Physicists gain new insights into the remote control of quantum systems

Aug 06, 2012
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: Copyright: University of Vienna

An international collaboration led by physicists of the University of Vienna shines new light on the question of the resources required for achieving quantum information processing. The scientists demonstrate that less demanding resources, which are easier to prepare and to control, can be used for quantum-enhanced technologies. In the experiment, which is published in Nature Physics, the researchers achieve remote quantum state preparation without requiring entanglement as a resource.

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 : 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 might be. This feature allows for the remote quantum state preparation, which is an essential ingredient for applications in quantum communication, , and quantum computation.

The degree of entanglement is often used as a figure of merit for determining its usefulness for . 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 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 Philip Walther. "In the experiment we observe how the quality of our remotely prepared 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.

Explore further: Physicists design quantum switches which can be activated by single photons

More information: "Quantum discord as resource for remote state preparation": Borivoje Dakic, Yannick-Ole Lipp, Xiaosong Ma, Martin Ringbauer, Sebastian Kropatschek, Stefanie Barz, Tomasz Paterek, Vlatko Vedral, Anton Zeilinger, Caslav Brukner, Philip Walther (Nature Physics 2012) DOI: 10.1038/NPHYS2377

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