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The core piece of the X-ray interferometer, made
from a silicon single crystal, furnishes the lattice parameter of
silicon as an utmost precisely determined length scale.
Image by PTB
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Up to now, a further spreading of this method
had, however, been impeded by the low translation velocities of only 1
nm/s to 10 nm/s. They are due to the limited intensity of typical
laboratory X-ray sources: the necessary filtering of the periodic
interference signal leads to a reduction in contrast which, in a
classic measurement, requires a slow translation of the interferometer.
In a quantum-mechanical sense, however, interference occurs also in a
strongly "diluted" stream of X-ray photons: Regarded as a wave packet,
even single photons follow in their temporal impact on the detector
the same probability which, in the case of sufficiently intense X-ray
light, leads to the continuous signal whose period one wants to
determine. This well-known quantum-mechanical fact is now exploited
for a specific purpose: if one protocols the times at which the single
photons hit the detector, one can, by means of a subsequent Fourier
transform of this time series, determine very precisely the frequency
at which the lattice periods were passed. At constant velocity, it is
then possible to reconstruct the path information, and one obtains the
same information as with the classic measurement, but in a much
shorter amount of time.
Thus, translation velocities of up to 1000 nm/s could be realised.
This method will in future not only be used in further improved
measuring arrangements for the determination of the lattice parameter
of silicon, but also for other length measurements in nanotechnology.
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