DEFINITION
A quantum
computer is a computer design which uses the principles of quantum physics to
increase the computational power beyond what is attainable by a traditional
computer. Quantum computers have been built on the small scale and work
continues to upgrade them to more practical models.
HOW A QUANTUM COMPUTER WOULD WORK
A quantum
computer, on the other hand, would store information as either a 1, 0, or a
quantum superposition of the two states. Such a "quantum bit," called
a qubit, allows
for far greater flexibility than the binary system. Specifically, a quantum computer would be able to
perform calculations on a far greater order of magnitude than traditional
computers ... a concept which has serious concerns and applications in the
realm of cryptography & encryption. Some fear that a successful &
practical quantum computer would devastate the world's financial system by ripping
through their computer security encryptions, which are based on factoring large
numbers that literally cannot be cracked by traditional computers within the
life span of the universe. A quantum computer, on the other hand, could factor
the numbers in a reasonable period of time.
ENTANGLEMENT
Entanglement
is a term used in quantum theory to describe the way that particles
of energy/matter can become correlated to predictably interact with each other
regardless of how far apart they are. Particles,
such as photons, electrons, or qubits that have interacted with each other
retain a type of connection and can be entangled with each other in pairs, in
the process known as correlation. Knowing the spin state of one entangled
particle - whether the direction of the spin is up or down - allows one to know
that the spin of its mate is in the opposite direction. Even more amazing is
the knowledge that, due to the phenomenon of superposition, the measured
particle has no single spin direction before being measured, but is
simultaneously in both a spin-up and spin-down state. The spin state of the
particle being measured is decided at the time of measurement and communicated
to the correlated particle, which simultaneously assumes the opposite spin direction
to that of the measured particle. Quantum entanglement allows qubits that are
separated by incredible distances to interact with each other immediately, in a
communication that is not limited to the speed of light. No matter how great
the distance between the correlated particles, they will remain entangled as
long as they are isolated. Entanglement
is a real phenomenon (Einstein called it "spooky action at a
distance"), which has been demonstrated repeatedly through
experimentation. The mechanism behind it cannot, as yet, be fully explained by
any theory. One proposed theory suggests that all particles on earth were once
compacted tightly together and, as a consequence, maintain a connectedness.
Much current research is focusing on how to harness the potential of
entanglement in developing systems for quantum cryptography and quantum
computing.
QUBIT
A qubit is a quantum bit ,
the counterpart in quantum computing to the binary digit or bit of
classical computing. Just as a bit is the basic unit of information in a
classical computer, a qubit is the basic unit of information in a quantum computer . In a quantum computer, a number of elemental
particles such as electrons or photons can be used (in practice, success has
also been achieved with ions), with either their charge or polarization acting
as a representation of 0 and/or 1. Each of these particles is known as a qubit;
the nature and behavior of these particles (as expressed in quantum theory ) form the basis of quantum computing.
The two most relevant aspects of quantum physics are the principles of superposition and entanglement.
QUANTUM GATES
Quantum computing and specifically the quantum
circuit model of computation, a quantum gate (or quantum logic gate) is a basic quantum circuit operating on
a small number of qubits. They are the building blocks of quantum
circuits, like classical logic gates are for conventional digital
circuits. Unlike many classical logic gates, quantum logic
gates are reversible. However, classical computing can be performed using
only reversible gates. For example, the reversibleToffoli gate can
implement all Boolean functions. This gate has a direct quantum equivalent,
showing that quantum circuits can perform all operations performed by classical
circuits. Quantum logic gates are represented by unitary
matrices. The most common quantum gates operate on spaces of one or two qubits,
just like the common classical logic gates operate on one or two bits. This
means that as matrices, quantum gates can be described by 2 x 2 or 4 x 4 unitary
matrices.
SHOR'S ALGORITHM
Shor's algorithm, named after mathematician Peter Shor,
is a quantum algorithm (an algorithm which runs on a quantum computer)
for integer factorization discovered
in 1994. Informally it solves the following problem: Given an integer N, find its prime factors. On a quantum computer,
to factor an integer N,
Shor's algorithm runs in polynomial time (the time taken is polynomial in log N, which is the size of the
input). Specifically it takes time O((log N)3), demonstrating
that the integer factorization problem can be efficiently solved on a quantum
computer and is thus in thecomplexity class BQP. This is
exponentially faster than the most efficient known classical factoring
algorithm, the general number field sieve, which works in sub-exponential time -- about O(e(log N)1/3 (log log N)2/3). The efficiency
lies in the efficiency of the quantum Fourier transform, and modular exponentiation by squarings.
SUMBER :
http://whatis.techtarget.com/
http://en.wikipedia.org/
