Breaking the law: Laser focus

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  • Published: Apr 15, 2014
  • Author: David Bradley
  • Channels: Atomic
thumbnail image: Breaking the law: Laser focus

Cool for nano

Researchers from the University of Vienna, the Institute of Photonic Sciences in Barcelona and the Swiss Federal Institute of Technology in Zürich previously succeeded in accurately predicting the likelihood of events transiently violating the second law of thermodynamics.

An international team has demonstrated that a nanoparticle trapped with a laser beam can temporarily violate the strict second law of thermodynamics, a feat that is impossible on the everyday time and length scales of our reality.

It seems that nanoscopic objects, the molecular building blocks of living cells or the components of nanotechnological devices undergo random collisions and fluctuations that mean they can for brief instants exist beyond the fundamental laws of thermodynamics. Specifically, the entropic second law of thermodynamics that precludes order arising spontaneously out of chaos in a closed system cannot be broken. Hard-boiled eggs cannot be unboiled, spilled milk does not coalesce and leap back into a glass tumbler and snowmen do not unmelt and grab their carrot nose and coal eyes.

However, even the sacrosanct second law is there to be broken, in some sense, at the sub-microscopic scale, albeit fleetingly. The impossibility of irreversibility at the nanoscale seems to be something that an international team of physicists was not happy with.

No limits

Researchers from the University of Vienna, the Institute of Photonic Sciences in Barcelona and the Swiss Federal Institute of Technology in Zürich previously succeeded in accurately predicting the likelihood of events transiently violating the second law of thermodynamics. They immediately put the mathematical fluctuation theorem they derived to the test using a tiny glass sphere with a diameter of less than 100 nm held in optical tweezers. Their experimental set-up allowed the research team to capture the nano-sphere and hold it in place, and, furthermore, to measure its position in all three spatial directions with exquisite precision. In the trap, the nano-sphere rattles around due to collisions with surrounding gas molecules.

The team used the laser trap to cool the nano-sphere below the temperature of the surrounding gas and, thereby, put it into a non-equilibrium state. They then turned off the cooling and watched the particle relaxing to the higher temperature through energy transfer from the gas molecules. The researchers observed that the tiny glass sphere sometimes, although rarely, does not behave as one would expect according to the second law: the nano-sphere effectively releases heat to the hotter surroundings rather than absorbing the heat.

"The probability of the glass sphere cooling further immediately after the cooling has been switched off is 50 per cent; a tenth of a second later there is still a 10 per cent chance and, after a second, it is vanishingly small. From then on, conventional thermodynamics apply,” explains ETH's Lukas Novotny. The theory derived by the researchers to analyse the results confirms the emerging picture on the limitations of the second law on the nanoscale.

Fundamentally Gaussian

Fundamentally, the team explains that a value such as the total energy of a system with a specific numbers of degrees of freedom follows a Gaussian distribution for the probability of energy flow. In the everyday world, the probability of flow from the colder system to the hotter bath is exponentially small compared with the probability of observing energy transfer in the "normal" direction. On the nanoscale, fluctuations within that distribution are on the same scale as the movements of the entities involved. In their laser-trapped nanosphere the probability of observing energy flowing from the colder system to the hotter one is exponentially small compared with the probability of observing energy transfer in the other direction.

By modulating the laser trap, the team essentially drives the nanoparticle out of equilibrium. "When we switch off the modulation and watch the particle as it relaxes back to equilibrium, it exchanges energy with its environment," explains ETH's Jan Gieseler. "Since it is initially colder than the environment, the second law of thermodynamics predicts that the particle heats up. However, we observe that sometimes the particle gets even colder. This would correspond to a water droplet spontaneously freezing." He adds that this is not observed in macroscopic everyday life because the probability of observing such an unexpected behaviour is exponentially small and scales with the size of the system. "As a consequence of this scaling, in small systems the thermal fluctuations become so large that we can actually observe these rare events. This allows us to test experimentally theoretical predictions from statistical physics," he adds.

"We envision that our approach of using highly controllable nanomechanical oscillators will open up experimental and theoretical studies of fluctuation theorems in complex settings, which arise, for instance, from the interplay of thermal fluctuations and non-linearities where detailed balance does not hold," the team concludes. The system might thus be used as an experimental simulator of quantum simulators based on ultracold gases, superconducting circuits or trapped ions.

"The aim of this research is to establish our experimental platform as a testbed for theoretical predictions from statistical physics," Gieseler adds. "The next step is to test other related predictions which so far have not been addressed experimentally. This advances our understanding of how nanoscale devices behave and ultimately will lead to improved technologies based on these devices."

Related Links

Nature Nanotech, 2014, online: "Dynamic relaxation of a levitated nanoparticle from a non-equilibrium steady state"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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