scientific correspondence extend back only to about 1982, are limited Network Daily Temperature and Precipita- index variability in eastern North America by cloud cover, and may not distinguish tion Data (CDIAC, Oak Ridge National (Fig. 2). between overstorey (the dominant trees in Laboratory). I calculated several indices for My results endorse empirical models as the forest) and understorey (shrubs and each year for all stations over their respec- useful partners for satellite-derived green- herbaceous plants on or near the forest tive periods of record. All indices showed wave measures. Studies of other regions, floor) phenology in forests2,5. considerable year-to-year variation, with- especially those with long-term warming Other biospheric phenomena have been out striking long-term trends, but with trends, can provide a context for evaluating assessed globally through strategic combina- significant shorter-period changes. For satellite measurements that indicate earlier tions of satellite and surface measurements3. example, from 1978 to 1990, first-leaf greenness onset over large areas. Energy The above limitations of remote-sensing spring index dates became earlier at a rate budget analyses coupled with surface and `phenology' highlight the complexity of the of about 1 day per year (trend adjusted satellite phenology (for example, examina 8 - green wave, and suggest the need for a simi- r2 0.43, -level 0.009; Fig. 2). tion of phenological effects on the lar strategy here. What the satellite senses The tendency of global phenology exchange of latent and sensible heat and what is observed on the ground are research to concentrate on satellite-based between the surface and the atmosphere) both integral parts of the green wave. Con- measurements makes this approach funda- should also prove rewarding in further ventional phenological data - carefully mentally incomplete. Satellite measurements understanding and monitoring spring selected according to species, and globally provide broad areal coverage but reveal only plant­climate interactions. distributed - should play a crucial role in one aspect of green-up. All measures - Mark D. Schwartz global green-wave research. As global-scale empirical models, native-species pheno- Department of Geography, phenological networks do not yet exist, logy, and appropriately calibrated satellite University of Wisconsin­Milwaukee, empirical green-wave models can partly fill indices - need to be understood and inter- Milwaukee, Wisconsin 53201-0413, USA the void. connected for maximum effectiveness5,6. e-mail: mds@uwm.edu Models that simulate phenology using Global-scale plant phenology networks 1. Moulin, S., Kergoat, L., Viovy, N. & Dedieu, G. J. Clim. 10, meteorological data offer several advan- should be established to strengthen this 1154­1170 (1997). tages, if based on appropriate plants10,11. research strategy. However, empirical mod- 2. Reed, B. C. et al. J. Veg. Sci. 5, 703­714 (1994). They allow reconstruction of green waves els offer a way to reconstruct past green 3. Sellers, P. J. et al. J. Clim. 9, 706­737 (1996). back through the period for which instru- waves, and allow regional comparisons. 4. White, M. A., Thornton, P. E. & Running, S. W. Glob. Biogeochem. Cycles 11, 217­234 (1997). mental records of weather are available, Although imperfect and of limited geo- 5. Schwartz, M. D. in Phenology of Seasonal Climates (eds Lieth, providing a context for short-interval satel- graphical application, these models provide H. & Schwartz, M. D.) 23­38 (Backhuys, Netherlands, 1997). lite data. Also, models serve as `anchor one of the few independent comparisons 6. Schwartz, M. D. Int. J. Biometeorol. 38, 18­22 (1994). points', binding together commonalities available for satellite measurements of plant 7. Running, S. W. & Hunt. E. R. in Scaling Physiological Processes, Leaf to Globe (eds Field, C. & Ehleringer, J.) 144­157 among records for native species in adjacent activity9. Connections between conven- (Academic, New York, 1993). biomes and remote-sensing observations. tional phenology and remotely sensed 8. Leopold, A. & Jones, E. Ecol. Monogr. 17, 81­122 (1947). Empirical models (spring indices) can greenness measures at the ecosystem level 9. Myneni, R. B., Keeling, C. D., Tucker, C. J., Asrar, G. & Nemani, R. R. Nature 386, 698­702 (1997). closely mimic actual plant first-leaf and are being confirmed, and further studies are 10.Schwartz, M. D. Phys. Geogr. 14, 536­550 (1993). first-bloom events, correlate with native- underway4. It is encouraging that a satellite- 11.Hopp, R. J. & Vittum, M. T. Organic Gard. Farm. 24, 127­129 species data, and reveal changes in lower- derived change in green-up9 (earlier by (1977). atmospheric processes at the ecosystem 8 3 days over the 1981­91 period) is con- 12.Schwartz, M. D. J. Clim. 9, 803­808 (1996). 13.Schwartz, M. D. Month. Weather Rev. 120, 2570­2578 (1992). scale5,6,10,12,13, illustrating their potential use sistent with the trend, over roughly the in longer-term studies (Fig. 1). same period, that I describe here (ten days As an example, I studied green-wave earlier during the 1980­90 period, inferred changes in eastern North America5 from from the 1978­90 regression line). But this A posteriori 1900 to 1995 (Fig. 2). Meteorological data short period appears unremarkable when teleportation were from the Historical Climatology compared with the overall first-leaf spring 10 10 The article by Bouwmeester et al.1 on experi- 8 8 mental quantum teleportation constitutes 6 6 an important advance in the burgeoning 4 als (days) 4 field of quantum information. The experi- 2 2 ment was motivated by the proposal of 0 0 ean date (days) Bennett et al.2 in which an unknown quan- m ­2 ­2 tum state is `teleported' by Alice to Bob. As ­4 1961­90 norm ­4 illustrated in Fig. 1, in the implementation ­6 ­6 of this procedure by Bouwmeester et al.1, an ­8 ­8 input quantum state is `disembodied' into eparture from ­10 ­10 D Lilac first leaf quantum and classical components, as in ­12 eparture from ­12 Spring index first leaf D the original protocol2. However, in contrast ­14 ­14 to the original scheme, Bouwmeester et al.'s 1962 1966 1970 1974 1978 1982 1986 1990 1994 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 procedure necessarily destroys the state at Year Year Bob's receiving terminal, so a `teleported' FFiigguurree 11 Comparison of annual departures from the FFiigguurree 22 Eastern North American5 departures from state can never emerge as a freely propagat- mean of spring index first-leaf dates (derived from the mean of spring index first-leaf dates, 1900­95, ing state for subsequent examination or an empirical model) and of Syringa chinensis `red with 1 standard error bars, and smoothed trend exploitation. In fact, teleportation is rothomagensis' (lilac) first-leaf dates (derived from produced by a nine-year, moving average, normal achieved only as a postdiction. actual data), 1962­94 (details of the network of the curve filter (dotted line) (details of the network of the Bouwmeester et al. used parametric 183 stations with relevant data, and methods, are 465 relevant stations, and methods, are available down-conversion from two sources (SI, SII available from the author). from the author). in Fig. 1) in an attempt to teleport the 840 NATURE | VOL 394 | 27 AUGUST 1998 Nature © Macmillan Publishers Ltd 1998 scientific correspondence (for example, by cascading conventional detectors) could provide an effective rem- edy. Appropriate selection could be imple- mented with a polarization-independent S quantum non-demolition measurement of the total photon number at Bob's end. Alternatively, pre-selection could be imple- Alice mented by enhancing the coupling betwee f2 f1 Classical 8n modes (2, 3) relative to modes (1, 4). Despite our comments, we believe that the experiment of Bouwmeester et al. is a information significant achievement in demonstrating the non-local structure of teleportation. Samuel L. Braunstein SEECS, University of Wales, 1 2 Bangor LL57 1UT, UK H. J. Kimble Norman Bridge Laboratory of Physics 12­33, SI SII d1 California Institute of Technology, Sources Pasadena, California 91125, USA 4 3 1. Bouwmeester, D. et al. Nature 390, 575­579 (1997). d2 2. Bennett, C. H. et al. Phys. Rev. Lett. 70, 1895­1899 (1993). p PBS Bouwmeester et al. reply - Braunstein and Bob Kimble observe correctly that, in the Inns- ? bruck experiment, one does not always observe a teleported photon conditioned on a coincidence recording at the Bell-state analyser. In their opinion, this affects the FFiigguurree 11 The teleportation set-up of ref. 1. PBS, polarizing beam splitter. fidelity of the experiment, but we believe, in contrast, that it has no significance, and polarization state of a single-photon wave- field 4 at detector p, which projects the field that when a teleported photon appears, it packet (in beam 1) from Alice's sending in beam 1 accordingly. Joint detection at has all the properties required by the tele- station to Bob's receiving station (in beam (f1, f2) then provides threefold coincidence portation protocol. These properties can 3). Statistics consistent with teleportation with p, yielding a statistical mixture for the never be achieved by "abandoning telepor- are obtained for events with a fourfold field 3 arriving at Bob's station. The fraction tation altogether and transmitting ran- coincidence (from detectors f1 and f2, d1 of the state in this mixture gives F. To domly selected polarization states" as or d2, and p). We ask whether the detec- the lowest order in the down-converter Braunstein and Kimble suggest. The fact tion of all four quanta is essential for tele- coupling strength, the Bouwmeester et al. that there will be events where no tele- portation in this scheme. To answer this scheme yields a 50:50 mixture of the vacu- ported photons are created merely affects question, we calculated the teleportation um state 0 and the desired state , with the efficiency of the experiment. This sug- fidelity, F, when the coincidence condi- F 1/2, so that there is never a physical state gests that the measure of fidelity used by tion is relaxed to exclude detection at Bob's with high teleportation fidelity. Indeed, Bob Braunstein and Kimble is unsuitable for station (d1, d2). could achieve this same fidelity, F 1/2, by our experiment. Under relaxed conditions, requiring abandoning teleportation altogether and During the detection of the teleported only threefold coincidence of detectors p transmitting randomly selected polariza- photons, no selection was performed based and (f1, f2), teleportation is achieved when tion states. Faced with this state of affairs, on the properties of these photons. There- the fields of beams 1 and 3 match with suf- the experiment of ref. 1 obtains a surrogate fore, no a posteriori measurement in the ficiently high fidelity. In the simplest for high fidelity by destructively recording usual sense as a selective measurement was approximation, type II parametric down- the field 3 at (d1, d2). performed. The detection of the teleported conversion of modes (i, j) generates We emphasize that the nature of the photon could have been avoided altogether wavepacket states as follows: mixture containing the vacuum state has if we had used a more expensive detector, p, definite physical implications, which can be that could distinguish between one- and A0 0 ij + A1 i,j + A2 i,j +..., (1) verified by more general measurements two-photon absorption. The inability of than photon counting (for example, by our teleportation experiment to perform where A0, A1 and A2 are the coefficients for quantum-state tomography). Moreover, the such refined detections does not, however, obtaining no (vacuum), one and two freedom of a potential consumer of the out- imply that "a teleported state can never down-converted pairs, respectively, and put from Bob's receiving station to select emerge as a freely propagating state...". (i, j) (1, 4) (2, 3). Of these terms, only alternative detection strategies means that Braunstein and Kimble do not, therefore, states corresponding to the second term are classical analogies fail. reveal a principal flaw in our teleportation selected by fourfold coincidence, as speci- To achieve conventional a priori telepor- procedure, but merely address a non-trivial fied by equations (2) and (3) of ref. 1. How- tation, the set-up in ref. 1 would have to be practical question. ever, anything less than complete modified to eliminate the vacuum from the D. Bouwmeester, J.-W. Pan, M. Daniell, destruction of the output 3 necessarily mixture. Because the vacuum appears when H. Weinfurter, M. Zukowski, A. Zeilinger leaves undesirable terms that reduce F. two pairs of (1, 4) photons are created, we Institut für Experimentalphysik, The initial input state to Alice's station, might seek to resolve one- and two-photon Universität, Innsbruck, Technikerstrasse 25, , is prepared by detecting the state in detection events at p. Upgraded detection A-6020 Innsbruck, Austria NATURE | VOL 394 | 27 AUGUST 1998 841 Nature © Macmillan Publishers Ltd 1998