References and Notes 1. A. Borst, J. Haag, D. F. Reiff, Annu. Rev. Neurosci. 33, 49–70 (2010). 2. M. S. Tu, M. H. Dickinson, J. Comp. Physiol. A. 178, 813–830 (1996).

3. F.-O. Lehmann, in Nature’s Versatile Engine: Insect Flight Muscle Inside and Out, J. O. Vigoreaux, Ed. (Landes Bioscience/Eurekah.com, Georgetown, TX, 2006), pp. 230–239. 4. M. A. Frye, M. H. Dickinson, Neuron 32, 385–388 (2001). 5. M. H. Dickinson, F.-O. Lehmann, S. P. Sane, Science 284, 1954–1960 (1999). 6. M. H. Dickinson, K. Götz, J. Exp. Biol. 174, 45–64 (1993). 7. L. Ristroph, G. J. Berman, A. J. Bergou, Z. J. Wang, I. Cohen, J. Exp. Biol. 212, 1324–1335 (2009). 8. G. Card, M. H. Dickinson, Curr. Biol. 18, 1300–1307 (2008). 9. S. A. Combes, D. E. Rundle, J. M. Iwasaki, J. D. Crall, J. Exp. Biol. 215, 903–913 (2012). 10. K. G. Götz, Kybernetik 77–92 (1964). 11. T. L. Hedrick, B. Cheng, X. Deng, Science 324, 252–255 (2009). 12. A. J. Bergou, L. Ristroph, J. Guckenheimer, I. Cohen, Z. J. Wang, Phys. Rev. Lett. 104, 148101 (2010). 13. Materials and methods are available as supplementary materials on Science Online. 14. M. B. Reiser, M. H. Dickinson, J. Neurosci. Methods 167, 127–139 (2008). 15. E. I. Fontaine, F. Zabala, M. H. Dickinson, J. W. Burdick, J. Exp. Biol. 212, 1307–1323 (2009). 16. S. E. J. de Vries, T. R. Clandinin, Curr. Biol. 22, 353–362 (2012). 17. J. A. Bender, M. H. Dickinson, J. Exp. Biol. 209, 3170–3182 (2006). 18. M. B. Reiser, M. H. Dickinson, J. Exp. Biol. 216, 719–732 (2013). 19. H. Fotowat, F. Gabbiani, Annu. Rev. Neurosci. 34, 1–19 (2011). 20. C. David, Physiol. Entomol. 3, 191–195 (1978). 21. K. G. Götz, U. Wandel, Biol. Cybern. 51, 135–139 (1984). 22. S. Vogel, J. Exp. Biol. 44, 567–578 (1966). 23. S. N. Fry, R. Sayaman, M. H. Dickinson, Science 300, 495–498 (2003). 24. P. Domenici, J. M. Blagburn, J. P. Bacon, J. Exp. Biol. 214, 2463–2473 (2011). 25. M. J. Elzinga, W. B. Dickson, M. H. Dickinson, J. R. Soc. Interface 9, 1685–1696 (2012).

26. J. A. Bender, M. H. Dickinson, J. Exp. Biol. 209, 4597–4606 (2006). 27. L. Ristroph et al., Proc. Natl. Acad. Sci. U.S.A. 107, 4820–4824 (2010). 28. A. Sherman, M. H. Dickinson, J. Exp. Biol. 206, 295–302 (2003). 29. W. B. Dickson, P. Polidoro, M. M. Tanner, M. H. Dickinson, J. Exp. Biol. 213, 3047–3061 (2010). 30. B. Cheng, X. Deng, IEEE Trans. Robot. 27, 849–864 (2011). 31. F.-O. Lehmann, M. H. Dickinson, J. Exp. Biol. 200, 1133–1143 (1997). 32. M. H. Dickinson, Philos. Trans. R. Soc. Lond. B Biol. Sci. 354, 903–916 (1999). 33. L. F. Tammero, M. H. Dickinson, J. Exp. Biol. 205, 327–343 (2002). 34. F. van Breugel, M. H. M. Dickinson, J. Exp. Biol. 215, 1783–1798 (2012). Acknowledgments: This work was supported by grants from the Air Force Office of Scientific Research (FA9550-10-1-0368) to M.H.D., the Paul G. Allen Family Foundation to M.H.D., Army Research Laboratory (DAAD 19-03-D-0004) to M.H.D., Swedish Research Council to F.T.M., and the Royal Physiographical Society in Lund to F.T.M. We thank S. Safarik, X. Zabala, and J. Liu for their technical support, and B. van Oudheusden for co-supervising J.M.M. The data reported in this paper are tabulated in the supplementary materials: The body and wing kinematics data for all reported flight sequences, as well as forces and torques from the robotic fly experiments, are stored in Database S1, and the Fourier series coefficients required to reconstruct the here analyzed wingbeat kinematics (using eq. S1) are available in table S1.

Supplementary Materials www.sciencemag.org/content/344/6180/172/suppl/DC1 Materials and Methods Figs. S1 to S6 Table S1 Movies S1 to S11 Database S1 References (35–37) 25 November 2013; accepted 11 March 2014 10.1126/science.1248955

REPORTS Ultrafast Switching to a Stable Hidden Quantum State in an Electronic Crystal L. Stojchevska,1,2 I. Vaskivskyi,1 T. Mertelj,1 P. Kusar,1 D. Svetin,1 S. Brazovskii,3,4 D. Mihailovic1,2,5* Hidden states of matter may be created if a system out of equilibrium follows a trajectory to a state that is inaccessible or does not exist under normal equilibrium conditions. We found such a hidden (H) electronic state in a layered dichalcogenide crystal of 1T-TaS2 (the trigonal phase of tantalum disulfide) reached as a result of a quench caused by a single 35-femtosecond laser pulse. In comparison to other states of the system, the H state exhibits a large drop of electrical resistance, strongly modified single-particle and collective-mode spectra, and a marked change of optical reflectivity. The H state is stable until a laser pulse, electrical current, or thermal erase procedure is applied, causing it to revert to the thermodynamic ground state. n condensed matter systems, laser photoexcitation may temporarily destroy groundstate ordering; the system typically reverts to the ground state in a few picoseconds, unless it passes though a transient metastable state. Such metastable states have been shown to persist on time scales between 10−9 and 10−3 s

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(1–9) before returning to the ground state by a combination of thermal, electronic, and lattice relaxation processes (2). Stability of photoinduced states has been demonstrated in a manganite (6) and in chalcogenide glasses (10), where switching occurs between neighboring equilibrium thermodynamic states. Here, we report on bi-

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stable switching to a hidden (H), spontaneously ordered macroscopic quantum state whose properties are distinct from those of any other state in the equilibrium phase diagram. The hidden state transition (HST) occurs in a layered quasi–twodimensional chalcogenide 1T-TaS2 crystal, which exhibits multiple competing ground states under equilibrium conditions. Near Tc0 = 550 K, 1T-TaS2 forms an incommensurate (IC) charge density wave (CDW) with an associated lattice distortion. Upon cooling, these modulations sharpen to form star-shaped polaron clusters (Fig. 1A). Their ordering is thought to be responsible for a variety of phases, causing a transition to a nearly commensurate (NC) state for T < Tc1 = 350 K, and a hysteretic first-order transition to a gapped commensurate (C) phase near Tc2 = 183 K. Upon

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and downstroke (12). This discrepancy with previous studies might reflect an interesting difference in the control of pitch and roll compared with yaw. It is also noteworthy that stroke angle exerts a stronger influence over forces and moments than either wing rotation or stroke deviation. This finding is consistent with many previous studies in tethered flight which show that flies robustly modulate stroke amplitude in response to sensory signals that elicit changes in flight force (31), as well as roll, pitch, and yaw (28, 32). Our results indicate that flies escape from looming objects by exhibiting a rapid banked turn. The motor basis of these rapid maneuvers are quite distinct from those previously described in that the change in direction is generated by a combination of pitch and roll, requiring active torque and countertorque generated by a fine-scaled, coordinated change in all aspects of wing motion. The changes in heading during these maneuvers are roughly 5 times as fast (5300° s−1) as those measured during voluntary saccadic turns (1000° s−1) (33, 34), suggesting that this strategy provides the animals with the fastest possible means for altering direction. Using the genetic and physiological approaches available in the closely related species D. melanogaster, it should be possible to elucidate the neural circuitry and muscle physiology that underlies these rapid behaviors.

1 Department of Complex Matter, Jozef Stefan Institute, Jamova 39, Ljubljana SI-1000, Slovenia. 2Jozef Stefan International Postgraduate School, Jamova 39, Ljubljana SI-1000, Slovenia. 3 LPTMS-CNRS, UMR8626, Université Paris-Sud, F-91405 Orsay, France. 4International Institute of Physics, 59078-400 Natal, Rio Grande do Norte, Brazil. 5CENN Nanocenter, Jamova 39, Ljubljana SI-1000, Slovenia.

*Corresponding author. E-mail: [email protected]

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a function of UW; below threshold fluence, the resistivity, AM frequency, and reflectivity revert to the C state values after the W pulse. Close to threshold fluence, the AM shows bimodal behavior (fig. S5D), which we interpret as incomplete switching. We observed no intermediate shift of the AM in different samples, indicating distinct two-phase behavior. The H state spectrum is quite different from the NC state spectrum (Fig. 2, A and B) or the T state spectrum (14), indicating that it is not related to the equilibrium states. We emphasize some notable features of the HST: (i) After photoexcitation, the H state spontaneously orders below TH, as indicated by the narrowness of the AM peak and the fact that no partial frequency shift is observed even when incomplete switching is caused by near-threshold excitation. (ii) The switching occurs only with

short pulses, and the threshold increases with increasing tW (Fig. 2C). (Note that the threshold can no longer be achieved with tW > 4 ps at any UW that we tried.) (iii) The H state is stable until erased or heated above ~70 K. Note that TH has no special importance under equilibrium conditions and is relevant only for describing the transition from the H state to the C state. To understand these unusual phenomena, we first introduce a scenario for switching based on the current understanding of the electronic ordering in 1T-TaS2 (11, 15, 17, 19), and then describe a phenomenological model consistent with the data. The relevant electronic states of 1T-TaS2 in the C state that are within reach of our 1.5-eV laser photons are shown in Fig. 3C. They are formed predominantly from a single Ta d band, which is split into subbands by the formation of

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heating, the system develops a triclinic (T) stripelike ordered state around 223 K, which reverts to the NC state at T = 283 K (11). Further nearby equilibrium states are revealed upon application of external pressure (12) or doping (13), both of which make 1T-TaS2 superconducting. To induce the HST, we use a single sub–35-fs write (W) pulse from an amplified Ti-sapphire laser at 800 nm with energy UW ≈ 1 mJ/cm2. After a HST is induced at 1.5 K, the four-probe resistance r(T) drops approximately three orders of magnitude and remains in this state indefinitely (verified up to 1 week) at this temperature (Fig. 1B). Upon heating, r(T ) is approximately constant up to 60 K, whereupon it starts increasing; above TH ~ 100 K, it merges approximately with the virgin r(T ) curve corresponding to the C state. The current-voltage characteristics remain linear throughout. Empirically we found that the H state can be completely erased (E) by a train of 104 50-ps pulses, each with energy UE ≈ 1 mJ/cm2. Alternatively, Joule heating can be used for erasure by passing a current of ~0.1 mA through the device (14). In both cases, the system reverts to the C state. Stable switching can also be achieved at intermediate temperatures up to T ~ 70 K (14). The effect is entirely reversible from cycle to cycle and from sample to sample, irrespective of the sample growth batch, and there appears to be no limit on the number of W-E cycles that can be performed. [See supplementary materials for experimental details on thermal protocols, including aging effects (15), and a description of the laser lithography used to manufacture the contacts.] To gain insight into the microscopic nature of the hidden state, we investigated the singleparticle and collective excitations by pump-probe spectroscopy, with the pump and probe pulse energies kept low (