## A ‘Quantum’ Leap

Computing took a “Quantum” leap recently when Jeremy O’Brien and two other researchers from university of Bristol (United Kingdom) successfully built, for the first time, an Optical Quantum computer chip and used it to perform mathematical calculations. This prototype is the first demonstration of the prophesied advantages of Quantum computers over classical computers in terms of efficiency. The Optical Quantum computer chip performed Shor’s Algorithm (a Quantum Algorithm invented by Peter Shor in 1994) and calculated the prime factors of 15. Though this looks a very small feat, it is a dramatic development in Quantum Computing considering that this domain is still in its nascent stage and this is the first practical step towards building large scale Quantum Computers. This operation also proved that Quantum Computers are exponentially faster than classical computers.

How are Quantum Computers different?

Quantum computers are computing systems which try to exploit the quantum mechanical properties of a particle namely, superposition and entanglement. In Classical Computers the information is stored and processed as binary bits, either a ‘1’ or a ‘0’. A Quantum Computer stores and processes the information as qu-bits (stands for quantum bits). A qu-bit can represent a ‘0’ or ‘1’ or a super-position of these two states; in other words an intermediate state. So if a two bit register in a classical computer can represent any one of these four states 00, 01, 10, 11 at a time, a two qu-bit register can be in more than one of these states at the same time, i.e., it can be in a superposition of these 4 states simultaneously. In general, n qu-bits can be in a superposition of 2n states simultaneously. To explain it more simply, consider a three bit classical register and three bit quantum register. There are eight states (000,001,010 … 111) in the classical computing system, while in the Quantum system a qu-bit state can be any point in a sphere drawn with radius one unit (this sphere is termed as the Bloch Sphere). This implies that the sum of square of probability coefficients of the states is always equal to one, whereas in classical system, the sum of probability coefficients of the states is one. The probability coefficient of a quantum state is used to measure the state of the qu-bit. So a qu-bit can store infinite amount of information, as long as it is not measured. When we try to measure the state of a qu-bit, we are trying to determine what ‘classical state’ this qu-bit represents (because the result should be a classical state which humans can understand and use). This measurement depends upon the probability coefficient of the qu-bit and when we measure we collapse the quantum state to a classical state. The key to the efficiency of Quantum Computers is exploiting this phenomenon of the qu-bits. Entanglement is another phenomenon which allows Quantum Computers to operate faster. If two qu-bits are allowed to interact physically, they become ‘entangled’. Entangled qu-bits influence each other even if they are far apart. So effectively, by measuring the state of one qu-bit, the state of the other qu-bit can be determined. This opens the doors to parallel processing of all the qu-bits at the same time, thus making quantum computation faster. However, the problem with the quantum computers is that we need a quantum algorithm for computing (it is not quite straight forward like classical computation). This is required to tell us what state our input qu-bit should be and how to measure the output qu-bit. One of the main drawbacks of Quantum Computing at its present state is that there are only a handful of algorithms existing; the first to be invented was Shor’s Algorithm to compute prime factors of a number; Grove’s Algorithm is for searching an unsorted database; and a few more. And Quantum algorithms are complex to develop as it involves manipulation of Quantum states of the qu-bit to get the desired result. The popular choice for qu-bits is photons, whose spin state can be relatively easily manipulated.