles characterize the telescope orientation in the polarization model.
- POLARIZATION COMPENSATION IN THE DOWNLINK: The CCR transformation compensates the uplink rotation, so the receiving polarization state is corrected by the downlink path.This follows from the relation σz R(θ) = R(−θ) σz.
- POLARIZATION COMPENSATION IN THE DOWNLINK: Adding a Faraday Rotator to the CCR produces an active transformation UCCR(φ) = R(−φ)σzR(φ).The retroreflector transformation is incorporated into the overall uplink–CCR–downlink channel.
- POLARIZATION COMPENSATION IN THE DOWNLINK: Modulating φ rotates an input horizontal polarization by 2φ and realizes the proposed two-way QKD protocol.The rotation is expressed in the laboratory reference frame.