NATURE|Vol 466|5 August 2010 NEWS & VIEWS Goulielmakis and colleagues1 characterized Figure 1 | The first attosecond probe the coherence, and thus the entanglement, of experiments. Goulielmakis et al.1 report a Kr+ and the lost electron. In their experiments, technique for observing electron motion in the intense, ultrashort pump pulse ensures real time. They irradiated krypton atoms (Kr) significant overlap of the two quantum states Kr+, 3d–1 with a ‘pump’ pulse of infrared light lasting a few femtoseconds, liberating electrons to of the removed electron that correlate with generate Kr+ ions in a superposition of two two different pathways in the ion’s subsystem states, 4p−1(J = 1/2) and 4p−1(J = 3/2), where J is (Fig. 1b), resulting in a low electron–ion entan- total angular momentum. Black arrows indicate glement, a high coherence of the hole’s wave the two ionization pathways. The authors then packet and high visibility of the interference Kr+, irradiated the ions with attosecond ‘probe’ pulses 4p–1(J=1/2) fringes. The ability to probe decoherence is a + of extreme-ultraviolet light, exciting them to a Kr , −1 very important aspect of the experiment. 4p–1(J=3/2) higher-energy 3d state; red and green arrows The authors’ experiment is reminiscent of a indicate the two possible excitation pathways. two-colour coherent-control scheme2. In such The complete system constitutes an entangled electron–ion pair. a, The different excitation schemes, population of a final state is controlled pathways taken by the ion to reach the 3d−1 by the relative phase between the two colours state may cause the liberated electrons to adopt of light needed to promote a system from two orthogonal quantum states. The spheres represent Kr intermediate states (J = 1/2, 3/2) to a final state. two states of the same electron, in which the red One might thus conclude that Goulielmakis sphere correlates with the J = 1/2 state of Kr+, and et al. could have made their measurements the green sphere correlates with the J = 3/2 state. without resorting to attosecond pulses — two The ‘hands’ on the spheres don’t touch, indicating ‘phase-locked’ colours, with controlled phase φ a that the states don’t overlap. b, In Goulielmakis between them, would have been enough. From and colleagues’ experiments, strong overlap of this perspective, the use of a time-delayed the two quantum states (indicated by the held hands of the spheres) lowers entanglement and attosecond probe can be viewed merely as a b allows the two possible excitation pathways of the convenient way to achieve this goal. Indeed, ion to interfere. By measuring the interference, both colours needed to promote the system to the authors tracked the motion of the hole (the −1 the final 3d state are naturally present in the absence of the liberated electron) in Kr+ in real ultrashort probe pulse used by Goulielmakis time, characterizing its coherence and the degree et al., and the relative phase between them of electron–ion entanglement. changes with the pump–probe delay. Are atto- second probe pulses really needed? direct time-domain measurements, such as in connected5 to the concept of charge-directed The answer is generally yes, if one deals with the experiment1, become indispensable — a reactivity6 — the idea that molecular bonds open systems. In the authors’ study, conducted two-colour coherent-control scheme operating break in places to which a hole has migrated. in the gas phase, decoherence arises only during with long pulses may not catch ultrafast changes This idea assumes coherence of the hole wave the preparation of the hole wave packet and does in electron coherence. packet when it is prepared. The technique of not evolve afterwards. But a notable strength Subfemtosecond hole migration across atto second transient absorption spectroscopy, of Goulielmakis and colleagues’ technique is many ångströms has been predicted to occur introduced by Goulielmakis and colleagues1, is that it can also be used in condensed phases in large molecules3,4. Such motion may have well suited to check this key assumption. (such as liquids and solids). Here, decoher- important implications5 for subsequent, femto- The coupling of hole motion to other elec- ence may quickly evolve during the time delay second-scale nuclear dynamics in these mol- tronic and vibrational modes in molecules, between the pump and the probe pulses, and so ecules. Thus, early-stage hole dynamics may be which is responsible for charge-directed NEUROANATOMY The labelled neurons could then be followed during development, From fin to forelimb crucially showing that they develop in situ rather than migrating to their The vertebrate invasion of land was cartilaginous fish such as sharks. final location. made possible in part by evolution Motor-neuron innervation in Baker and colleagues’ extension of the tetrapod forelimb from the tetrapods (forelimb) and fish here in a species called the plainfin of their study to lobe-finned fish pectoral fin. But what changes (pectoral fin) arises from the midshipman fish, attached to and cartilaginous fish provided occurred in neural control during spinal cord. But for ray- and lobe- its egg yolk), the authors also evidence that, in these groups too, this transition? finned fish, there is evidence that demonstrated that the motor pectoral-fin motor-neuron control Robert Baker and colleagues these nerves also originate in the neurons project from both the is exercised from the hindbrain have tackled this question using a hindbrain. In following up that hindbrain and the spinal cord. as well as the spinal cord. Overall, thorough application of comparative evidence, the authors looked at the Studies with transgenic zebrafish, the authors conclude that this neuroanatomy (L.-H. Ma et al. gross anatomy of the developing containing a fluorescently tagged dual contribution is the ancestral Nature Commun. 1, 49, doi:10.1038/ pectoral fin buds of various ray- enhancer that reports the activity condition in vertebrates. As to the ncomms1045; 2010). Their study finned fish. They found that they all of the developmental gene hoxb4a functional context, they speculate centred on the developmental have a similar organization of the in motor neurons, confirmed the that the advent of spinal-only motor biology of several species of ray- buds themselves, of the myotomes mapping of pectoral-fin neurons. innervation of the forelimb allowed finned fish, which are by far the that give rise to muscles, and of the Further work involved injection another notable characteristic of largest group of extant fish. But it neuroepithelium that generates of the messenger RNA for a tetrapods compared with fish — their also included lobe-finned fish (a pectoral motor neurons. Using photoactive fluorescent protein, greater freedom of head movement. lineage that led to tetrapods) and dye-labelled fin buds (pictured kaede, into zebrafish embryos. Katie Ridd 701 © 2010 Macmillan Publishers Limited. All rights reserved.