A regular computer bit is either a 1 or 0,
on or off. A quantum state can be much
more complex than that because, as we
know, things can be both particle and
wave at the same times, and the
uncertainty around quantum states allows
us to encode more information into a
much smaller computer. So that's what's
exciting about quantum computing ...
In normal computers, bits are the
smallest units of information. Quantum
computers use qubits, which can also be
set to one of two values. A qubit can be
any two-level quantum system, such as a
spin in a magnetic field or a single
photon. 0 and 1 are this system's possible
states, like the photon's horizontal or
vertical polarization. In the quantum
world, the qubit doesn't have to be in
just one of those. It can be in any
proportions of both states at once. This
is called superposition, but as soon as
you test its value, say by sending the
photon through a filter, it has to decide
to be either vertically or horizontally
polarized. So as long as it's unobserved,
the qubit is in a superposition of
probabilities for 0 and 1, and you can't
predict which it will be,
but the instant you measure it, it
collapses into one of the definite
states. Superposition is a game
changer. Four classical bits can be in 1
of 2 to the power of 4 different
configurations at a time.
That's 16 possible combinations out of
which you can use just one. Four qubits in
superposition, however, can be in all of
those 16 combinations at once. This
number grows exponentially with each
extra qubit. 20 of them can already store
a million values in parallel. A really
weird and unintuitive property qubits
can have is entanglement, a close
connection that makes each of the qubits
react to a change in the other state
instantaneously, no matter how far they
are apart. This means that when measuring
just one entangled qubit,
you can directly deduce properties of
its partner's without having to look.
Qubit manipulation is a mind bender as
well. A normal logic gate gets a simple
set of inputs and produces one definite
output. A
quantum gate manipulates an input of
superpositions, rotates probabilities, and
produces another superposition as its
output, so a quantum computer sets up
some qubits, applies quantum gates to
entangle them and manipulate
probabilities, then finally measures the
outcome, collapsing superpositions to an
actual sequence of zeros and ones. What
this means is that you get the entire
lot of calculations that are possible
with your setup all done at the same
time. Ultimately, you can only measure one
of the results, and it will only probably
be the one you want, so you may have to
double check and try again, but by
cleverly exploiting superposition and
entanglement,
this can be exponentially more efficient
than would ever be possible on a normal
computer. And if this still doesn't make
much sense, just remember, as Richard
Feynman once said, if you think you
understand quantum mechanics, you don't
understand quantum mechanics.
