BOOKS ET AL.

29 OCTOBER 2010 VOL 330 SCIENCE www.sciencemag.org 588

CR

ED

IT: A

NT

ON

ZE

ILIN

GE

R/C

OU

RT

ES

Y F

AR

RA

R, S

TR

AU

S, A

ND

GIR

OU

X

Ever since the inception of the theory

of quantum mechanics at the start

of the previous century, debate has

raged about how to interpret what the theory

tells us about the fundamental workings of

nature. For example, what is the origin for the

seemingly impossible correlations between

two entangled particles? What happens to a

quantum particle in a superposition of two

locations during a measurement of that loca-

tion? The debates have often impinged on

philosophy and religion. Einstein famously

objected to the inherent randomness of the

theory, remarking that “God does not play

dice.” He nevertheless made great contribu-

tions to our understanding of quantum phys-

ics (perhaps most nota-

bly through his question-

ing of the completeness

of quantum mechanics),

contributions that high-

lighted the strangeness

of entanglement. Despite

such intense questioning

of the interpretation of

quantum mechanics, its

utility is beyond doubt:

its predictive power is

unmatched by any other

scientifi c theory.

Anton Zeil inger’s

Dance of the Photons

begins in a tunnel under-

neath the Danube. There,

scientists are harnessing

the strange quantum properties of superpo-

sition and entanglement for something use-

ful—secure communication via the transmis-

sion of single particles of light (photons) in

optical fi bers running through the tunnel. The

security comes from the fact that encoding

information in a quantum system enables two

legitimate parties, typically designated Alice

and Bob, to detect the presence of an eaves-

dropper because any information the latter

gains necessarily results in a detectable dis-

turbance of the quantum system. And so we

begin an exploration of this fascinating scien-

tifi c fi eld that lays the foundations for technol-

ogies designed specifi cally to employ quan-

tum effects to gain new

functionality and power

in information process-

ing, communication, and

measurement.

Early in the book,

Zeilinger (a physicist

at the University of Vienna) introduces his

Alice and Bob—two undergraduate physics

students. Wanting to know more about quan-

tum mechanics than Physics 1.01 has taught

them, they approach Professor Quantinger

(the author’s fi ctionalized alter ego). Rather

than recommending a few books, he gets

them involved in an experiment set up by one

of his graduate students. Working in separate

labs connected by opti-

cal fi bers to a source of

(what they later learn

are) pairs of entangled

photons, they observe

f irsthand the bizarre

“nonclassical” corre-

lations. This narrative

style gets the reader

involved and excited

to learn more. As Alice

and Bob experience the

strangeness of quantum

physics firsthand, the

reader feels like they are

there with them as they

work in the lab, attend

lectures, and have infor-

mal conversations with

the professor and his graduate student. Inter-

leaved with this narrative are sections in Zeil-

inger’s own voice, which work extremely well

to complete the story.

Alice and Bob discover that when they

each measure the polarization (the hori-

zontal, vertical, etc. direction of the electric

fi eld) of their photon from an entangled pair,

the correlations they see cannot be explained

by any “classical” physics theory. More pre-

cisely, these correlations violate local real-

ism. [This was what was bothering Einstein

in the 1935 “EPR” paper, “Can quantum-

mechanical description of physical reality be

considered complete?” ( 1)] Zeilinger com-

ments, “Some of the predictions of quantum

physics question central cherished aspects

of our view of the world.” But “people were

happy that quantum mechanics gave such an

exact description of nature, and they were

busy applying it to all kinds of phenomena.”

But this all changed dramatically in 1964,

when Irish physicist John Bell “showed that

it is not possible to understand the

phenomenon of entangled sys-

tems if one starts from rather ‘rea-

sonable’ assumptions of how the

world should work, assumptions

that one might even be tempted to

call self-evident.” Zeilinger asserts

that Aristotle could have derived

“Bell’s inequality” had he known

that it was an interesting problem.

As Zeilinger notes, when French physi-

cist Alain Aspect thought about testing these

ideas, Bell’s fi rst question was “Do you have

a permanent position?” Only after Aspect’s

affi rmative answer did Bell encourage him

to carry out the experiment. Confi rmation

of Bell’s ideas by Aspect, Philippe Grang-

ier, and others forces us to abandon at least

one of these cherished views. Whereas most

physicists hold that quantum mechanics is

nonlocal (the “spooky” property that Ein-

stein objected to), Zeilinger argues that it

is just as logical to “give up the picture of a

world that exists in all its properties indepen-

dent of us.” Either way, nature is far stranger

than we typically assume. Remarkably, John

Clauser, who teamed with Stuart Freedman to

carry out the earliest tests of Bell’s inequal-

ity, expected that the inequality would not be

violated. He didn’t consider it at all possible

“that the world could be so crazy that local

realism could be wrong.”

In recent years, some researchers have put

aside philosophical concerns to explore the

possibilities of technologies that take advan-

tage of quantum features such as entangle-

ment. Perhaps one of the most surprising and

powerful of such applications is quantum

teleportation. By making a joint entangling

measurement of a photon in an unknown state

and one of two photons in an entangled state,

it is possible to teleport the unknown state to

the third photon in the entangled state. Zeil-

inger and co-workers carried out the original

experiment in 1997, and they have continued

to push the distance over which such experi-

ments are performed.

Zeilinger concludes by highlighting the

path to quantum technologies: the fundamen-

tal curiosity of Einstein and others, the coin-

cidence of the discovery of Bell’s inequality

with the invention of the laser, and fi nally,

beginning in the 1990s and still unfolding, the

ideas about new ways to transmit and process

information. In a broad perspective, he notes,

“all teleportation experiments are part of a

research program to realize quantum comput-

ers.” Of the various approaches to developing

Exploiting Entanglement

QUANTUM PHYSICS

Jeremy L. O’Brien

The reviewer is at the Centre for Quantum Photonics and the Department of Electrical and Electronic Engineering, University of Bristol, Bristol BS8 1UB, UK. E-mail: Jeremy.OBrien@bristol.ac.uk

Dance of the Photons

From Einstein to Quantum

Teleportation

by Anton Zeilinger

Farrar, Straus, and Giroux,

New York, 2010. 313 pp. $26.

ISBN 9780374239664.

Published by AAAS

on

Oct

ober

5, 2

012

ww

w.s

cien

cem

ag.o

rgD

ownl

oade

d fr

om

BOOKS ET AL.

www.sciencemag.org SCIENCE VOL 330 29 OCTOBER 2010

such a device ( 2), using photons as quantum

bits (or qubits) is particularly promising ( 3):

“[I]n principle, it should be possible to build

future quantum computers that are based on

photons, that is, quantum light only,” and

there are great prospects for doing this in a

silicon chip ( 4– 6).

Those seeking an accessible popular

account of this fascinating field will find

their search over. No one has thought longer

or harder about the fundamental science and

practical applications of superposition and

entanglement than Zeilinger. It is clear that

he has also thought long and hard about how

to give lucid and entertaining expositions of

these challenging concepts. Readers should

not be fooled by the simple cartoon diagrams.

They refl ect the beauty of Dance of the Pho-

tons: Taking some of the most complex ideas

from cutting-edge science, Zeilinger provides

simple and clear explanations that in no way

compromise the fundamental concepts.

References

1. A. Einstein, B. Podolsky, N. Rosen, Phys. Rev. 47, 777 (1935).

2. T. D. Ladd et al., Nature 464, 45 (2010). 3. J. L. O’Brien, Science 318, 1567 (2007). 4. A. Politi, M. J. Cryan, J. G. Rarity, S. Yu, J. L. O’Brien, Sci-

ence 320, 646 (2008). 5. A. Politi, J. C. Matthews, J. L. O’Brien, Science 325, 1221

(2009). 6. A. Peruzzo et al., Science 329, 1500 (2010).

10.1126/science.1196975

Where is the mind? “In the head” or

“in the brain,” most people might

respond. The philosopher Gilbert

Ryle gave a different answer:

The statement “the mind is in its own

place,” as theorists might construe it, is not

true, for the mind is not even a metaphori-

cal “place.” On the contrary, the chess-

board, the platform, the scholar’s desk, the

judge’s bench, the lorry, the driver’s seat,

the studio and the football fi eld are among

its places. ( 1)

Recently, this idea of the mind not being

confi ned to the head has been reinvigorated

by philosophers and cognitive scientists,

who see the mind as “spread-

ing out” or “extending” into

the world. “How do you know

the way to San José?” phi-

losopher John Haugeland has

famously asked ( 2). Chances

are you don’t have some

inner analog of a printed

map. Rather, you know where

you should enter the highway, and then you

get there by following the road signs. Your

knowledge seems to be partially “imple-

mented” in the environment. There is now

a blooming fi eld of research into “situation

cognition,” which explores how cognitive or

mental phenomena such as problem solving

or remembering can be strongly dependent

on interactions between subjects and their

environments.

The possible far-reaching implications of

a situated view of cognition were brought into

sharp focus by Andy Clark and David Chalm-

ers in their 1998 paper “The extended mind”

( 3). There they defend the idea that the mind

“extends” into the environment in cases in

which a human organism and the environ-

ment become cognitively coupled systems.

Their by now iconic illustration of cognitive

coupling involves “Otto,” a “slightly amne-

sic” person, who uses a notebook to write

down important facts that he is otherwise

likely to forget. Unlike a person who remem-

bers the address of the Museum of Mod-

ern Art by relying on natu-

ral memory, Otto recalls it by

accessing his notebook. If one

supposes that the notebook is

constantly available to Otto

and that what is written in it is

endorsed by Otto, it becomes

plausible—so Clark and Chal-

mers argue—that Otto’s mem-

ory extends to include the notebook. After all,

they notice, Otto’s notes seem to play exactly

the same role as memory traces in other peo-

ple. Wouldn’t it be chauvinistic to restrict the

mind’s extent to what’s natural and inner?

Clark and Chalmers’s paper has triggered

a vigorous and continuing debate. Nonbeliev-

ers concede that numerous tight causal cou-

plings between minds and environments exist,

but they deny that it therefore makes sense to

speak of an extended mind instead of a mind

Unbounding the Mind

NEUROPHILOSOPHY

Erik Myin

The reviewer is at the Centre for Philosophical Psychology, Department of Philosophy, University of Antwerp, 2000 Antwerpen, Belgium. E-mail: erik.myin@ua.ac.be

The Extended Mind

Richard Menary, Ed.

MIT Press, Cambridge, MA,

2010. 390 pp. $40, £29.95.

ISBN 9780262014038.

Published by AAAS

on

Oct

ober

5, 2

012

ww

w.s

cien

cem

ag.o

rgD

ownl

oade

d fr

om