Writing Words on Atoms
This computer simulation shows the electron cloud of a hydrogen atom
sculpted to read the word “optics”. Predicted and theoretically studied
over the last decade, the first step towards realization has now been made
by Jake Bromage and C.R. Stroud Jr. at the University of Rochester in the
United States1. They “wrote” on an atom via the controlled excitation of
its electrons. Applications may range from Quantum Computing to a
controlled manipulation of chemical reactions.
To catch the electron
The history of the elementary particles has been written and rewritten during the last
centuries. Where Bohr at the beginning of this century proposed an electron as a
particle orbiting around the nucleus, DeBroglie in the 1920s postulated that every
particle should have associated with it a wavelength according to the formula =h/mv
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(h is the so-called Planck’s constant and m and v the mass and velocity of the
electron, respectively). On this basis Schrodinger founded the modern Quantum
Mechanics with his wave equation. The wave properties of the electron have been
confirmed by diffraction experiments, and since that time the dual nature of matter is
accepted among scientists.
The apparatus used by Bromage and Stroud
Strange electronic states
Usually scientists look at an atom in its “ground state”, where all electrons occupy the
lowest energy levels – because of attractive forces between nuclei and electrons these
are the ones closest to the nuclei.
Electromagnetic waves (e.g. laser light) can transfer electrons to “excited states” with
higher energy. Where the diameter of a hydrogen atom is of the order of 10-10 m these
excited atoms are of the size of 10-6 m – this is like blowing a cherry to the size of a
three-storied house. In these “Rydberg atoms” the electrons are no longer a charge
cloud enshrouding the nucleus but a “wave packet” that circles within a finite spatial
area around the nucleus –similar to the classical picture of the electrons as particles.
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A picture of the electron of a Rydberg atom,
as Bromage and Stroud measured it –
somewhere between particle and wave.
The observation – between particle and wave
Waves behave additionally – two maxima amplify, a minimum and a maximum
cancel each other (everyone knows this behaviour from water waves). If you excite
electrons with laser pulses you can create a set of wave packets where every single
packet is moving – but they are moving correlatively so that the sum of all amplitudes
is constant at a certain point. The resulting wave stands still – demonstrated in the
picture above.
A.B.: Prof. Stroud, this experiment has a long history – five years ago it was proposed
in “Nature”. Why did it take so long to realize it?
C.R.S.: There were two principal hurdles to overcome. First, we needed an
electromagnetic pulse one picosecond [i.e. 0.000 000 000 001 s] in duration that did
not oscillate, but simply turned on and off while pointed in one direction in space. The
technology for generating such “half-cycle” pulses existed, but we had to set it up in
our laboratory, and develop techniques for measuring the shape of the pulse
accurately. Secondly, five years ago there were no techniques for measuring the shape
of the wave packet to be sure that we had produced what we actually wanted.
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A.B.: In your article you mentioned that this “offers a launching point for further
research in quantum control”. So there may result applications from this experiment?
C.R.S.: Our original impetus for research in this area was to investigate the question:
Can a single atom be prepared in a state in which it appears to be completely
classical? We have answered this question in a negative, but along the way we
developed techniques for controlling atoms that may well turn out to be useful for
manipulating chemical reactions, producing large optical nonlinearities, or perhaps
even storing and manipulating information.
A.B.: What will be your next steps in this area?
C.R.S.: We are developing techniques for moving our three-dimensionally localized
electron wave packet around arbitrary paths in space. We are also attempting to
develop techniques for conveniently and accurately measuring the shape of an
arbitrary electron wave packet.
A.B.: Finally – what do you think will be the groundbreaking developments in
physics during the next decade?
C.R.S.: Prognostication is always a perilous pursuit, but I am excited about the
surprising features Einstein and Schrodinger were never able to accept and the field of
quantum optics spent the last 30 years studying. This field may become so well
controlled and understood that it becomes a field of engineering. There is real reason
to believe that such a field of “quantum-optical engineering” would allow another
revolution in computing and communication just as dramatic as the one we are
currently living through.
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A.B.: Thank you for this interview, Prof. Stroud.
1
Jake Bromage and C. R. Stroud, Jr. Physical Review Letters 1999 (83) 4963,
a good review can be found in the April issue of Optics and Photonic News, p. 35-38
(779 words with endnotes, without descriptions of figures)
Written by Andreas Bender, bender@jyi.org
Dublin, Wednesday, 01 March 2000
This document is available for download as a Word-Document at
http://www.andreasbender.de
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