Quantum network

A simulation of an entangled quantum system on a classical computer cannot simultaneously provide the same security and speed.

Third, to make maximum use of communication infrastructure, one requires optical switches capable of delivering qubits to the intended quantum processor.

[5] Telecommunication lasers and parametric down-conversion combined with photodetectors can be used for quantum key distribution.

[9] Another possible platform are quantum processors based on ion traps, which utilize radio-frequency magnetic fields and lasers.

Alternately, free space networks can be implemented that transmit quantum information through the atmosphere or through a vacuum.

In both cases, the telecom fiber can be multiplexed to send non-quantum timing and control signals.

In 2020 a team of researchers affiliated with several institutions in China has succeeded in sending entangled quantum memories over a 50-kilometer coiled fiber cable.

[15] However, over long distances, free space communication is subject to an increased chance of environmental disturbance on the photons.

The experimental exchange of single photons from a global navigation satellite system at a slant distance of 20,000 km has also been reported.

[17] These satellites can play an important role in linking smaller ground-based networks over larger distances.

In free-space networks, atmospheric conditions such as turbulence, scattering, and absorption present challenges that affect the fidelity of transmitted quantum states.

To mitigate these effects, researchers employ adaptive optics, advanced modulation schemes, and error correction techniques.

[18] The resilience of QKD protocols against eavesdropping plays a crucial role in ensuring the security of the transmitted data.

Specifically, protocols like BB84 and decoy-state schemes have been adapted for free-space environments to improve robustness against potential security vulnerabilities.

Long-distance communication is hindered by the effects of signal loss and decoherence inherent to most transport mediums such as optical fiber.

That is, to implement an amplifier, the complete state of the flying qubit would need to be determined, something which is both unwanted and impossible.

This means that when making encryption keys, the sender and receiver are secure even if they do not trust the quantum repeater.

[19] In this case, the quantum network consists of many short distance links of perhaps tens or hundreds of kilometers.

It can be seen that a network of such repeaters can be used linearly or in a hierarchical fashion to establish entanglement over great distances.

However, there are also hardware platforms specific only[22] to the task of acting as a repeater, without the capabilities of performing quantum gates.

Due to technological limitations, however, the applicability is limited to very short distances as quantum error correction schemes capable of protecting qubits over long distances would require an extremely large amount of qubits and hence extremely large quantum computers.

In these cases, the goal of the quantum communication is to securely transmit a string of classical bits.

In general, quantum entanglement is well suited for tasks that require coordination, synchronization or privacy.

Examples of such applications include quantum key distribution,[25][26] clock stabilization,[27] protocols for distributed system problems such as leader election or Byzantine agreement,[5] extending the baseline of telescopes,[28][29] as well as position verification,[30][31] secure identification and two-party cryptography in the noisy-storage model.

Furthermore, qubits can be encoded in a variety of materials, including in the polarization of photons or the spin states of electrons.

The network located in Bristol used already deployed fibre-infrastructure and worked without active switching or trusted nodes.

[40] In February 2025, researchers from Oxford University experimentally demonstrated the distribution of quantum computations between two photonically interconnected trapped-ion modules.

Each module contained dedicated network and circuit qubits, and they were separated by approximately two meters.

The team achieved deterministic teleportation of a controlled-Z gate between two circuit qubits located in separate modules, attaining an 86% fidelity.

This requires a sophisticated, super-cooled yttrium silicate crystal to sandwich erbium in a mirrored environment to achieve resonance matching of infrared wavelengths found in fiber optic networks.