NEWS VIEWS NATUREIVol 437120 October 2005 The second avenue is phylogenetic.The Graham E.Budd is in the Department of interferometer,in which a nanoscale grating is evolutionary scheme we have outlined implies Earth Sciences,University of Uppsala, used to diffract atomic waves,thus acting as a that the transition from great appendage to Norbyvagen 22,Uppsala SE-752 36,Sweden. matter-beam splitter.The authors inserted labrum happened once in the common ances- e-mail:graham.budd@pal.uu.se a further 250-nm-thick membrane with tor of all living arthropods apart from the Maximilian J.Telford is in the Department of thousands of 50-nm-wide slits into one branch pycnogonids,which must therefore be very Biology,University College London,Gower Street of this interferometer.As they pass through basal in evolutionary terms.But if the pyc- London WC1E 6BT,UK. this additional membrane,the atoms experi- nogonids truly are the sister group of the e-mail:m.telford@ucl.ac.uk ence a weak,attractive van der Waals force spiders and scorpions(which some molecular through electronic coupling with the mem- data suggest),then the results of Maxmen 1.Bourlat,S.J.,Nielsen,C.Lockyer,A.E Littlewood,D.T.& brane's walls.This interaction speeds up the et al.will be hard to square (Fig.2).Testing Telford,M.J.Nature 424,925-928(2003). atoms'pulse-the phase of the atom-wave the phylogenetic position of pycnogonids is 2.Maxmen,A.,Browne,W.E.,Martindale,M.Q.&Giribet,G. Nature437,1144-1148(2005). becomes shifted with respect to the free-atom therefore crucial. 3. Telford,M.J.&Thomas,R.A.Proc.NatlAcad.Sci.USA 95, wave in the interferometer's other branch. The conclusions of Maxmen et al.overturn 10671-10675(1998). From the measured interference,the phase entrenched ideas about the body plan of the 4 Winter,G.Z.Zool.Syst Evol 18,27-61 (1980). shift caused by the atom-surface interaction sea spiders and,furthermore,lend support 5.Rempel,J.G.Q.Entomol 11,7-25 (1975). b. Scholtz,G.Zoology 103,99-111(2001). can be exactly quantified. to some controversial theories of arthropod 7 Budd.G.E.Nature417,271-275(2001). This measurable change in the interference evolution.Unlike their terrestrial analogues, 8.Chen,J.-Y.,Waloszek,D.&Maas,A.Lethaio 37,1-17 pattern arises from an atomic interaction that sea spiders lack a poisonous bite,but this (2004). occurs over a distance up to 1,000 times that of paper is bound to inject venom into what is 9.Regier,J.C.,Shultz,J.W.Kambic,R.E.Proc.R Soc.Lond.B 272,395-401(2005). an atomic diameter.Perreault and Cronin are, already one of the most controversial of all 10.Vilpoux,K.Waloszek,D.Arthropod Struct Dev.32, to their knowledge,the first to determine zoological topics. ● 349-383(2003). directly the phase shift caused by the van der Waals interaction between an atom and a sur- face.The acceleration towards the surface of the channels experienced by the sodium atom QUANTUM PHYSICS waves is more than a million times that caused Atom waves in passing by Earth's gravitational field.The channels are, however,very short,so the actual time differ- ence measured by the interferometer is only Maarten DeKieviet and Joerg Schmiedmayer about 100 attoseconds(10-1seconds).Con- trasting this with the overall flight time Matter-wave interferometers are unique tools for exposing particles acting through the device of around one millisecond like waves-one of the stranger facets of quantum theory.They can even gives an idea of the exquisite sensitivity possi- measure the quickening of an atom's'pulse'as it flies past a surface. ble with interference experiments.This experi- ment is a beautiful example of the many tools that are being developed in a true renaissance Particles sometimes act like waves,and waves each other.A simple analogy is a zip-fastener in the study of atom-surface interactions sometimes act like particles.This phenom- for proper zipping,the teeth ofone strand must The potential impact of such work stems enon,known as wave-particle duality,may fit perfectly with those of the other.If,however, from its connection to the fields of nanotech- seem to confuse what are(to everyday experi- they are shifted such that the teeth oppose each nology and atom optics.Nanometre-scale ence at least)two separate and unambiguous other,the zipper won't close.Analogously,ifthe structures could lead to smaller transistors and concepts.But 100 years after Albert Einstein peaks ofone wave are next to the troughs of the motors,or the ability to assemble molecules first introduced the idea of waves behaving like other,the waves are perfectly out of phase,and atom by atom.Exploiting the wave behaviour particles to explain the photoelectric effect, their amplitudes cancel out-they interfere of atoms could lead the way to more precise and more than 80 years after the French physi- 'destructively.Conversely,if the two waves are gyroscopes for navigation,gravity gradiome- cist Louis de Broglie proposed the converse perfectly in phase,with the peaks and troughs ters for subterranean mapping and other field behaviour,wave-particle duality has become matching,they interfere constructively to pro sensors.The work of Perreault and Cronin a staple food of the quantum diet.Writing in duce a net amplitude that is the sum of the two lies at the intersection of these two fields, Physical Review Letters',John Perreault and individual amplitudes. putting a limit on how small nanotechnologi- Alexander Cronin expose a further experi- In an interferometer,an incoming wave is cal and atom-optical devices can be made mental ramification of the effect,by measur- split into two branches.One of these branches before the van der Waals interaction disrupts ing the shift in phase-a wave property-of is subjected to an outside influence that slows their operation. an atom as it flies past,and interacts with, down or speeds up the atom-wave's cycle,or Maarten DeKieviet and Joerg Schmiedmayer are a surface. pulse,thus shifting its phase relative to that at the Physikalisches Institut der Universitat In doing so,they take advantage of matter- of the other branch.These two branches are Heidelberg,Philosophenweg 12,D-69120 wave interferometry,a technique that has in then brought back together and interfere, Heidelberg,Germany. recent decades given fresh impetus to studies the amplitude of the resulting wave being pro- e-mail:Schmiedmayer@physi.uni-heidelberg.de of the wave-like nature of particles.Pioneered portional to the degree to which the two waves for simple particles such as electrons'and are in phase.Generally in wave mechanics 1.Perreault,J.D.&Cronin,A.D.Phys.Rev.Lett.95,133201 (2005). neutrons(ref.3 and references therein),the only intensities-the squares of the ampli- 2.Tonomura,A.etal.Electron Holography(Springer,Berlin, technique has since been extended to larger tudes-can be measured,so phase informa- 1999). particles such as atoms and molecules (ref.4 tion is lost.The power ofinterferometry is that 3.Rauch,H.Werner,S.A.Neutron Interferometry:Lessons in Experimental Quantum Mechanics(Clarendon,Oxford, and references therein). it transforms a shift in phase to a change in 2000). Interferometry as a generalized technique amplitude,which can be measured as change 4.Berman,P.R (ed.)Atom Interferometry (Academic,San involves the superposition of two waves to in intensity. Diego,1997刀. gain information about their relative phase 5.Keith,D.W.et al.Phys.Rev.Lett.66,2693-2696(1991). And so it is in Perreault and Cronin's ex- 6.Proc.Conf.Atoms and Molecules near Surfaces VoL.19: -where in their cycle they are in relation to periment'.They make use ofa Mach-Zehnder ww.iop.org/EI/toc/1742-6596/19/1(2005). 1102 2005 Nature Publishing Group
© 2005 Nature Publishing Group NEWS & VIEWS NATURE|Vol 437|20 October 2005 1102 interferometer, in which a nanoscale grating is used to diffract atomic waves, thus acting as a matter-beam splitter5 . The authors inserted a further 250-nm-thick membrane with thousands of 50-nm-wide slits into one branch of this interferometer. As they pass through this additional membrane, the atoms experience a weak, attractive van der Waals force through electronic coupling with the membrane’s walls. This interaction speeds up the atoms’ pulse — the phase of the atom-wave becomes shifted with respect to the free-atom wave in the interferometer’s other branch. From the measured interference, the phase shift caused by the atom–surface interaction can be exactly quantified. This measurable change in the interference pattern arises from an atomic interaction that occurs over a distance up to 1,000 times that of an atomic diameter. Perreault and Cronin are, to their knowledge, the first to determine directly the phase shift caused by the van der Waals interaction between an atom and a surface. The acceleration towards the surface of the channels experienced by the sodium atomwaves is more than a million times that caused by Earth’s gravitational field. The channels are, however, very short, so the actual time difference measured by the interferometer is only about 100 attoseconds (1016 seconds). Contrasting this with the overall flight time through the device of around one millisecond gives an idea of the exquisite sensitivity possible with interference experiments. This experiment is a beautiful example of the many tools that are being developed in a true renaissance in the study of atom–surface interactions6 . The potential impact of such work stems from its connection to the fields of nanotechnology and atom optics. Nanometre-scale structures could lead to smaller transistors and motors, or the ability to assemble molecules atom by atom. Exploiting the wave behaviour of atoms could lead the way to more precise gyroscopes for navigation, gravity gradiometers for subterranean mapping and other field sensors. The work of Perreault and Cronin1 lies at the intersection of these two fields, putting a limit on how small nanotechnological and atom-optical devices can be made before the van der Waals interaction disrupts their operation. ■ Maarten DeKieviet and Joerg Schmiedmayer are at the Physikalisches Institut der Universität Heidelberg, Philosophenweg 12, D-69120 Heidelberg, Germany. e-mail: Schmiedmayer@physi.uni-heidelberg.de 1. Perreault, J. D. & Cronin, A. D. Phys. Rev. Lett. 95,133201 (2005). 2. Tonomura, A. et al. Electron Holography (Springer, Berlin, 1999). 3. Rauch, H. & Werner, S. A. Neutron Interferometry: Lessons in Experimental Quantum Mechanics(Clarendon, Oxford, 2000). 4. Berman, P. R. (ed.) Atom Interferometry (Academic, San Diego, 1997). 5. Keith, D. W. et al. Phys. Rev. Lett. 66, 2693–2696 (1991). 6. Proc. Conf. Atoms and Molecules near SurfacesVol. 19; www.iop.org/EJ/toc/1742-6596/19/1 (2005). Graham E. Budd is in the Department of Earth Sciences, University of Uppsala, Norbyvägen 22, Uppsala SE-752 36, Sweden. e-mail: graham.budd@pal.uu.se Maximilian J. Telford is in the Department of Biology, University College London, Gower Street, London WC1E 6BT, UK. e-mail: m.telford@ucl.ac.uk 1. Bourlat, S. J., Nielsen, C., Lockyer, A. E., Littlewood, D. T. & Telford, M. J. Nature 424, 925–928 (2003). 2. Maxmen, A., Browne, W. E., Martindale, M. Q. & Giribet, G. Nature 437,1144–1148 (2005). 3. Telford, M. J. & Thomas, R. A. Proc. Natl Acad. Sci. USA95, 10671–10675 (1998). 4. Winter, G. Z. Zool. Syst. Evol.18, 27–61 (1980). 5. Rempel, J. G. Q. Entomol.11, 7–25 (1975). 6. Scholtz, G. Zoology 103, 99–111 (2001). 7. Budd, G. E. Nature 417, 271–275 (2001). 8. Chen, J.-Y., Waloszek, D. & Maas, A. Lethaia 37,1–17 (2004). 9. Regier, J. C., Shultz, J. W. & Kambic, R. E. Proc. R. Soc. Lond. B 272, 395–401 (2005). 10. Vilpoux, K. & Waloszek, D. Arthropod Struct. Dev. 32, 349–383 (2003). The second avenue is phylogenetic. The evolutionary scheme we have outlined implies that the transition from great appendage to labrum happened once in the common ancestor of all living arthropods apart from the pycnogonids, which must therefore be very basal in evolutionary terms. But if the pycnogonids truly are the sister group of the spiders and scorpions (which some molecular data suggest9 ), then the results of Maxmen et al. will be hard to square (Fig. 2). Testing the phylogenetic position of pycnogonids is therefore crucial. The conclusions of Maxmen et al. overturn entrenched ideas about the body plan of the sea spiders and, furthermore, lend support to some controversial theories of arthropod evolution. Unlike their terrestrial analogues, sea spiders lack a poisonous bite, but this paper is bound to inject venom into what is already one of the most controversial of all zoological topics. ■ Particles sometimes act like waves, and waves sometimes act like particles. This phenomenon, known as wave–particle duality, may seem to confuse what are (to everyday experience at least) two separate and unambiguous concepts. But 100 years after Albert Einstein first introduced the idea of waves behaving like particles to explain the photoelectric effect, and more than 80 years after the French physicist Louis de Broglie proposed the converse behaviour, wave–particle duality has become a staple food of the quantum diet. Writing in Physical Review Letters1 , John Perreault and Alexander Cronin expose a further experimental ramification of the effect, by measuring the shift in phase — a wave property — of an atom as it flies past, and interacts with, a surface. In doing so, they take advantage of matterwave interferometry, a technique that has in recent decades given fresh impetus to studies of the wave-like nature of particles. Pioneered for simple particles such as electrons2 and neutrons (ref. 3 and references therein), the technique has since been extended to larger particles such as atoms and molecules (ref. 4 and references therein). Interferometry as a generalized technique involves the superposition of two waves to gain information about their relative phase — where in their cycle they are in relation to each other. A simple analogy is a zip-fastener: for proper zipping, the teeth of one strand must fit perfectly with those of the other. If, however, they are shifted such that the teeth oppose each other, the zipper won’t close. Analogously, if the peaks of one wave are next to the troughs of the other, the waves are perfectly out of phase, and their amplitudes cancel out — they interfere ‘destructively’. Conversely, if the two waves are perfectly in phase, with the peaks and troughs matching, they interfere constructively to produce a net amplitude that is the sum of the two individual amplitudes. In an interferometer, an incoming wave is split into two branches. One of these branches is subjected to an outside influence that slows down or speeds up the atom-wave’s cycle, or pulse, thus shifting its phase relative to that of the other branch. These two branches are then brought back together and interfere, the amplitude of the resulting wave being proportional to the degree to which the two waves are in phase. Generally in wave mechanics only intensities — the squares of the amplitudes — can be measured, so phase information is lost. The power of interferometry is that it transforms a shift in phase to a change in amplitude, which can be measured as change in intensity. And so it is in Perreault and Cronin’s experiment1 . They make use of a Mach–Zehnder QUANTUM PHYSICS Atom waves in passing Maarten DeKieviet and Joerg Schmiedmayer Matter-wave interferometers are unique tools for exposing particles acting like waves — one of the stranger facets of quantum theory. They can even measure the quickening of an atom’s ‘pulse’ as it flies past a surface. 20.10 N&V 1097 new McP 17/10/05 10:31 AM Page 1102 ©2005 NaturePublishingGroup