Google says it now has a quantum computer which can solve complex problems in minutes that would take today's most powerful supercomputer thousands of years to crack.

The long-awaited moment of 'quantum supremacy' heralds a new dawn in the age of computing - but just how does quantum computing differ from what we already have?

Here's what you need to know.

What is 'quantum supremacy'?
Quantum supremacy is a holy grail for researchers in the field: It's when the superior potential of quantum computing is finally achieved in practice - effectively superseding existing forms of computing.

What's different about quantum computing?
Conventional computers use 'bits' - or ones and zeroes - as the basis of their calculations. Quantum computing makes use of a quantum bit, or 'qubit', which can be both one and zero at the same time, known as superposition.

What makes quantum computers so powerful?
Another property of sub-atomic particles is for them to become 'entangled' - meaning that they can influence each other's behaviour in an observable way. Combining entanglement with superposition leads to exponential increases in computing power with each additional qubit.

Why did it take so long to get here?
The challenges of making these computers work are daunting. The qubits need to be cooled to just above absolute zero to reduce 'noise' - or vibration - that introduces errors into the computers' calculations.

Does this mean that old computers are finished?
Critics, including rival IBM, say Google is creating a misleading impression that quantum computers render conventional computers obsolete but argue that quantum computing is still a long way away from practical use.

So what's next?
Google's AI research team see its potential use in machine learning and materials science and chemistry. Cryptographers are, meanwhile, already looking at how quantum computers might be used to crack the codes used, for example, to secure online access to bank accounts.

Is Google in the lead?
Google is staking a claim to leadership in the field but IBM is a close rival. China, which has invested heavily in fields such as artificial intelligence, is also backing quantum computing.