Just as bits are the fundamental object of information in classical computing, qubits (quantum bits) are the fundamental object of information in quantum computing.

Qubits can take the same value simultaneously. States can be superposed. This characteristic expands the possibility of parallel calculations.

Qubits may be pictured in 3D using the Bloch sphere representation.

The Bloch sphere is a way to describe a single-qubit quantum state (which is a two-dimensional complex vector) as a three-dimensional real-valued vector.

When a qubit is measured along its vertical axis, it ‘collapses’ to either 0 or 1 with certain probabilities — a fundamental feature of quantum measurements. In the diagram above, the qubit state (the point indicated by the black arrow) is in the northern hemisphere so it will probably collapse to 0 (the North pole). The closer the qubit is to a pole, the more likely it is to settle there upon measurement.

The number of states needed to describe the multi-qubit system increases exponentially with the number of qubits. For 2 qubits, we need 4 states; for 3 qubits, 8 states, and so forth.

The challenge is to make sure all the information is manipulated in the right way. The more qubits used in a computation, the more challenging it is to ensure that the desired outcome is achieved.

It is also a challenge to ensure that quantum computations are not overtaken by errors which is a difficult task since qubits are highly sensitive to their external environments and can lose their information easily.