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A qubit (quantum bit) is the fundamental unit of information in quantum computing. The qubit (CUE-bit) was first used by Benjamin Schumacher and it represents the core building block for the future quantum computers.

In classical computing the unit of information is the bit which can have one of the two values: 0, 1.  A qubit, on the other hand, can have states 0, 1, or a combination of 0 and 1.  The combination of both states is called superposition, an important concept of quantum computing.  Whereas the state of a bit is like flipping a coin resulting in one of the two options,, the state of a qubit is like the position of an electron in its spherical (circular) orbit.

• The position of the electron in the “north” position could represent 1 and the electron in the “south” position could represent 0.
• Any other location in the orbit is a combination of 0 and 1 and is the superposition mentioned above (somewhere between 0 and 1).

Image source: Wikimedia Commons

These superpositions enable the qubit to have a virtually infinite number of states. The superposition attribute of the qubit enables quantum computer algorithms to use other quantum mechanics properties such as entanglement and interference.

Qubit Entanglement

In a quantum computer, entanglement among multiple qubits enable them to form a single system and stay correlated in such a way as impact on one qubit would impact the state of the rest.  Also, when two entangled qubits are moved far apart from each other they remain entangled thus being able to measure one cubit and know the state of the other.

Qubit Interference

Interference is a quantum mechanical effect which can be observed at the microscopic level. One such example would be watching the waves created in water by dropping two objects at the same time.  When the two crests meet, they cancel each other.  Qubit interference is used by the quantum computers to interfere the superpositions of a qubit with the resulting state being the result of the computation of the given quant circuit.

Qubit technologies

In classical computing,the bit is implemented by one of the two levels of two direct current voltage levels.  In quantum computing, the technology is still emerging so there have been a number of different options proposed for  the physical implementation of qubits to construct quantum computers

Qubits could be implemented or represented by making use of:

• Polarization of a photon
• Energy levels of an ion
• Coherent state of light
• Superconducting charge qubit
• Quantum dot spin
• The nuclear spin states of an atom
• Spin state of an electron

Image source: IBM Research