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A
quantum computer based on quantum particles instead of classical bits,
can in principle outperform any classical computer. However, it still
remains an open question, how fast and how efficient quantum computers
really may be able to work. A critical limitation will be given by the
velocity with which a quantum signal can spread within a processing
unit. For the first time, a group of physicists from the Quantum
Many-Body Systems division at the Max-Planck Institute of Quantum Optics
(Garching near Munich) in close collaboration with theoretical
physicists from the University of Geneva (Switzerland) has succeeded in
observing such a process in a solid-state like system.
The
physicists generated a perfectly ordered lattice of rubidium atoms and
then induced a quantum excitation—an “entangled” pair of a doubly
occupied lattice site next to a hole. With the aid of a microscope they
observed how this signal moved from lattice site to lattice site. “This
measurement gives insight into very elementary processes involved in the
communication and processing of quantum information,” Professor
Immanuel Bloch points out.
The
communication and processing of information in a quantum computer is
based on concepts that are inherently different from those used in
classical computers. This is due to the fundamental differences between
quantum particles and classical objects. Whereas the latter are, for
example, either black or white, quantum particles can take on both
colours at the same time. It is only at the process of measurement that
the particles decide on one of the two possible properties. As a
consequence of this peculiar behaviour, two quantum objects can form one
entangled state in which their properties are strictly connected, i.e.
quantum correlated. At present there is no general model for predicting
how fast a quantum correlation can travel after it is generated.
Now
physicists from the Quantum Many-Body Systems division have been able
to directly observe such a process. They start the experiment by
generating an extremely cold gas of rubidium atoms. The ensemble is then
kept in a light field which divides it into several parallel
one-dimensional tubes. Now the tubes are superimposed with yet another
light field, a standing laser light wave. By the periodic sequence of
dark and bright areas, the atoms are forced to form a lattice structure:
exactly one atom is trapped in each bright spot, and is separated from
the neighboring atom by a dark area which acts as a barrier.
Changing
the intensity of the laser light controls the height of this barrier.
At the beginning of the experiments, it is set to a value that prevents
the atoms from moving to a neighboring site. Then, in a very short
time, the height of the barrier is lowered such that the system gets out
of equilibrium and local excitations arise: Under the new conditions
one or the other atom is allowed to “tunnel” through the barrier and
reach its neighboring site. If this happens, entangled pairs are
generated, each consisting of a doubly occupied site, a so-called
doublon, and a hole, named holon. According to a model developed by
theoretical physicists from the University of Geneva around Professor
Corinna Kollath, both doublon and holon move through the system – in
opposite directions—as if they were real particles (see figure).
“Regarding
one entangled pair, it is not defined whether the doublon sits on the
right or on the left side of the holon. Both constellations are present
at the same time,” Dr. Marc Cheneau, a scientist in the Quantum
Many-Body Systems division, explains. “However, once I observe a doubly
occupied or an empty site, I exactly know where to find its counterpart.
This is the correlation we are talking about.”
Now
the scientists observe how the correlations are carried through the
system. Using a new microscopic technique, they can directly image the
single atoms on their lattice sites. In simplified terms, they make a
series of snapshots each showing the position of the doublons and the
holons at that very moment. The propagation velocity of this correlation
can be deduced from the distance the two partners have moved apart in a
certain period of time. The experimental results are in very good
agreement with the predictions of the model mentioned above.
“As
long as quantum information is communicated with light quanta, we know,
that this is done with the speed of light,” Dr. Cheneau points out.
“If, however, quantum bits or quantum registers are based on solid-state
structures, things are different. Here the quantum correlation has to
be passed on directly from bit to bit. Once we know how fast this
process can happen, we have the key to understand, what will limit the
velocity of future quantum computers.”
Light-cone-like spreading of correlations in a quantum many-body system
