Quantum Computers work by harnessing the properties of quantum mechanics. 

• As the reality of a quantum computer comes closer, it is useful for us to understand both how one functions and how it’s different from a traditional computer. The first thing to bear in mind is that they use different basic units of data: ‘bits’ and ‘qubits’.  
• Every element of a classical computer is written in binary code (1s and 0s) and is translated into electricity: high voltage is represented by 1, and low voltage by 0. In quantum computing, qubits are the basic unit and their value can be 1, 0, or 1 and 0 simultaneously, overlapping (superposition) and intertwining (entanglement) according to the laws of physics.  
• This means that qubits, as opposed to bits, can take on various values at one time and can perform calculations that a conventional computer cannot. 

• The classical computer, if there is interference with the system; the system can correct itself and continue running. For the time being, this is not the case with quantum computers.  
• “External disturbances force the system to define itself as 1 or 0, causing it to lose its quantum coherence.  
• To avoid this kind of external ‘noise,’ the system has to be completely isolated: the atoms have to be very quiet, ensuring nothing makes them collide or interact with the surroundings.  
• Quantum computers have to be at a temperature of -273 °C (-459 °F) with hardly any atmospheric pressure and isolated from Earth’s magnetic field. 

Source: The Hindu