This is a graphic representation of remotely prepared quantum states. In their experiment, the researchers demonstrated that it is possible to prepare quantum states remotely by means of quantum discord (outside the circle) which might not always be possible to achieve by means of entanglement (inside the circle). Credit: University of Vienna

A
fundamental characteristic of quantum physics is the fact that two or
more particles can exhibit correlations stronger than classically
allowed. This unique characteristic applies particularly to quantum
entanglement: as soon as the quantum state of a particle is measured the
state of its entangled partner changes accordingly, regardless of how
far apart the two entangled particles might be. This feature allows for
the remote quantum state preparation, which is an essential ingredient
for applications in quantum communication, quantum cryptography, and
quantum computation.

   

The
degree of entanglement is often used as a figure of merit for
determining its usefulness for quantum technologies. Strongly entangled
systems, however, are very sensitive to extrinsic influence and
difficult to prepare and to control. A team of researchers headed by the
physicists Caslav Brukner (theory) and Philip Walther (experiment) at
the University of Vienna have been able to show that in order to achieve
successful remote state preparation entanglement is not the only way
forward. Under certain circumstances, non-entangled states can
outperform their entangled counterparts for such tasks – as long as they
have a significant amount of so-called “quantum discord”. This novel
and not yet fully understood measure of quantum correlations quantifies
the disturbance of correlated particles when being measured.

   

In their experiments, the researchers used a variety of two-photon states with different polarization correlations.

“By
measuring the polarization state of a certain photon we prepare the
state of the respective partner photon remotely,” explains Walther. “In
the experiment we observe how the quality of our remotely prepared
quantum state is affected by changes in the quantum discord.”

This
work provides an important and significant step towards future quantum
information processing schemes that would rely on less demanding
resources.  

The
research was undertaken as a collaboration between the Faculty of
Physics at the University of Vienna and the Vienna Center for Quantum
Science and Technology (VCQ), the Institute for Quantum Optics and
Quantum Information (IQOQI) of the Austrian Academy of Sciences, the
Centre for Quantum Technologies at the National University of Singapore
and the University of Oxford.

Quantum discord as resource for remote state preparation

Source: University of Vienna