X-ray gun. This string of
magnets at the SLAC National Accelerator Laboratory’s Linac Coherent
Light Source controls a fast beam of electrons that generates an intense
x-ray beam. A theoretical proposal shows how individual x-ray photons
from such a source could be stored for 100 nanoseconds or more and then
be released at a later time with their quantum properties preserved.
In the field of photonics, researchers dream of performing all manner
of electronics operations with photons instead of electrons, but so far
they have only used visible and infrared light. Building on an earlier
experiment that showed how x rays could be briefly stored in nuclear
excitations, a team writing in Physical Review Letters
now proposes a technique that would store a single x-ray photon and
allow it to be released on demand with its quantum properties unchanged.
The work is a first step toward x-ray photonic systems, which could
exploit the shorter wavelengths of x rays to pack more active elements
into a given space.
In 1996, a team led by Yuri Shvyd’ko, then at the
University of Hamburg, Germany, succeeded in delaying the decay of an
excited nuclear state of iron-57 with an energy of 14.4
kilo-electron-volts (keV) [1].
The team fired a short pulse from a polarized beam of 14.4-keV x rays
at an iron-57 target in the presence of a magnetic field perpendicular
to the beam. The field splits the nuclear ground state into two levels
with slightly different energies and splits the excited state into four
levels. The geometry of the experiment allowed a transition from each
ground state only to a specific upper state, so that just two of the
four excited levels were populated. The resulting nuclear excitation was
a quantum combination (superposition) of these two states, and because
of their different magnetic properties, it turns out that the
probability for the nucleus to decay back to the ground state oscillates
in time.
A few nanoseconds after the excitation, the team turned
on a second magnetic field, perpendicular to the first. By changing the
orientation of the magnetic field, this second field, in effect, mixed
up the identity of the upper states, so that the excitation was spread
among all four of them. Shvyd’ko and his colleagues showed that this
procedure, when done at the right time in the oscillation cycle of the
excitation, transformed it into a state that could not easily decay.
Turning off the second field allowed the nucleus to decay and emit an
x-ray photon.
In this experiment, the photons that emerged after being
“stored” had the same energy as the original photons, but other quantum
properties were not preserved. To overcome that deficiency, and to allow
single-photon manipulation, Adriana Pálffy and her colleagues at the
Max Planck Institute for Nuclear Physics in Heidelberg, Germany, suggest
a variation on the technique. As before, they use polarized x-ray
photons to excite iron-57 nuclei immersed in a magnetic field. By
adjusting the intensity of the beam and the concentration of iron-57
atoms, it can be arranged that most of the time, just a single nuclear
excitation will be created by the x-ray pulse.
Instead of adding a second field, the team proposes
turning off the first altogether at a specific moment in the oscillation
of the excited state, about 10 nanoseconds after the x-ray pulse. The
team’s calculations show that the excitation would then be “frozen” in a
quantum state that does not allow decay by the usual route. Reapplying
the magnetic field after an arbitrary time would allow the excitation to
decay and emit a photon identical in all of its quantum
properties—energy, polarization, and phase—to the photon that created
the excitation. It should be possible to store an x-ray photon for 100
nanoseconds or more with its full quantum properties intact, according
to Pálffy.
The new proposal also offers a bonus: if the magnetic
field is reapplied with opposite polarity, the phase of the emitted
photon is exactly reversed. Although this is a modest capability
compared to what can be done with visible photons, Pálffy and her
colleagues argue that their proposed technique is a first step toward
photonics systems operating at much shorter wavelengths.
Gennady Smirnov of the Kurchatov Institute in Moscow says
that the proposed storage method certainly preserves polarization and
phase, but he has some doubts about its practicality, particularly in
the need to switch a magnetic field off and on so quickly. But Norman
Sherman, of the National Research Council of Canada in Ottawa, says the
proposed method “opens the door to coherent quantum optics for x rays,”
which might be used for future quantum computers.
–David Lindley
David Lindley is a freelance writer in Alexandria, Virginia, and author of Uncertainty: Einstein, Heisenberg, Bohr and the Struggle for the Soul of Science (Doubleday, 2007).
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