Quantum computing represents one of the most significant technological developments of the modern era, with the potential to fundamentally reshape both computational capability and Cyber Security risk. Unlike incremental advances in classical computing, quantum computing introduces an entirely new paradigm, based on the principles of quantum mechanics. While still an emerging technology, its trajectory – driven by both commercial innovation and nation state investment – positions it as a strategic risk that organisations must begin to address today. 

At its core, quantum computing differs from classical computing in how it processes information. Classical computers operate using binary bits, which exist in a state of either 0 or 1. In contrast, quantum computers use quantum bits, or qubits, which can exist in multiple states simultaneously through a phenomenon known as superposition. This allows quantum systems to explore multiple computational pathways at once. Combined with entanglement, where qubits become intrinsically linked regardless of distance, quantum computing enables the solving of certain classes of problems at a scale and speed that would be infeasible for classical systems.

However, despite its theoretical power, quantum computing remains constrained by significant practical limitations. Current systems require extremely controlled environments to function, including temperatures close to absolute zero, electromagnetic shielding, and isolation from vibration and contamination. These factors mean that quantum computers are not yet widely deployable and are unlikely to replace classical computing for general-purpose workloads. Instead, they are expected to remain specialised tools, applied to specific high-complexity problems such as drug discovery, financial modelling and cryptanalysis.

The most significant impact of quantum computing from a cyber security perspective lies in its ability to undermine modern cryptographic systems. Today’s digital security infrastructure relies heavily on Public Key Encryption (PKE), including algorithms such as RSA and Elliptic Curve Cryptography (ECC). These systems depend on mathematical problems—such as integer factorisation and discrete logarithms—that are computationally infeasible for classical computers to solve within a practical timeframe. Quantum computing, however, changes this assumption. Using algorithms such as Shor’s, a sufficiently powerful quantum computer would be able to solve these problems efficiently, rendering current encryption, digital signatures, and authentication mechanisms ineffective. While public key cryptography faces the most severe impact, other cryptographic mechanisms are also affected, albeit to a lesser degree. Symmetric encryption algorithms, such as AES, are susceptible to quantum optimisation techniques like Grover’s algorithm, which effectively reduces their strength but does not render them completely insecure. Similarly, cryptographic hashing functions remain viable, though their security margins are diminished.

A critical concept in understanding the quantum threat is the notion of “Q-Day”—the point at which a quantum computer becomes sufficiently powerful to break current cryptographic systems. At this stage, the integrity of global digital trust would be fundamentally compromised, with immediate consequences for secure communications, identity systems, and data protection. However, one of the most important insights for organisations is that quantum risk is not confined to this future milestone. The concept of “Harvest Now, Decrypt Later” (HNDL) highlights the reality that adversaries may already be collecting encrypted data today, with the intention of decrypting it once quantum capabilities become available. This creates a present-day risk for any data that requires long-term confidentiality, including personally identifiable information, intellectual property, and sensitive government or defence-related data.

In response to this emerging threat, the cyber security community has been actively developing and standardising new forms of cryptography designed to withstand quantum attacks. Collectively referred to as Post-Quantum Cryptography (PQC), these algorithms are based on mathematical problems that are believed to remain difficult even for quantum computers, such as lattice-based and hash-based constructions. Leading standards bodies, including NIST, have begun formalising these approaches, with algorithms such as ML-KEM (Kyber) for key exchange and ML-DSA (Dilithium) for digital signatures representing the foundation of future cryptographic systems.

Despite their promise, PQC solutions will introduce new challenges. Key sizes are significantly larger than those used in current cryptographic standards, resulting in increased demands on processing power, storage, and network bandwidth. Furthermore, many legacy systems are not designed to accommodate these changes, meaning that organisations will need to carefully manage a transition period involving hybrid cryptographic models that combine classical and quantum-resistant approaches. This transition will require coordination across technology, vendors, and supply chains, particularly as cloud providers and platform vendors begin to introduce PQC capabilities into their offerings.

For organisations, particularly those operating in regulated or high-risk environments, the implications of quantum computing are both strategic and operational. The challenge is not only technical but also organisational, requiring improved visibility of cryptographic assets, stronger governance, and proactive engagement with suppliers and technology partners. A structured approach is essential, beginning with awareness at the leadership level, followed by discovery and assessment of cryptographic dependencies across systems and data assets. From there, organisations must prioritise risk based on data sensitivity and longevity, developing phased migration plans that incorporate principles of crypto-agility—the ability to rapidly adapt cryptographic mechanisms as standards evolve.

In conclusion, quantum computing should be viewed not as a distant or theoretical concern, but as a developing reality with tangible implications for cyber security today. While large-scale quantum systems capable of breaking cryptography may still be several years away, the existence of HNDL risk and the pace of technological advancement mean that organisations must act now to prepare. Early investment in awareness, assessment, and cryptographic resilience will enable a more controlled and effective transition, ensuring that security, compliance, and trust can be maintained in the face of this transformative technological shift.