What Is a Qubit?
A qubit is a unit of quantum information. Learn why measuring it in the zero-or-one basis gives one result, not every possibility.
Free video lesson · 1:16
What Is a Qubit?
Quantum Computing & Encryption for Beginners
Music: “Thinking Music” by Kevin MacLeod. Creative Commons Attribution 4.0.
A qubit can have a superposition state, but measuring it in the described way returns one bit.
Read the video transcript
A qubit is a unit of information in a quantum computer. An ordinary bit is read as zero or one. Measure a qubit in the same two-choice way, and you also get zero or one.
The difference is what can happen before you read it. A qubit can be prepared in a quantum combination of its zero and one states. That combination is called superposition.
That does not let you read zero and one together as two answers. Each measurement gives one result. If you prepare the same state again and again, the results can build up a pattern.
For example, one prepared state gives zero about half the time and one about half the time. Other preparations change those chances. Quantum calculations change the state before the result is read.
A random coin also has chances, but a qubit is not just a hidden coin toss. Its quantum state allows effects that an ordinary random bit does not. Those effects are what quantum calculations make use of.
Explore What Is a Quantum Computer? Or follow the security question: How Many Qubits Would It Take to Break RSA?
The short answer
A qubit is a unit of information in a quantum computer. An ordinary bit is read as zero or one. Measure a qubit in the same two-choice way, and you also get zero or one.
A simple example
The difference is what can happen before you read it. A qubit can be prepared in a quantum combination of its zero and one states. That combination is called superposition.
That does not let you read zero and one together as two answers. Each measurement gives one result. If you prepare the same state again and again, the results can build up a pattern.
For example, one prepared state gives zero about half the time and one about half the time. Other preparations change those chances. Quantum calculations change the state before the result is read.
A random coin also has chances, but a qubit is not just a hidden coin toss. Its quantum state allows effects that an ordinary random bit does not. Those effects are what quantum calculations make use of.
Preparing is different from reading
A qubit has a quantum state that can be changed before it is measured. Repeating a prepared experiment gives a pattern of results. That pattern is useful, but one measurement does not print out every possibility in the state.
Check your understanding
When you measure a qubit in the zero-or-one basis, do you get both numbers at once?
Answer
No. That measurement gives one result: zero or one.
Keep learning
Follow the free video course one question at a time. For help applying these ideas at work, explore team training or a cryptography audit.