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

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 pathways for global quantum communication and thus for the quantum internet.

The distribution of quantum state entanglement is a key component for future quantum-based communication and computing networks. Ground-satellite systems are considered a promising path to realize such connections over long distances. So far, the focus has mainly been on satellites generating entangled photon pairs in space and sending them down to Earth (downlink). This paper reverses that approach and systematically investigates the feasibility of the so-called uplink configuration: Two ground stations each generate a photon pair, send one photon from each pair to orbit, where a Bell measurement entangles the remaining qubits on the ground stations. This method theoretically offers advantages, such as reduced satellite hardware requirements and a shift in power demand to the ground, but has been considered practically unfeasible until now. The authors now show through numerical modeling that this uplink variant is, in principle, feasible.

Draft of the protocol

In the described setup, two ground stations are 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 (in the model, perfect Bell pairs with Fidelity = 1 are assumed) and encodes one photon of the pair into the polarization of a photon, while retaining the other as a stationary qubit. Both photons are sent simultaneously to the orbiting satellite. In the satellite, an optical Bell measurement is performed using a combination of a polarization beam splitter (PBS), a 45° wave plate (Hadamard), and polarization splitting with detectors. A successful detection is registered if exactly two clicks are observed—one in each of the two spatially resolved modes—signifying that the remaining stationary qubits at the two ground stations are entangled.

Modeling of significant 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 function correctly, the two photons must arrive at the satellite perfectly overlapped in time and space. Any deviation increases the distinguishability of the photons and lowers the fidelity. The use of time gating reduces the proportion of unsynchronized photons but at the cost of success probability.

Beam widening and wandering: During the ascent to orbit, the photon beam increasingly spreads ("beam widening") and its center shifts randomly ("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 leads to absorption and scattering – in the uplink variant, already at the beginning of the path, making the impact stronger than in the downlink variant.

Noise photons and background interference: The receiving area in the satellite is exposed not only to targeted photons but also to numerous unwanted photons, such as reflected sunlight, moonlight, or from 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") indicating a successful Bell measurement, accounting for all losses and noise sources. Fidelity, on the other hand, quantifies how "close" the generated entangled state is to an ideal Bell state, and it directly depends on the probability that a legitimate set of photon events is detected (relative to error events caused by noise). Simulations reveal a trade-off between a larger time-gating window (which allows for more photon detections) and higher fidelity: while a larger window increases the success probability, it also permits more noise or unsynchronized photons, thereby reducing fidelity.

Further simulations show that as the satellite altitude and the distance between ground stations increase, both fidelity and success probability decrease significantly. Initially, increasing altitude improves the situation because photons traverse less atmosphere (due to a decreasing zenith angle). However, at higher altitudes, beam divergence becomes the dominant factor. Additionally, as the distance between ground stations increases, the photon path length grows, which amplifies losses and noise.

Under optimized parameters – e.g., time gating of 40 ns, wave packet width 10 ns, satellite altitude 200 km (approx. 124 miles), ground station distance 300 km (approx. 186 miles)—a fidelity of approximately 0.972 with a success probability of around 1.5 × 10⁻⁴ can be achieved. For more realistic altitudes (500 km) and distances (1,000 km, approx. 620 miles), still acceptable values of fidelity ~ 0.84 and η ≈ 2.4 × 10⁻⁶ are obtained.

Discussion and outlook

The study demonstrates that the uplink variant of entanglement distribution is theoretically feasible, but under specific conditions: nighttime operation, shorter distances between ground stations, low orbital altitude, and excellent synchronization and optics are required. During daytime, noise levels are so high that fidelity drops to approximately 0.25, rendering the operation practically inefficient. However, the significant advantage lies in the fact that ground stations can provide much higher photon generation power than satellites. This opens up possibilities for more compact satellite architectures that primarily serve as measurement stations, while high-powered ground stations handle the main workload.

For future work, the hardware implementation will be particularly critical: the study assumes on-demand photon sources as a model, but these are not yet experimentally available with sufficient quality. Pulsed sources, which produce photons probabilistically, introduce additional synchronization challenges between the two ground stations. The multiplexing of the protocol—essential for achieving higher rates—also requires thorough investigation, as does a synchronization protocol to correctly tag the photons for entanglement.

Final remark

In summary, the analysis demonstrates that the use of satellites for uplink methods in quantum optical entanglement distribution is not merely a theoretical variation but a realistic option. Although the achieved success probabilities are very low—around 10⁻⁶—they provide new avenues for the global networking of quantum stations when sufficient fidelity is ensured. This represents another building block for the future realization of the "quantum internet." (mr)