快猫短视频

SETH LLOYD has an extraordinary goal. He wants to set up a network to
distribute one of the Universe鈥檚 strangest resources, a commodity that exists
only in the quantum world, and one that could prove more valuable than gold or
silver.

This bizarre commodity is called 鈥渆ntanglement鈥�. 快猫短视频s believe that
establishing a network for creating, storing and distributing entanglement could
be the first step towards developing the kind of teleport system dreamed up by
science fiction writers. It would also open up a way to make superfast quantum
computers and link them together into a quantum internet. As well as helping
researchers understand the strange role that quantum mechanics plays in the
universe, quantum computers would also crack the most secret codes currently in
use鈥攐ne of the reasons the army is funding Lloyd鈥檚 project at the
Massachusetts Institute of Technology in Cambridge to the tune of several
million dollars.

Entanglement is a ghostly, almost telepathic link between particles that have
interacted at some time in the past. The connection is instantaneous and works
even if the particles are on opposite sides of the Universe. Entanglement is
already being used to carry out quantum cryptography and very small-scale
quantum computing and teleportation. If entangled particles could be sent around
the world through a 鈥渜uantum internet鈥�, they could spark a revolution in
computing, communications and our understanding of the universe.

Quantum computers would be linked up to make hugely powerful number-crunching
machines. Information would wing its way around the globe at speeds far
exceeding those that are even theoretically possible using today鈥檚 technologies.
Anyone wanting to run quantum computations would be able to download all the
quantum software they needed. And physicists would be able to get their hands on
鈥渙ff-the-shelf鈥� samples of quantum material. Each of these potential
applications would be reason enough to build a quantum internet.

There is a problem however: quantum particles are fragile and spill their
information easily. Merely looking at them can destroy it. So building a network
to distribute quantum information poses some serious difficulties. But last
month Lloyd and his colleagues published details of a plan to build a quantum
internet, and announced that the technology to create it is available now. 鈥淎ll
the different pieces have been done before,鈥� says Lloyd. He plans to have the
first three nodes of his network set up within 3 years.

Building a quantum internet would mean rebuilding the intellectual
foundations of the communications industry. Every phone call, TV broadcast and
Internet connection relies crucially on the work of Claude Shannon. At Bell Labs
in New Jersey, in the 1940s, Shannon laid the foundations of classical
information theory. He determined the theoretical capacity of any communications
channel 鈥攖he maximum amount of information that can be reliably sent along
it鈥攁nd outlined the compression techniques that let engineers send their
messages more efficiently.

Shannon also showed how to deal with noisy channels, where bits get flipped.
The sender might repeat every bit three times, for example. He called his
solution channel coding.

Shannon also worked out the theoretical limit of efficiency. The next
generation of mobile phones will use recently developed channel codes that
approach this 鈥淪hannon limit鈥�.

But Shannon鈥檚 ideas apply only to classical information. The grand challenge
now is to rewrite Shannon鈥檚 theories for the quantum world, paving the way for
the quantum internet.

Information in the quantum world is weird. A good example of classical
information is a sequence of 0s and 1s, which might be encoded by varying the
voltage across a wire. Above a certain level, the bit is a 1; otherwise it鈥檚 a
0. But encoding a piece of information in a quantum particle such as a photon is
quite different.

Photons can exist in two or more states at the same time. For example, a
photon鈥檚 electric field can be filtered so it oscillates in one particular
plane, a phenomenon known as polarisation. Make the plane vertical, and you can
call it a 鈥�0鈥�. Make it horizontal, and it鈥檚 a 鈥�1鈥�.

But because of 鈥渜uantum superposition鈥�, the photon can be both vertically and
horizontally polarised at the same time, and so can be both 0 and 1
simultaneously. The polarisation of this photon is called a qubit鈥攕hort
for quantum bit (and pronounced 鈥渃ue bit鈥�).

With superposition, engineers might think it possible to immediately double
the capacity of communications channels using these qubits in superposition.
Unfortunately, that鈥檚 not the case. The strange and fragile nature of quantum
information simply doesn鈥檛 allow it.

The problem is not just about the amount of information that can be stored,
but the amount that can be retrieved. That has to be done by taking a
measurement from the photon, and in the quantum world, measurements change
everything. With a single photon it is only possible to measure the polarisation
in one direction. When this measurement is made, the rest of the information the
photon contains is lost and cannot be retrieved. So it is only possible to
extract one bit of information from a qubit. The capacity of a quantum channel
for sending classical information, it would seem, cannot exceed the capacity of
a classical channel.

But entanglement changes all this. The curious thing about entangled
particles is that measuring one of the pair immediately determines the
outcome of a measurement on the other, no matter how far apart they may be. This
ability to magically connect two points in space-time means that entanglement
can revolutionise communication.

In 1992, Charles Bennett of IBM鈥檚 Thomas J. Watson Research Center near New
York, and Stephen Wiesner of Tel Aviv University showed that entanglement has a
profound effect on the capacity of a quantum channel鈥攁t the very least, it
boosts it by a factor of two.

This is great news for Alice and Bob, the characters that quantum researchers
use to play out their ideas. Alice spends her time sending quantum messages to
Bob. So far they have been unable to communicate any faster than they could over
the telephone. But what if Alice and Bob share a pair of entangled photons?

Each photon can have either a horizontal (1) or a vertical (0) polarisation,
so together the pair can be in one of four states: either both vertically
polarised, both horizontally polarised or either one vertically polarised and
the other horizontally polarised. In binary terms this corresponds to 00, 11, 01
or 10: the decimal numbers 0 to 3. The curious thing is that Alice can determine
which of these four states the whole system adopts simply by tweaking the one
photon she possesses.

Alice and Bob each have one of the entangled pair, which is initially in a
superposition of all four states. Alice then puts her photon through one of four
very simple optics devices. And because of the strange nature of entanglement,
her action also affects Bob鈥檚 photon: Alice鈥檚 operation simultaneously writes
information onto her photon and Bob鈥檚.

Alice then sends her photon to Bob, who still knows nothing about either of
the photons. When Alice鈥檚 photon arrives, Bob reads the data by looking at the
optical properties of the pair. This tells him which of the four operations
Alice actually carried out. The extraordinary thing is that Alice has used
entanglement to send two bits of information using one photon. Entanglement
doubles the capacity of her channel, a phenomenon known as quantum superdense
coding.

And even better things might be on the way. Physicists have recently begun to
play with entangled triplets (qutrits) and entangled quads (ququarts) with more
complex properties. Larger combinations are inevitable. These states might allow
quantum information to streak through networks at breakneck speeds.

All this beautiful innovation will count for nothing, however, if physicists
can鈥檛 correct the errors that will inevitably arise in their quantum networks.
Quantum states are so fragile that any outside influence can destroy them.
Because of this, many physicists believed it would be impossible to send quantum
information reliably. Last year, however, two physicists came up with a perfect
solution.

Isaac Chuang at IBM Almaden in San Jose, California, and Daniel Gottesman of
Microsoft Research in Redmond, Washington, have worked out a way to make
software that carries out quantum computations, protects the content of quantum
messages and keeps quantum bits error-free. And the crucial ingredient that
makes this possible is entanglement.

Chuang and Gottesman鈥檚 idea is based on quantum teleportation. In
teleportation, researchers perform a measurement on the qubit to be sent, and
the same measurement on one half of an entangled pair (快猫短视频,
14 March 1998, p 26). This sends some information about the qubit to the other
half of the entangled pair, allowing the original qubit to be reconstructed. But
Chuang and Gottesman pointed out that this scheme allows much more than simple
reconstruction.

Quantum computation is simply the result of one quantum state (called the
gate) acting on another (the qubit). By preparing the state of the entangled
pair in a particular way, what gets teleported can be the result of a
computation performed on the original qubit
(see Diagram).

Research published independently by Peter Shor of AT&T Laboratories in
New Jersey and Andrew Steane of Oxford University in 1995 showed that quantum
errors can be corrected simply by performing a certain series of computations on
the data. So, according to Chuang and Gottesman, you could download a set of
pre-fabricated 鈥渆rror-correcting entangled photons鈥� from a quantum website.

In their paper, Chuang and Gottesman predicted that the entangled pairs might
one day become a commercial resource that can be bought and sold by researchers
carrying out quantum computation over a quantum internet. John Preskill of
Caltech agrees: if you need a gate with a really complex quantum state, he says,
it would certainly be easier to download it from the quantum internet rather
than make it yourself. He foresees a flourishing quantum software industry with
manufacturers designing particularly valuable quantum states, creating and
storing multiple copies, then allowing consumers to download them for a fee.

Such schemes could allow those with rather basic quantum computers to
download extra capabilities. And the secretive nature of entangled
photons鈥攚hich change their state if anyone tries to look at
them鈥攃ould be used by a company to develop downloadable quantum software
for private communications.

Most exciting of all, the quantum internet could be the ideal tool for
teleporting complex molecules around the world. Currently, researchers can only
teleport simple things like the quantum state of a photon; going further means
using more complicated entanglements. So the quantum internet and its quantum
software is just what researchers need to begin teleporting atoms, molecules
and鈥攅ventually, perhaps鈥攖he components of life.

This will only work if physicists can sort out the practicalities of a
quantum internet. Ignacio Cirac and Peter Zoller of Innsbruck University came up
with the first plan for the quantum internet in 1997. In March this year, Lloyd
and Selim Shahriar at MIT and Philip Hemmer at the Air Force Research Laboratory
in Lincoln, Massachusetts brought the idea much closer to reality. Their idea is
to create a pair of entangled photons and send them along two optical fibres:
one to Alice and one to Bob. Alice and Bob both have laser traps containing
supercooled atoms, which would absorb the photons. Lloyd and his colleagues say
you can determine when an atom has absorbed a photon without disturbing it, and
by checking for simultaneous absorptions Alice and Bob can find out when the
atoms have absorbed an entangled pair. When this happens the atoms themselves
become entangled, and Alice and Bob now share a pair of entangled particles. As
the atoms have no electric charge they are immune to electric and magnetic
fields, and so are easy to protect from the outside world. For the first time,
entanglement would become a resource which physicists draw upon at leisure.

Alice and Bob could pull this software 鈥渙ff the shelf鈥� and use it to send
their messages. Physicists could use the pairs of atoms to send quantum
information between quantum microcomputers鈥攋ust what they need to scale up
quantum computers to carry out useful calculations.

Today鈥檚 quantum computers are essentially individual molecules that process
quantum information but cannot share it: the most powerful is a 7-qubit machine
at Los Alamos National Laboratories in New Mexico. Connecting many of these
machines in parallel would allow useful quantum computations to be carried out.
With a network as widespread as the Web, massive computations could be carried
out on computers all over the world.

A global quantum internet might appear sooner rather than later. Lloyd hopes
to have his source of entanglement running within 6 months and to be sending
entanglement within 2 years. The three-node internet should come a year
later.

Start investing now for the trading opportunity of the 21st
century鈥攅ntangled information. And there鈥檚 good news for Microsoft and
anyone else thinking of supplying entanglement to the world. Quantum software
can be used only once: entanglement and the rules of quantum measurement mean
that using it destroys it. For the first time in computing history, software
pirates are set to vanish in a puff of entangled magic.