02-03-2013, 01:59 PM
Quantum Computing and Quantum Computers
Quantum Computing.pptx (Size: 1.17 MB / Downloads: 41)
Limit of today’s Computer Architecture
There’s a joke about personal computers that has been around since they came in the market : You buy a new computer, take it home and just as you finish unpacking it you see an advertisement for a new computer that makes yours obsolete.
Speed has always been one of the prime objectives to be improved. And we’ve moved from vaccume tubes to transistors to reduce the size and increase the performance.
If you make a chart of the evolution of the computer in terms of processing power, you would see that progress has been exponential. The man who first made this famous observation is Gordon Moore, a co-founder of the microprocessor company Intel.
Why Quantum Computing ?
In the search for ever smaller and faster computational devices, and the researches on computational power of biological systems such as the brain, one is naturally led to consider the possibility of computational devices of the size of cells, molecules, atoms, or of even smaller scales.
At such scales, quantum effects become important whether we want them or not. The natural laws that govern the behavior of particles on extremely small scales.
It has been pointed out that if trends over the last forty years continue, we may reach atomic-scale computation by the year 2010. Transistors with “gate lengths” of 10nm can already be fabricated.
Wave-like quantum properties of electrons become important on this length scale.
Transistors switchable by a single electron predicted by 2015 or so.
How is Quantum Mechanics different ?
A classical system is always (in principle) in a definite state; we “just” have to specify which one. We consider or want a classical bit to be in either 1 or 0 state.
The state of a quantum system can involve many different possibilities simultaneously. Quantum bit or ‘qubit’ is 0 and 1 simultaneously, superposition of both.
What is Quantum Computing ?
QC can be defined as a device for computation that makes direct use of quantum mechanical phenomena, such as superposition and entanglement, to perform operations on data. The basic principle behind quantum computation is that quantum properties can be used to represent data and perform operations on these data.
Quantum Algorithms
Algorithm design is a highly complicated task, and in quantum computing it becomes even more complicated due to the attempts to harness quantum mechanical features to reduce the complexity of computational problems and to "speed-up" computation. Before attacking this problem, we should first convince ourselves that quantum computers can be harnessed to perform standard, classical, computation without any "speed-up".
In some sense this is obvious, given the belief in the universal character of quantum mechanics, and the observation that any quantum computation that is diagonal in the computational basis, i.e., involves no interference between the qubits, is effectively classical.
Yet the demonstration that quantum circuits can be used to simulate classical circuits is not straightforward. Indeed, quantum circuits cannot be used directly to simulate classical computation, but the latter can still be simulated on a quantum computer using an intermediate gate, namely the Toffoli gate.
This gate has three input bits and three output bits, two of which are control bits, unaffected by the action of the gate. The third bit is a target bit that is flipped if both control bits are set to 1, and otherwise is left alone. This gate is reversible (its inverse is itself), and can be used to simulate all the elements of the classical irreversible circuit with a reversible one. Consequently, using the quantum version of the Toffoli gate one can simulate, although rather tediously, irreversible classical logic gates with quantum reversible ones. Quantum computers are thus capable of performing any computation which a classical deterministic computer can do.
What comes after Quantum Computing ?
Once scientists can use atoms to complete complex computations, the day when a computer will be the size of 1 atom. These new machines can be deployed in ways that are not available today. For example, a nanomachine can be built and programmed to enter human cells to fight diseases or even resuscitate those who have just died. Yet, this approach will not be available until scientists can manipulate atoms using the quantum physics approaches of entanglement and superposition.