USTC版 - 两项科大成果入选 MIT Technology Review:Best of 2012 |
|---|
|
|
|
|
|
|
q**w
发帖数: 782 | 1 http://www.technologyreview.com/view/509106/best-of-2012-chines
Best of 2012: Chinese Physicists Smash Distance Record For Teleportation
The ability to teleport photons through 100 kilometres of free space opens
the way for satellite-based quantum communications, say researchers
Teleportation is the extraordinary ability to transfer objects from one
location to another without travelling through the intervening space.
The idea is not that the physical object is teleported but the information
that describes it. This can then be applied to a similar object in a new
location which effectively takes on the new identity.
And it is by no means science fiction. Physicists have been teleporting
photons since 1997 and the technique is now standard in optics laboratories
all over the world.
The phenomenon that makes this possible is known as quantum entanglement,
the deep and mysterious link that occurs when two quantum objects share the
same existence and yet are separated in space.
Teleportation turns out to be extremely useful. Because teleported
information does not travel through the intervening space, it cannot be
secretly accessed by an eavesdropper.
For that reason, teleportation is the enabling technology behind quantum
cryptography, a way of sending information with close-to-perfect secrecy.
Unfortunately, entangled photons are fragile objects. They cannot travel
further than a kilometre or so down optical fibres because the photons end
up interacting with the glass breaking the entanglement. That severely
limits quantum cryptography’s usefulness.
However, physicists have had more success teleporting photons through the
atmosphere. In 2010, a Chinese team announced that it had teleported single
photons over a distance of 16 kilometres. Handy but not exactly Earth-
shattering.
Now the same team says it has smashed this record. Juan Yin at the
University of Science and Technology of China in Shanghai, and a bunch of
mates say they have teleported entangled photons over a distance of 97
kilometres across a lake in China.
That’s an impressive feat for several reasons. The trick these guys have
perfected is to find a way to use a 1.3 Watt laser and some fancy optics to
beam the light and receive it.
Inevitably photons get lost and entanglement is destroyed in such a process.
Imperfections in the optics and air turbulence account for some of these
losses but the biggest problem is beam widening (they did the experiment at
an altitude of about 4000 metres). Since the beam spreads out as it travels,
many of the photons simply miss the target altogether.
So the most important advance these guys have made is to develop a steering
mechanism using a guide laser that keeps the beam precisely on target. As a
result, they were able to teleport more than 1100 photons in 4 hours over a
distance of 97 kilometres.
That’s interesting because it’s the same channel attenuation that you’d
have to cope with when beaming photons to a satellite with, say, 20
centimetre optics orbiting at about 500 kilometres. “The successful quantum
teleportation over such channel losses in combination with our high-
frequency and high-accuracy [aiming] technique show the feasibility of
satellite-based ultra-long-distance quantum teleportation,” say Juan and co.
So these guys clearly have their eye on the possibility of satellite-based
quantum cryptography which would provide ultra secure communications around
the world. That’s in stark contrast to the few kilometres that are possible
with commercial quantum cryptography gear.
Of course, data rates are likely to be slow and the rapidly emerging
technology of quantum repeaters will extend the reach of ground-based
quantum cryptography so that it could reach around the world, in principle
at least.
But a perfect, satellite-based security system might be a useful piece of
kit to have on the roof of an embassy or distributed among the armed forces.
Something for western security experts to think about.
http://www.technologyreview.com/view/509096/best-of-2012-first-
Best of 2012: First Teleportation from One Macroscopic Object to Another
Physicists have teleported quantum information from one ensemble of atoms to
another 150 metres away, a demonstration that paves the way towards quantum
routers and a quantum Internet
One of the enabling technologies behind a quantum internet will be quantum
routers capable of transmitting quantum information from one location to
another without destroying it.
That’s no easy task. Quantum bits or qubits are famously fragile—a single
measurement destroys them. So it’s not all obvious how macroscopic objects
such as routers in a fibre optics network can handle qubits without
demolishing them.
However, physicists have a trick up their sleeve to help send qubits safely.
This trick is teleportation, a standard tool in any decent quantum optics
lab.
It relies on the strange phenomenon of entanglement in which two quantum
objects share the same existence. That link ensures that no matter how far
apart they are, a measurement on one particle instantly influences the other.
It is this ‘influence’ that allows physicists to transmit quantum
information from one point in space to another without it passing through
the space in between.
Of course, teleportation is tricky, but physicists are getting better at it.
They’ve teleported quantum information from one photon to another, from
ions to photons and even from a macroscopic ensemble of atoms to a photon.
Today, Xiao-Hui Bao at the University of Science and Technology of China in
Hefei and a few buddies say they’ve added a new and important technique to
this box of tricks.
These guys have teleported quantum information from ensemble of rubidium
atoms to another ensemble of rubidium atoms over a distance of 150 metres
using entangled photons. That’s the first time that anybody has performed
teleportation from one macroscopic object to another.
“This is interesting as the first teleportation between two macroscopic-
sized objects at a distance of macroscopic scale,” say Xiao-Hui and co.
Quite right. The goal in a quantum internet is that ensembles of atoms will
sit at the heart of quantum routers, receiving quantum information from
incoming photons and then generating photons that pass this information on
to the next router.
So clearly the first teleportation from one of these hearts to another is an
important advance.
Of course, there are hurdles ahead. Xiao-Hui and co want to increase the
probability of success for each instance of teleportation, to increase the
amount of time that the atomic ensemble can store quantum information before
it leaks away (currently just over 100 microseconds) and to create a chain
of atomic ensembles that will better demonstrate the potential of the
technique for quantum routing.
None of those challenges seem like showstoppers. Which means that practical
quantum routers and the quantum internet that relies on them are just around
the corner. |
|
|
|
|
|
|
|
