Classical vs Quantum Computers

We’ve established that the public-key mailbox is threatened by “a quantum computer.” Time to meet the machine itself — and to be honest about what it can and cannot do, because the popular picture is half right and half misleading.

The computer you know: bits and switches

Every ordinary computer — your phone, your laptop, the servers behind every site you visit — stores information in bits. A bit is the simplest thing imaginable: a switch that is either off (0) or on (1). Nothing in between. Every photo, song, spreadsheet, and video is, at bottom, billions of these switches, each holding one definite value, being flipped in careful patterns.

To keep things straight, we call this a classical computer — “classical” just meaning “the normal kind, following everyday physics.” For almost everything you will ever do, the classical computer is exactly the right tool, and it has been improving relentlessly for eighty years.

The new machine: qubits and “both at once”

A quantum computer replaces the bit with a qubit (“quantum bit”). A qubit can be 0, or 1, or — here is the strangeness — a blend of both at the same time, right up until the moment you look at it. Physicists call that blend superposition (we’ll explore it properly in the next article).

The everyday image is a spinning coin. While it spins in the air, it is meaningfully neither heads nor tails but a mix of both possibilities; only when it lands — when you measure it — does it settle on one face. A qubit is like a coin that can be kept spinning, and only becomes a definite 0 or 1 when read.

The useful half-truth of “trying everything at once”

You will hear, everywhere, that a quantum computer “tries every possibility at once.” With many qubits in superposition together, they can represent an astronomical number of combinations simultaneously — 300 qubits can, in a sense, hold more combinations than there are atoms in the observable universe. So far, so wondrous.

The real trick: cancel the wrong, reinforce the right

So how does it ever help? Through a second quantum property that behaves like waves. When waves meet, they can add up (crest meets crest — louder) or cancel out (crest meets trough — silence). Quantum states can do the same, an effect called interference.

The real art of quantum algorithm design — which took brilliant people decades — is to arrange the calculation so that the paths leading to wrong answers cancel each other out, while the paths leading to the right answer reinforce. Then, when you finally measure, the answer you want is overwhelmingly the one most likely to appear. The magic isn’t “check everything”; it’s “make the wrong answers destroy themselves.”

Crucially, this only works for problems with a hidden structure that allows such a cancelling pattern to be built. Most problems have no such structure, so no quantum speed-up exists for them.

A specialist, not a faster everything-machine

This is the single most important correction to make to the popular image:

For the overwhelming majority of computing — the things you do every day — a quantum computer offers no benefit whatsoever, and is in practice clumsier, colder, and far more error-prone than the phone in your pocket. It is a delicate laboratory instrument, not a replacement for the data centre. But for its few special problems, the speed-up isn’t incremental — it can turn “longer than the age of the universe” into “an afternoon.”

So why should security care?

Because of a spectacular piece of bad luck. One of those rare problems with exactly the right hidden structure — the kind where wrong answers can be made to cancel — is the “one-way street” that public-key cryptography depends on: factoring a large number back into its prime pieces, and the closely related elliptic-curve problem. A classical computer would need longer than the lifetime of the sun to reverse it. A large enough quantum computer, running the right recipe, could do it in hours.

That recipe has a name — Shor’s algorithm — and there is a second, milder one for symmetric keys called Grover’s algorithm. Together they decide exactly which locks break and which merely bend. We meet them in the fifth Foundations article. First, in the next one, we’ll get a little more comfortable with the strangeness itself: superposition and entanglement.

Bit

Classical unit of information: 0 or 1, one definite value.

Qubit

Quantum unit: a blend of 0 and 1 (superposition) until measured, when it becomes a definite 0 or 1.

Superposition

A qubit being multiple values at once until read.

Interference

Quantum states adding up or cancelling like waves — the mechanism that makes useful answers stand out.

Classical computer

An ordinary computer using bits and everyday physics.

NIST (National Institute of Standards and Technology)

The US standards body behind the post-quantum algorithms; cited here for its plain-English quantum explainers.

NSA (National Security Agency) & CISA (Cybersecurity and Infrastructure Security Agency)

US agencies that co-publish quantum-readiness migration guidance.

What to carry forward

• Classical = bits (definite 0/1). Quantum = qubits (a blend until measured).
• “Tries everything at once” is a half-truth — measuring collapses it all to one answer.
• Real power = arranging wrong answers to cancel and right ones to reinforce — only possible for special problems.
• A quantum computer is a specialist, useless for most tasks — but one of its special tasks is breaking public-key crypto.

Next: Qubits, Superposition & Entanglement — a closer, still-gentle look at the three quantum ideas that make it all work.

Understand it in your own words

Paste into any AI assistant to check yourself:

I'm learning what makes a quantum computer different, before studying
how it breaks cryptography. Quiz me one question at a time, correcting
me gently:

1. What's the difference between a bit and a qubit?
2. People say a quantum computer "tries everything at once." Why is that
   a misleading half-truth? What happens when you MEASURE the qubits?
3. If it doesn't just check every answer, how does a quantum computer
   actually produce the right one? (Hint: waves, cancelling, reinforcing.)
4. Why is a quantum computer a "specialist," not a faster version of my
   laptop? Name tasks where it gives NO advantage.
5. Why does cryptography care about this machine at all?