Quantum communications and networks share quantum information to achieve tasks beyond the reach of classical systems. At NPL, researchers help develop the measurements and standards needed to make these systems reliable and secure. NPL is a key partner in the UK Integrated Quantum Networks Hub that is playing a foundational role in building the science and hardware for future quantum networks (the quantum internet).
Quantum communication sends information using quantum states, often carried by single discrete packets of light called photons. Unlike conventional digital communication, which uses bits with values that are either 0 or 1, quantum systems use quantum bits (qubits) where the value of the qubit can be in a combination of 0 and 1 at the same time.
Quantum key distribution (QKD) can share encryption keys between two separated parties to enable secure classical communications between them. This is important because QKD can detect eavesdropping and shares keys whose security relies on proven physical laws rather than on the unproven impossibility of breaking mathematical encryption. Networks that distribute quantum states can be used to connect quantum computing and sensing nodes and implement new secure communication protocols, leading to the future quantum internet.
NPL helps build the measurement infrastructure behind quantum technologies. This includes testing devices, improving accuracy, reducing signal loss and helping develop standards so that quantum communication systems can work reliably in the real world.
In quantum networks, information is carried by photons travelling primarily through optical fibres, but also through free space. These photons must be generated very precisely, transmitted with minimal loss, and detected with highly sensitive detectors.
Single-photon sources can be used to create entangled-photon sources which are essential for quantum networks. Single-photon sources can also enhance technologies such as QKD, because attenuating a laser source so that there is on average one photon per pulse means that there are sometimes pulses with more than one photon, requiring some of the transmitted states to be consumed in countermeasures to eavesdropping attacks.
Single-photon detectors register the arrival of individual photons. They need to be extremely sensitive and carefully calibrated. NPL can measure the efficiency of these detectors and their noise (false signal) characteristics.
As photons travel through fibres or free space some are absorbed or scattered. In ordinary communications, weak signals can be amplified, but quantum states cannot simply be copied and boosted without affecting their characteristics.
The no-cloning theorem says that an unknown quantum state cannot be copied perfectly. This matters because it prevents quantum information from being duplicated like ordinary digital data.
The no-cloning theorem helps make quantum communication secure, because an eavesdropper cannot secretly copy the signal without sometimes changing its properties, which can be detected by legitimate users.
Quantum repeaters are devices that extend the range of quantum networks. Because quantum information cannot be copied and amplified in the usual way, long distance quantum communication must be achieved by a different approach. A repeater breaks the route into shorter sections and links them together using quantum effects such as entanglement.
Quantum memory stores a quantum state temporarily so that shorter sections can be linked together. This is important because photons required to interact with one another may not arrive at the interaction point simultaneously. Materials such as specially engineered crystals or defects in solid-state materials can help hold this information briefly.
Entanglement swapping is a process that connects shorter quantum links into a longer one. By making the right measurement between photons arriving from adjacent links at an intermediate node, two distant users can become linked through a shared quantum state even though they have never interacted directly.
Quantum processes are the only ones known to be genuinely random, in contrast to the classical world which is deterministic. Quantum random number generators use these random quantum processes to create unpredictable numbers. For example, a single photon sent to a beam splitter may end up either transmitted or reflected and the outcome cannot be predicted in advance. This randomness is useful for generating secure encryption keys.
Quantum devices are only useful if they can be trusted. Small biases in sources, detectors, beam splitters or fibres can affect performance and security. NPL’s metrology helps check that devices behave as expected using measurements that are traceable and trusted.
Quantum communications combine physics, engineering and measurement science. The key ideas are single photons, qubits, security through quantum effects, signal loss in fibres, and the need for accurate testing. NPL’s role is to make sure the devices and standards behind these technologies are good enough for practical use. To send quantum information securely, a network needs a hardware device that can generate exactly one photon at a time. If a device accidentally transmits two identical photons, a hacker could steal one without being detected.
Quantum communications - NPL
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