Think of a coin spinning on a table. While it spins, it is neither heads nor tails. When it falls, it lands on one side. The image has a limit, though. A spinning coin already has a side facing up; we just cannot see it. A qubit hides no answer. The result is only decided at the moment it is read.
Qubits are fragile: the slightest disturbance makes them lose their information. To get one qubit that can be relied on, you have to combine many fragile ones, sometimes hundreds. So when a company announces a computer with 1,000 qubits, ask how many of them are reliable. The answer is always far fewer.
This matters for data security. In 2019, experts estimated that breaking the encryption protecting much of the internet would take 20 million qubits. In 2025, that estimate fell below one million.
A quantum computer does not test every possible answer at the same time. It works by making the wrong answers disappear so that the right one comes out. That is why it is only faster for certain problems.
The most advanced machines now have hundreds or thousands of qubits, but far fewer reliable ones. IBM aims to reach 200 reliable qubits by 2029. The question is no longer how many qubits a machine has, but how many it can trust.
Superposition · Quantum entanglement · Quantum computer · Quantum mechanics
It is the quantum version of the bit. A bit is 0 or 1. A qubit can be both while a calculation runs, and becomes 0 or 1 when it is read.
When you read a qubit, you get exactly one bit: a 0 or a 1. Before it is read, describing it takes more than one number, but you can never get more than one bit out of it.
Not necessarily. A qubit can be an atom, a particle of light or a tiny electrical circuit cooled close to absolute zero. No technology has yet become the standard.
A physical qubit is a single piece of hardware, and it makes errors. A logical qubit is a reliable qubit made by combining many physical ones. What a quantum computer can really do depends on its logical qubits.
It depends on their quality. Fifty perfect qubits could handle calculations that ordinary computers struggle to simulate. Fifty fragile qubits, like the ones built today, do not yet solve any useful business problem. That is why a qubit count alone says very little.
Any contact with the outside world, such as heat, vibration or stray light, can disturb a qubit and erase its information. Physicists call this decoherence. Depending on the technology, a qubit keeps its information for anything from a millionth of a second to a few minutes.
1. Gidney, C. & Ekerå, M., How to factor 2048-bit RSA integers in 8 hours using 20 million noisy qubits, 2019.
2. Gidney, C., How to factor 2048-bit RSA integers with less than a million noisy qubits, Google Research, May 2025.
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