A quantum computer from A to Z: a simple explanation for laymen

A quantum computer sounds like sci-fi and most articles about it are full of complicated physics. Yet the basic idea can be explained without formulas. Let us do it simply and at the same time honestly, where the reality is and where it is still only a promise.

What a quantum computer is

A quantum computer is a computer that computes according to the laws of quantum physics, and therefore in a completely different way than an ordinary computer. It is not just a “faster PC”. It is a different way of computing that is exceptional only for certain types of tasks, not for ordinary work.

Important right at the start: a quantum computer will not replace your laptop or phone. It is not suited to email, the web or games, and an ordinary computer is better for that.

A bit versus a qubit

The difference is best understood through the basic unit of information:

• A bit in an ordinary computer has a value of either 0 or 1. Always one of two.
• A qubit (a quantum bit) can be 0, 1, or both at once in a certain ratio. This “at once” state is called superposition.

It is precisely the qubit and its ability to be in several states at the same time that gives a quantum computer its strange power.

Quantum phenomena simply

• Superposition. A qubit can be both 0 and 1 at the same time, until you measure it. Thanks to this the computer can “explore” many possibilities at once.
• Entanglement. Two qubits can be linked so that their states relate to each other, even when far apart. This enables computations that ordinary bits cannot manage.
• Interference. With a clever procedure the correct answers are amplified and the wrong ones cancel each other out.
• Measurement. When you measure a qubit, the superposition “collapses” to a clear 0 or 1. That is why algorithms have to be designed very cunningly.

Why it could be powerful

For certain problems a quantum computer can explore a huge space of possibilities far more efficiently than an ordinary one. Simply put: with each additional qubit, the number of states it can capture grows twofold. With enough qubits it could handle tasks that would take an ordinary computer an unimaginably long time.

Beware, this does not apply generally. It is an advantage only for specific tasks, not for everything.

What it would be good for

• Simulating molecules and materials. Designing drugs, better batteries and new materials. This is its most promising use.
• Optimization. Finding the best solution in transport, logistics or finance.
• Cryptography. It could potentially break some of today’s encryption, which we will talk about below.
• Research in AI and machine learning, still at an early stage. It relates to the article machine learning.

What the reality is today

Let us be fair, quantum computers are still at the beginning:

• Qubits are extremely fragile. They are easily disturbed by their surroundings and make errors (this is called decoherence).
• They need extreme cold, near absolute zero, and demanding error correction.
• Current machines are “noisy” and small. They have a limited number of qubits and are not yet practically useful for most real problems.
• Access is through the cloud. You will not have one at home, companies like IBM or Google offer remote access to them.

In other words, it is an extraordinarily promising technology, but its real practical benefit is still to come.

The security impact: encryption and post-quantum cryptography

This is a part that also concerns companies. A sufficiently powerful quantum computer could one day break some of today’s encryption (for example the one on which SSL/TLS certificates and secure connections stand). There is also the “store now, decrypt later” scenario, where an attacker collects encrypted data today and waits for a future quantum computer.

That is why post-quantum cryptography is already being prepared, that is new ways of encryption resistant even to quantum computers. For an ordinary company this is not yet urgent, but it is good to know about it and to keep it in mind for sensitive long-term data. It relates to cybersecurity principles.

Practical for a layman and a business

• It will not change your everyday computing, at least not in the foreseeable future.
• It is not a competitor to your PC, but a tool for specific scientific, optimization and cryptographic tasks.
• The most real impact is on security, the gradual transition to post-quantum encryption.

Conclusion

A quantum computer computes according to the laws of quantum physics and, thanks to qubits, superposition and entanglement, can do for certain tasks what an ordinary computer cannot. But it is not a faster PC or a replacement for your laptop, it is a specialized tool and still at the beginning of its development. Its most practical impact will be in security, where we will gradually move to encryption resistant to quantum computers.

This article is part of our Hardware and components overview.