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PITY the most famous
feline in physics, Schrödinger's cat. Sealed in a box with a vial of
poison, the unfortunate animal faces an uncertain future. No one knows
if it is alive or dead - killed by a quantum event that causes toxic
fumes to spew from the shattered vial. It is only when we open the box
that we discover the cat's condition.
Thankfully
Schrödinger's cat is all in the mind, a bizarre thought experiment
proposed by Austrian physicist Erwin Schrödinger in 1935 to highlight
the weirdness of quantum theory. Quantum mechanics says that while the
cat remains unobserved in the box, it is simultaneously both dead and
alive. It is the act of looking inside the box that determines its
fate. But imagine you opened the box and found Schrödinger's cat lying
with its eyes shut. Is it dead, or just sleeping?
This may sound like a scene from a Monty Python
sketch, but physicists wrestling with the vagaries of the quantum world
find themselves in a similarly ridiculous position. And it is no
abstract difficulty: thanks to the discovery of a fundamental problem
with observing quantum phenomena, researchers are now wondering whether
we need to rethink how we put quantum theory to work.
We are getting used to
hearing extremely upbeat predictions about how weird quantum behaviour
can change the way we communicate. These forecasts have thrilled
researchers in the field of optics and telecommunications because they
seem to herald strange feats, such as teleporting particles across huge
distances.
The first stage of
teleportation relies on a seemingly supernatural link that ties quantum
particles together over any distance. Known as entanglement, this
intimate connection arises when two particles bump into each other or
come into existence in the same process. For ever after, it is
impossible to tease apart the quantum characteristics of the two
particles. So if you do something to the quantum state of one particle,
it inevitably, and instantaneously, affects the state of the other, no
matter how far apart the particles are.
So entangled particles
should be useful in quantum communication by tapping this connection
between them to send encrypted messages. More exotic applications could
include quantum teleportation and quantum computing.
But some theoretical
physicists have recently dented the hopes researchers have for
entanglement. Among them is physicist Howard Wiseman of Griffith
University in Queensland, Australia. The mathematical descriptions for
quantum entanglement might look fine on paper, he argues, but many of
these descriptions do not relate to anything practical. Instead they
refer to a kind of phantom entanglement that appears in theoretical
calculations but cannot actually be measured. Such phantoms have been
given the derisory name "fluffy bunnies". In effect, we are looking at
Schrödinger's cat but unable to decide whether it is alive or dead.
Suddenly, quantum researchers are facing the embarrassing possibility
that they haven't got as close to controlling the quantum world as they
thought.
So how do you overcome
this problem? How do you make sure that the thing you are talking about
can actually be measured? With a cat, the answer is easy: you check its
breathing or try waking it up. Maybe you check for a heartbeat, or even
call a vet. At least you would know what to do to assess its condition.
But in the world of quantum mechanics, the solution is not always as
clear-cut.
This realisation is
having serious repercussions for quantum technology. To exploit
entanglement, two people at distant points - conventionally dubbed
Alice and Bob - need to measure the properties of those linked
particles. This is where the trouble starts. Most measurements rely on
discovering whether each particle is spinning with its axis pointing up
or down. How does Bob know his notion of up is the same as Alice's?
Usually, of course,
Alice and Bob are on the same planet and even in the same lab. They
know which way is up because it is the opposite direction to gravity's
pull. They also agree on other things, like what time it is. In other
words, they share the same frame of reference. "You have a reasonably
good idea of time. You can count seconds. You know the difference
between your head and your feet, and in a lab we have objects like
clocks and rules that we can share also," says Terry Rudolph of
Imperial College London.
But is that always
true? It is possible to imagine a situation in which Alice and Bob
don't have much in common. Maybe Alice is driving around the Sahara
desert without a GPS locator, while Bob is sitting at a computer in
Taiwan. Now his idea of up is directly opposite to hers because they
are on opposite sides of the planet.
If quantum technologies
are ever going to make it out of the lab and become useful, these
measurement questions need to be cleared up. "This is why we are
getting people interested in this now," Rudolph says, "because we can
point out the relevance for current technology."
Recent work by Nicolas
Gisin of the University of Geneva in Switzerland, and the company id
Quantique, which sells quantum communication technologies, has shown
that there can be serious problems with communication via optical
fibres. Take a photon travelling down a fibre spinning one way. Gisin
has shown the axis of its spin can drift considerably as it travels.
And a person measuring a photon at one end of the fibre cannot be sure
that what they measure as up is the same as the person who sent the
photon.
But with so many
successful quantum experiments already, can this really be an issue?
Yes, says Jeff Kimble of the California Institute of Technology in
Pasadena, one of the leading researchers in quantum technology (see "The key to teleportation").
"It is clear now when one is doing teleportation that everyone has to
agree on a frame of reference," he says. "That is an essential
requirement."
Kimble's own use of
reference frames survived scrutiny, but Wiseman's crusade has targeted
several other groups where researchers have discussed clever quantum
effects without paying proper attention to how they might be used. But
now some groups are starting to think up ways to exploit entanglement
that are not so susceptible to the measuring problem. If Alice and Bob
cannot agree on which way is up, why not have them communicate in terms
that do not depend on sharing that information?
In March, Konrad
Banaszek at the University of Oxford and colleagues at the University
of Warsaw in Poland did just that in an experiment for the first time.
They sent information between two points but instead of encoding it in
terms of the up and down spins of photons relative to the laboratory's
frame of reference, they used the angle between the spins of entangled
photons. So for example, a binary 1 might correspond to a pair of
photons with spins at 180 degrees to each other, while a 0 described
spins at 45 degrees. Banaszek's group sent two such photons along the
same optical fibre 6 nanoseconds apart. After they had both arrived,
their spins were measured at exactly the same time. From the
measurement, the group could work out what bits had been received and
compare their message with what had been sent.
Sometimes, of course,
the spins of the photons were disturbed in transit, corrupting the
message. But Banaszek's group showed that a third fewer messages are
garbled using their method compared with the technique that relies on
knowing the difference between up and down. That, says Banaszek, is
because imperfections due to the uncertainty of which way is up no
longer matter.
An even more dramatic
example has come from a group led by Harald Weinfurter of the Max
Planck Institute for Quantum Optics in Garching, Germany. The team has
an even more complicated communication scheme that relies on the
relative spins of four entangled photons. The researchers sent the four
photons through a material that randomises the spins of individual
photons, deliberately messing up any chance of Alice and Bob agreeing
on which way is up. Despite this, they found that the information
carried by their four photons arrived unharmed (Physical Review Letters,
vol 92, p 107901). That is because although each individual spin was
random, the relationship between them survived the transit.
Rob Spekkens of the
Perimeter Institute for Theoretical Physics in Waterloo, Canada, says
their success was no coincidence. The difficulty of aligning Alice and
Bob's positions would introduce noise to any messages they share. But
if they don't need to share the same reference frame, they should be
protected from this. If quantum communication technologies are to
develop to the point where they can be used in a worldwide network, or
even between distant points in the solar system, it may make sense to
encode the information in terms of angles.
Spekkens is so
intrigued by the possibilities that in July he and Stephen Bartlett of
the University of Queensland in Brisbane, Australia, held the first
workshop on solving reference-frame problems.
But there is still
controversy over how far these solutions can go, says Mohamed
Bourennane also at the Max Planck Institute, who worked on Weinfurter's
experiment. He points out that while the experiment made it unnecessary
for Bob and Alice to share a reference for up and down, they still
needed to share an idea of time because they had to agree in advance
the order in which the four photons were sent.
This raises a profound
limitation that quantum researchers are still struggling to accept.
Perhaps, to get the most from quantum mechanics, people will always
have to share classical reference information about their set-up,
something that has been agreed in advance. As fast as the tentacles of
quantum weirdness creep into our comfortable world, reality seems to
find a way to slap them back.
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Lost reality
When the theory of quantum mechanics was first developed in the
1920s, Niels Bohr and Albert Einstein had a series of prominent debates
on how to make measurements. Bohr won, by arguing that the very act of
measuring a thing gives it a reality that it previously did not possess.
It is this argument that Erwin Schrödinger mockingly illustrated
with his infamous cat in a box. But as time went by, says Terry
Rudolph, who studies the role of measurement in quantum theory at
Imperial College London, people began to forget about the importance of
measurement. "It got abandoned," he says. "We ended up with this very
clean mathematical formulism." And it does not necessarily correspond
to reality.
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The key to teleportation
Jeff Kimble at the California Institute of Technology knew from the
start that the success of his team's attempt to teleport a quantum
state would depend on being able to prove that teleportation had indeed
taken place. But he only realised later just how important it was to
define a reference frame for the experiment.
In 1998 Kimble's group was the first to teleport a quantum state
across a laboratory. But three years later, the claim came under
attack. Rudolph and Barry Sanders of the University of Calgary in
Canada argued that the team had, without meaning to, cheated. The link
between quantum-entangled particles was not the only connection between
Alice (the sender) and Bob (the receiver) in Kimble's lab.
In the teleportation experiment, Alice and Bob shared entangled
photons of light from the same laser beam. Alice then wrote an unknown
quantum state, fed to her on another laser, onto her entangled photons.
The theory behind entanglement says that Bob's beam should acquire the
same quantum state because his photons are entangled with Alice's. In
this way, the unknown state of Alice's photons is teleported to his.
However, to help them prove that the quantum state had indeed been
teleported, Kimble's group needed to make careful measurements of Alice
and Bob's photons. To do this, the researchers passed a laser beam
between Alice and Bob which helped to synchronise the measurements
being made. Perhaps this was not spooky action at a distance after all,
critics contended. Maybe the synchronisation beam had helped to
communicate the unknown quantum state between Alice and Bob. Rather
than teleportation, this was straightforward communication by laser.
Others leapt quickly to the Kimble group's defence. They showed that
the laser beam was not acting as a communication device. It was just a
way of making sure that clocks being used at both ends of the
experiment were telling the same time. The critics eventually backed
off, but Kimble says he realised later, thanks to them and Wiseman, how
fundamental that laser beam actually was to all of teleportation.
Without it, the teleportation itself had no meaning, because it is the
act of measurement that gives reality to the quantum world.
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