Quantum Communications Explained: When the Laws of Physics Become Part of the Network

This is Part 1 of Bridge Connect’s six-part insight series, Quantum Communications: From Quantum Physics to Critical Infrastructure. The series follows the development of quantum communications from its underlying principles through quantum key distribution, post-quantum security choices and operational deployment to real-world results and the longer-term prospect of a quantum internet. Part 2 will examine how quantum key distribution can reveal eavesdropping—and where the promise of physics-based security meets the realities of operational networks.

For more than a century, telecommunications has been concerned with moving information by manipulating physical signals. Copper networks represented speech as electrical variations. Radio systems encoded information into electromagnetic waves. Fibre-optic networks turned data into pulses of light.

Quantum communications also uses physical signals, frequently photons travelling through optical fibre or free space. But it treats those signals in a fundamentally different way.

In a conventional communications system, the physical properties of the signal are principally a means of carrying classical information: ones and zeros that can be detected, copied, amplified, stored and retransmitted. In a quantum communications system, the quantum state of the signal is itself an essential part of the information being communicated.

This creates properties that have no direct equivalent in conventional networks. A quantum state cannot generally be measured without affecting it. An unknown quantum state cannot be copied perfectly. Two quantum systems can also share correlations that cannot be reproduced by classical physics.

These properties could support new ways of distributing cryptographic keys, linking quantum computers, connecting advanced sensors and protecting information whose confidentiality must survive for decades.

They do not, however, produce instantaneous communications, unbreakable networks or a wholesale replacement for the internet.

Understanding that distinction is the starting point for assessing what quantum communications can realistically deliver.

From bits to quantum states

All conventional digital communications ultimately reduce information to bits. A bit has one of two definite values: zero or one.

The physical representation can vary. A bit may be encoded as a voltage, a radio waveform, a pulse of light, a magnetic state or a charge within a semiconductor. Whatever the medium, the network interprets the signal as one of two classical alternatives.

A quantum bit, or qubit, is different. It can be prepared in one of two basis states—commonly described as zero and one—but it can also exist in a quantum combination of those states known as a superposition.

This is sometimes described too casually by saying that the qubit is “both zero and one at the same time”.

That phrase conveys some of the strangeness, but it can also be misleading. A qubit is not simply a classical bit with two simultaneous values. It is a quantum state defined by a combination of possibilities, including their relative amplitudes and phases.

When the qubit is measured in a particular basis, the measurement produces a definite result. The original superposition does not emerge intact from that measurement.

This matters because quantum information is not merely a more complicated version of conventional data. It behaves according to different rules.

Photons are especially useful for quantum communications because aspects of a photon—such as its polarisation, phase, arrival time or path—can be used to encode a quantum state. Photons also travel naturally through fibre or free space and interact relatively weakly with their environment.

That does not make them easy to use. The same sensitivity that gives quantum communications its distinctive security properties also makes quantum states fragile. Loss, noise, imperfect sources, detector errors and interaction with the environment can all degrade the state being transmitted.

The engineering challenge is therefore to exploit quantum behaviour while controlling—or at least accurately characterising—the imperfections of real equipment and networks.

Measurement changes the system

In a classical network, an adversary may be able to copy a signal without visibly changing it.

Someone who gains access to a fibre can potentially divert and record some of the optical signal. An intelligence service can collect encrypted traffic today and retain it for future analysis. If the interception is sufficiently well executed, the legitimate users may never know that a copy was made.

Quantum information behaves differently. Measuring an unknown quantum state generally affects that state.

The effect depends on how the state was prepared and how it is measured. If a photon is prepared using one basis but measured using an incompatible basis, the measurement does not reliably reveal the original information. It instead produces a probabilistic result and changes the state.

This creates the possibility of identifying interception. If an eavesdropper attempts to measure a sequence of quantum states and then forwards replacements, the intervention will normally introduce detectable discrepancies into the results observed by the legitimate participants.

It is important to describe this accurately. The principle does not mean that every interaction with a quantum channel generates a simple alarm saying that a named attacker has entered the network.

Legitimate loss, component imperfections and environmental noise can also produce errors. A practical system must estimate whether the observed error rate remains within an acceptable threshold.

Nor does the principle mean that the entire communications system becomes immune to attack. It applies to the relevant quantum states and protocol. It does not inherently secure the end devices, network-management platform, key-management system, conventional data channel or people operating the service.

Nevertheless, it creates a significant departure from classical communications. In the right circumstances, an attempt to acquire the quantum information changes the evidence available to the communicating parties.

The no-cloning principle

Reliable copying is one of the foundations of telecommunications.

Signals are regenerated.

Packets are replicated.

Data is backed up.

Routers inspect and retransmit traffic.

Mobile networks hand communications between different cells and network functions.

Cloud platforms reproduce information across multiple systems and locations.

Quantum information cannot be treated in precisely the same way.

The no-cloning theorem states that it is not possible to make a perfect independent copy of an arbitrary unknown quantum state.

This is not simply the result of inadequate equipment.

It is a consequence of quantum mechanics.

The restriction is specific and should not be exaggerated.

Known quantum states can be prepared repeatedly.

Classical information extracted through measurement can be copied. Some carefully defined sets of states can be distinguished under particular conditions.

What cannot be built is a universal machine that takes any unknown quantum state and produces a flawless duplicate while leaving the original unchanged.

For an eavesdropper, this removes an intuitively attractive attack. The attacker cannot simply capture each unknown quantum state, retain an exact copy and pass on an undisturbed original for the intended recipient.

The US National Institute of Standards and Technology describes this as the important difference between quantum information and classical bits: classical bits can be copied perfectly, whereas an attempt to measure or copy an unknown quantum state changes or destroys the information encoded in it.

The no-cloning principle also has major consequences for network design. A conventional optical signal can be detected, regenerated and retransmitted by an amplifier or repeater.

A quantum signal cannot simply be read, copied and amplified without losing the quantum information that the network is supposed to preserve.

This is one reason why distance and signal loss remain central obstacles to large-scale quantum networking.

Entanglement: correlation without instant messaging

Entanglement is perhaps the most famous - and most frequently misrepresented - feature of quantum mechanics:

• Two quantum systems are entangled when their states must be described jointly rather than independently.

• Measurements performed on them can exhibit correlations that cannot be explained by assigning each particle a complete set of predetermined classical properties.

This remains true even when the entangled systems are separated.

Einstein famously objected to what he called “spooky action at a distance”, and entanglement certainly challenges everyday intuition. Yet it does not provide a way to send an intelligible message instantaneously.

The result observed when one member of an entangled pair is measured is intrinsically probabilistic. The observer cannot choose the result in order to encode a message. A conventional communications channel is still needed to compare measurement choices and results.

Entanglement therefore does not overturn the speed of light or allow a network to operate without classical connectivity.

What it can provide is a shared quantum resource. Properly created, distributed and verified entanglement could allow geographically separated systems to perform tasks that are impossible - or significantly more difficult - with classical communications alone.

These may eventually include linking quantum processors, coordinating distributed quantum computations, supporting enhanced clock synchronisation, enabling certain forms of secure computation and connecting quantum sensors into networks with capabilities beyond those of individual devices.

Entanglement is therefore not simply another encryption technique. It could become a building block of a different class of network.

What does a quantum communications system actually transmit?

The term “quantum communications” covers several related but distinct technologies.

The most mature is quantum key distribution, or QKD. In a typical QKD system, quantum states are transmitted so that two parties can establish shared cryptographic key material and identify whether the quantum exchange has been excessively disturbed.

The resulting key is then used by conventional encryption equipment to protect ordinary data. The customer’s voice, video, database transaction or control message is not necessarily sent as quantum information. The quantum channel supports key establishment; the operational data normally remains classical.

This is an important distinction because phrases such as “quantum-encrypted communications” can imply that the entire message travels in an exotic quantum form. In most current deployments, that is not what happens.

A second field concerns quantum random-number generation. Quantum processes can provide sources of randomness for cryptographic operations.

This is closely associated with quantum-secure communications, although it does not require a quantum network.

A third area involves quantum direct communication, in which information rather than merely cryptographic key material is encoded through a quantum protocol. This remains much less mature as a practical telecommunications proposition.

The most ambitious objective is the development of entanglement-based quantum networks, potentially forming a future quantum internet. Such networks would distribute and manipulate entanglement between remote nodes and could connect quantum computers, memories and sensors.

These categories should not be merged into a single technology-readiness claim.

The commercial availability of point-to-point QKD does not mean that a general-purpose quantum internet is close to deployment. They depend on different levels of component maturity, network control and system integration.

Quantum communications and quantum computing are not the same thing

Quantum computing and quantum communications share an underlying scientific foundation, but they address different functions.

Quantum computing seeks to manipulate quantum information to perform computation. Quantum communications seeks to transfer quantum states or establish quantum resources between locations.

The relationship between the two is nevertheless strategically important.

First, sufficiently capable quantum computers could threaten widely used public-key cryptography. Shor’s algorithm could, in principle, undermine encryption and signature systems based on integer factorisation and discrete logarithms. This creates the need to migrate to post-quantum cryptography and has also contributed to interest in QKD.

Second, distributed quantum computers may eventually need quantum networks. A future system could link multiple smaller quantum processors rather than relying exclusively on one very large machine. That would require far more than an ordinary low-latency data connection. The network would need to preserve and distribute quantum states or entanglement.

Third, access to quantum computing could itself become network-based. Organisations may use remote quantum processors through cloud services rather than owning the hardware. Most such access today uses conventional networks: the customer sends a classical description of a problem and receives a classical result. More advanced future services could require quantum communications between the customer and the remote processor.

Quantum communications is therefore simultaneously part of the response to the cryptographic threat from quantum computing and a potential enabler of future distributed quantum computation.

The first practical application: quantum key distribution

QKD is where the principles begin to turn into operational systems.

In a simplified prepare-and-measure protocol, one participant—conventionally called Alice—prepares photons in a sequence of quantum states. Another participant, Bob, measures them. The two then communicate over an authenticated classical channel to compare selected information about their preparation and measurement choices.

They do not disclose the entire sequence that could become the key. Instead, the public comparison allows them to discard incompatible results and estimate the error level within the remaining data.

If the level of disturbance is acceptable, they perform further classical processing, including error correction and privacy amplification, to derive matching secret keys. If the disturbance is too high, they reject the exchange rather than trusting potentially compromised key material.

In entanglement-based QKD, pairs of entangled photons are distributed and measured by the participating parties. The observed quantum correlations support the establishment and verification of the key.

In either case, QKD does not encrypt the operational data by itself. It generates or distributes key material.

A separate encryption system—often using a well-established symmetric cipher such as the Advanced Encryption Standard—uses those keys to protect the data traffic.

QKD also requires authentication. Alice and Bob must know that they are communicating with each other rather than with an attacker impersonating both sides. The quantum properties do not make identity management disappear.

The UK government’s 2026 QKD research report describes the technology as a method of generating and distributing cryptographic keys through quantum-mechanical effects over an optical channel. It also emphasises that this quantum channel is typically distinct from the communications channel carrying the encrypted data. UK government QKD research report

These qualifications do not diminish QKD. They define what the technology actually does and allow its value to be assessed properly.

Why quantum communications is difficult to scale

The internet works partly because classical information can be regenerated, stored and routed with extraordinary reliability and at low incremental cost.

Quantum networks cannot assume the same operating model.

Photons are lost as they travel through optical fibre. Quantum states can be degraded by interaction with the surrounding environment. Detectors are imperfect. Sources may emit pulses that do not correspond exactly to the theoretical model. Conventional optical amplifiers cannot simply copy and boost an unknown quantum state.

Current QKD networks often address distance limitations by using trusted nodes. One QKD link terminates at a secure location, where keys are processed before another link continues towards the destination.

This can extend the reach of the service, but the intermediate node must genuinely be trusted. The end-to-end security claim now depends partly on protecting that site, its hardware, software, personnel and key-management processes.

Future quantum repeaters are intended to extend entanglement without requiring the intermediate node to access the underlying information in the same way. They would depend on capabilities such as entanglement swapping, quantum memories and error management.

These technologies are progressing, but they are not yet equivalent to the mature, mass-produced repeaters and routers of conventional networks.

Satellites offer another way to overcome terrestrial distance limitations because photons experience relatively low loss while travelling through space. Most of the loss occurs in the atmosphere and at the optical terminals. Satellite systems, however, introduce requirements involving line of sight, cloud cover, orbital availability, precision pointing, ground stations and space infrastructure.

The real future network is consequently likely to be hybrid: quantum and classical, terrestrial and space-based, with different technologies used according to distance, assurance requirement, cost and operational context.

Security based on physics is not the same as perfect security

One of the strongest claims made for quantum communications is that security can be based on the laws of physics rather than the assumed difficulty of a mathematical problem.

At the level of an ideal protocol, that is a profound advantage. An attacker cannot overcome a fundamental quantum principle simply by purchasing a faster computer.

Practical systems, however, are not ideal protocols. They contain lasers, detectors, electronics, control software, network interfaces, key stores, management systems and conventional cryptographic functions. Those components may behave differently from the theoretical assumptions.

Side-channel attacks can exploit information leaked through implementation details. A detector may respond differently under manipulated illumination. A source may emit unintended additional photons. An attacker may target the management interface rather than the quantum exchange. Deliberate disruption can force the system to abandon key generation, producing a denial-of-service condition.

ETSI’s work on QKD therefore includes interfaces, component characterisation, implementation-security requirements and protection profiles. Standardisation and assurance are needed precisely because invoking quantum physics does not remove the need for secure engineering.

The most accurate conclusion is not that quantum communications is automatically secure. It is that quantum mechanics can provide distinctive security properties when those properties are realised through correctly designed, implemented, integrated and operated systems.

What quantum communications could change

The immediate importance of quantum communications lies in high-assurance security and the development of national quantum capability.

Governments, defence organisations, financial institutions, telecommunications operators and critical-infrastructure providers may have information that must remain confidential for several decades. For them, the possibility that encrypted traffic could be collected today and decrypted in the future is a strategic concern.

QKD offers one possible additional layer of protection for selected connections. It may also provide evidence of physical-channel interference that conventional key exchange does not reveal.

But quantum communications has a longer horizon.

Networks capable of distributing entanglement could connect quantum computers into larger systems. Quantum sensors might operate collaboratively across multiple locations. Remote users might access quantum resources while revealing less information about their data or computation. Time and frequency services could gain new forms of assurance or precision.

Many of these applications remain experimental. Their eventual value may prove greater than that of QKD alone, just as the value of the early internet ultimately extended far beyond the first applications demonstrated over its underlying networks.

That analogy should be treated cautiously. Technological history is littered with systems presented as “the next internet”. Quantum networking faces difficult physics, demanding infrastructure and an uncertain timetable.

Nevertheless, the strategic proposition is credible enough for governments and network operators to invest in testbeds, standards, skills and supply chains.

The board-level “so what?”

Boards do not need to become quantum physicists. They do need to distinguish among three different questions.

The first is whether quantum computing will require changes to current cryptography. The answer is yes. Organisations should identify cryptographic dependencies, develop crypto-agility and prepare for migration to post-quantum cryptography.

The second is whether they should deploy QKD. The answer depends on the use case. QKD may be appropriate for a relatively small number of high-value, controlled connections. It is not currently a universal substitute for conventional key establishment or post-quantum algorithms.

The third is whether quantum networks could create new capabilities beyond cybersecurity. The answer is potentially—but the technologies, applications and commercial models remain at an earlier stage.

The best strategic approach is therefore neither dismissal nor unquestioning enthusiasm. It is to separate the time horizons:

• Act now on cryptographic discovery, risk assessment and post-quantum migration.

• Evaluate selectively where QKD could add an independent layer of protection.

• Experiment intelligently with quantum-network technologies where there is strategic or operational relevance.

• Monitor patiently the development of repeaters, memories, standards and genuinely useful network applications.

Quantum communications matters because it changes the relationship between information and the physical network carrying it. Instead of treating physics merely as a medium to be engineered around, it makes physical law part of the network’s security model and, eventually, part of its functionality.

That is a substantial shift. But its significance will be determined not by the elegance of quantum theory alone. It will depend on whether the industry can translate fragile quantum effects into services that are interoperable, manageable, resilient and economically justified.

The next article in this series will examine the most mature expression of that effort: how quantum key distribution works, what an eavesdropper can and cannot do, and where the promise of physics-based security meets the realities of operational networks.

Bridge Connect helps boards, investors, governments and critical-infrastructure organisations understand the strategic implications of emerging technologies without losing sight of operational and commercial reality. We can support organisations in assessing their exposure to quantum-related security risks, distinguishing between post-quantum cryptography and quantum communications, identifying credible use cases and developing proportionate technology and investment roadmaps. We also provide independent briefings, strategic advisory support and specialist insight to help decision-makers separate genuine capability from market hype.