what is the difference between a physical qubit and a logical qubit?

A physical qubit is one piece of hardware, such as a single superconducting loop or a single trapped atom. A logical qubit is a steady qubit built from many physical ones plus constant error checking. Logical qubits are what real programs need, and they are far scarcer than physical ones.

what are qubits actually made of?

Several designs work. The common ones are superconducting loops on a cold chip, single charged atoms held by electric fields, neutral atoms held in laser light, single particles of light, and single electrons trapped in silicon. Each trades speed against how long it can hold its state.

why do qubits need to be kept so cold?

Heat is movement, and movement disturbs the delicate state a qubit holds. Superconducting qubits only work at temperatures near the lowest possible, so they sit inside a large staged fridge. Some other designs, such as trapped ions and photons, do not need the same deep cold.

What is a qubit?

Q: is a qubit really 0 and 1 at the same time?

That phrase is close and slightly misleading. A qubit holds two numbers, one linked to the answer 0 and one linked to the answer 1. Those numbers set how likely each result is, and how the qubit adds to or cancels with other paths. When you measure it you always see one plain value.

Spin a coin on a table and watch it. While it spins you cannot call it heads or tails. It is neither, and it is somehow both. Slap your hand down and it lands, and now it is one or the other.

A qubit is the closest thing a computer has to that spinning coin. It is the basic unit of information in a quantum computer, and it holds a mix of 0 and 1 until you look.

The coin is a good start, and it is not the whole story. Two spinning coins on a table are still two separate coins. However, two qubits can be tied together so tightly that they stop being separate at all. That part has no everyday copy.

What a bit is, and what a qubit adds

Start with the ordinary bit. A bit is a switch on a wall, and it is off or on, 0 or 1. Line up eight of them and you can count past two hundred. Line up billions and you have a phone.

A qubit keeps those two labels and adds two things. First, it can sit in a mix of 0 and 1. That mix is called superposition. Second, the mix carries a timing, called phase, in the way that a wave has a crest and a trough.

Phase is the part most short explanations skip. Phase is what lets two paths through a sum cancel each other out. Without phase, a qubit would be no more than a coin you had not looked at yet.

What superposition really means

Here is the honest version. A qubit does not quietly hold a 0 or a 1 that you have simply not seen yet.

It holds two numbers, one attached to the answer 0 and one attached to the answer 1. Those numbers set how likely each answer is when you measure. They also set how this qubit will add to, or cancel with, other paths along the way.

Measuring ends it. Look at the qubit and you get a single 0 or 1, and the mix is gone. You cannot peek and then carry on. You also cannot copy an unknown qubit, because that is a proven rule of physics, and better tools will not get around it.

How does entanglement fit in?

Take two qubits and put them through the right operation. They can end up sharing one joint state, in which neither qubit has a value of its own.

Measure the first and you get a random answer. Measure the second and it matches, every time, however far apart the two are. This is entanglement.

Think of a pair of dice that always land on the same face, even in different rooms. Each roll looks random on its own, and the pairing only shows up when the two people compare notes. Note that you cannot send a message this way, because each result on its own is still random.

Entanglement is what lets a group of qubits behave as one large system rather than as a shelf of small ones. That is where the power comes from.

Why counting qubits is confusing

You will see headlines counting qubits. Read them with care, because there are two kinds.

A physical qubit is one piece of hardware. A logical qubit is a steady, error-free qubit built out of many physical ones working together. Estimates vary by design, and it can take hundreds or even thousands of physical qubits to hold one logical qubit.

THE NUMBER THAT MATTERS IS LOGICAL QUBITS. This is because everything useful is measured there. Our guide to quantum error correction explains how the many become one.

What are qubits made of?

Several kinds work, and none has won yet.

Superconducting loops. Tiny circuits on a chip, cooled to near the lowest temperature there is. They are fast, and they lose their state quickly.
Trapped ions. Single charged atoms held in place by electric fields and poked with lasers. They are steady, and they are slower.
Neutral atoms. Atoms held in a grid of laser light, which makes large arrays easier to build.
Photons. Single particles of light, which travel well and are hard to hold still.
Spins in silicon. Single electrons trapped in a chip, which could borrow the tools of the existing chip industry.

Every design therefore trades speed against steadiness. A fast qubit that forgets quickly and a slow qubit that remembers well can end up equally useful.

Why are qubits so fragile?

A qubit is a very small thing holding a very exact state. Therefore, almost anything can disturb it. Heat, stray magnetic fields, the shake of a passing lorry, even a cosmic ray from space.

When that happens, the qubit drifts out of its mix, and the word for this is decoherence. Some kinds of qubit hold their state for only a tiny fraction of a second. Others hold on for seconds or longer.

This is the central problem of the whole field. It is also why a quantum computer is a room of cooling, shielding and control wiring wrapped around a chip the size of a coin.

How many qubits are enough?

It depends entirely on the job.

Small chemistry problems may need a few hundred logical qubits. Breaking the encryption that guards bank traffic would need many thousands, plus a long run with no errors. As of 2026 no machine is close to that second target.

The word “close” is doing real work there. Progress is genuine, and the gap is still wide. That gap is also why the world has begun moving to post-quantum cryptography already.

What to check when you read a headline

Ask four questions of any qubit claim.

Are these physical qubits or logical ones?
How long did the qubits hold their state?
What was the error rate for each operation?
Could a normal computer have done the same job?

Try this now, and find any recent news story that names a qubit count. Search that same page for the word “logical”. If the word is missing, the story is describing hardware rather than useful computing power. Our explainer on quantum computing shows what that power would be used for.

Just Out Tech explains new research in plain language. This article was drafted with AI assistance and checked by a human against the original source.