Next Step Towards a Quantum Internet Quantum Entanglement: The Network of the Future

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Researchers demonstrate for the first time that entangled quantum states can also be realized via the uplink—from Earth to satellite. Despite atmospheric disturbances and extremely low success rates, this method opens up new avenues for global quantum communication and thus for the quantum internet.

The distribution of quantum state entanglement represents a central component for future quantum-based communication and computing networks. Ground-satellite systems are considered a promising way to establish such connections over long distances. So far, the focus has primarily been on satellites that generate decoupled photon pairs in space and send them to Earth (downlink). This article reverses that approach and systematically investigates the feasibility of the so-called uplink configuration: Two ground stations each generate a photon pair, send one part of each photon pair to orbit, and there a Bell measurement is conducted, entangling the remaining qubits on the ground stations. This procedure theoretically offers advantages – for example, regarding ground power vs. satellite hardware – but has so far been considered practically hardly feasible. The authors now demonstrate through numerical modeling that this uplink variant is, in principle, feasible.

Draft of the Protocol

In the outlined setup, there are two ground stations separated by a distance (D_G). A satellite in low Earth orbit (LEO) is equidistant from both ground stations. Each ground station generates a Bell pair (the modeling assumes perfect Bell pairs with fidelity = 1) and encodes one photon of the pair into the polarization of a photon, while retaining the other part as a stationary qubit. Both photons are simultaneously sent into orbit. On the satellite, an optical Bell measurement is carried out using a combination of a polarization beam splitter (PBS), a 45° wave plate (Hadamard), and polarization splitting with detectors. A successful detection occurs when exactly two clicks are registered – one in each of the two spatially resolved modes – at which point the remaining stationary qubits at the two ground stations are entangled.

Modeling of Key Error Sources

The authors consider several physical effects that could impact the success and quality (fidelity) of the protocol:

Mode mismatch: For the Bell measurement to work correctly, the two photons must arrive at the satellite in perfect temporal and spatial overlap. Any deviation increases the distinguishability of the photons and lowers the fidelity. The use of time gating reduces the proportion of unsynchronized photons, albeit at the cost of success probability.

Beam widening and wandering: As the photons ascend to orbit, the radiation leads to an increasing spread of the beam ("beam widening") as well as random shifts of its center ("beam wandering") due to atmospheric turbulence. These effects reduce the coupling to the satellite telescope and thus the overall efficiency.

Atmospheric attenuation: The path through Earth's atmosphere results in absorption and scattering—more significantly at the start of the path in the uplink version, making the impact greater than in the downlink version.

Noise photons and background noise: The receiving area in the satellite is exposed not only to the targeted photons but also to numerous unwanted photons, such as those from reflected sunlight or moonlight as well as the Earth's surface. These can cause erroneous measurements, thereby reducing the effective fidelity and success probability.

Success Probability and Quality

The overall performance of the protocol is characterized by two key metrics: the success probability (η_tot) and the resulting fidelity (F). The success probability is determined by the likelihood of obtaining a correct detection pattern ("signature") that indicates a valid Bell measurement, taking into account all losses and noise sources. Fidelity, in turn, reflects how "close" the generated entangled state is to an ideal Bell state and directly depends on the probability of detecting a legitimate photon event set (relative to error events caused by noise). Simulations reveal a trade-off between a large time-gating window (allowing more photons to be detected) and high fidelity: a larger window increases the success probability but also allows more noise or non-optimally synchronized photons to pass through, reducing fidelity.

Further simulations show that with increasing satellite altitude and growing distance between ground stations, fidelity and success probability decrease significantly – initially, the situation improves with increasing altitude because the photons have to traverse less atmosphere (due to the decreasing zenith angle), but later, beam divergence becomes dominant. As the distance between ground stations increases, the photon path lengthens, amplifying loss and noise.

Under optimized parameters – e.g., time gating of 40 ns, wave packet width of 10 ns, satellite altitude of 200 km, ground station distance of 300 km – a fidelity of approximately 0.972 can be achieved with a success probability of about 1.5 × 10⁻⁴. For more realistic altitudes (500 km) and distances (1,000 km), acceptable values of fidelity ~ 0.84 with η ≈ 2.4 × 10⁻⁶ can still be achieved.

Discussion and Outlook

The study demonstrates that the uplink variant of entanglement distribution is, in principle, feasible – but under clear conditions: nighttime operation, shorter ground station distances, low orbit altitude, and excellent synchronization and optics are required. During the day, noise levels are so high that fidelity drops to approximately 0.25, making the operation practically inefficient. However, the significant advantage lies in the fact that ground stations can provide significantly higher photon generation capacity than satellites. This opens up prospects for more compact satellite architectures that primarily serve as measurement stations, while high-performance ground stations bear the main workload.

For future work, hardware implementation will be particularly critical: the study assumes on-demand photon sources as a model assumption, but these are not yet experimentally available in sufficient quality. With pulsed sources that generate photons probabilistically, additional synchronization problems arise between the two ground stations. The multiplexing of the protocol – to achieve higher rates – also needs to be thoroughly investigated, as does a synchronization protocol for correctly tagging the photons for entanglement.

Final Remark

In summary, the analysis shows that the use of satellites for uplink methods in quantum optical entanglement distribution is not merely a theoretical variation but a realistic option. While the achieved success probabilities are very low – in the range of 10⁻⁶ – they offer new pathways for the global networking of quantum stations with sufficient fidelity. This adds another building block for the future realization of the "quantum internet." (mr)