through Heisenberg’s uncertainty principle,as one is measured more precisely the infor-mation on the other is degraded. Thisincompatibility severely limits the fidelity of storage and retrieval of information in asimple ‘classical’memory, in which X

^

L and P^

L

are measured and these values are thenimprinted on X

^

A and P^

A.Julsgaard et al. have achieved a faithful

copy with a more elaborate scheme, reminis-cent of their recent experiment on atomicentanglement10.It proceeds in two steps.Oneof the light observables is first directly copiedonto the atomic system through a non-resonant laser–atom interaction.The secondstep is then more akin to the classical memoryoperation: the other light observable is measured and the measurement result is fedback onto the atomic system, completing thememory operation.

To follow the process in more detail,imagine that the initial quantum propertiesof the input light pulse are described by X

^

Lin

and P^

Lin, and those of the atomic sample by

X^

Ain and P

^

Ain. The laser pulse crosses the cell of

caesium vapour. It is not absorbed, but X^

A —which is now X

^

Ain�P

^

Lin — stores the P

^

L infor-mation (with a bit of added noise, owing tothe initial quantum uncertainty X

^

Ain). The

laser observable X^

L is cast into X^

Lin�P

^

Ain. In

the next step of the process, X^

L is measured(destroying P

^

L, but that is no longer impor-tant, because it is already stored in an atomicvariable). The measured value is made nega-tive and fed back onto P

^

A by an electronic cir-cuit and a magnetic field acting on the atoms.This achieves two goals at once: the initialquantum noise P

^

Ain is cancelled and the

observable is replaced by �X^

Lin. Finally, X

^

Lin

and P^

Lin are mapped onto �P

^

A and X^

A, com-pleting the storage operation.

In principle, the storage operation can bereversed and a light pulse identical to theinput one can be regenerated. Julsgaard et al.preferred instead to measure the atomicobservables with additional laser pulses.Through careful calibration of the quantumnoise, and by comparing the probability dis-tributions of the input and memory observ-ables, they assessed the storage fidelity — it issignificantly higher than the best possibleperformance of the ‘measure and imprint’classical approach. But it is still not perfect,limited by experimental imperfections andinitial quantum noise on X

^

A.The latter couldbe combated in more elaborate versions ofthe experiment by preparing the atoms ini-tially in a ‘squeezed state’, with considerablyreduced fluctuations on X

^

A (at the expense ofincreased ones on P

^

A). There should then beno limit to the fidelity.

This experiment suggests a basis for aquantum-information network operatingwith faint laser pulses. Obviously, there ismuch more work to do. The fidelity shouldbe pushed up and the storage time increasedabove the present value of a few milliseconds.

single ocelli can have both types of receptorcell3. The small size and sporadic occurrenceof such structures has discouraged any systematic study of their function, however,so they have remained little more thananomalies in an otherwise broadly acceptedgeneral pattern.

The advantage of molecular techniques,as applied to the problem by Arendt et al.1,is twofold: first, their ability to reveal gene-expression patterns in individual cells; sec-ond, the inferences one can make regardingfunction based on the known function ofhomologous genes (orthologues) in otheranimals. The results show that there are two forms of the gene for the photopigmentopsin in Platynereis, one ciliary, previouslyunknown from protostomes, and one rhab-domeric. The former is expressed in twosmall clusters of apical cells with internalizedcilia, located in the developing brain. Thecells also express an orthologue of the rxgene,an upstream controller of ciliary photorecep-tor differentiation in vertebrates, and eitherthey or adjacent cells show rhythmic expres-sion of a bmal/cycle gene, a key component ofthe circadian clock.An unanswered questionis the relation between the cells that expressthese genes and larval apical tuft cells, whichare internalized intact during developmentin some marine worms4. An assortment of other, possibly related structures — apicalciliated pits and ampullary organs — alsooccur in molluscan larvae5.

Assuming that Platynereis does indeedpreserve something of the ancestral condi-tion (that is, of the common ancestor ofprotostomes and deuterostomes), the resultsare best explained by an early origin of twoseparate types of photoreceptor. Rhabdo-meric ones would have been used for moni-toring light direction, and ciliary ones forphotoperiod.As image-forming eyes evolved,

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454 NATURE | VOL 432 | 25 NOVEMBER 2004 | www.nature.com/nature

An encouraging point is that this schemeuses rather simple elements, being based onclever ideas rather than heavy technology,and could therefore be turned into a practi-cal device. It is also a clear demonstrationthat a large ensemble of atoms can be used asa quantum system — a line of research that isbound to generate many more excitingresults. ■

Jean-Michel Raimond is in the Laboratoire KastlerBrossel, 24 Rue Lhomond, Paris 75005, France.e-mail: jmr@lkb.ens.fr

Evolutionary biology

Light on ancient photoreceptors Thurston Lacalli

Early multicellular organisms had two distinct types of photoreceptorcells, apparently with different functions. How these cells combined toform modern eyes turns out to be a complicated story.

The image-forming eyes, simple eyes(ocelli) and other photoreceptororgans of animals are structurally

diverse. But their photoreceptor cells arebasically of two types only — either ‘ciliary’or ‘rhabdomeric’, depending on whetherthey use cilia or arrays of microvilli for lightreception (Fig. 1). In a study of Platynereis,a marine segmented worm, published in Science, Arendt et al.1 provide convincingevidence from gene-expression studies andsequence comparisons that the last commonancestor of bilaterally symmetric animalshad both types. Their proposal for the functions the two performed, specifically the role of ciliary receptors in monitoringphotoperiod, advances our understandingof the ancestral condition, before the originof divergent types of advanced, image-form-ing eyes.

Our own eyes, like those of other verte-brates, have ciliary photoreceptors; so doesthe pineal ‘third eye’,a structure that is buriedin the brain and is involved in circadianrhythmicity, and which still, in lower verte-brates, functions directly as a photoreceptor.The various ocelli and image-forming eyes ofinvertebrates, in contrast, are rhabdomeric.This, for a while, provided a useful generalrule that, along with embryological differ-ences, distinguished between the two maingroups of animals: protostomes (diverseworms, molluscs and arthropods) use rhab-domeric photoreceptors; deuterostomes(vertebrates and their kin) have ciliary ones.

The person most closely associated withthe idea of a dichotomy is the late RichardEakin of the University of California, Ber-keley, who carried out an extensive study of comparative eye structure using the then relatively new technique of electronmicroscopy2.Exceptions to the rule do occur,and in some marine flatworm larvae even

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74, 145–195 (2002).

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which would have been a later event proba-bly coincident with the Cambrian radiation6,some 540 million years ago, protostomes and deuterostomes followed different paths,coopting rhabdomeres and cilia respectivelyas visual receptors. The presence of a rem-nant of the ciliary system in Platynereis sug-gests that such cells may occur more widelyamong invertebrates than is currently recog-nized, which should provide an impetus forfurther investigations.

Do vertebrates have any remnant of therhabdomeric system? Arendt et al. suggestthat they do,proposing retinal ganglion cells,the neurons whose fibres form the opticnerve, as the best candidates. They also sug-gest that homology explains similarities inthe pattern of optical pathways in proto-stome and deuterostome brains, but that ismuch more speculative.

Speculations aside, the new work pro-vides a solid starting point for further study of the evolution of photoreceptororgans during the diversification of bilateral

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NATURE | VOL 432 | 25 NOVEMBER 2004 | www.nature.com/nature 455

observed in recent physics experiments hasbeen truly breathtaking. One of the newestspecies to be added to the menagerie is the‘discrete breather’,or intrinsic localized mode.Briefly characterized, discrete breathers arespatially localized, time-periodic, stable exci-tations that exist and propagate in spatiallyextended,perfectly periodic,discrete systems.On page 486 of this issue, Sato and Sievers1

report the sighting of a particularly elusiveform of discrete breather that exists at theatomic scale in a magnetic solid. Coupledwith other recent observations in systemsranging from Josephson-junction arrays2,3,through micromechanical systems4, to photonic crystals5 and optical-switchingwaveguide arrays6, this new observationunderscores the ubiquity of these nonlinearexcitations and the importance of under-standing their role in determining the proper-ties of these widely different physical systems7.

To understand Sato and Sievers’ elegantexperiment and its implications, it is best tostart with the core principles of solid-statephysics at the linear level, and then build up (to use a deliberate oxymoron) somecounter-intuitive ‘nonlinear’ intuition. Atthe atomic level, crystalline solids are madeup of discrete arrays — lattices — of atoms(or molecules) that typically form a regular,periodic structure, like the sleepers (ties)supporting railway tracks. As a result, linearexcitations — be they electrons or phonons(the particles that carry sound) — movingthrough a solid will experience a periodicenergy potential. This implies, by generalmathematical theorems devised by Blochand Floquet, the existence of ‘forbidden’ and‘allowed’bands of frequency and velocity fortheir motion. Linear excitations can propa-gate through the solid only in the allowedbands, which cover a finite range of frequen-cies and thus have a highest and a lowestallowed frequency. Importantly, the dis-creteness of the lattice determines this high-est frequency; to continue the analogy, thefarther apart the railway sleepers, the smallerthe highest frequency allowed.

What are the consequences of nonlinear-ity? Indeed, what does ‘nonlinear’ mean inthis context? To answer these questions, let usrecall one of the hallmarks of the dawn ofmodern science: Galileo’s observation of theoscillations of the censers in the cathedral inPisa. This is now replicated in all elementaryphysics courses as the ‘plane pendulumexperiment’. It is a classic example in at leasttwo senses. First, it is a problem that allnovice physicists solve. Second, it is a typicalexample of how we mislead ourselves andour students about the prevalence andimportance of nonlinearity. Without goinginto the mathematics, just remember thatthe result we are supposed to discover fromthis experiment is that the frequency of oscil-lation of the pendulum does not depend onits amplitude (that is, how far the censer

Figure 1 Rhabdomeric ocellusfrom a Platynereis larva. Here, aparallel array of microvilli (therhabdome, asterisk) is employedto increase the surface availablefor light reception. In contrast

(inset), the vertebrateeye contains ciliaryphotoreceptors, inwhich membranouslamellae are stackedwithin the body of amodified cilium, thebase of which isarrowed. Arendt et al.1

show that Platynereiscontains two forms ofthe gene for thephotopigment opsin: arhabdomeric one, asexpected, but also aciliary one.

Nonlinear physics

Fresh breather David K. Campbell

The direct observation of highly localized, stable, nonlinear excitations— known as discrete breathers — at the atomic level underscores theirimportance in physical phenomena at all scales.

Stanislaw Ulam, the celebrated Polishmathematician and godfather of thefield now known as nonlinear science,

famously remarked that using the term ‘non-linear science’ was like “calling the bulk ofzoology the study of non-elephants”. Hemeant that linear processes are the exceptionrather than the rule; that most phenomena

are inherently nonlinear; and that the effectsof nonlinearity are apparent everywhere innature, from the synchronized flashing offireflies through clear-air turbulence to tor-nadoes and tsunamis.

Perhaps Ulam should have carried themetaphor of a nonlinear ‘zoo’a bit further, forthe remarkable taxonomy of ‘non-elephants’

multicelled animals. In evolutionary terms, it is a long way from a simple ocellus,involving no more than a few cells, to thecomplexity of an optimally constructedimage-forming eye. Evolution seems to have accomplished this transition piece-meal,by myriad small steps,each an adaptiveimprovement over what went before7. Adetailed accounting of the steps is as yetbeyond us, but clarifying the nature of theancestral condition is a useful beginning. ■

Thurston Lacalli is at the Department of Biology,University of Victoria, Victoria, British ColumbiaV8W 3N5, Canada.e-mail: lacalli@uvic.ca1. Arendt, D., Tessmar-Raible, K., Snyman, H., Dorresteijn, A. W.

& Wittbrodt, J. Science 306, 869–871 (2004).

2. Eakin, R. M. in Visual Cells in Evolution (ed. Westfall, J. A.)

91–105 (Raven, New York, 1982).

3. Eakin, R. M. & Brandenberger, J. L. Science 211, 1189–1190

(1981).

4. Lacalli, T. C. Proc. R. Soc. Lond. B 212, 381–402 (1981).

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(2000).

6. Parker, A. R. Proc. R. Soc. Lond. B 265, 967–972 (1998).

7. Lenski, R. E. et al. Nature 423, 139–144 (2003).

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