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Home|Physics
& Math|
News

Dodge ban on quantum clones to trap
Schrödinger's cat

13 April 2012 by Lisa
Grossman

Magazine issue 2860.
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save

For similar stories, visit the Quantum
World Topic Guide

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full article

Continue reading page |1 |2

FROM the photocopier to the fax to a computer's "copy and
paste" function, the ability to replicate information is something we take for
granted. At the quantum scale, however, making exact copies should be
impossible. Now there's a way to dodge this cloning ban that could also help map
the border between the "classical", everyday world and the quantum realm - a
goal illustrated by the Schrödinger's cat paradox.

The ban arises because, unlike large objects, two or more
quantum particles can be entangled, a weird state in which they remain
intimately linked even when physically separated. If entangled objects could
also be cloned, it would be possible to use them to send a message across space:
changing one object would automatically cause an identical change in the other.
And this quantum-telegraph message would travel faster than light, violating
Einstein's relativity and posing problems for causality.

Luckily for physicists, in the 1980s a mathematical proof
emerged showing that some of the information stored in the original quantum
state is always lost: the best you can do is make a sort of grainy,
black-and-white photocopy. "Copying classical information is a really important
thing that we do all the time," says Christoph Simon of the University of Calgary in Alberta,
Canada. "So it's interesting that for quantum information, you can't do it."

One way to make the quantum equivalent of the grainy photocopy
is to shine a single photon at a crystal. If the photon is in a particular,
excited state, it can prod the atoms into emitting many more photons that all
approximate the original, but none of which is identical.

In 2008, a group led by Francesco De Martini of the Sapienza University of Rome,
Italy, made about 10,000 of these imperfect clones in order to probe a different
question raised by quantum mechanics: how big can an object be yet still show
quantum activity?

Their original photon shared its quantum state with another
photon via entanglement.
The team claimed that the whole army of imperfect clones was therefore also entangled with this quantum partner. Such large-scale
entanglement is reminiscent of Schrödinger's cat. In that thought experiment, a
cat sits in a box with a radioactive atom linked to a poisoning device. If the
atom decays, the cat dies; if not, the cat lives. Being a quantum object, until
someone opens the box the atom can exist in both states at once - physicists see
examples of such quantum behaviour in little things all the time. But then the
cat must also be both dead and alive, and no one ever sees evidence of a
cat-scale object in such a superposition of states.

"Schrödinger originally was using that to say, 'If you take
quantum physics to this extreme size, it's absurd'," Simon says. "Nowadays,
we're not so sure."

De Martini's group argued that they had attacked this problem
because the army of imperfect clones is a macroscopic object behaving in a
quantum way, by being entangled with a single photon. But it proved tough to
show that all the clones were entangled with the original photon's entangled
partner.

Now Simon and his colleagues suggest a way to settle this, by
converting the ensemble of imperfect clones back into an exact copy of the
original photon that spawned them. This would provide a way to dodge the ban on
quantum cloning without creating a relativity-defying quantum telegraph, they
suggest.

The key to their proposal is a second crystal that acts as an
inverted version of the first - instead of amplifying the original photon and
distributing its quantum state, it takes the clone army and boils it back down
into a single photon, ideally preserving all the information in the process (see
diagram).

This could be done using lasers, says Simon. A laser is
already used to turn the first crystal into an amplifier, which then awaits the
initial single photon. His team proposes pumping the second crystal with a laser
of the opposite phase to prime it to do exactly the opposite - convert the
clones back into a single photon (Physical Review Letters, DOI:
10.1103/PhysRevLett.108.120404).

The new set-up also provides an easier way to test whether a
single photon can be entangled with the entire clone army, as De Martini's team
claimed. Only if this is possible can the second emitted photon be entangled
with the first photon's quantum partner.

So to prove entanglement between the quantum and macroscopic
realms, all you need to do is measure entanglement between two photons, not
thousands. "If you can show that after all of this you still have entanglement,
then that implies you had entanglement in between," Simon says.

To discover where
the laws of the macroscopic world start dominating over those of quantum
mechanics, his team suggest increasing the size of the clone army one photon
at a time and noting when entanglement breaks down. This would help to explain
why we see atoms but not cats in two or more states at once.

Read
full article

Continue reading page |1 |2

From issue 2860 of
New Scientist magazine, page 8-9.

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Calm Light

Thu Apr 12 08:12:26 BST 2012 by Eric
Kvaalen

"If the photon is in a particular, excited state..."

What does that
mean? A photon does not have "excited states". Maybe it should have said "if the
crystal is in a particular excited state"?

I don't think it's correct to
say that we observe atoms in two or more states at once (but not cats). Whenever
we observe an atom, we get just one response.

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Life's a box, then you die (or don't) (Image: A.
Aleksandravicius/Getty)

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