### computers size is reducing day by day..!!! started with a room size one n know we r working on laptops , heard of palmtops n probably v wil use it..!!!

wid ‘Quantum tech.’ on road v can even expect a small piece n v wil experience a new knid of computation , so r “quantum computers”….

*Introduction:*

Civilization has advanced as people discovered new ways of exploiting various physical resources such as materials, forces and energies. The history of computer technology has involved a sequence of changes of physical realization – from gears to relays to valves to transistors to integrated circuits and so on. Today’s advanced lithographic techniques can squeeze fraction of micron wide logic gates and wires onto the surface of silicon chips. Soon they will yield even smaller parts and inevitably reach a point where logic gates are so small that they are made out of only a handful of atoms.

On the atomic scale matter obeys the rules of quantum mechanics, which are quite different from the classical rules that determine the properties of conventional logic gates. So if computers are to become smaller in the future, quantum technology must replace or supplement what we have now. The point is, however, that quantum technology can offer much more than cramming more and more bits to silicon and multiplying the clock-speed of microprocessors. It can support entirely new kind of computation with qualitatively new algorithms based on quantum principles!

*So what is a ‘Quantum Computer’?*

A *Quantum Computer* is a computer that harnesses the power of atoms and molecules to perform memory and processing tasks. It has the potential to perform certain calculations billions of times faster than any silicon-based computer.

The classical desktop computer works by manipulating bits, digits that are *binary *— i.e., which can either represent a zero or a one. Everything from numbers and letters to the status of your modem or mouse are all represented by a collection of bits in combinations of ones and zeros. These bits correspond very nicely with the way classical physics represents the world. Electrical switches can be on or off, objects are in one place or they’re not, etc. Quantum computers aren’t limited by the binary nature of the classical physical world, however — they depend on observing the state of *quantum bits* or *qubits *that might represent a one or a zero, might represent a combination of the two or might represent a number expressing that the state of the qubit is somewhere *between 1 and 0.*

**How does a quantum computer work?**

In the classical model of a computer, the most fundamental building block – the bit, can only exist in one of two distinct states, a ‘0’ or a ‘1’. In a quantum computer the rules are changed. Not only can a qubit, exist in the classical ‘0’ and ‘1’ states, but it can also be in a superposition of both! In this coherent state, the bit exists as a ‘0’ and a ‘1’ in a particular manner. Let’s consider a register of three classical bits: it would be possible to use this register to represent any one of the numbers from 0 to 7 at any one time. If we then consider a register of three qubits, we can see that if each bit is in the *superposition or coherent state*, the register can represent all the numbers from 0 to 7 simultaneously!

A processor that can use registers of qubits will in effect be able to perform calculations using all the possible values of the input registers simultaneously. This phenomenon is called** **quantum parallelism, and is the motivating force behind the research being carried out in quantum computing.

*Analysis:*

Quantum computers are advantageous in the way they encode a bit, the fundamental unit of information. A number – 0 or 1, specifies the state of a bit in a classical digital computer. An n-bit binary word in a typical computer is accordingly described by a string of *n* zeros and ones. A qubit might be represented by an atom in one of two different states, which can also be denoted as 0 or 1. Two qubits, like two classical bits, can attain four different well-defined states (0 and 0, 0 and 1, 1 and 0, or 1 and 1).

But unlike classical bits, qubits can exist simultaneously as 0 and 1, with the probability for each state given by a numerical coefficient. Describing a two-qubit quantum computer thus requires four coefficients.* *In general, *n *qubits demand 2* ^{n}* numbers, which rapidly become a sizeable set for larger values of n. For example, if

*n*equals 50, about 10

^{50}numbers are required to describe all the probabilities for all the possible states of the quantum machine–a number that exceeds the capacity of the largest conventional computer. A quantum computer promises to be immensely powerful because it can be in superposition and can act on all its possible states simultaneously. Thus, a quantum computer could naturally perform myriad operations in parallel, using only a single processing unit.

*Challenges:*

The current challenge is not to build a full quantum computer right away but rather to move from the experiments in which we merely observe quantum phenomena to experiments in which we can control these phenomena. This is a first step towards quantum logic gates and simple quantum networks.

Experimental and theoretical research in quantum computation is accelerating worldwide. New technologies for realizing quantum computers are being proposed, and new types of quantum computation with various advantages over classical computation are continually being discovered and analyzed and we believe some of them will bear technological fruit. From a fundamental standpoint, however, it does not matter how useful quantum computation turns out to be, nor does it matter whether we build the first quantum computer tomorrow, next year or centuries from now. The quantum theory of computation must in any case be an integral part of the worldview of anyone who seeks a fundamental understanding of the quantum theory and the processing of information.

*Today’s Quantum Computers:*

Quantum computers could one day replace silicon chips, just like the transistor once replaced the vacuum tube. But for now, the technology required to develop such a quantum computer is beyond our reach. Most research in quantum computing is still very theoretical. The most advanced quantum computers have not gone beyond manipulating more than 7 qubits, meaning that they are still at the “1 + 1” stage. However, the potential remains that quantum computers one day could perform, quickly and easily, calculations that are incredibly time-consuming on conventional computers.

*The advantages of Quantum Computing:*

There are several reasons that researchers are working so hard to develop a practical quantum computer. First, atoms change energy states very quickly — much more quickly than even the fastest computer processors. Next, given the right type of problem, each qubit can take the place of an entire processor — meaning that 1,000 ions of say, barium, could take the place of a 1,000-processor computer. The key is finding the sort of problem a quantum computer is able to solve.

If functional quantum computers can be built, they will be valuable in factoring large numbers, and therefore extremely useful for decoding and encoding secret information. If one were to be built today, no information on the Internet would be safe. Our current methods of encryption are simple compared to the complicated methods possible in quantum computers. Quantum computers could also be used to search large databases in a fraction of the time that it would take a conventional computer.

It has been shown in theory that a quantum computer will be able to perform any task that a classical computer can. However, this does not necessarily mean that a quantum computer will outperform a classical computer for all types of task. If we use our classical algorithms on a quantum computer, it will simply perform the calculation in a similar manner to a classical computer. In order for a quantum computer to show its superiority it needs to use new algorithms which can exploit the phenomenon of quantum parallelism.

The implications of the theories involved in quantum computation reach further than just making faster computers. Some of the applications for which they can be used are –

#### · *Quantum Communication-*

Quantum communication systems allow a sender and receiver to agree on a code without ever meeting in person. The uncertainty principle, an inescapable property of the quantum world, ensures that if an eavesdropper tries to monitor the signal in transit it will be disturbed in such a way that the sender and receiver are alerted.

#### · *Quantum Cryptography-*

The expected capabilities of quantum computation promise great improvements in the world of cryptography. Ironically the same technology also poses current cryptography techniques a world of problems. They will create the ability to break the RSA coding system and this will render almost all current channels of communication insecure.

#### · *Artificial Intelligence-*

The theories of quantum computation suggest that every physical object, even the universe, is in some sense a quantum computer. As Turing’s work says that all computers are functionally equivalent, computers should be able to model every physical process. Ultimately this suggests that computers will be capable of simulating conscious rational thought. And a quantum computer will be the key to achieving true artificial intelligence.

*Conclusion:*

Although the future of quantum computing looks promising, we have only just taken our first steps to actually realizing a quantum computer. There are many hurdles, which need to be overcome before we can begin to appreciate the benefits they may deliver. Researchers around the world are racing to be the first to achieve a practical system, a task, which some scientists think, is futile. *David Deutsch* – one of the groundbreaking scientists in the world of quantum computing – himself said, “Perhaps, their most profound effect may prove to be theoretical”.

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yestina (mgit ece 2nd yr)

not having it information…..