Quantum entanglement in a macroscopic system at high temperatures

“Quantum entanglement has been observed at low temperatures in both microscopic and macroscopic systems. It now seems that the effect can also occur at high temperatures if the systems are not in thermal equilibrium.” The spanish scientist Fernando Galve and colleagues show that quantum entanglement can appear in macroscopic systems at high temperatures. “Decoherence due to contact with a hot environment typically restricts quantum phenomena to the low temperature limit, kBT/ℏω≪1 (ℏω is the typical energy of the system). But a nonequilibrium state for two coupled, parametrically driven, dissipative harmonic oscillators could have stationary entanglement at high temperatures.” Vlatko Vedral, “Quantum physics: Hot entanglement,” Nature 468: 769–770, 09 December 2010, viewpoints the technical paper by Fernando Galve (CSIC-Universitat Illes Balears, Palma de Mallorca, Spain), Leonardo A. Pachón (Universidad Nacional de Colombia, Bogotá, Colombia), and David Zueco (CSIC-Universidad de Zaragoza, Zaragoza, Spain), “Bringing Entanglement to the High Temperature Limit,” Phys. Rev. Lett. 105: 180501, 25 October 2010 [ArXiv 1002.1923].

“Quantum physics and quantum entanglement are usually thought to apply to small systems at low temperatures. Entanglement refers to a state of two or more quantum systems in which the systems are so intertwined that they behave like one. A general rule says that if the interaction strength between the subsystems is larger than the thermal energy due to their coupling to the environment, entanglement should exist between these subsystems provided that they are in thermal equilibrium with the environment. Now Galve et al. prove that this relationship between temperature and entanglement is not valid for systems that are not in thermal equilibrium. In fact, they predict that nanomechanical oscillators can be entangled at much higher temperatures than previously thought possible.

When a system is not in thermal equilibrium, the temperature no longer provides the relevant energy scale against which to compare the system’s quantum behaviour. What matters instead is an effective temperature, which can be much lower than the absolute one. This effective temperature is obtained by multiplying the absolute temperature by the rate at which the system approaches equilibrium divided by the driving frequency, the frequency of the signal with which the system is made to oscillate. If we can drive the system to oscillate within a shorter timescale than the time it takes to reach thermal equilibrium, then an entangled steady state can be attained at higher temperatures than the absolute one.

Galve et al. investigate two macroscopic (harmonic) oscillators coupled to each other. In the past high-temperature entanglement has been achieved in nanomechanical oscillators at temperatures of about 20 kelvin. Galve and colleagues’ new ideas suggest how to push upwards this result up to 100 kelvin. This would eliminate the current need for expensive and elaborate cryogenics to cool the oscillators.”

The most exciting macroscopic and ‘hot’ non-equilibrium systems we know are, of course, the living ones. Recent experiments show a quantum effect leading to entanglement in some photosynthetic complexes. Such entanglement might yield an increased efficiency in the transfer and processing of energy in photosynthesis. The overall mystery of photosynthesis remains, but there is now evidence that quantum physics has something to do with it in a profound way. And there are other instances in biology in which quantum entanglement could be important. If this is a general trend in the biological world (and it is a big ‘if’), could it be that life does not just keep its entropy low, but rather, also aims to keep its quantum entanglement high if and when needed for an increased efficiency of energy transport?”

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