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Measuring a qubit yields either a zero or a one, just like the classical bit. A qubit, however, is different from a classical bit in the sense it can carry ‘infinite’ information, whereas the classical bit carries ‘only one piece’ of information.
Mathematically, a qubit is defined as a two-state system with a state vector. Another way of saying the same thing is that a qubit is defined by the two quantum basis states and an infinite number of linear combinations of those two states. So technically, a qubit can store an infinite amount of information.
Yet, as one of the most mystifying rules of the quantum world shows, measurement changes everything. Any attempt to measure this infinite information residing in a qubit leads to the collapse of the quantum state carrying this information onto the result of the measurement.
This phenomenon stems from the special property of quantum states known as ‘superposition’. This is possible because at sub-atomic scales, matter shows the wave-particle duality.
When two qubits are combined together under certain circumstances, another special property arises — that of quantum entanglement. When two qubits are entangled, their measurements are perfectly correlated.
‘Superposition’, ‘quantum entanglement’ and a third property of ‘quantum tunneling’ lends brute heft to quantum computing. Together, these properties unique to qubits allow a kind of parallel processing of information which is far more efficient than the best super computers of today.
The number of qubits used in computing systems is exponentially linked to its computational ability. Typically, ‘n’ qubits can represent 2ⁿ states. Countries and companies are spending large amounts on building quantum computers using higher and higher numbers of qubits.
And the current aspirational milestone was set by IBM in September 2020. And that was to build a 1000-qubit system by 2023.