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Complex Organic Molecules in Nebulae 2 Mercury’s Plasma Belt 3 Reaching the Quantum Noise Limit 4 Mirrors That Reflect Matter 5 Neutrino- Antineutrino Violation 5 Optical Black Holes 6 tm BriefingS | Space & Physics 2

versity of Antwerp and col- ASTROCHEMISTRY leagues from Austria, the Neth- Briefings erlands and Canada have now A gift from the demonstrated an electron vortex stars beam with a diameter of less Scientific American Briefings: Space & Physics than 1.2 Å. consists of summaries of recent peer-reviewed articles from the scientific literature. It draws Observatories around the Electron vortex beams were these summaries from the journals of Nature world have recorded infrared first created by passing a plane Publishing Group, including Nature, Nature Nanotechnology, Nature Photonics, Nature emission spectra at wavelengths wave beam through a graphite Physics and the Nature Reviews journals. of 3–20 micrometers from stars film that spontaneously formed

Mariette DiChristina and nebulae. These spectra were tockphoto a spiral structure, and acted as a Senior Vice President and Editor-in-Chief, initially considered characteris- phase plate. This was difficult to Scientific American tics of polycyclic aromatic hydro- i S g/ K eatin reproduce and gave limited con- Philip Yam carbons — fused-ring molecules trol over the resulting beam. Ver-

Managing Editor, Online, Scientific American ourtney commonly found in burnt food, C beeck and co-workers had im- John Rennie Contributing Editor, Scientific American tar, coal and oil. Tightly focused laser beams, proved on this approach by cre- Michael Mrak Sun Kwok and Yong Zhang at known as optical tweezers, have ating a vortex beam with a Design Director, Scientific American the University of Hong Kong been used to manipulate micro- holographic mask inside a trans- Philip Campbell have now proposed an alterna- scopic objects for two decades. mission electron microscope. Editor-in-Chief, Nature tive hypothesis — that these Andrey Lutich, Jochen Feld- However, the effective beam di- Steven Inchcoombe emission spectra are character- mann and their co-workers at ameter was several micrometers. Managing Director, Nature Publishing Group, and President, Scientific American istic of complex organic mole- the Ludwig–Maximilians Univer- Verbeek and colleagues have Mike Florek cules with disorganized struc- sity in Munich used a similar ap- now reduced this beam diameter Executive Vice President, Scientific American tures. They examined the emis- proach: they suspended a to atomic dimensions by placing a Bruce Brandfon sion spectra from several 60-nanometre gold nanoparticle holographic mask in the con- Vice President and Publisher, Scientific American nebulae and pro-planetary neb- in water in the focal spot of a denser plane of a state-of-the-art Wendy Elman ulae, including the Orion Bar laser beam. They sent sound microscope with double aberra- Vice President, Digital Solutions, Scientific American nebula, and found that the most waves through the water and tion correction. At 1.2 Å, the beam Michael Voss Vice President and Associate Publisher, Marketing fitting model would be if the or- measured changes in the parti- size is comparable to the size of and Business Development, Scientific American ganic molecules contained a cle’s constrained motion. The au- the 2p orbital in a nitrogen atom Christian Dorbandt mixture of aliphatic branches thors calculate that the nanopar- (see image; left and right panels Managing Director, Consumer Marketing, and aromatic rings, and not just ticle can pick up sounds as low show the beam and the 2p orbital Scientific American aromatic rings alone. as −60 decibels, making the set- respectively, drawn approximately Matt Hansen Senior Production Editor, Nature Publishing Group In fact, recent laboratory anal- up a million times more sensi- to scale). The tiny vortex beam Kerrissa Lynch yses have identified organic mole- tive than the human ear. may allow atomic-resolution Web Production Editor, Scientific American cules with a chemical structure —Eugenie Samuel Reich, mapping of magnetic states. How to contact us similar to that described above in Nature —Michael Segal, For subscription correspondence, including the Murchison meteorite. The re- Nature Nanotechnology change of e-mail addresses: ■■ Phys. Rev. Lett. doi: 10.1103/ U.S. and Canada: 800-333-1199 searchers suggest that organic Outside the North America: +1-515-248-7684 molecules might exist throughout PhysRevLett.108.018101 (2012) ■■ Appl. Phys. Lett. doi: 10.1063/1.3662012 Email: [email protected] (2011) Postal address: Scientific American Briefings, Box the universe, and that meteorites 3187, Harlan, IA 51537 might have carried these mole- Annual subscription (12 issues): $19.95 (USD) ELECTRON BEAMS For editorial comments: cules from distant galaxies to our TECHNIQUES Email: [email protected] solar system. An atom-sized www.ScientificAmerican.com —Edward Duca, Cooling by Nature China vortex Scientific American is a trademark of Electron beams are typically numbers Scientific American, Inc., used with permission. ■■ Nature doi: 10.1038/nature10542 (2011) plane waves. This means that the Like electrons trapped in the beam phase is identical for all potential of a nucleus, bosonic PHYSICS points in a plane perpendicular atoms residing at the individual tm to the beam direction. The phase sites of an optical lattice — a peri- ‘Nanoear’ hears of an electron vortex beam, on odic structure defined by inter- the other hand, describes a fering laser beams — can occupy small sounds spiral. As a result, vortex beams discrete motional orbitals. By using laser beams to trap a carry orbital angular moment Waseem Bakr and colleagues gold nanoparticle, researchers in and magnetic moment, which have now implemented a tech- Germany have developed a sensi- leads to unique interactions with nique for deterministically con- tive sound-wave detector. matter. Jo Verbeeck of the Uni- trolling the distribution of atoms SUBSCRIBE >> tm BriefingS | Space & Physics 3

across these different energy water in a rotating magnetic trons at a distance of about half states, and establish it as a new field, which triggered nickel the planet’s radius from the means of cooling quantum gases. nanowires in the tank to rotate surface. The key is a mechanism in turn, generating microvor- The authors compare the belt Bakr et al. call “orbital excitation tices. The vortices trapped poly- to Earth’s ring current and say blockade.” They transfer ground- styrene microbeads in the water. that it could influence the pat- state atoms to a higher orbital by By controlling the movement of tern of surface weathering and modulating the lattice depth at a the nanowires, the authors could the formation of a thin atmo- suitable frequency; but once one tightly control the movement of sphere around Mercury. atom is transferred, it shifts the the beads. —Emma Maris, Nature On the Cover energy levels to a degree that They also successfully manip- ■■ Geophys. Res. Lett. doi: Light from the Orion Nebula, other atoms at the same site are ulated Escherichia coli bacteria 10.1029/2011GL049629 (2011) one of the most arresting pushed off-resonance — the first using a pair of microspheres in sights in the night sky, excitation blocks further ones. place of the nanowires. suggests that some highly Bakr and colleagues use this —Daniel Cressey, Nature interesting mixture of mechanism to reshuffle and se- ■■ Nano Lett. 10.1021/nl2032487 (2011) WAVEGUIDES complex organic molecules lectively remove atoms from a can be found near these lattice with random occupation Luminous stellar remnants. See page 2. numbers. As atom-number fluc- PLANETARY SCIENCE tuations across the lattice are the repulsion Credit: ESO/J. Emerson/ main source of entropy, levelling Mercury’s enhanced VISTA/Cambridge the number of atoms at each site Astronomical Survey Unit amounts to cooling the gas. The plasma belt When a dielectric particle is ex- authors expect that in the future posed to a strong optical gra- their method can also serve in dient it becomes electrically po-

quantum computations on op- ton larized and subject to an attrac-

tical-lattice systems. ashin g tive force in the direction of

—Andreas Trabesinger, W o f increasing field strength. This is hysics P A pplied Nature Physics the principle by which optical tweezers are able to trap and ma- ■■ Nature doi: 10.1038/nature10668 (2011) I nstitution g ie nipulate living cells and other arne

/C microscopic objects. NANOTECHNOLOGY A similar force can be gener-

aboratory ated between two light-carrying U niversity H opkins ohns NASA/J L Swirls move Mercury. microscopic (and nanoscopic) waveguide structures. Moreover, tiny objects Like Earth, Mercury has a the sign of the force can be Individual cells can be ma- magnetosphere, a zone of inter- tuned from attractive to repul- nipulated by tiny vortices gener- action between its magnetic field sive by shifting the phase of the ated in fluids, rather than by the and incoming plasma from the light travelling in one waveguide potentially harmful lasers or Sun. But scientists have been un- relative to the other — which is electric fields typically used. The sure about whether Mercury also potentially useful for optome- concept is the brainchild of Li has the concentration of charged chanical sensing and switching Zhang and his colleagues at the particles around its equator that applications. Unfortunately, the Swiss Federal Institute of Tech- Earth does. magnitude of such forces is nology in Zurich, who used the David Schriver at the Univer- much less than the force exerted vortices to control the movement sity of California, Los Angeles, by optical tweezers. And at small of microscopic objects. and his colleagues argue that separations the sign is invariably The team placed a tank of Mercury does have such a quasi- attractive.

Scientific AmericanBriefings, Space & Physics, Volume 1, Number 2, trapped particle belt, citing sim- Ardavan Oskooi and col- February 2012, published monthly by Scientific American, a division of Nature America, Inc., 75 Varick Street, 9th Floor, New York, NY 10013-1917. ulations of the planet’s dynamics leagues propose a way to in- Subscription rates: 1 year (12 issues) $19.95 (USD). crease the repulsion between Please send subscription correspondence, including change and data from instruments of e-mail and postal addresses to: aboard the MESSENGER space- waveguides. Their calculations Scientific American Briefings, Box 3187, Harlan, IA 51537. E-mail address for subscription inquires:

tockphoto craft, which entered into orbit suggest that the key is to cut [email protected]. / i S E-mail address for general inquires: around Mercury in March 2011. each waveguide in half, as semi-

[email protected]. vans Subscription inquires: E The craft ploughed through an circular waveguides perform U.S. and Canada: 800-333-1199; other: +1-515-248-7684. : M ark : equatorial population of 1–10-ki- much better than circular. This Copyright © 2012 Scientific American, a division of Nature America, Inc.

photonic crystal waveguides, quency is higher than the reso- also demonstrated that the per- which show a 30-fold increase in nance frequency of the cavity, formance of their diode is not repulsive force that remains re- and vice versa. And if a weak sensitive to the intrinsic dissipa- pulsive even at small probe signal is sent into the tion of the quantum impurity. separations. system when energy is being —James Baxter, —Ed Gerstner, transferred to the resonator, this Nature Photonics Nature Physics probe can also be amplified. Massel and co-workers show ■■ Phys. Rev. Lett. doi: 10.1103/ ■■ Opt. Lett. doi: 10.1364/OL.36.004638 that approximately 20 noise PhysRevLett.107.173902 (2011) (2011) quanta are added to the signal, and predict that it should be MATHEMATICS NANOELECTROMECHANICAL SYSTEMS possible to reach the quantum limit of adding just half a What da Vinci Keeping the quantum of noise. —Peter Rodgers, saw in trees noise down Nature Nanotechnology In his notebooks, the fifteenth- century Italian polymath Leon- ■■ Nature doi: 10.1038/nature10628 (2011) ardo da Vinci observed that the total cross-section of a tree re- OPTICAL ISOLATORS mains the same along its height every time it branches. Physicists Single-photon have searched for years for an ex- planation for this phenomenon, performance but have yet to find one that is Unwanted feedback caused widely accepted. tockphoto

/ i S by reflections between optical orres components can have a delete-

T arlos rious effect on quantum optical C uis

L devices. Avoiding feedback re- quires the use of optical diodes, Any device that amplifies a also known as isolators, which signal inevitably adds noise, and support the propagation of pho- quantum mechanics prevents tons in one direction while pro- this added noise being reduced hibiting propagation in the op- below a certain value. It is pos- posite direction. Traditional sible to approach this quantum solid-state optical isolators in- limit by using superconducting clude Faraday rotators, which devices to amplify electrical sig- provide isolation by rotating the nals, but these devices are com- polarization of a light beam. plex. Now Francesco Massel and Now, Jung-Tsung Shen and col- co-workers at Aalto University leagues at Washington Univer- L ibrary oyal and the VTT Technical Research sity in St Louis, USA, have dem-

Centre of Finland have shown onstrated that near-complete op- R T urin , that nanomechanical resonators tical isolation can be achieved at Da Vinci’s self-portrait. can amplify microwave signals, the single-photon level by cou- Christophe Eloy of Aix-Mar- and that it may be possible to pling a quantum impurity to a seilles University in France reach the quantum limit with passive, linear waveguide that shows that the observation fol- this approach. has a locally planar, circular po- lows from two assumptions — The Finnish team starts by larization. Their single-photon first, that trees are fractal, or using lithography and focused optical diode operates on indi- self-similar in nature, and, ion-beam etching to define a me- vidual photons, thus enabling second, that their growth is chanical resonator and a micro- unidirectional propagation. Fur- determined by the need to wave cavity in a 150-nm-thick thermore, this configuration withstand wind stress on their layer of aluminum on a silica does not rely on the use of bulk branches. Eloy says that the surface. When a pump signal is nonlinear materials or quasi- most relevant property of wind fed into this system, energy is phase-matching and can be im- loads is the way it diverges to- transferred from the cavity to plemented in various types of wards the tips of branches. the resonator if the pump fre- waveguide. The researchers have Static loads from fruit, snow SUBSCRIBE >> tm BriefingS | Space & Physics 5

or ice would have a similar tire region is devoid of a strong effect. magnetic signal, including areas PARTICLE PHYSICS —Eugenie Samuel, Nature not affected by impacts. Rather than undergoing an Spot the ■■ Phys. Rev. Lett. doi: 10.1103/ impact-induced removal of the difference PhysRevLett.107.258101 (2011) signature, the entire South Prov- ince may have instead formed Neutrinos oscillate — PLANETARY SCIENCE during a period of weak mag- that is, each of the three types netic field strength on young of neutrino (electron, muon Mars Mars. and tau) can evolve into one of —Amy Whitchurch, the others, a behavior that is demagnetized Nature Geoscience captured in a matrix of mixing angles. This also brings the ■■ Icarus doi: 10.1016/ possibility of ‘CP violation’ in j.icarus.2011.10.023 (2011) the neutrino sector, comple- menting that seen in the QUANTUM PHYSICS mixing of quarks, which could be spotted through a detectable Guided matter difference in the oscillation Interfering laser beams probabilities for neutrinos and can reflect particles — a phe- antineutrinos. nomenon now demonstrated by Discrepancies between neu- Charlotte Fabre and colleagues, trino and antineutrino data who have created a matter have been noticed by some ex- mirror by crossing two infrared periments, notably MINOS and beams. MiniBooNE, which both use Optical interference creates a neutrino beams generated at periodic energy potential where Fermilab, in the United States. the two laser beams meet. By K. Abe and colleagues have

/NASA T elescope S pace H ubble launching a cloud of rubidium now searched data collected for A view of the southern hemisphere of Mars. atoms towards this mirror and atmospheric neutrinos over an then imaging the atoms after 11-year period by the Super-Ka- The South Province of Mars is they had interacted with the miokande apparatus, located in almost devoid of a magnetic sig- laser light, Fabre et al. were able the Mozumi mine under Mount nature. Analyses of crustal struc- to measure what fraction of the Kamioka in Japan. ture, combined with demagneti- incident cloud was reflected, and In Super-Kamiokande’s huge zation models, suggest that the how many atoms were trans- underground tank, lined with lack of a magnetic signal here mitted. The reflectivity of the photomultiplier tubes and may indicate a weak magnetic mirror could be controlled by in- filled with 50,000 tons of water, field early in Mars’s history. creasing the laser power. Abe et al. sought evidence of Jafar Arkani-Hamed of the Periodic mirrors are already the disappearance of muon University of Toronto and Daniel well known in optics, where they neutrinos and muon antineu- Boutin of McGill University, are called distributed Bragg re- trinos as they oscillated into Montreal, tested whether large flectors. Alternating layers of the other particle types. asteroid impacts on Mars may high- and low-refractive-index Through a challenging analysis have re-melted parts of the crust materials can trap photons in that allows for 120 sources of long after the magnetic field had cavities, to construct lasers, for systematic uncertainty, the au- died out, erasing the signature in example, or guide light propaga- thors conclude that, as yet, the the South Province. They com- tion, as in optical fibres. The de- mixing parameters for neu- bined analyses of crustal struc- velopment of distributed Bragg trinos and antineutrinos from ture with models of impact-in- reflectors for matter waves will atmospheric sources are en- duced demagnetization and aid the translation of these con- tirely consistent with each found that asteroid impacts in cepts from light to matter. other. the South Province would have —David Gevaux, —Alison Wright, been sufficient to erase any ex- Nature Physics Nature Physics isting magnetic signature around the impact basin. How- ■■ Phys. Rev. Lett. doi: 10.1103/ ■■ Phys. Rev. Lett. doi: 10.1103/ ever, they also show that the en- PhysRevLett.107.230401 (2011) PhysRevLett.107.241801 (2011) SUBSCRIBE >> tm BriefingS | Space & Physics 6

SENSORS QUANTUM OPTICS Photon counting Coherent photon conversion ommons

Entangled photons are key C elements of quantum information processing and communica-

tockphoto tions technology such as ikimedia M nol f/W quantum key distribution. Un- A conventional ion trap.

/ i S J onsson fortunately, current schemes for achieving entangled photon gen- trap both ions and cold atoms. H enrik Optical fiber. eration are inefficient because of The apparatus combines a linear the probabilistic processes in- radiofrequency Paul trap for ions Distributed temperature volved. Now, Nathan Langford with a six-laser-beam magneto- sensing can be useful for moni- and co-workers from Austria, optical trap for atoms. Spatial toring structures such as build- UK, Canada and Japan have pro- overlap between the traps allows ings and pipelines. Michael posed a deterministic process the investigation of ion–atom in- Tanner and colleagues from Her- called coherent photon conver- teractions at low temperatures. iot-Watt University in the UK and sion that could be more suitable The researchers cooled the the National Institute of Stan- for practical applications. The atoms by laser Doppler cooling dards and Technology (NIST) in key principle of coherent photon using light from an amplified Colorado, USA, have now built a conversion is the use of classi- homemade external-cavity diode distributed fiber Raman sensor cally pumped nonlinearities to laser. They tested their design that can simultaneously measure induce coherent oscillations be- with 85Rb atoms and 85Rb+ ions, absolute temperature at over 100 tween different multi-excitation which were made by using two- 1.2-cm-spaced positions along a states. The resulting approach photon ionization to strip elec- single-mode optical fiber. Al- offers a new way to generate and trons from 85Rb atoms. The re- though distributed fiber sensors process quantum states for searchers say that the scheme have been around for a while, the quantum information processing also lends itself to any species researchers claim that previous tasks such as multiphoton entan- that can be laser-cooled in a research focused on using multi- glement and the realization of vapor cell, and that experiments mode fibers and avalanche photo- optically switched quantum cir- with multiple species may even diodes, which limited detection cuits. The researchers demon- be possible. wavelengths to around 800–900 strated their approach by using a —Oliver Graydon, nm. In contrast, this latest work four-color nonlinear process in a Nature Photonics uses pulses of 1,550 nm light and standard commercial, polariza- measures the single-photon level tion-maintaining photonic ■■ Appl. Phys. B doi: 10.1007/s00340-011- Raman backscattered signal crystal fiber with a 532 nm 4726-6 (2011) using superconducting nanowire pulsed laser and a 710 nm diode single-photon detectors. The re- laser. PLASMONICS searchers used a time-of-flight ap- —Noriaki Horiuchi, proach to determine the tempera- Nature Photonics Optical black ture profile along the fiber. They recorded temperature measure- ■■ Nature doi: 10.1038/nature10463 (2011) hole ments at various positions along Although the use of transfor- the fiber with an uncertainty of TRAPPING mation optics and metamaterials less than 3 K over a period of 1 to create optical analogues of minute. They hope that a 1-km- A marriage of black holes has already been sug- range distributed sensor will be gested, experimental realization possible in the near future. atoms and ions of the required material struc- —David Pile, Scientists from the Raman tures remains a significant Nature Photonics Research Institute in India and hurdle. Khachatur Nerkararyan the Johannes Gutenberg Univer- and co-workers from Yerevan ■■ Appl. Phys. Lett. doi: sity in Germany have unveiled a State University in Armenia and 10.1063/1.3656702 (2011) device that can simultaneously the University of Southern Den- SUBSCRIBE >> tm BriefingS | Space & Physics 7

mark have now suggested that spherical metal surfaces brought SUPER-RESOLUTION IMAGING into contact may provide an alternative approach for investi- Acoustic help gating optical black holes. The The resolving capabilities of researchers showed that plas- far-field optical imaging schemes mons with an appropriate trajec- are limited by their inability to tory in the gap between two collect rapidly decaying evanes- metal spheres (or between a cent waves, which contain de- sphere and a planar surface) can tailed spatial information. To be concentrated into a “black overcome this restriction, Leonid hole” around the point of struc- Alekseyev, Evgenii Narimanov tural contact. The team used an and Jacob Khurgin have now pro- tockphoto

effective-index approximation to / i S posed a time-multiplexed ap- simplify the problem, thereby re- proach for recovering evanescent ducing the three-dimensional tephens waves in the far-field. In their S acom problem down to two dimen- J scheme, the object to be imaged is sions. In one example, they con- Electron microscope. placed in the near-field of an sidered a 100-μm-radius gold tistically significant sample of a acousto-optic modulator and illu- sphere in contact with a planar system that contains nanoparti- minated with a plane wave from a surface and 800-nm-wavelength cles with a variety of different light source. Waves scattered from light. The effect relies on the fact compositions, such as a hetero- the object strike a phonon grating that reducing the gap width in- geneous catalyst, is impractical. formed by the acousto-optic mod- creases the effective permittivity David Muller, Zhongyi Liu and ulator, thereby causing the eva- experienced by the gap plas- colleagues at Cornell University, nescent components to be shifted mons, thanks to an increasing General Motors and Florida In- in both frequency and transverse Coulomb attraction across the ternational University have now wave vector. The researchers dis- gap. This increased effective per- shown that the improved elec- covered that for a sufficiently mittivity draws the light towards tron optics of an aberration-cor- large phonon wave vector, the ev- the point of contact, which rected electron microscope can anescent components with high therefore acts as an optical black allow hundreds of platinum–co- spatial frequency can be con- hole. The researchers hope that balt nanoparticles to be chemi- verted into propagating waves, this configuration can be real- cally mapped. which can then be collected and ized in practice to allow the The US team used a scanning imaged. Super-resolved finger- table-top exploration of phe- transmission electron micro- printing and digital holography nomena such as Hawking scope that can correct up to the are suggested as two example ap- radiation. fifth-order of aberrations and al- plications of this scheme. Simula- —David Pile, lows data to be collected around tions show that mixing the fre- Nature Photonics a thousand times faster than on quency-shifted fields with a refer- a conventional microscope. With ence wave can create a ■■ Opt. Lett. doi: 10.1364/ the instrument, the platinum– high-spatial-frequency beat note OL.36.004311 (2011) cobalt nanoparticles — which photocurrent at the detector, and are promising as a fuel-cell cata- that true super-resolved imaging ELECTRON MICROSCOPY lyst but are known to degrade can be achieved when the refer- over time — were mapped at var- ence signal is Bragg-shifted. Al- Mapping ious stages of ageing in a proton- though this scheme is particularly exchange-membrane fuel cell. By suitable for super-resolution im- ensembles mapping ensembles of nanopar- aging at infrared and terahertz Electron microscopy is ticles, the precise structure and wavelengths, the researchers say routinely used to characterize composition of the catalyst could that it might also be able to work the structure of metal nanoparti- be linked to its bulk electro- at optical frequencies by replacing cles, and with the help of elec- chemical performance with sta- the acousto-optic medium with a tron energy-loss spectroscopy, tistical confidence. moving nanostructured grating. chemical maps with atomic reso- —Owain Vaughan, —Rachel Won, lution can also be obtained. A Nature Nanotechnology Nature Photonics chemical map of a single particle can, however, take hours to re- ■■ Nano Lett. doi: 10.1021/ ■■ Opt. Express doi: 10.1364/ cord. Therefore, acquiring a sta- nl203975u (2011) OE.19.022350 (2011) SUBSCRIBE >> tm BriefingS | Space & Physics 8

mance of solar cells. One idea is ergy. Theoretical analysis of this SOLAR CELLS to split degenerate excited en- scheme suggests that the photo- ergy levels by exploiting tun- current and peak power of a pho- Fano-enhanced neling between two adjacent tovoltaic cell employing fully co- performance quantum dots. The two split en- herent Fano interference could ergy levels (known as an energy be improved by up to 50 percent. Quantum coherence be- level doublet) both couple to the The researchers also suggest the tween competing energy-level conduction state. Fano interfer- possibility to engineer the cell pathways is known to help opti- ence between the two pathways such that the Fano interference mize photosynthesis and enable minimizes unwanted radiative is robust against environmental phenomena such as lasing transitions and helps to maxi- decoherence. The challenge now without inversion. Researchers mize the generated photocur- is for the research community to from Texas A&M University and rent. An alternative approach is realize such a cell in practice. Princeton University in the USA to create an energy level doublet —Oliver Graydon, have now suggested that from two lower-energy levels Nature Photonics quantum coherence could also and use Fano interference to en- be used to enhance the perfor- hance the absorption of solar en- ■■ Phys. Rev. A doi: 10.1103/ PhysRevA.83.053818 (2011)

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