Gamma-ray shaping could lead to 'nuclear' quantum computers
A way of modulating the waveforms of
individual, coherent high-energy photons at room temperature has been
demonstrated by researchers in the US and Russia. The advance opens the
way for new quantum-optics technologies capable of extremely
high-precision measurements, as well as the possibility of
quantum-information systems based on nuclear processes. The new approach
could also be useful for those doing fundamental research in a variety
of areas, ranging from the role of quantum phenomena in biological
processes to fundamental questions in quantum optics itself.
The technique was developed by Olga Kocharovskaya,
Farit Vagizov and colleagues at Texas A&M University and the Kazan
Federal University. Their set-up bears some similarity to a Mössbauer
spectroscopy experiment. A sample of radioactive cobalt-57 decays to an
excited state of iron-57, which then decays by emitting a 14.4 keV
"soft" gamma-ray photon. This photon can then be absorbed and re-emitted
by a nearby stainless-steel foil containing iron-57. Because of the
Mössbauer effect, no energy is lost in the recoil of the stainless-steel
lattice and the photon is emitted at 14.4 keV with very little spectral
blurring.
As the foil absorbs and re-emits the photons, it is vibrated at
megahertz frequencies. By making clever use of the Doppler effect, the
team is able to shape a single photon into a double pulse and even a
train of ultrashort pulses. This makes it possible to use the gamma-ray
photons to encode quantum information in a "time-bin qubit" – quantum
bits in which information is encoded in terms of the relative arrival
time of pulses.
Easier to detect
The new method offers advantages over schemes for encoding quantum
information in lower-energy optical or microwave photons. Photons in the
10–100 keV energy range – soft gamma rays from nuclear transitions and
"hard" X-rays from atomic transitions – penetrate deeper into materials
and can be detected with greater efficiency. And in the case of the
Mössbauer photons, the recoil-less nature of the effect means that the
re-emitted photons retain their quantum coherence at room temperature.
Recent developments towards the generation of entangled gamma-photon
pairs, combined with the team's new method, could lead to
quantum-computing applications that use nuclei as qubits. "Entangled
nuclear ensembles may be produced via resonant interactions with the
entangled photons, using quantum-memory protocols," explains
Kocharovskaya.
The new technique gives physicists a more versatile way of controlling
gamma rays than current methods of manipulating high-energy photons. "In
our approach, we have more parameters to manipulate," Kocharovskaya
says. Indeed, the team can alter the amplitude, frequency and phase of
the radiation and change the nature and temporal profile of the
modulating wave. Arbitrary waveforms can be created "on demand" by
changing the absorber depth and frequency of resonance. "Besides, our
method allows one to manipulate single gamma-photons, which is
impossible with the existing nonlinear techniques," she says.
Kocharovskaya and colleagues are now working to improve their technique
as well as pursuing a number of potential applications.
Deterministic pulse shaping
"This study opens for the first time possibilities for deterministic
pulse shaping in the X-ray regime," says X-ray optics expert Ralf Röhlsberger
of DESY in Hamburg, Germany. "Unfortunately, the positive aspects come
with some drawbacks." He points out that photons in this energy range
are difficult to manipulate using conventional optical devices and that
photon loss through absorption and scattering will limit performance.
"These technologies, however, have nevertheless the potential to boost
our understanding of fundamental aspects of the light–matter
interactions," he says.
Christoph Keitel
of the Max Planck Institute for Nuclear Physics in Heidelberg, Germany,
agrees that the work of Kocharovskaya and colleagues is a significant
accomplishment. He points out that "this is a demanding field with
numerous serious challenges: the radiation sources are weak and often
relatively dirty, and the couplings to the nuclei are generally small.
This renders this absolutely clean and innovative way of coherently
adapting gamma waves so impressive."
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