news and views Particles driven to diffraction Philip H.Bucksbaum Almost 70 years after it was first proposed,an experiment shows that electrons can be diffracted by light waves.This result highlights the interchangeable roles of matter and light. ave-particle duality is the concept that all particles can behave as waves. 0 and vice versa.This intellectually challenging notion,which is a fundamental prediction of quantum theory,has been tested in a new way by Herman Batelaan and co-workers,in an experiment reported on page 142 of this issue The debate over the particle versus wave B character of light is far older than quantum 22 theory.Newton was an early and active advocate for the corpuscular nature of light. 11 But it was in the first decades of the twentieth century that quantum mechanics brought E this discussion to a new plane by including matter as just another form of energy subject to the wave-particle dichotomy. What does it mean to say that matter Figure 1 Making light of the matter.Below left,diffraction of light forms a rainbow pattern on the behaves like a wave?We know waves have surface of a compact disk.Above,a drawing from Kapitza and Dirac's 1933 paper'that describes a ripples but,because atoms and electrons are proposed method for the diffraction of electrons(from the pathAE toAE)from an optical standing so small,if they are waves then their ripples wave formed by a light source,O,a collimating lens,D,and a mirror,C.Batelaan and co-workers'use must be tiny.The quantum ripples of an elec- a similar geometry in their experiment. tron in an atom are typically less than an angstrom,or one ten-billionth of a metre,in rainbow pattern of colours that you see when as described in the previous paragraph,even size.But we don't need to see ripples to detect you look at the surface of a compact disk is when the light waves are replaced by particles waves.The accepted evidence for wave-like caused by light waves diffracting from the and the material grating by light.Think behaviour is the phenomenon of diffraction. regularly spaced bands of shiny material that about how to make that compact disk out of Diffraction is easy to demonstrate for make up the tracks.This effect can be seen alight beam fora moment.Don't panicif you light.The because the wavelength of light,although haven't come upwithasolution:the Batelaan small,is large enough to be comparable to group has done it for you. the spaces between adjacent tracks. Batelaan and colleagues used a method When light from a lamp or the sun originally proposed by two brilliant physi- strikes a compact disk,each compo- cists,Paul Dirac and P.L.Kapitza,in a classic nent of colour in the 'white'light is paper2 written in 1933.Dirac and Kapitza deflected in a direction dictated by each won Nobel prizes later,but not for this the ratio of its wavelength to the work.This is the only paper that they wrote track spacing.Specifically,for together,and it seems to be an isolated light with wavelength A inci- curiosity.It was not written to resolve the dent at 90on a grating with wave-particle debate,because by the early track spacing d.diffraction 1930s this had been decided by numerous occurs at an angle given by experiments in favour of...well,both parti- sin0=λ/d2Wd3 Vd and cles and waves,as quantum theory predicts. so on.Light waves scatter- Nonetheless they wrote that the diffraction ing from all of the tracks of electrons by light would be a very interest- add coherently only at ing experiment. these special angles.The The figure in the Kapitza-Dirac paper wavelengths of visible light reveals the trick for creating a regular lattice are tiny (just 400-700 of optical radiation (Fig.1).Kapitza and nanometres)but,if the Dirac reasoned that an optical standing grating spacing d is small wave would have the correct properties.A enough,the separation of the standing wave is just wave-like motion that colours (due to the angle 6)is oscillates but doesn't travel,such as the oscil- easy to detect. lations ofa vibrating violin string.Anoptical Wave-particle duality in quan- standing wave has an oscillating electric field tum mechanics means that we should made by two counterpropagating and over- be able to perform the same observation lapping light beams.Kapitza and Dirac NATURE|VOL41313SEPTEMBER 2001www.nature.com 2001 Macmillan Magazines Ltd 117
Wave–particle duality is the concept that all particles can behave as waves, and vice versa. This intellectually challenging notion, which is a fundamental prediction of quantum theory, has been tested in a new way by Herman Batelaan and co-workers, in an experiment reported on page 142 of this issue1 . The debate over the particle versus wave character of light is far older than quantum theory. Newton was an early and active advocate for the corpuscular nature of light. But it was in the first decades of the twentieth century that quantum mechanics brought this discussion to a new plane by including matter as just another form of energy subject to the wave–particle dichotomy. What does it mean to say that matter behaves like a wave? We know waves have ripples but, because atoms and electrons are so small, if they are waves then their ripples must be tiny.The quantum ripples of an electron in an atom are typically less than an ångström, or one ten-billionth of a metre, in size.But we don’t need to see ripples to detect waves. The accepted evidence for wave-like behaviour is the phenomenon of diffraction. Diffraction is easy to demonstrate for light. The rainbow pattern of colours that you see when you look at the surface of a compact disk is caused by light waves diffracting from the regularly spaced bands of shiny material that make up the tracks. This effect can be seen because the wavelength of light, although small, is large enough to be comparable to the spaces between adjacent tracks. When light from a lamp or the sun strikes a compact disk, each component of colour in the ‘white’ light is deflected in a direction dictated by the ratio of its wavelength to the track spacing. Specifically, for light with wavelength incident at 90° on a grating with track spacing d, diffraction occurs at an angle given by sin/d, 2/d, 3/d and so on. Light waves scattering from all of the tracks add coherently only at these special angles. The wavelengths of visible light are tiny (just 400–700 nanometres) but, if the grating spacing d is small enough, the separation of the colours (due to the angle ) is easy to detect. Wave–particle duality in quantum mechanics means that we should be able to perform the same observation as described in the previous paragraph, even when the light waves are replaced by particles and the material grating by light. Think about how to make that compact disk out of a light beam for a moment.Don’t panic if you haven’t come up with a solution;the Batelaan group1 has done it for you. Batelaan and colleagues used a method originally proposed by two brilliant physicists, Paul Dirac and P. L. Kapitza, in a classic paper2 written in 1933. Dirac and Kapitza each won Nobel prizes later, but not for this work. This is the only paper that they wrote together, and it seems to be an isolated curiosity. It was not written to resolve the wave–particle debate, because by the early 1930s this had been decided by numerous experiments in favour of… well, both particles and waves, as quantum theory predicts. Nonetheless they wrote that the diffraction of electrons by light would be a very interesting experiment. The figure in the Kapitza–Dirac paper reveals the trick for creating a regular lattice of optical radiation (Fig. 1). Kapitza and Dirac reasoned that an optical standing wave would have the correct properties. A standing wave is just wave-like motion that oscillates but doesn’t travel, such as the oscillations of a vibrating violin string.An optical standing wave has an oscillating electric field made by two counterpropagating and overlapping light beams. Kapitza and Dirac NATURE|VOL 413 | 13 SEPTEMBER 2001 |www.nature.com 117 news and views Particles driven to diffraction Philip H. Bucksbaum Figure 1 Making light of the matter. Below left, diffraction of light forms a rainbow pattern on the surface of a compact disk. Above, a drawing from Kapitza and Dirac’s 1933 paper2 that describes a proposed method for the diffraction of electrons (from the path AE to AE) from an optical standing wave formed by a light source, O, a collimating lens, D, and a mirror, C. Batelaan and co-workers1 use a similar geometry in their experiment. STEVE PERCIVAL/SPL Almost 70 years after it was first proposed, an experiment shows that electrons can be diffracted by light waves. This result highlights the interchangeable roles of matter and light. © 2001 Macmillan Magazines Ltd
news and views suggested that a standing wave of light could so the light has to be much more intense to peaks to build electron interferometers. be constructed from radiation produced by have any effect.Continuous lasers simply These ideas are worth pursuing.They belong mercury atoms in an arc lamp,which fluor- cannot be made strong enough,but pulsed to a new field of physicalresearch devoted to esce in intense and sharp wavelength bands. lasers can bridge this gap easily.The effect of manipulating quantum phenomena using If you use a compact disk to diffract light light on free electrons was first observed the exquisite control we now have over laser from a fluorescent lamp,you should be able using large pulsed lasers in the 1960s.and it fields.Quantum computing.slow light, to see the diffraction lines they were think- was studied in detail in the 1980s,partly by atom lasers and similar subjects that have ing about,because most fluorescent lamps some experiments using standing waves appeared in these pages in the recent past produce their light from mercury atoms. These experiments did not use sufficiently belong to this new field of quantum control. An electron beam in which the electrons collimated electrons to see the individual The Batelaan experiment helps to tie these follow parallel paths (are collimated)and peaks at different angles that are the hall- new advances to the foundations of quan- have similar velocities would diffract from mark of electron diffraction from a standing tum theory. the standing wave at angles given by the wave.The full experimental test of this idea Philip H.Bucksbaum is at the FOCUS Center, same formulae that describe the diffraction has had to wait until now,40 years after the Department of Physics,University of Michigan. of light from a grating-that is,at an angle invention of the laser,and nearly 70 years Ann Arbor.Michigan 48109-1120.USA determined by the ratio of the electron's since the Kapitza-Dirac paper. e-mail:phb umich.edu wavelength to the period of the standing Batelaan and colleagues'experiment'is 1.Frelmund D.L..Aflatooni.K.Batelaan,H.Nature 413. wave.(The period of the light grating is half well executed and the series of electron peaks 142-143(2001). the optical wavelength,because there are at different scattering angles seen in Fig.2 2.Kapltza.P.L&Dirac,P.A.M.Proc Camb.Philos Soc.29 297-300(1933). two intensity peaks per cycle in a standing on page 143 is in beautiful agreement with 3.Gould,P.L,Ruff,G.E Pritchard.D.E Phrys Rev.Lett.56, wave.The electron's quantum mechanical the Kapitza-Dirac theory.A larger question, 827-830(1986). de Broglie wavelength,according to quan- though,is where this advance leads us in 4.Bartell,L.S..Roskos.R.R.Thompson,H.B.Phys.Rev.166 1494-1504(1968. tum theory,is inversely proportional to its physics.Batelaan's group proposes usingit as 5.Buckshaum.P.H.Schumacher.D.W.Bashkansky.M.Phys momentum p:Add=h/p.where h is a spectroscopic tool,or using the multiple Rev.Lett.61,1182-1185(1988). Planck's constant.So the angle of deviation for an electron beam incident at 90 should be integer multiples of 2h/(Ap),where A is Physiology the wavelength of the light.This is one- hundredth of a degree or so for a grating of green light and electrons with 380 electron Nitric oxide and respiration volts of energy.as in the Batelaanexperiment. Stuart A.Lipton But there is a problem:the force exerted by optical radiation on free electrons is The theory that haemoglobin evolved to carry oxygen around the body incredibly weak.In other words,returning may need a rethink in light of another way in which molecules related to to our original experiment with the compact nitric oxide,released from haemoglobin,help the brain control respiration. disk.it is as though the disk were nearly invisible because it was made of something ur ability to breathe,and to modify our loaded onto haemoglobin together;SNOs with nearly the same optical properties as breathing according to the amount of are released from haemoglobin when it is the air around it.In that case the light would oxygen in the air and the demands of deoxygenated,dilating the small blood ves- pass right through it,and never diffract.This our bodies,is essential for survival.Failure to sels that deliver oxygen directly to tissues is why the Kapitza-Dirac thought experi- breathe more often when oxygen levels are Remarkably.then,SNOs seem to control ment remained untested for many decades. low can contribute to respiratory distress in all three parts of the respiratory cycle:the The situation improved following the newborn mammals and to sleep apnoea- oxygenation of haemoglobin in lungs,the invention of the laser.Continuous semi- temporary inability to breathe-in adults. delivery of oxygen to tissues,and the control conductor diode lasers like the ones that read How is the increase in breathing in the face of breathing by the brain. compact disks are still not powerful enough ofashortage ofoxygen controlled?As Lipton In an elegant series of experiments,rang- to demonstrate the Kapitza-Dirac effect for (no relation!)and colleagues'tell us on page ing from physiological to chemical analyses. free electrons,but they can be used to diffract 171 of this issue.the answer seems to be Lipton et al.'show that increased breathing beams of atoms if their wavelength is close not by the mere lack of oxygen,but rather ('minute ventilation)is controlled in part by to an atomic transition line(like the spectral by molecules related to a different gas,nitric SNOs acting in an area called the nucleus trac- lines you get from fluorescent mercury oxide (NO),which affect respiratory centres tus solitarius in the brainstem.The authors atoms).Diffraction of matter waves by at the base of the brain. also tracked down the exact molecule a optical standing waves was therefore first The molecules in question are S-nitroso- metabolite of S-nitrosoglutathione-that demonstrated using a beam of neutral atoms thiols(SNOs)-complexes of NO bound to probably exerts this ventilatory effect.The passing through the optical standing wave a thiol (sulphydryl)group in the amino acid metabolite is S-nitrosocysteinylglycine,which of a continuous laser.In this experiment cysteine.This is not the first time that these is produced in neuronal tissue through cleav- Gould.Ruff and Pritchard confirmed the molecules have been found to be involved age of S-nitrosoglutathione by the enzyme y- Kapitza-Dirac formula,and in so doing in respiration.They also have a crucial role glutamyl transpeptidase.This requirement helped to stimulate interest in the kind of in matching ventilation to perfusion in the for S-nitrosoglutathione and y-glutamyl particle-wave physics known today as atom lungs (that is,in matching the diameter of transpeptidase may distinguish the effects of optics.Techniques of atom optics have led the airways to the diameter of the lung blood SNOs in the brainstem from their oxygen- to atomic Bose-Einstein condensates,atom vessels).This ensures that haemoglobin,the regulated effects on blood vessels-the latter lasers and developments in direct-write blood's oxygen-carrying molecule,is fully effects are not reproduced by applicationofS- atom lithography. loaded with its cargo. nitrosoglutathione.In future.it might be The scattering force on free electrons by Moreover.SNOs are involved in control- possible to take advantage of this distinction laser light is more than a billion timessmaller ling the supply of oxygenated blood to tis- to inhibit or augment the effects of specific than the carefully tuned laser-atom force. sues.In the lungs.SNOs and oxygen are SNOs on particular aspects of respiration. 118 2001 Macmillan Magazines Ltd NATURE VOL413 13SEPTEMBER 2001www.nature.com
suggested that a standing wave of light could be constructed from radiation produced by mercury atoms in an arc lamp, which fluoresce in intense and sharp wavelength bands. If you use a compact disk to diffract light from a fluorescent lamp, you should be able to see the diffraction lines they were thinking about, because most fluorescent lamps produce their light from mercury atoms. An electron beam in which the electrons follow parallel paths (are collimated) and have similar velocities would diffract from the standing wave at angles given by the same formulae that describe the diffraction of light from a grating — that is, at an angle determined by the ratio of the electron’s wavelength to the period of the standing wave. (The period of the light grating is half the optical wavelength, because there are two intensity peaks per cycle in a standing wave.) The electron’s quantum mechanical de Broglie wavelength, according to quantum theory, is inversely proportional to its momentum p: deBroglieh/p, where h is Planck’s constant. So the angle of deviation for an electron beam incident at 90° should be integer multiples of 2h/(p), where is the wavelength of the light. This is onehundredth of a degree or so for a grating of green light and electrons with 380 electron volts of energy,as in the Batelaan experiment. But there is a problem: the force exerted by optical radiation on free electrons is incredibly weak. In other words, returning to our original experiment with the compact disk, it is as though the disk were nearly invisible because it was made of something with nearly the same optical properties as the air around it. In that case the light would pass right through it, and never diffract. This is why the Kapitza–Dirac thought experiment remained untested for many decades. The situation improved following the invention of the laser. Continuous semiconductor diode lasers like the ones that read compact disks are still not powerful enough to demonstrate the Kapitza–Dirac effect for free electrons,but they can be used to diffract beams of atoms if their wavelength is close to an atomic transition line (like the spectral lines you get from fluorescent mercury atoms). Diffraction of matter waves by optical standing waves was therefore first demonstrated using a beam of neutral atoms passing through the optical standing wave of a continuous laser3 . In this experiment, Gould, Ruff and Pritchard confirmed the Kapitza–Dirac formula, and in so doing helped to stimulate interest in the kind of particle–wave physics known today as atom optics. Techniques of atom optics have led to atomic Bose–Einstein condensates, atom lasers and developments in direct-write atom lithography. The scattering force on free electrons by laser light is more than a billion times smaller than the carefully tuned laser–atom force, so the light has to be much more intense to have any effect. Continuous lasers simply cannot be made strong enough, but pulsed lasers can bridge this gap easily. The effect of light on free electrons was first observed4 using large pulsed lasers in the 1960s, and it was studied in detail in the 1980s, partly by some experiments using standing waves5 . These experiments did not use sufficiently collimated electrons to see the individual peaks at different angles that are the hallmark of electron diffraction from a standing wave. The full experimental test of this idea has had to wait until now, 40 years after the invention of the laser, and nearly 70 years since the Kapitza–Dirac paper. Batelaan and colleagues’ experiment1 is well executed and the series of electron peaks at different scattering angles seen in Fig. 2 on page 143 is in beautiful agreement with the Kapitza–Dirac theory. A larger question, though, is where this advance leads us in physics.Batelaan’s group proposes using it as a spectroscopic tool, or using the multiple peaks to build electron interferometers. These ideas are worth pursuing.They belong to a new field of physical research devoted to manipulating quantum phenomena using the exquisite control we now have over laser fields. Quantum computing, slow light, atom lasers and similar subjects that have appeared in these pages in the recent past belong to this new field of quantum control. The Batelaan experiment helps to tie these new advances to the foundations of quantum theory. ■ Philip H. Bucksbaum is at the FOCUS Center, Department of Physics, University of Michigan, Ann Arbor, Michigan 48109-1120, USA. e-mail: phb@umich.edu 1. Freimund, D. L., Aflatooni, K. & Batelaan, H. Nature 413, 142–143 (2001). 2. Kapitza, P. L. & Dirac, P. A. M. Proc. Camb. Philos. Soc. 29, 297–300 (1933). 3. Gould, P. L., Ruff, G. E. & Pritchard, D. E. Phys. Rev. Lett. 56, 827–830 (1986). 4. Bartell, L. S., Roskos, R. R. & Thompson, H. B. Phys. Rev. 166, 1494–1504 (1968). 5. Bucksbaum, P. H., Schumacher, D. W. & Bashkansky, M. Phys. Rev. Lett. 61, 1182–1185 (1988). news and views 118 NATURE|VOL 413 | 13 SEPTEMBER 2001 |www.nature.com Our ability to breathe,and to modify our breathing according to the amount of oxygen in the air and the demands of our bodies,is essential for survival.Failure to breathe more often when oxygen levels are low can contribute to respiratory distress in newborn mammals and to sleep apnoea — temporary inability to breathe — in adults. How is the increase in breathing in the face of a shortage of oxygen controlled? As Lipton (no relation!) and colleagues1 tell us on page 171 of this issue, the answer seems to be not by the mere lack of oxygen, but rather by molecules related to a different gas, nitric oxide (NO), which affect respiratory centres at the base of the brain. The molecules in question are S-nitrosothiols (SNOs) — complexes of NO bound to a thiol (sulphydryl) group in the amino acid cysteine. This is not the first time that these molecules have been found to be involved in respiration. They also have a crucial role in matching ventilation to perfusion in the lungs (that is, in matching the diameter of the airways to the diameter of the lung blood vessels)2 . This ensures that haemoglobin, the blood’s oxygen-carrying molecule, is fully loaded with its cargo. Moreover, SNOs are involved in controlling the supply of oxygenated blood to tissues. In the lungs, SNOs and oxygen are loaded onto haemoglobin together; SNOs are released from haemoglobin when it is deoxygenated, dilating the small blood vessels that deliver oxygen directly to tissues3,4. Remarkably, then, SNOs seem to control all three parts of the respiratory cycle: the oxygenation of haemoglobin in lungs, the delivery of oxygen to tissues, and the control of breathing by the brain. In an elegant series of experiments, ranging from physiological to chemical analyses, Lipton et al.1 show that increased breathing (‘minute ventilation’) is controlled in part by SNOs acting in an area called the nucleus tractus solitarius in the brainstem. The authors also tracked down the exact molecule — a metabolite of S-nitrosoglutathione — that probably exerts this ventilatory effect. The metabolite is S-nitrosocysteinyl glycine,which is produced in neuronal tissue through cleavage of S-nitrosoglutathione by the enzyme - glutamyl transpeptidase. This requirement for S-nitrosoglutathione and -glutamyl transpeptidase may distinguish the effects of SNOs in the brainstem from their oxygenregulated effects on blood vessels — the latter effects are not reproduced by application of Snitrosoglutathione3,4. In future, it might be possible to take advantage of this distinction to inhibit or augment the effects of specific SNOs on particular aspects of respiration. Physiology Nitric oxide and respiration Stuart A. Lipton The theory that haemoglobin evolved to carry oxygen around the body may need a rethink in light of another way in which molecules related to nitric oxide, released from haemoglobin, help the brain control respiration. © 2001 Macmillan Magazines Ltd
