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## A Nobel Prize for quantum optics

Quantum mechanics predicts the bizarrest things. Tiny particles
like electrons can simultaneously be in two
places, or, more generally, in two states that would seem mutually
exclusive in our everyday experience of physics. Similarly weirdly,
particles that have once interacted can remain *entangled* even
when they're moved far apart and then
influence each other instantaneously, something which Einstein called "spooky action
at a distance". These seemingly magical properties could be
exploited for exciting real-world applications, if it wasn't for
another strange consequence of quantum mechanics: that by simply looking at a
quantum system you destroy many of its properties. (Find out more in this *Plus* article.)

Serge Haroche.

The 2012 Nobel Prize for Physics has been awarded to Serge Haroche
and David J. Wineland for (independently) finding ways of observing certain aspects
of quantum systems without destroying them. Haroche, of the Collège de France and Ecole Normale Supérieure in Paris, found a way of trapping individual
photons (particles of light)
for a record-breaking amount of time. Using
extremely reflective mirrors which bounce the photons back and forth, Haroche was
able to keep the photons "alive" for almost a tenth of a second, during
which time they would have travelled around 40,000km. Cleverly devised
experiments then allowed him to measure and count individual photons
without destroying them. They also allowed him to use quantum
entanglement to trace how a quantum
system changes from a state of *superposition* — being in two states at
once — to the state of definite existence we expect based on our
everyday experience.

David Wineland, from the University of Colorado, Boulder, used carefully tuned laser pulses to put electrically charged atoms in a state of superposition, for example occupying two different energy levels at once.

Haroche and Wineland's work is interesting to theorists and experimentalists alike. On the theoretical side, it gives some insight into one of the greatest mysteries of quantum mechanics: exactly how the act of measuring interferes with a quantum system, so that a particle which is in a state of superposition collapses into a single state.

David Wineland.

On the practical side, their work
may result in superfast quantum computers. While ordinary computers
store information in bits which take on either the value 0 or the value 1, a
quantum computer would exploit the phenomenon of superposition to
allow a *quantum bit* to take on both values at
once. If a single quantum bit can simultaneously take on two values,
then two of them can simultaneously take on four values, three can
simultaneously take on eight values, and so on. In general,
*n* quantum bits can simultaneously take on
2^{n} values. It's this increased capacity to represent information that may one day lead to computers much faster
than anything around today. Wineland and his team were the first to
show that a quantum operation involving two quantum bits is possible,
thus paving the way towards the superfast computers of the future.

Wineland has also used his lab techniques to build a clock that's
100 times more accurate than the clocks currently setting our
time standards. Time can be defined in terms of the frequencies of electromagnetic radiation emitted by atoms.
Wineland's clock measures radiation that's within the visible light range of the spectrum, and it's therefore called an *optical clock*. Optical clocks are incredibly accurate: if you had set one running at the moment of the Big Bang, it would now only be out by about five seconds.

According to Royal Swedish Academy of Sciences, who awards the Nobel Prizes, Haroche and Wineland have "opened the door to a new era of experimentation with quantum mechanics". Their methods for probing the physical world at the smallest scales may one day help lift the veil on some of the biggest mysteries in physics.

You can find out more in this excellent write-up on the Nobel Prize website and read more about quantum mechanics on *Plus*.