What two frames buy
At the SQS instrument of the European XFEL, two undulator sections with a total length of 120 metres were tuned to about 1.0 and 1.2 kiloelectronvolts, and the second X-ray pulse was emitted tens to hundreds of femtoseconds after the first. Both pulses reached a free-flying, xenon-doped helium nanodroplet, and the scattered fields overlapped on the same detector. The reason is plain: light detectors cannot be triggered or read out quickly enough to capture two flashes this close together, so the sum of the two signals is recorded in a single diffraction image.[1]
Until now a process was followed by observing different samples at different delays and lining the images up afterwards. That works when the samples are identical, and free-flying particles differ from one another in size, shape, orientation and internal structure, so every frame of such a sequence belongs to a different object. Two views of one droplet remove that substitution. The opposing reading deserves naming too: successful reconstructions were achieved for a subset of optimal diffraction patterns rather than for all of them, so the gain may stay narrower than it sounds.[1]
What the instrument does not yet show
The two images the method returns show xenon-doped helium nanodroplets with 20 nm spatial resolution, taken at delays of 50 and 750 fs. The consistency of the structures observed in both images is treated as evidence that, under these illumination conditions, significant structural damage only occurs at longer timescales. The same sentence can be read the other way: the first two-frame sequence of a nanoparticle mostly shows a nanoparticle doing very little. That is usually the honest shape of a method demonstration.[1]
That stage is normal for an instrument, and what it really decides is who can use it. The method was also tested on pairs of silver nanoparticles intercepted by the same light pulse, where the imaging problem is mathematically the same and the light is more plentiful. That testbed is not affected by the limited pulse energies of XFELs in two-color operation mode, and for exactly that reason it shows where the constraint sits: in the machine itself. The same shape held for the connectome that made a brain the size of a poppy seed readable end to end; the gain was real, and so were the single specimen and the incomplete completion rate.[1], [2]
Earning a word
Alessandro Colombo's phrasing carries its own hedge: if by movie one means multiple frames of the same object, these are the fastest nanoscale movies ever recorded. Two frames is where a movie starts. The distance from here to a sequence is measured in countable things: how many frames one pattern can be separated into, how many delays fit between them, how ordinary a successful reconstruction becomes. The first thing to watch is a number too, in a later study: whether a reconstruction made from patterns that were not preselected gets published.[1]
Most images we have at this scale have been single still images retrieved from a single diffraction image, with motion assembled from averages through pump-probe schemes across different samples. That average has taught a great deal and goes on teaching. But the first time two of those stills belong to the same object, the average stops being the only way to see. For someone reading the news tonight, the change is small and specific: objects at this scale can now be asked about one at a time, inside their own clocks.[1]