(July 13, 2012) LMU/MPQ
scientists have demonstrated a method of heralding entanglement between distant
stationary quantum systems without destroying this particular state. The
experimental set-up paves the way towards quantum communication over large
distances.
In contrast to classical
objects that are, for example, either black or white, quantum particles take on
both “colours” at the same time. It is at the very last moment, at the process
of measurement that the particles decide on one of the two possible properties.
This peculiar behaviour becomes even more surprising, when two quantum objects
form one entangled state in which their properties are strictly connected, i.e.
quantum correlated. When the state of one particle, e.g. the polarization state
of a photon, is determined in a measurement, we know immediately which one of
the two states will be observed in the partner particle, independent of the
distance between the particles. This seeming “action at a distance” is
incompatible with the classical communication of information which obeys the
rules of local causality. In 1964 the Irish physicist John Bell described this
incompatibility in the form of a mathematical inequality.
At present, theoretical and
experimental physicists think of quantum networks as made of stationary nodes
between which the quantum information is mediated by photons via optical
fibres. Quantum mechanical entanglement between the stationary quantum systems
plays a key role for large distance communication. However, due to the loss of
photons in the glass fibres the extension of such networks is limited. A
solution to this problem is offered by a so-called quantum repeater which
passes the entanglement on over a sequence of many small sections, thus
extending an entangled state over a long way. A team of scientists around Dr.
Wenjamin Rosenfeld (Max-Planck-Institute of Quantum Optics in Garching, MPQ),
Dr. Markus Weber (Ludwig-Maximilians-Universität München, LMU) and Prof. Harald
Weinfurter (LMU and MPQ) has now developed a fundamental component of such a
device. In their experiment two rubidium atoms, 20 metres apart, are entangled
in such a way that a signal is generated each time entanglement is achieved (Science,
6 July 2012; DOI:10.1126/science.1221856).