What Is Quantum Superposition?

What you will learn

• Distinguish a superposition state from a measured result.
• Explain how a qubit uses superposition, compared with an ordinary bit.
• State why not all results can be read at once.

Before you start

It helps to know that an ordinary bit takes one of two values. Other terms are explained as they appear.

Is superposition a state or a measured result?

Superposition is a state, not a measured result. A person on a ladder has one definite energy, depending on the rung. An atom is different: it can act as though it has two or more distinct amounts of energy at once. For a person that would be like standing on the lowest and highest rungs at the same time, which makes no sense.

Once in that mixed state, the atom stays there until it is measured or disturbed by the outside world. On measurement the atom “collapses” to a single energy state, comparable to dropping to the low or the high rung.

Separate the two. Before the measurement, the system is in superposition. After it, you see one definite result. That result is random: the system must collapse instantly to one of two states, 0 or 1.

How does a qubit use this idea?

An ordinary bit encodes information with the digits 1 and 0 and sits in one of two states. A qubit is a quantum bit and can be put into superpositions of several states. In other words, a qubit can be in state 0, state 1, or a mix of the two.

The number of combinations grows fast. Two qubits can hold a superposition of 4 combinations of 0 and 1 at once, three qubits hold 8 combinations, and four qubits hold 16. Each additional qubit doubles the number of combinations.

To use this power, the operator first entangles the qubits with each other and then carries out operations on them. Depending on the type of machine, electromagnetic signals or lasers create the entanglement and the operations.

Why can the results not all be read at once?

A quantum computer can carry out many different computations in superposition. But at the end, the measurement can extract only a small amount of information about the results of all those computations.

That is why describing a quantum computer as “trying every solution at once” misleads. Superposition also does not allow an efficient brute-force search over all potential solutions. The key is to design the measurement so that it extracts useful information about the whole set of results.

There is one more limit. Temperature fluctuations or electric fields can disrupt the state of a qubit, losing the superposition before it is needed.

Real-world example

Picture a three-step routine. Prepare: put the system in a superposition. Measure: a “ruler” cuts across and the system collapses to one result. Result: you record exactly one value, 0 or 1.

Now ask: if you repeat the routine, will the result be the same? Because each measurement result is random, not necessarily. A useful quantum program has to arrange its operations so that the information you want shows up through the measurement, instead of relying on one lucky reading.

Common misunderstandings

Superposition does not mean you can read every answer at once. A measurement gives one result.

Superposition is not a measured result. It is the state before measurement, and measuring makes it collapse to a single level.

A quantum computer does not try every solution and pick the best. The measurement extracts only a small amount of information, so it has to be designed with care.

Frequently asked questions

How does a qubit differ from a bit?

A bit is in one of two states. A qubit can be in state 0, state 1, or a mix of the two.

How many combinations do several qubits hold?

Two qubits hold 4 combinations, three hold 8 and four hold 16. Each additional qubit doubles the number.

What destroys a superposition?

A measurement makes the system collapse to one state. Disturbances such as temperature fluctuations or electric fields can also disrupt a qubit’s state.

Summary

• Superposition is a state; a measurement gives one definite result.
• Qubits use superposition, and the number of combinations doubles with each added qubit.
• Measurement extracts little information, so all results cannot be read at once.