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.
Superposition
Think
of a qubit as an electron in a magnetic field. The electron's spin may be
either in alignment with the field, which is known as a spin-up state, or
opposite to the field, which is known as a spin-down state. Changing the
electron's spin from one state to another is achieved by using a pulse of
energy, such as from a laser - let's say that we use 1 unit of laser energy.
But what if we only use half a unit of laser energy and completely isolate the
particle from all external influences? According to quantum law, the particle
then enters a superposition of states, in which it behaves as if it were in
both states simultaneously. Each qubit utilized could take a superposition of
both 0 and 1. Thus, the number of computations that a quantum computer could
undertake is 2^n, where n is the number of qubits used. A quantum computer
comprised of 500 qubits would have a potential to do 2^500 calculations in a
single step. This is an awesome number - 2^500 is infinitely more atoms than
there are in the known universe (this is true parallel processing - classical
computers today, even so-called parallel processors, still only truly do one
thing at a time: there are just two or more of them doing it). But how will
these particles interact with each other? They would do so via quantum
entanglement.
Entanglement
Particles
that have interacted at some point retain a type of connection and can be
entangled with each other in pairs, in a process known as correlation. Knowing
the spin state of one entangled particle - 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. This is a real phenomenon (Einstein called it
"spooky action at a distance"), the mechanism of which cannot, as
yet, be explained by any theory - it simply must be taken as given. Quantum
entanglement allows qubits that are separated by incredible distances to
interact with each other instantaneously (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.
Taken
together, quantum superposition and entanglement create an enormously enhanced
computing power. Where a 2-bit register in an ordinary computer can store only
one of four binary configurations (00, 01, 10, or 11) at any given time, a
2-qubit register in a quantum computer can store all four numbers
simultaneously, because each qubit represents two values. If more qubits are
added, the increased capacity is expanded exponentially.
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