Free-space and Satellite-Based Quantum Communication: Principles, Implementations, and Challenges

TL;DR

This review analyzes the trajectory of satellite-based quantum communication, detailing how discrete-variable and continuous-variable QKD protocols are implemented over free-space and space-ground links. It synthesizes key experimental milestones, including Micius, daylight operations, moving receivers, and integrated space-to-ground networks, while outlining atmospheric turbulence, standardization, and the role of quantum repeaters as primary challenges. The work highlights advances in error correction, finite-key security, and high-dimensional QKD concepts such as Fourier-qubits, framing a path toward global quantum networks. The collected insights emphasize practical architectures, adaptive strategies, and standardization efforts essential for realizing a secure, scalable quantum internet powered by satellites and ground stations.

Abstract

Satellite-based quantum communications represent a critical advancement in the pursuit of secure, global-scale quantum networks. Leveraging the principles of quantum mechanics, these systems offer unparalleled security through Quantum Key Distribution (QKD) and other quantum communication protocols. This review provides a comprehensive overview of the current state of satellite-based quantum communications, focusing on the evolution from terrestrial to space-based systems. We explore the distinct advantages and challenges of discrete-variable (DV) and continuous-variable (CV) quantum communication technologies in the context of satellite deployments. The paper also discusses key milestones such as the successful implementation of quantum communication via the Micius satellite and outlines the primary challenges, including atmospheric turbulence and the development of quantum repeaters, that must be addressed to achieve a global quantum internet. This review aims to consolidate recent advancements in the field, providing insights and perspectives on the future directions and potential innovations that will drive the continued evolution of satellite-based quantum communications.

Figures (20)

• Figure 1: Experimental setup for satellite-to-ground quantum key distribution (QKD) using the Micius satellite liao2017satellite. The Micius satellite, weighing 635 kg, operates in a Sun-synchronous orbit approximately 500 km above Earth and carries three payloads designed for space-based quantum experiments including QKD, Bell tests, and quantum teleportation. The satellite’s QKD transmitter employs eight laser diodes emitting attenuated pulses at around 850 nm, which pass through a BB84 encoding module composed of polarizing beam splitters, a half-wave plate, and a beam splitter. The encoded quantum signals are co-aligned with a 532 nm green laser used for system tracking and time synchronization, then transmitted via a 300-mm-aperture Cassegrain telescope. Beam control is achieved through a two-axis gimbal mirror for coarse tracking and fast steering mirrors for fine tracking, while a low-power 671 nm laser serves as a polarization reference. On the ground, the Xinglong station features a 1,000-mm-aperture telescope that separates the incoming 532 nm tracking laser and 850 nm quantum signals using a dichroic mirror. The tracking beam is monitored by a camera for alignment, while the quantum signals are analyzed by a BB84 decoder consisting of beam splitters and four single-photon detectors. The ground station also sends a 671 nm laser beam back to the satellite for reciprocal tracking. This dual-wavelength synchronization and hybrid tracking system enable precise alignment and polarization compensation, facilitating high-rate QKD over distances up to 1,200 km and demonstrating a significant advancement in space-based quantum communication.
• Figure 2: Schematic diagram of a QKD system based on the BB84 protocol with a Free-space communication channel. Two parties, typically named Alice and Bob, wish to communicate securely over a public channel. Alice transmits a series of individual photons to Bob, each with a random polarization state (horizontal, vertical, diagonal, or anti-diagonal). Bob then measures the polarization of each photon in a randomly chosen basis (horizontal, vertical or diagonal, anti-diagonal) and records the result. Afterwards, Alice and Bob compare a subset of their measurements to detect any eavesdropping attempts.
• Figure 3: High-dimensional quantum key distribution (HD-QKD) using qubit-like states (Fourier-qubits). (a) Conceptual illustration of Fourier-qubit (F-qubit) states in a $d=4$ time-bin encoding. Logical basis states occupy distinct time bins, while F-qubits are constructed as equal-weight superpositions of only two logical states with a discrete relative phase $\omega_d^m = e^{2\pi i m/d}$, enabling phase-error estimation without full mutually unbiased bases. (b) Experimental realization of the F-qubit protocol using orbital angular momentum (OAM) modes of light in a noisy Free-space channel. Spatial light modulators (SLMs) generate and project qubit-like superpositions of Laguerre--Gaussian modes, while adaptive optics compensate turbulence-induced distortions prior to detection. (c) Measured probability outcome matrices for the F-qubit basis in four dimensions, showing the ideal theoretical distribution (left) and experimentally observed distribution after propagation through a turbulent channel (right). These statistics enable indirect reconstruction of the phase error rate and demonstrate secure key generation exceeding one bit per sifted photonscarfe2025fast
• Figure 4: Simulated amplitudes of Hermite Gauss bases of dimensions $d=2$ (a) and $d=4$ (c) as well as their MUBs (b,d) meyer2025analogy.
• Figure 5: Field-deployed high-dimensional QKD over multicore fiber. Hybrid time-path encoded states are transmitted through a 52 km loop formed by two spatial cores of a deployed four-core fiber. The system incorporates decoy-state modulation and active phase stabilization using a dual-band phase-locked loop. Adapted from: Nature Communications, 2024.