Quantum Computing Dr. Kuntal Roy kuntal[AT]iiserb.ac.in Quantum computers directly utilize the quantum mechanical phenomena, e.g., superposition and entanglement. The basic idea of quantum information processing is that the superposition of huge number of wave functions can be manipulated in parallel, thereby achieving a massive speedup in computation compared to conventional computers. In fact, such quantum computers can be capable of cracking problems that are inaccessible to the most powerful classical computers in foreseeable future. The examples where quantum computation can be useful are determining two prime factors of a number and breaking a cryptographic key. While a classical binary bit can have two allowed states, 0 and 1 (ON/OFF states of a transistor, UP/DOWN spins in a nanomagnet), a qubit, the phase information of a quantum state, can exist as arbitrary superpositions of 0 and 1. However, a qubit is extremely sensitive to environment, which is the major bottleneck behind the implementation of quantum computers. While a strong interaction is required between a qubit and the external field to manipulate the quantum states, it needs to switch off these interactions to maintain phase coherence during a computation, which are contradictory requirements. The 2012 Nobel Prize in Physics (awarded to Haroche and Wineland) recognizes the advancement of the field of quantum information processing.