Physics Today De Broglie's meter stick:Making measurements with matter waves Markus Arndt Citation:Physics Today 67(5),30(2014);doi:10.1063/PT.3.2381 View online:http://dx.doi.org/10.1063/PT.3.2381 View Table of Contents:http://scitation.aip.org/content/aip/magazine/physicstoday/67/5?ver=pdfcov Published by the AlP Publishing JANIS Does your research require low temperatures?Contact Janis today. Our engineers will assist you in choosing the best system for your application. 10 mK to 800 K LHe/LN2 Cryostats Cryocoolers Magnet Systems Dilution Refrigerator Systems Micro-manipulated Probe Stations sales@janis.com www.janis.com Click to view our product web page. This article is copyrighted as indicated in the article.Reuse of AIP content is subject to the terms at:http://scitation aip.org/termsconditions.Downloaded to IP: 2021202300n:Thu01May2014232612
Physics Today De Broglie’s meter stick: Making measurements with matter waves Markus Arndt Citation: Physics Today 67(5), 30 (2014); doi: 10.1063/PT.3.2381 View online: http://dx.doi.org/10.1063/PT.3.2381 View Table of Contents: http://scitation.aip.org/content/aip/magazine/physicstoday/67/5?ver=pdfcov Published by the AIP Publishing This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 202.120.2.30 On: Thu, 01 May 2014 23:26:12
De Broglie's meter stick: Making measurements with matter waves Markus Arndt Interfering atoms and molecules serve as exquisite rulers that sharpen our knowledge of physical laws,measure tiny forces,and reveal subtle properties of matter. n 1923 Louis de Broglie proposed an idea that was as revolutionary as it was simple:!that one can 'associate a periodical phenome- non with any isolated portion of matter or energy"and that a fixed observer will associate with that phenomenon a wave of wavelength AaB=h/mo that scales with Planck's constant h,the object's mass m,and its velocity v. De Broglie's waves came as a surprise because they represent states of matter that seem to defy classical logic.For instance,the text- book example of electron diffraction at a double slit can only be explained by assuming a delocalized quantum wave in free propagation-even though the electrons themselves are detected as localized clicks.Because every single measuring minute fields,inertial forces,and the particle can be described as a sum of two or more properties of atoms themselves.Lately the effort has waves whose centers may be clearly separated,it is been extended to molecules and macromolecules. tempting to speak of the object as "being"in many Soon even large biomolecules may be studied in places at once.The wave-particle duality is partic- quantum-enhanced sensors.Eventually the sensi- ularly conspicuous when observed with large mol- tivity and accuracy of such matter-wave sensors is ecules,which can be inspected under a microscope expected to exceed that of classical techniques.Here as localized composite particles before and after I focus on a question that's being asked by several their wave-like evolution through the experiment.? groups across the globe:What kinds of matter-wave Such observations have spurred interesting and sensors are conceivable with atoms and molecules? ongoing debates about the meaning of words like reality,locality,space,and time. Setting up a quantum ruler Independent of those philosophical puzzles,it's In quantum physics,both massless photons and clear that the quantum wave nature of matter is massive particles are described by a wavefunction. firmly rooted in the Schrodinger equation,which For simplicity,let's restrict ourselves to a scalar func- has been perfectly confirmed in uncountable exper- tionψ-中exp[iφ(t)]with amplitude vo and time- iments.How,then,can we exploit the wave nature dependent phase o(t).The wave's phase-the posi- of matter in emergent quantum technologies?In re- tion of its crests and troughs relative to some cent decades various groups have devised strategies reference point in space and time-defines a natural to use interfering atoms as fine rulers capable of ruler.Common detectors,however,are sensitive only to the intensity How can one then extract Markus Arndt is a physics professor at the University of any phase? Vienna,where he leads the quantum nanophysics Interferometers provide the solution.They con- research group. vert phase differences between two waves into in- 30 May 2014 Physics Today www.physicstoday.org
30 May 2014 Physics Today www.physicstoday.org I n 1923 Louis de Broglie proposed an idea that was as revolutionary as it was simple:1 that one can “associate a periodical phenomenon with any isolated portion of matter or energy” and that a fixed observer will associate with that phenomenon a wave of wavelength λdB = h/mv that scales with Planck’s constant h, the object’s mass m, and its velocity v. De Broglie’s waves came as a surprise because they represent states of matter that seem to defy classical logic. For instance, the textbook example of electron diffraction at a double slit can only be explained by assuming a delocalized quantum wave in free propagation—even though the electrons themselves are detected as localized clicks. Because every single particle can be described as a sum of two or more waves whose centers may be clearly separated, it is tempting to speak of the object as “being” in many places at once. The wave–particle duality is particularly conspicuous when observed with large molecules, which can be inspected under a microscope as localized composite particles before and after their wave-like evolution through the experiment.2 Such observations have spurred interesting and ongoing debates about the meaning of words like reality, locality, space, and time. Independent of those philosophical puzzles, it’s clear that the quantum wave nature of matter is firmly rooted in the Schrödinger equation, which has been perfectly confirmed in uncountable experiments. How, then, can we exploit the wave nature of matter in emergent quantum technologies? In recent decades various groups have devised strategies to use interfering atoms as fine rulers capable of measuring minute fields, inertial forces, and the properties of atoms themselves. Lately the effort has been extended to molecules and macromolecules. Soon even large biomolecules may be studied in quantum-enhanced sensors. Eventually the sensitivity and accuracy of such matter-wave sensors is expected to exceed that of classical techniques. Here I focus on a question that’s being asked by several groups across the globe: What kinds of matter-wave sensors are conceivable with atoms and molecules? Setting up a quantum ruler In quantum physics, both massless photons and massive particles are described by a wavefunction. For simplicity, let’s restrict ourselves to a scalar function ψ = ψ0exp[iϕ(t)] with amplitude ψ0 and timedependent phase ϕ(t). The wave’s phase—the position of its crests and troughs relative to some reference point in space and time—defines a natural ruler. Common detectors, however, are sensitive only to the intensity ∣ψ∣ 2 . How can one then extract any phase? Interferometers provide the solution. They convert phase differences between two waves into inMarkus Arndt is a physics professor at the University of Vienna, where he leads the quantum nanophysics research group. Interfering atoms and molecules serve as exquisite rulers that sharpen our knowledge of physical laws, measure tiny forces, and reveal subtle properties of matter. De Broglie’s meter stick: Making measurements with matter waves Markus Arndt This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 202.120.2.30 On: Thu, 01 May 2014 23:26:12
tensity modulations and,in turn,into measurable ties-mass,polarizability,and so forth-in all acces- detector clicks.Dozens of interferometer types have sible places.Atomic beam-splitting is actually about been developed in classical and quantum optics dividing an individual atom's quantum wavefunc- The key idea is always the same:An incident beam tion.We may,in particular,distinguish between is divided by a beamsplitter into at least two wavefront and amplitude beamsplitters. wavelets;those wavelets-steered by mirrors,grat- Wavefront beamsplitters modify the wave- ings,waveguides,or even gravity-travel along dif- function as a function of its position.In quantum ferent paths before being recombined with a second physics,position and momentum are related beamsplitter. through a Fourier transform.A modulation of the The sensitivity of the interferometer derives wavefront-for instance,by a mechanical grating of from the fact that different parts of the same wave period d,as shown in figure 2a-therefore imprints explore different regions of spacetime.They may a coherent superposition of transverse momenta travel different distances or be exposed to different Ap=n.h/d,which leads to diffraction peaks in the forces or potentials.As a result,two wavelets far field(at distances much greater than A2/A,where iexp[io (t)]and vzexp[io2(t)],acquire different A is the width of the beam-constricting aperture) phases.In linear optics as well as in quantum me- behind the grating.The peaks correspond to well- chanics,we have to add the amplitudes rather than separated wavelets having fixed phase relations but the intensities when two wavelets are superposed. traveling their own unique paths through space- Provided the wavelets remain indistinguishable in time;n is a natural number describing the diffrac- all degrees of freedom,they interfere and modulate tion order.To achieve a wide separation of the in- the detected intensity according to their phase dif- terferometer arms,the grating period should be ference:l地aP=lp,P+l+2ψ2cos(p1-p2)To small.Many atom and molecule interferometers use ensure that intensity modulations of individual gratings with 100 nm<d<1 um.Such gratings can matter waves add constructively to form an inter- be produced in nanofabrication laboratories?3 and ferogram-that is,to ensure that the contributions even occur naturally in the skeletons of nanoporous of individual particles don't wash each other out- algae.2 the beam must remain spectrally and spatially co- Wavefront beamsplitters can also be realized herent for the duration of the experiment with optical gratings(see figure 2b).Phase gratings In an optical interferometer,such as the Mach- couple the polarizability of matter to the electric Zehnder interferometer illustrated in figure 1a,vari- ations in the difference between two paths can be measured to a fraction of the light's wavelength.The a Detector sensitivity is limited only by the signal-to-noise ratio and the overall stability of the setup.State-of- the-art optical interferometers that are used to hunt for gravitational waves can even detect length changes as small as 10-18 m. Modern matter-wave interferometers cover de Broglie wavelengths ranging from 1013 m for macromolecules to more than 106 m for ultracold atoms.That corresponds roughly to the range be- Mirror tween x rays and IR radiation in optics.A key ad- vantage of matter waves over photons is that they couple to a plethora of external perturbations. b G Atoms and molecules have mass and rich internal electromagnetic spectra.Molecules and clusters add vibrational,rotational,and conformational dynam- ics.Thus matter waves are particularly sensitive to inertial and electromagnetic forces or collisions with many kinds of particles.As a result,matter-wave in- terferometers realize both force transducer and ruler in the same element.But how does one split a matter wave in the first place? Dividing the indivisible The word "atom"derives from the ancient Greek for Figure 1.Interferometry's ingredients.(a)In an optical Mach- indivisible.The splitting of an atomic beam,how- Zehnder interferometer,a beamsplitter(BS.divides an incident beam ever,must certainly be more than the random sort- of light into a superposition of wavelets;mirrors steer the wavelets ing of particles in one direction or the other.That along two distinct paths toward a second beamsplitter(BS,),and a pair possibility is excluded by the fact that interference of detectors records the intensity of the superposed wavelet fields. is based on the superposition of wavelets with a (b)In the matter-wave version of the interferometer,two nanoscale well-defined phase.We can also rule out the idea diffraction gratings(G,and G)act as the beamsplitters.A third grating that each atom is physically split in any naive sense; (G,)plays the part of the mirrors.The three-grating arrangement has all experiments confirm that each particle always contributes with the entirety of its internal proper- become the basis for many atom and macromolecule interferometers www.physicstoday.org May 2014 Physics Today 31
tensity modulations and, in turn, into measurable detector clicks. Dozens of interferometer types have been developed in classical and quantum optics. The key idea is always the same: An incident beam is divided by a beamsplitter into at least two wavelets; those wavelets—steered by mirrors, gratings, waveguides, or even gravity—travel along different paths before being recombined with a second beamsplitter. The sensitivity of the interferometer derives from the fact that different parts of the same wave explore different regions of spacetime. They may travel different distances or be exposed to different forces or potentials. As a result, two wavelets, ψ1exp[iϕ1(t)] and ψ2exp[iϕ2(t)], acquire different phases. In linear optics as well as in quantum mechanics, we have to add the amplitudes rather than the intensities when two wavelets are superposed. Provided the wavelets remain indistinguishable in all degrees of freedom, they interfere and modulate the detected intensity according to their phase difference: ∣ψtot∣ 2 = ∣ψ1∣ 2 + ∣ψ2∣ 2 + 2ψ1ψ2cos(ϕ1 − ϕ2). To ensure that intensity modulations of individual matter waves add constructively to form an interferogram—that is, to ensure that the contributions of individual particles don’t wash each other out— the beam must remain spectrally and spatially coherent for the duration of the experiment. In an optical interferometer, such as the Mach– Zehnder interferometer illustrated in figure 1a, variations in the difference between two paths can be measured to a fraction of the light’s wavelength. The sensitivity is limited only by the signal- to- noise ratio and the overall stability of the setup. State-ofthe-art optical interferometers that are used to hunt for gravitational waves can even detect length changes as small as 10−18 m. Modern matter- wave interferometers cover de Broglie wavelengths ranging from 10−13 m for macromolecules to more than 10−6 m for ultracold atoms. That corresponds roughly to the range between x rays and IR radiation in optics. A key advantage of matter waves over photons is that they couple to a plethora of external perturbations. Atoms and molecules have mass and rich internal electromagnetic spectra. Molecules and clusters add vibrational, rotational, and conformational dynamics. Thus matter waves are particularly sensitive to inertial and electromagnetic forces or collisions with many kinds of particles. As a result, matter- wave interferometers realize both force transducer and ruler in the same element. But how does one split a matter wave in the first place? Dividing the indivisible The word “atom” derives from the ancient Greek for indivisible. The splitting of an atomic beam, however, must certainly be more than the random sorting of particles in one direction or the other. That possibility is excluded by the fact that interference is based on the superposition of wavelets with a well-defined phase. We can also rule out the idea that each atom is physically split in any naive sense; all experiments confirm that each particle always contributes with the entirety of its internal properties—mass, polarizability, and so forth—in all accessible places. Atomic beam-splitting is actually about dividing an individual atom’s quantum wavefunction. We may, in particular, distinguish between wavefront and amplitude beamsplitters. Wavefront beamsplitters modify the wave - function as a function of its position. In quantum physics, position and momentum are related through a Fourier transform. A modulation of the wavefront—for instance, by a mechanical grating of period d, as shown in figure 2a—therefore imprints a coherent superposition of transverse momenta Δp = n · h/d, which leads to diffraction peaks in the far field (at distances much greater than A2 /λ, where A is the width of the beam-constricting aperture) behind the grating. The peaks correspond to wellseparated wavelets having fixed phase relations but traveling their own unique paths through spacetime; n is a natural number describing the diffraction order. To achieve a wide separation of the interferometer arms, the grating period should be small. Many atom and molecule interferometers use gratings with 100 nm < d < 1 μm. Such gratings can be produced in nanofabrication laboratories2,3 and even occur naturally in the skeletons of nanoporous algae.2 Wavefront beamsplitters can also be realized with optical gratings (see figure 2b). Phase gratings couple the polarizability of matter to the electric www.physicstoday.org May 2014 Physics Today 31 BS2 BS1 b a G1 G2 G3 Mirror Detector Figure 1. Interferometry’s ingredients. (a) In an optical Mach– Zehnder interferometer, a beamsplitter (BS1) divides an incident beam of light into a superposition of wavelets; mirrors steer the wavelets along two distinct paths toward a second beamsplitter (BS2), and a pair of detectors records the intensity of the superposed wavelet fields. (b) In the matter- wave version of the interferometer, two nanoscale diffraction gratings (G1 and G3) act as the beamsplitters. A third grating (G2) plays the part of the mirrors. The three- grating arrangement has become the basis for many atom and macromolecule interferometers. This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 202.120.2.30 On: Thu, 01 May 2014 23:26:12
Matter-wave metrology field of a standing light wave to imprint a position- (G)grating can be arranged to redirect and recom- dependent phase onto the matter-wave field.Pho bine those wavelets to obtain quantum interference todepletion gratings mask portions of the wave- The intensity profiles of the two recombined beams function-for instance,by transferring particles behind G:are complementary and depend on the that pass through the grating's antinodes to unde- phase shift between the two arms of the inter- tectable states.Light masks may ionize atoms,dis- ferometer.Additional interference pathways arise sociate molecular clusters,or change a particle's in- through higher diffraction orders.They can be spa- ternal state to make the particle undetectable if it tially separated with a slit. passes through an antinode of the light field.The Well-collimated atomic beams have been split effect closely resembles that of nanomechanical into arms separated by as much as 27 um,sufficient gratings. to expose two wavelets to different electric fields or A collimated particle source,a single grating, to regions of different gas pressure.That approach and a detecting screen can be interpreted as a full produced the most precise measurement of an interferometer.If the source is sufficiently small or atomic ground state polarizability to date and a so- distant to the grating,a coherent wave field may phisticated analysis of atom-atom scattering cross evolve that covers transverse distances larger than sections.3 the grating period d.The wavelets emerging from neighboring slits will then interfere on a screen in Interfering incoherent beams the far field,even without the help of additional Mach-Zehnder interferometry can be generalized beamsplitters and beam-steering elements.Modern to many atoms in the periodic table and to some precision experiments,however,typically include molecules.It requires,however,an intense and three or four optical elements to achieve larger beam transversely coherent beam,which can be routinely separation,longer coherence times,and higher sen- achieved for many atoms but remains an open chal- sitivity to external perturbations lenge for molecules and nanoparticles.Atomic In 1991 a group led by David Pritchard com- beams are typically formed by evaporation or sub- bined three equidistant,nanofabricated gratings, limation,sometimes at temperatures exceeding configured as shown in figure 1b,to realize a Mach- 1000 K.Massive molecules tend to be less volatile Zehnder interferometer for sodium atoms.3The first and would require still higher temperatures.Many grating (G)splits the wavefront into wavelets of molecules,atomic and molecular clusters,and various diffraction orders.A second(G2)and third nanoparticles,however,need to be studied at cool a Collimator le,p+hk gP) b a lBP+hk1-k2》 Collimator BP P〉 Figure 2.Splitting matter waves.(a)A nanomechanical diffraction mask splits an atomic or molecular beam by dividing the matter wavefront into beams of varying diffraction order.(Beams of diffraction order 0 and+1 are shown here.)(b)A beam can be similarly split using a standing light wave,which may remove parts of the matter wave via ionization or fragmentation or may modulate the wavefront's phase via dipole interaction. (c)Amplitude beamsplitters use laser pulses to create a coherent superposition between an atom's ground statelg)and a resonantly coupled excited state le).Upon excitation,the momentum p of the atom is enhanced by the momentum hk of the photon.The atom therefore ends up in an entangled state,where neither the internal nor the center-of-mass state is known but where the two are strictly correlated.(d)When the superposition is created between two hyperfine ground states Ig,)and Ig,)via a two-photon Raman transition,the apparatus is known as a Raman beamsplitter.Wider arm separations can be achieved with higher-order Raman transitions.(Panels c and d adapted from ref.16.) 32May2014 Physics Today www.physicstoday.org
32 May 2014 Physics Today www.physicstoday.org Matter-wave metrology field of a standing light wave to imprint a position- dependent phase onto the matter- wave field. Photodepletion gratings mask portions of the wavefunction—for instance, by transferring particles that pass through the grating’s antinodes to undetectable states. Light masks may ionize atoms, dissociate molecular clusters, or change a particle’s internal state to make the particle undetectable if it passes through an antinode of the light field. The effect closely resembles that of nanomechanical gratings. A collimated particle source, a single grating, and a detecting screen can be interpreted as a full interferometer. If the source is sufficiently small or distant to the grating, a coherent wave field may evolve that covers transverse distances larger than the grating period d. The wavelets emerging from neighboring slits will then interfere on a screen in the far field, even without the help of additional beamsplitters and beam- steering elements. Modern precision experiments, however, typically include three or four optical elements to achieve larger beam separation, longer coherence times, and higher sensitivity to external perturbations. In 1991 a group led by David Pritchard combined three equidistant, nanofabricated gratings, configured as shown in figure 1b, to realize a Mach– Zehnder interferometer for sodium atoms.3 The first grating (G1) splits the wavefront into wavelets of various diffraction orders. A second (G2) and third (G3) grating can be arranged to redirect and recombine those wavelets to obtain quantum interference. The intensity profiles of the two recombined beams behind G3 are complementary and depend on the phase shift between the two arms of the inter - ferometer. Additional interference pathways arise through higher diffraction orders. They can be spatially separated with a slit. Well- collimated atomic beams have been split into arms separated by as much as 27 μm, sufficient to expose two wavelets to different electric fields or to regions of different gas pressure. That approach produced the most precise measurement of an atomic ground state polarizability to date and a sophisticated analysis of atom–atom scattering cross sections.3 Interfering incoherent beams Mach–Zehnder interferometry can be generalized to many atoms in the periodic table and to some molecules. It requires, however, an intense and transversely coherent beam, which can be routinely achieved for many atoms but remains an open challenge for molecules and nanoparticles. Atomic beams are typically formed by evaporation or sublimation, sometimes at temperatures exceeding 1000 K. Massive molecules tend to be less volatile and would require still higher temperatures. Many molecules, atomic and molecular clusters, and nanoparticles, however, need to be studied at cool Figure 2. Splitting matter waves. (a) A nanomechanical diffraction mask splits an atomic or molecular beam by dividing the matter wavefront into beams of varying diffraction order. (Beams of diffraction order 0 and ±1 are shown here.) (b) A beam can be similarly split using a standing light wave, which may remove parts of the matter wave via ionization or fragmentation or may modulate the wavefront’s phase via dipole interaction. (c) Amplitude beamsplitters use laser pulses to create a coherent superposition between an atom’s ground state ∣g〉 and a resonantly coupled excited state ∣e〉. Upon excitation, the momentum p of the atom is enhanced by the momentum ħk of the photon. The atom therefore ends up in an entangled state, where neither the internal nor the center- of- mass state is known but where the two are strictly correlated. (d) When the superposition is created between two hyperfine ground states ∣g1〉 and ∣g2〉 via a two- photon Raman transition, the apparatus is known as a Raman beamsplitter. Wider arm separations can be achieved with higher-order Raman transitions. (Panels c and d adapted from ref. 16.) This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 202.120.2.30 On: Thu, 01 May 2014 23:26:12
a Grating Screen 3L-2 Gratings 3L2 Figure 3.Talbot-Lau interferometry.(a)Coherent radiation of wavelength A that's diffracted by a transmission grating of period d produces self-images of the grating's intensity distribution at multiples of the Talbot distance L=d2/A.Such a self-image is visible in the interference pattern at right,collected by scanning a detection screen along the z direction.(Fractional images of the grating occur at fractions of the Talbot distance.)(b)To achieve the same effect for monochromatic but spatially incoherent radiation,one can introduce a second,identical grating.Although the first grating does not produce visible interference fringes, it creates coherent illumination for the second grating,which then images the first onto a detection screen. The pattern shows interferograms for symmetric arrangements in which the spacing L between the two gratings is equal to that between the second grating and the detection screen.(Adapted from ref.16.) temperatures:Biomolecules usually denature at tem- each incident wave expands coherently by virtue of peratures in excess of 330 K,nanoparticles can decom- quantum physics. pose when heated,and clusters often only form by In 1948 Ernst Lau suggested exploiting that aggregation at temperatures of a few kelvin.Various finding in the optics setting by combining two grat- methods developed for mass spectrometry have ings:Diffraction at any one slit in the first grating, proven potent for generating charged or fast neutral G,will produce wavelets that expand sufficiently beams of macromolecules or clusters,but methods to overlap several slits in G,.Multipath interference for forming intense,directed,and neutral particle behind G,then creates a self-image of G,at a certain beams of uniform mass,low velocity,and low tem distance farther downstream.If the spacing be- perature still require extensive development. tween the gratings is chosen properly,typically on In 1997 John Clauser suggested a scheme that the order of the Talbot distance,the interferograms could substantially increase the signal even for a caused by all the wavelets emerging from G,over- weak,spatially incoherent beam source.Known as lap at the same distance behind G,and add con- a Talbot-Lau interferometer,it is based on William structively to form an interference pattern such as Henry Fox Talbot's observation,nearly two cen- that shown in figure 3b.The pattern can be recorded turies ago,that spatially coherent light can image on a screen,but for practical reasons,it's often de- periodic structures even in the absence of lenses.As tected behind a third grating. shown in figure 3a,coherent light passing through The above strategy has been realized with var- a grating produces self-images of the grating at mul- ious beamsplitters for clusters and large molecules. tiples of the Talbot distance L=d2/A.At first glance, The version in which all three gratings are mechan- that seems to contradict our textbook knowledge ical is known as a Talbot-Lau interferometer;when that the intensity pattern behind a grating peaks at the central grating is replaced by an optical phase sin 0=nA/d.The discrepancy is explained by the grating,it is called a Kapitza-Dirac-Talbot-Lau fact that Talbot's observation holds only in the near (KDTL)interferometer.5 The all-optical version, field behind the grating (A2/A);the textbook which uses three pulsed photo-depletion gratings, equations are given for the far field (zA2/A). has been established as the optical time-domain ion- So far,it seems we've gained nothing;we still izing matter-wave (OTIMA)interferometer. need coherent light.However,the physics that leads to diffraction under coherent illumination also cre- Entanglement-based beamsplitting ates coherence under incoherent illumination.That Amplitude beamsplitters divide the center-of-mass result from light optics also holds for matter waves. wavefunction of particles independent of their lateral If each slit is sufficiently narrow,the momentum of position.A particularly important implementation, www.physicstoday.org May 2014 Physics Today 33
www.physicstoday.org May 2014 Physics Today 33 temperatures: Biomolecules usually denature at temperatures in excess of 330 K, nanoparticles can decompose when heated, and clusters often only form by aggregation at temperatures of a few kelvin. Various methods developed for mass spectrometry have proven potent for generating charged or fast neutral beams of macromolecules or clusters, but methods for forming intense, directed, and neutral particle beams of uniform mass, low velocity, and low temperature still require extensive development. In 1997 John Clauser suggested a scheme that could substantially increase the signal even for a weak, spatially incoherent beam source. Known as a Talbot–Lau interferometer,4 it is based on William Henry Fox Talbot’s observation, nearly two centuries ago, that spatially coherent light can image periodic structures even in the absence of lenses. As shown in figure 3a, coherent light passing through a grating produces self-images of the grating at multiples of the Talbot distance LT = d2 /λ. At first glance, that seems to contradict our textbook knowledge that the intensity pattern behind a grating peaks at sin θ = nλ/d. The discrepancy is explained by the fact that Talbot’s observation holds only in the near field behind the grating (z ≪ A2 /λ); the textbook equations are given for the far field (z ≫ A2 /λ). So far, it seems we’ve gained nothing; we still need coherent light. However, the physics that leads to diffraction under coherent illumination also creates coherence under incoherent illumination. That result from light optics also holds for matter waves. If each slit is sufficiently narrow, the momentum of each incident wave expands coherently by virtue of quantum physics. In 1948 Ernst Lau suggested exploiting that finding in the optics setting by combining two gratings: Diffraction at any one slit in the first grating, G1, will produce wavelets that expand sufficiently to overlap several slits in G2. Multipath interference behind G2 then creates a self-image of G1 at a certain distance farther downstream. If the spacing between the gratings is chosen properly, typically on the order of the Talbot distance, the interferograms caused by all the wavelets emerging from G1 overlap at the same distance behind G2 and add constructively to form an interference pattern such as that shown in figure 3b. The pattern can be recorded on a screen, but for practical reasons, it’s often detected behind a third grating. The above strategy has been realized with various beamsplitters for clusters and large molecules. The version in which all three gratings are mechanical is known as a Talbot–Lau interferometer; when the central grating is replaced by an optical phase grating, it is called a Kapitza- Dirac- Talbot- Lau (KDTL) interferometer.5 The all- optical version, which uses three pulsed photo- depletion gratings, has been established as the optical time- domain ionizing matter- wave (OTIMA) interferometer.6 Entanglement-based beamsplitting Amplitude beamsplitters divide the center- of- mass wavefunction of particles independent of their lateral position. A particularly important implementation, L Screen Screen Grating Gratings x x z x y a b L L LT /2 LT /2 3 /2 LT 3 /2 LT LT LT L L Figure 3. Talbot–Lau interferometry. (a) Coherent radiation of wavelength λ that’s diffracted by a transmission grating of period d produces self- images of the grating’s intensity distribution at multiples of the Talbot distance LT = d2 /λ. Such a self-image is visible in the interference pattern at right, collected by scanning a detection screen along the z direction. (Fractional images of the grating occur at fractions of the Talbot distance.) (b) To achieve the same effect for monochromatic but spatially incoherent radiation, one can introduce a second, identical grating. Although the first grating does not produce visible interference fringes, it creates coherent illumination for the second grating, which then images the first onto a detection screen. The pattern shows interferograms for symmetric arrangements in which the spacing L between the two gratings is equal to that between the second grating and the detection screen. (Adapted from ref. 16.) This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 202.120.2.30 On: Thu, 01 May 2014 23:26:12
Matter-wave metrology Figure 4.Amplitude beamsplitters.(a)Pulsed laser beams at fixed /2 /2 positions and oriented orthogonal to an atom beam can form what's known as a Ramsey-Borde interferometer:A n/2 pulse-which places the beam in a superposition of entangled electronic and momentum states,as depicted in figure 2c-splits the beam into two arms;two successive n/2 pulses steer the arms back together;and a fourth 7/2 pulse recombines Beam the arms to produce the interference pattern.(b)An interferometer can also be realized with a three-pulse,7/2-7-7/2 sequence,as shown here for the case in which Raman pulses-counterpropagating beams whose frequency difference,w,-w,,is tuned to a hyperfine transition-are π/2 元/2 applied to an atomic cloud at intervals of time T.In the presence of gravity, the interferometer can be used to infer the acceleration due to gravity b π/2 2 based on the relative phase of the interferometer arms at time 27.(Red and green paths indicate atom-cloud trajectories in the absence and in the presence of gravity,respectively.) illustrated in figure 2c,is based on the entanglement between the internal and the external states of atoms:When resonant light irradiates an atom having an effective two-level energy system,the 0 2T light couples the ground state lg)to the excited state le)and induces population oscillations between photon transitions is almost double that of single- them.The oscillations occur at the Rabi frequency photon beamsplitters.The researchers also simpli- =d.E/h,where d is the dipole moment between fied the pulse sequence:As illustrated in figure 4b, Ig)and le),E is the light's electric field,and h is the three pulses-a n/2 pulse,followed by a n pulse that reduced Planck's constant.If the interaction time t interchanges the momenta of the two wave compo- is chosen to be dt =n/2,an atom initially in state nents,followed by a second n/2 pulse-suffice to Ig)is transferred into a coherent superposition of lg) form a Mach-Zehnder interferometer,now often re- and le).At the same time,the atom is transferred ferred to as a Raman interferometer.Entanglement- from its initial momentum state p)to the coherent based beamsplitters have become the basis for the superposition of lp)and lp +hk),where k is the pho- most sensitive and most accurate matter-wave sen- ton wavevector.Because there is a strict correlation sors to date.10.1 between the two processes-the photon absorption that electronically excites the atom also gives it a Atoms as quantum sensors momentum kick-the atom ends up in an entangled Atom interferometers are particularly useful for state o lg,p)-exp(io)le,p+hk),and the wave- measuring inertial forces,such as Earth's gravity or function can no longer be factorized into its internal rotation;in ultrahigh vacuum,atoms are essentially and external degrees of freedom.? unperturbed test masses.In three-beamsplitter inter- Sequences of n/2 pulses were originally used in ferometers like the ones shown in figures 1b and 4b, high-resolution spectroscopy to determine transi- wavelets traveling along the two arms acquire a rel- tion frequencies in atoms and small molecules.In ative phase shift△p=△p·a,TIh,where a is the ac- 1989 Christian Borde realized that the method,illus- celeration,Ap the momentum transfer in each beam- trated in figure 4a,also implements a matter-wave splitter,and T the time of flight between successive interferometer,now often referred to as a Ramsey- beamsplitters.Typical experiments are optimized to Borde interferometer.s During a first pulse,an atom detect either Earth's gravitational acceleration, may absorb a photon and receive the accompanying a=g=9.81 m/s2,or the rotational acceleration, momentum kick,or it may ignore the incident light. a.=2v x,due to the Coriolis effect,where v is the Asecond n/2 pulse hits the atom from the same side; atom's velocity and is the vector of Earth's angu- if the atom is still in its ground state,it may stay put, lar frequency as shown in the figure,or get electronically excited. In 1999 Achim Peters and colleagues used If the atom is already in the excited state,it may Raman interferometry in a fountain configura- ignore the arriving light or it may again receive the tion'2-in which a cold cloud of atoms is launched recoil of one photon,this time via stimulated emis- against gravity and returns in free fall-to measure sion in the direction opposite the beam source.A g with an accuracy of Ag/g=3x109.Recently, third and fourth pulse repeat the processes,except Susannah Dickerson and colleagues in Kasevich's with inverted signs of momentum transfer,to form lab have developed a 10-m fountain!that could a closed interferometer. potentially measure gravitational acceleration with In 1991 Mark Kasevich and Steven Chu realized a sensitivity of 3 x 10 g/Hz. an entanglement-based atom interferometer by cou- When two atom gravimeters are combined, pling two hyperfine ground states via two-photon they measure gravity gradients.Guglielmo Tino's Raman transitions(see figure 2d).That leads to a group recently used that approach'3 to measure boost in precision,because hyperfine ground states Newton's gravitational constant G in the presence of do not decay during the time scale of the experi- a moving test mass with a statistical uncertainty of ment.Moreover,the momentum transfer using two- 0.011×1011m3kgs2. 34 May 2014 Physics Today www.physicstoday.org
illustrated in figure 2c, is based on the entanglement between the internal and the external states of atoms: When resonant light irradiates an atom having an effective two- level energy system, the light couples the ground state ∣g〉 to the excited state ∣e〉 and induces population oscillations between them. The oscillations occur at the Rabi frequency ΩR = d · E/ħ, where d is the dipole moment between ∣g〉 and ∣e〉, E is the light’s electric field, and ħ is the reduced Planck’s constant. If the interaction time τ is chosen to be ∫0 τ ΩRdt = π/2, an atom initially in state ∣g〉 is transferred into a coherent superposition of ∣g〉 and ∣e〉. At the same time, the atom is transferred from its initial momentum state ∣p〉 to the coherent superposition of ∣p〉 and ∣p + ħk〉, where k is the photon wavevector. Because there is a strict correlation between the two processes—the photon absorption that electronically excites the atom also gives it a momentum kick—the atom ends up in an entangled state ∣ψ〉 ∝ ∣g, p〉 − exp(iϕ)∣e, p + ħk〉, and the wavefunction can no longer be factorized into its internal and external degrees of freedom.7 Sequences of π/2 pulses were originally used in high- resolution spectroscopy to determine transition frequencies in atoms and small molecules. In 1989 Christian Bordé realized that the method, illustrated in figure 4a, also implements a matter- wave interferometer, now often referred to as a Ramsey– Bordé interferometer.8 During a first pulse, an atom may absorb a photon and receive the accompanying momentum kick, or it may ignore the incident light. A second π/2 pulse hits the atom from the same side; if the atom is still in its ground state, it may stay put, as shown in the figure, or get electronically excited. If the atom is already in the excited state, it may ignore the arriving light or it may again receive the recoil of one photon, this time via stimulated emission in the direction opposite the beam source. A third and fourth pulse repeat the processes, except with inverted signs of momentum transfer, to form a closed interferometer. In 1991 Mark Kasevich and Steven Chu realized an entanglement- based atom interferometer by coupling two hyperfine ground states via two- photon Raman transitions (see figure 2d). That leads to a boost in precision, because hyperfine ground states do not decay during the time scale of the experiment. Moreover, the momentum transfer using two- photon transitions is almost double that of single- photon beamsplitters.9 The researchers also simplified the pulse sequence: As illustrated in figure 4b, three pulses—a π/2 pulse, followed by a π pulse that interchanges the momenta of the two wave components, followed by a second π/2 pulse—suffice to form a Mach–Zehnder interferometer, now often referred to as a Raman interferometer. Entanglement- based beamsplitters have become the basis for the most sensitive and most accurate matter- wave sensors to date.10,11 Atoms as quantum sensors Atom interferometers are particularly useful for measuring inertial forces, such as Earth’s gravity or rotation; in ultrahigh vacuum, atoms are essentially unperturbed test masses. In three- beamsplitter interferometers like the ones shown in figures 1b and 4b, wavelets traveling along the two arms acquire a relative phase shift Δϕ = Δp · a · T2 /ħ, where a is the acceleration, Δp the momentum transfer in each beamsplitter, and T the time of flight between successive beamsplitters. Typical experiments are optimized to detect either Earth’s gravitational acceleration, ag = g ≃ 9.81 m/s2 , or the rotational acceleration, ac = 2v × ΩE, due to the Coriolis effect, where v is the atom’s velocity and ΩE is the vector of Earth’s angular frequency. In 1999 Achim Peters and colleagues used Raman interferometry in a fountain configuration12—in which a cold cloud of atoms is launched against gravity and returns in free fall—to measure g with an accuracy of Δg/g = 3 × 10−9. Recently, Susannah Dickerson and colleagues in Kasevich’s lab have developed a 10-m fountain11 that could potentially measure gravitational acceleration with a sensitivity of 3 × 10−11 g/√―Hz. When two atom gravimeters are combined, they measure gravity gradients. Guglielmo Tino’s group recently used that approach13 to measure Newton’s gravitational constant G in the presence of a moving test mass with a statistical uncertainty of 0.011 × 10−11 m3 kg−1 s−2. 34 May 2014 Physics Today www.physicstoday.org Matter-wave metrology π/2 π/2 π/2 π π/2 z π/2 π/2 z x t ω1 ω2 0 T 2T a b Beam Figure 4. Amplitude beamsplitters. (a) Pulsed laser beams at fixed positions and oriented orthogonal to an atom beam can form what’s known as a Ramsey–Bordé interferometer: A π/2 pulse—which places the beam in a superposition of entangled electronic and momentum states, as depicted in figure 2c—splits the beam into two arms; two successive π/2 pulses steer the arms back together; and a fourth π/2 pulse recombines the arms to produce the interference pattern. (b) An interferometer can also be realized with a three-pulse, π/2-π-π/2 sequence, as shown here for the case in which Raman pulses—counterpropagating beams whose frequency difference, ω2 − ω1, is tuned to a hyperfine transition—are applied to an atomic cloud at intervals of time T. In the presence of gravity, the interferometer can be used to infer the acceleration due to gravity based on the relative phase of the interferometer arms at time 2T. (Red and green paths indicate atom-cloud trajectories in the absence and in the presence of gravity, respectively.) This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 202.120.2.30 On: Thu, 01 May 2014 23:26:12
Figure 5.Matter-wave diffraction started in 1930 with diatomic particles,but it didn't gain momentum until the early 1990s.Since then the technique has been extended to progressively larger and more complex molecules;some are depicted here along with their mass in atomic mass units,the number of atoms they comprise,and the year they were first successfully used in interference experiments.To date,a functionalized tetraphenylporphyrin,synthesized by Marcel Mayor and colleagues,is the most massive object for which matter-wave interference has been seen.In the future, bionanomatter such as hemoglobin proteins will likely be studied using matter-wave-enhanced measurements. H He2,Na,I,:Buckminsterfullerene Tetraphenylporphyrin Functionalized Hemoglobin 2 amu,2 atoms 8-254amu, 720 amu,60 atoms 614 amu,78 atoms Tetraphenylporphyrin >60000amu,-10000 atoms (1930) 2 atoms (1999) (2003) 10 123 amu,810 atoms (199495) (2013) Atom gravimeters will be important for envi- equivalence principle,for instance,would be to ronmental monitoring applications:Earthquakes compare matter of vastly different composition or and volcanoes may give telltale warnings in the to systematically explore the mass dependence of form of weak local accelerations.Earthbound and phase shifts.Almost certainly,those studies will satellite-based inertial sensors may permit a more happen in the future. precise determination of the geoid-Earth's gravita- For now,however,atom interferometry contin- tional equipotential surface-and thereby provide ues to lead the way in sensing,because hundreds of climatologists with information about changes of research groups have spent more than three water tables and glaciers. decades on the development of cold and ultracold Quantum gravimeters with a sensitivity of 1 part atom technologies.The development of intense in 10 or better can assist in the prospection for deeply sources of neutral macromolecules and highly effi- buried natural resources.Closer to the surface,small cient detectors is still an open challenge. caves or sewage pipes might be detectable via quan- In my group at the University of Vienna,we've tum gravity gradiometry. chosen to focus our molecular interferometry efforts Atomic inertial sensors are also of interest for on questions that no other experiment could ad- navigation systems.Global positioning systems rely dress with the same precision and scope.On the one on satellite signals,which are inaccessible in deep hand are tests of the linearity of quantum mechanics water and may be jammed by adversaries in the case in the limit of ultrahigh masses.On the other hand of a military conflict.In the future,inertial naviga- is the study of delocalized particles in well-defined tion systems that combine gravitational and rota- fields,which yields information about the particles' tional accelerometers may enable precise position- internal properties.In KDTL and OTIMA interfe- ing by integrating all accelerations along a path rometry,the particles may be atoms,molecules,or Only the occasional correction with an external even clusters of molecules. control signal would be necessary.Many of those Molecular-beam deflection,in which molecules' applications are expected to flourish as soon as internal properties are inferred from changes to their devices become compact and portable. trajectory in an applied external field,is a well- Ultrasensitive atom gravimeters will set new established method in physical chemistry.In con- bounds on deviations from the weak equivalence trast to classical machines,which typically realize principle-the conjecture that an object's freefall molecular beam widths around 100 um,our quan- trajectory is independent of its composition and tum interferometers can create molecular nano- structure-on violations of local Lorentz invariance, patterns with periods as small as 80 nm.Beam shifts on the gravitational redshift,and on the postulated can therefore be seen with more than a thousand- existence of a "fifth force."They may also serve as fold increase in position sensitivity. detectors for low-frequency gravitational waves. In 2007 we used a Talbot-Lau interferometer to Whereas inertial sensing uses atoms as test determine the static polarizability of the fullerenes masses to measure external fields,precision meas- carbon-60 and carbon-70.With KDTL interferome- urements of the photon recoil in a Raman beam- try,it is now possible to study the influence of per- splitter allow extraction of the ratio of h to atomic manent and even dynamic electric dipole moments mass.4 That ratio,along with precise mass meas- in a large variety of organic molecules.5 It will be urements,yielded the fine structure constant possible to explore the magnetic world of molecules a1=137.035 999 037 with a relative uncertainty of and clusters:aromaticity,magnetic dipole moments, 6.6×10-10. and,eventually,phase transitions in clusters. Most recently we have explored the possibility Molecules as quantum sensors of using molecule interferometers to measure ab- In principle,a molecule interferometer can detect solute optical absorption cross sections:The recoil both inertial forces and relativistic effects,just as an imparted on each molecule due to the absorption of atom interferometer can.An interesting test of the a single photon is sufficient to noticeably shift the www.physicstoday.org May 2014 Physics Today 35
www.physicstoday.org May 2014 Physics Today 35 Atom gravimeters will be important for environmental monitoring applications: Earthquakes and volcanoes may give telltale warnings in the form of weak local accelerations. Earthbound and satellite- based inertial sensors may permit a more precise determination of the geoid—Earth’s gravitational equipotential surface—and thereby provide climatologists with information about changes of water tables and glaciers. Quantum gravimeters with a sensitivity of 1 part in 108 or better can assist in the prospection for deeply buried natural resources. Closer to the surface, small caves or sewage pipes might be detectable via quantum gravity gradiometry. Atomic inertial sensors are also of interest for navigation systems. Global positioning systems rely on satellite signals, which are inaccessible in deep water and may be jammed by adversaries in the case of a military conflict. In the future, inertial navigation systems that combine gravitational and rotational accelerometers may enable precise positioning by integrating all accelerations along a path. Only the occasional correction with an external control signal would be necessary. Many of those applications are expected to flourish as soon as devices become compact and portable. Ultrasensitive atom gravimeters will set new bounds on deviations from the weak equivalence principle—the conjecture that an object’s freefall trajectory is independent of its composition and structure—on violations of local Lorentz invariance, on the gravitational redshift, and on the postulated existence of a “fifth force.” They may also serve as detectors for low- frequency gravitational waves. Whereas inertial sensing uses atoms as test masses to measure external fields, precision measurements of the photon recoil in a Raman beam - splitter allow extraction of the ratio of h to atomic mass.14 That ratio, along with precise mass meas - urements, yielded the fine structure constant α−1 = 137.035 999 037 with a relative uncertainty of 6.6 × 10−10. Molecules as quantum sensors In principle, a molecule interferometer can detect both inertial forces and relativistic effects, just as an atom interferometer can. An interesting test of the equivalence principle, for instance, would be to compare matter of vastly different composition or to systematically explore the mass dependence of phase shifts. Almost certainly, those studies will happen in the future. For now, however, atom interferometry continues to lead the way in sensing, because hundreds of research groups have spent more than three decades on the development of cold and ultracold atom technologies. The development of intense sources of neutral macromolecules and highly efficient detectors is still an open challenge. In my group at the University of Vienna, we’ve chosen to focus our molecular interferometry efforts on questions that no other experiment could address with the same precision and scope. On the one hand are tests of the linearity of quantum mechanics in the limit of ultrahigh masses. On the other hand is the study of delocalized particles in well- defined fields, which yields information about the particles’ internal properties. In KDTL and OTIMA interfe - rometry, the particles may be atoms, molecules, or even clusters of molecules. Molecular- beam deflection, in which molecules’ internal properties are inferred from changes to their trajectory in an applied external field, is a well- established method in physical chemistry. In contrast to classical machines, which typically realize molecular beam widths around 100 μm, our quantum interferometers can create molecular nano - patterns with periods as small as 80 nm. Beam shifts can therefore be seen with more than a thousandfold increase in position sensitivity. In 2007 we used a Talbot–Lau interferometer to determine the static polarizability of the fullerenes carbon-60 and carbon-70. With KDTL interferometry, it is now possible to study the influence of permanent and even dynamic electric dipole moments in a large variety of organic molecules.5 It will be possible to explore the magnetic world of molecules and clusters: aromaticity, magnetic dipole moments, and, eventually, phase transitions in clusters. Most recently we have explored the possibility of using molecule interferometers to measure absolute optical absorption cross sections: The recoil imparted on each molecule due to the absorption of a single photon is sufficient to noticeably shift the Figure 5. Matter- wave diffraction started in 1930 with diatomic particles, but it didn’t gain momentum until the early 1990s. Since then the technique has been extended to progressively larger and more complex molecules; some are depicted here along with their mass in atomic mass units, the number of atoms they comprise, and the year they were first successfully used in interference experiments. To date, a functionalized tetraphenylporphyrin, synthesized by Marcel Mayor and colleagues, is the most massive object for which matter- wave interference has been seen. In the future, bionanomatter such as hemoglobin proteins will likely be studied using matter- wave-enhanced measurements. This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 202.120.2.30 On: Thu, 01 May 2014 23:26:12
Matter-wave metrology interference pattern.s Such measurements can be year of functionalized organic molecules composed used to analyze very dilute samples of species hav- of 810 atoms(see figure 5),matter-wave interferom- ing unknown vapor pressure.Applied to medium- etry has already crossed the 10 000-amu threshold.5 sized biomolecules with and without hydration shells,the technology could,in the future,uncover I am grateful for support from my research group and from effects of solvation on structure,conformation,and the European Research Council,the European Commission, photo-induced conformation changes. the Austrian Science Fund,and the University of Vienna. Quantum interferometry with complex nano- References particles is still vast uncharted territory.Thousands 1.L.de Broglie,Nature 112,540(1923). of quantum objects,dozens of particle properties, 2.T.Juffmann,H.Ulbricht,M.Arndt,Rep.Prog.Phys.76, and many new manipulation techniques remain to 086402(2013) be explored.Often those quantities are intrinsically 3.A.D.Cronin,J.Schmiedmayer,D.E.Pritchard,Rev. more dynamic in complex,warm molecules than Mod.Phys.81,1051(2009). they would be in static atoms. 4.J.F.Clauser,in Experimental Metaphysics:Quantum Me- Ongoing high-mass interference experiments at chanical Studies for Abner Shimony,Volume One,R.S. Cohen,M.Horne,J.Stachel,eds.,Kluwer Academic, the University of Vienna allow handling of de Broglie Boston (1997),P.1. wavelengths as small as 200 fm.Taking that as the 5.K.Hornberger et al.,Rev.Mod.Phrys.84,157 (2012). current standard,a source that delivers nanoparticles 6.P.Haslinger et al.,Nat.Phys.9,144 (2013). of 10s atomic mass units(1.66x10-24 g)at a velocity 7.C.J.Bordé，Phys.Lett.A140,10(1989). of 1 m/s would be suitable for interferometry exper- 8.S.Eibenberger et al.,http://arxiv.org/abs/1402.5307v1. iments.Various new sources are being developed to 9.M.Kasevich,S.Chu,Phys.Rev.Lett.67,181 (1991). prepare tailor-made molecules and nanoparticles of 10.S.-Y.Lan et al.,Plrys.Rev.Lett.108,090402 (2012); H.Muintinga et al.,Plrys.Rev.Lett.110,093602 (2013); 10 amu or more for interferometry experiments Z.-K.Hu et al.,Phys.Rev.A 88,043610 (2013); Quantum interferometry with biological nano- R.Geiger et al.,Nat.Commun.2,474 (2011). materials is a fascinating short-term goal.Quantum 11.S.M.Dickerson et al.,Plnys.Rev.Lett.111,083001(2013). physics with proteins or DNA having masses in the 12.A.Peters,K.Y.Chung,S.Chu,Nature 400,849(1999). range 10 000-100 000 amu would open a new field 13.G.Lamporesi et al.,Phrys.Rev.Lett.100,050801(2008). of research and technology at the interface between 14.R.Bouchendira et al.,Phys.Rev.Lett.106,080801 quantum optics,physical chemistry,and biomolec- (2011). 15.S.Eibenberger et al.,Phys.Chem.Chem.Phys.15,14696 ular physics.The goal is challenging but not beyond (2013). reach:With the successful quantum interference last 16.W.B.Case et al.,Opt.Express 17,20966(2009). Optical Fiber Redesigning the Coating Service Physicist: Low-stress multilayer metalization -Barrier layer to prevent Au diffusion Physics Innovation and Entrepreneurship Education for the 21st Century Re-coating FBGs June5-6,2014 Superior adhesion College Park.MD ·End-face coating on connectorized APS is holding a conference for physics department or bare fibers leaders,targeted at developing physics innovation -Filters and entrepreneurship(PIE)programs at their AR coatings institutions.Sessions will feature successfully implemented PIE programs,and resources such as Shift-free coating the National Collegiate Innovators and Inventors Second surface Gold mirrors Alliance(NCIIA),which offer funding,guidance,and Other services include access to a community of practitioners. -Diamond-like Carbon To register,please visit: (DLC)for IR Optics www.aps.org/programs/education/ Transparent conferences/innovation.cfm Conductive Coatings APS :IntlVc intlvac.com NCIIA Carthagc T H I N F I L M sales@intlvac.com
Matter-wave metrology interference pattern.8 Such measurements can be used to analyze very dilute samples of species having unknown vapor pressure. Applied to medium- sized biomolecules with and without hydration shells, the technology could, in the future, uncover effects of solvation on structure, conformation, and photo- induced conformation changes. Quantum interferometry with complex nano - particles is still vast uncharted territory. Thousands of quantum objects, dozens of particle properties, and many new manipulation techniques remain to be explored. Often those quantities are intrinsically more dynamic in complex, warm molecules than they would be in static atoms. Ongoing high-mass interference experiments at the University of Vienna allow handling of de Broglie wavelengths as small as 200 fm. Taking that as the current standard, a source that delivers nanoparticles of 106 atomic mass units (1.66 × 10−24 g) at a velocity of 1 m/s would be suitable for interferometry experiments. Various new sources are being developed to prepare tailor- made molecules and nanoparticles of 107 amu or more for interferometry experiments. Quantum interferometry with biological nanomaterials is a fascinating short-term goal. Quantum physics with proteins or DNA having masses in the range 10 000−100 000 amu would open a new field of research and technology at the interface between quantum optics, physical chemistry, and biomolecular physics. The goal is challenging but not beyond reach: With the successful quantum interference last year of functionalized organic molecules composed of 810 atoms (see figure 5), matter-wave interferometry has already crossed the 10 000-amu threshold.15 I am grateful for support from my research group and from the European Research Council, the European Commission, the Austrian Science Fund, and the University of Vienna. References 1. L. de Broglie, Nature 112, 540 (1923). 2. T. Juffmann, H. Ulbricht, M. Arndt, Rep. Prog. Phys. 76, 086402 (2013). 3. A. D. Cronin, J. Schmiedmayer, D. E. Pritchard, Rev. Mod. Phys. 81, 1051 (2009). 4. J. F. Clauser, in Experimental Metaphysics: Quantum Mechanical Studies for Abner Shimony, Volume One, R. S. Cohen, M. Horne, J. Stachel, eds., Kluwer Academic, Boston (1997), p. 1. 5. K. Hornberger et al., Rev. Mod. Phys. 84, 157 (2012). 6. P. Haslinger et al., Nat. Phys. 9, 144 (2013). 7. C. J. Bordé, Phys. Lett. A 140, 10 (1989). 8. S. Eibenberger et al., http://arxiv.org/abs/1402.5307v1. 9. M. Kasevich, S. Chu, Phys. Rev. Lett. 67, 181 (1991). 10. S.-Y. Lan et al., Phys. Rev. Lett. 108, 090402 (2012); H. Müntinga et al., Phys. Rev. Lett. 110, 093602 (2013); Z.-K. Hu et al., Phys. Rev. A 88, 043610 (2013); R. Geiger et al., Nat. Commun. 2, 474 (2011). 11. S. M. Dickerson et al., Phys. Rev. Lett. 111, 083001 (2013). 12. A. Peters, K. Y. Chung, S. Chu, Nature 400, 849 (1999). 13. G. Lamporesi et al., Phys. Rev. Lett. 100, 050801 (2008). 14. R. Bouchendira et al., Phys. Rev. Lett. 106, 080801 (2011). 15. S. Eibenberger et al., Phys. Chem. Chem. Phys. 15, 14696 (2013). 16. W. B. Case et al., Opt. Express 17, 20966 (2009). ■ Redesigning the Physicist: Physics Innovation and Entrepreneurship Education for the 21st Century June 5 - 6, 2014 College Park, MD APS is holding a conference for physics department leaders, targeted at developing physics innovation and entrepreneurship (PIE) programs at their institutions. Sessions will feature successfully implemented PIE programs, and resources such as the National Collegiate Innovators and Inventors Alliance (NCIIA), which offer funding, guidance, and access to a community of practitioners. To register, please visit: www.aps.org/programs/education/ conferences/innovation.cfm This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 202.120.2.30 On: Thu, 01 May 2014 23:26:12