August 6th, 2012 by James Ayre
One of the fundamental characteristics of quantum physics is that two or more particles, sometimes quite distant, can interact much more strongly than allowed by classical physics.
This fundamental characteristic especially applies 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 quality “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.”
The research was done by using 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 quantum state is affected by changes in the quantum discord.”
This research provides a very important and significant step “towards future quantum information processing schemes that would rely on less demanding resources.”
Some Background Information:
“Quantum entanglement occurs when particles such as photons, electrons, molecules as large as buckyballs, and even small diamonds interact physically and then become separated; the type of interaction is such that each resulting member of a pair is properly described by the same quantum mechanical description (state), which is indefinite in terms of important factors such as position, momentum, spin, polarization, etc.”
“According to the Copenhagen interpretation of quantum mechanics, their shared state is indefinite until measured. Quantum entanglement is a form of quantum superposition. When a measurement is made and it causes one member of such a pair to take on a definite value (e.g., clockwise spin), the other member of this entangled pair will at any subsequent time be found to have taken the appropriately correlated value (e.g., counterclockwise spin). Thus, there is a correlation between the results of measurements performed on entangled pairs, and this correlation is observed even though the entangled pair may have been separated by arbitrarily large distances.”
“This behavior is consistent with quantum mechanical theory and has been demonstrated experimentally, and it is accepted by the physics community. However there is some debate about a possible underlying mechanism that enables this correlation to occur even when the separation distance is large. The difference in opinion derives from espousal of various interpretations of quantum mechanics.”
“Research into quantum entanglement was initiated by a paper of Albert Einstein, Boris Podolsky and Nathan Rosen in 1935; the EPR paradox, and several papers by Erwin Schrödinger shortly thereafter. Although these first studies focused on the counterintuitive properties of entanglement, with the aim of criticizing quantum mechanics, eventually entanglement was verified experimentally, and recognized as a valid, fundamental feature of quantum mechanics. The focus of the research has now changed to its utilization as a resource for communication and computation.”
Image Credits: University of Vienna; D Wave System via Wikimedia Commons
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