MODULE 5 · LESSON 3
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Sign in to track progress / enrolWhat Is Not Quantum Computing
Several substantial technologies carry the word quantum and are not quantum computing. The distinctions matter commercially, because products from these categories are regularly sold as though they addressed problems they do not touch.
Quantum annealing
The most frequent confusion, largely because D-Wave sold machines with thousands of qubits years before gate based systems reached a hundred, and coverage compared the numbers directly.
Annealing is not a general purpose quantum computer. It is a special purpose device for one job: finding low energy configurations of a particular kind of optimisation problem. You encode your problem so its lowest energy state is the answer, and let the system settle.
Three things follow.
It cannot run Shor's or Grover's algorithm. These require gate based operations. An annealer poses no threat to encryption whatsoever, which is worth knowing when someone cites qubit counts in a discussion about cryptography.
Its qubits are not comparable. An annealer's qubits are far less capable individually than gate based qubits, so a five thousand qubit annealer and a hundred qubit gate based processor are not on the same scale in any meaningful sense.
Its advantage is contested. Whether annealers outperform good classical optimisation on real problems has been debated for over a decade, with many claimed advantages subsequently matched classically. Some users report practical value; the general claim remains unsettled.
Annealing is a legitimate technology with real users. It is a different thing from what this course has described.
Quantum key distribution
A communications technology, not a computing one, and Module 2 introduced it briefly.
Quantum key distribution establishes a shared secret key between two parties, with security resting on no-cloning: an eavesdropper cannot copy the transmitted states without disturbing them, so interception is detectable.
The critical commercial point: it does not address the problem Shor's algorithm creates.
Shor threatens the public key cryptography that secures the internet at large. Quantum key distribution requires a dedicated physical link, fibre or line of sight to a satellite, between exactly two parties. It cannot secure a connection to a website you have never contacted, which is precisely what public key cryptography does.
It also has practical constraints: distance limits, dedicated infrastructure, no authentication of its own, so it still needs conventional cryptography to verify who is at the other end.
Most national security agencies recommend post-quantum cryptography, the subject of Module 6, rather than quantum key distribution as the general answer, on grounds of cost and practicality. Quantum key distribution has genuine niche applications, typically fixed high value links between two known sites.
If a vendor offers quantum key distribution as protection against quantum computers breaking your encryption, that is a category error worth catching.
Quantum inspired algorithms
Entirely classical algorithms that borrow mathematical structure from quantum mechanics and run on ordinary computers.
Some are genuinely good. Techniques such as tensor networks, developed for simulating quantum systems, have found real use in classical machine learning and optimisation.
The confusion is purely naming. These run on classical hardware, require no quantum computer, and offer no quantum speedup. When a vendor describes a quantum inspired solution, the accurate translation is a classical algorithm that borrows some quantum mathematics, which may be excellent and is not quantum computing.
Notably, several quantum inspired algorithms emerged directly from dequantisation: researchers examining a proposed quantum algorithm found the essential idea worked classically. Useful outcome, and it undercuts the quantum claim that motivated it.
Quantum sensing
The least confused and arguably the most commercially mature, so it deserves mention.
Quantum sensing uses quantum effects to measure things with extraordinary precision: magnetic fields, gravity, time, rotation. Applications include medical imaging, underground surveying, navigation without satellite positioning, and improved atomic clocks.
It is unrelated to computation and it works today at commercial quality. In terms of near term deployed value, quantum sensing is arguably ahead of quantum computing.
Because the vocabulary is shared across unrelated fields, a short procedure helps.
Is it gate based? If a vendor sells quantum computing, ask whether the machine executes a circuit of gates. If not, it is an annealer or a special purpose simulator, and general claims about quantum algorithms do not apply to it.
Does it compute or communicate? Quantum key distribution, quantum networking and quantum repeaters are communications technologies. They do not run algorithms and are irrelevant to whether anyone can break your encryption.
Does it need a quantum computer at all? If the product runs on classical hardware, it is quantum inspired, which means classical. This is often disclosed accurately in the technical documentation and obscured in the sales material.
What problem does it claim to solve, and is that problem in the short list? Module 3 gave the list: factoring and discrete logarithms, unstructured search with a quadratic gain, and simulating quantum systems. A claim outside these deserves the cancellation question from Module 2.
The commercially expensive error is the second one. Organisations have bought quantum key distribution equipment believing it addressed the Shor threat, when the actual mitigation is a post-quantum cryptography migration that costs a fraction as much and protects everything rather than one link.
If you take a single practical point from this module, make it that one: the answer to quantum computers breaking encryption is new mathematics deployed in software, not new hardware on your network.
A vendor proposes quantum key distribution hardware as your organisation's protection against quantum computers breaking encryption. What is the problem?
Quantum annealing
Click to flipSpecial purpose hardware for a class of optimisation problems. Cannot run gate based algorithms, so it poses no threat to encryption.
Click to flip backSeveral serious technologies share the word quantum and are not quantum computing. Annealers are special purpose optimisation devices that cannot run Shor or Grover, so their large qubit counts are irrelevant to encryption. Quantum key distribution is a communications technology needing a dedicated two party link and does not address the Shor threat at all. Quantum inspired methods are classical algorithms. Quantum sensing is precision measurement and is commercially further along than computing. The expensive error is buying hardware for a problem whose answer is a software migration.