Don't know why the subject didn't show up. On Fri, Mar 8, 2013 at 2:37 PM, Kevin O'Malley <kevmol...@gmail.com> wrote:
> Ex nihilo: Dynamical Casimir effect in metamaterial converts vacuum > fluctuations into real photons > March 8, 2013 by Stuart Mason Dambrot > > Copyright © PNAS, doi:10.1073/pnas.1212705110 (Phys.org) > > —In the strange world of quantum mechanics, the vacuum state (sometimes > referred to as the quantum vacuum, simply as the vacuum) is a quantum > system's lowest possible energy state. While not containing physical > particles, neither is it an empty void: Rather, the quantum vacuum contains > fluctuating electromagnetic waves and so-called virtual particles, the > latter being known to transition into and out of existence. In addition, > the vacuum state has zero-point energy – the lowest quantized energy level > of a quantum mechanical system – that manifests itself as the static > Casimir effect, an attractive interaction between the opposite walls of an > electromagnetic cavity. Recently, scientists at Aalto University in Finland > and VTT Technical Research Centre of Finland demonstrated the dynamical > Casimir effect using a Josephson metamaterial embedded in a microwave > cavity. They showed that under certain conditions, real photons are > generated in pairs, and concluded that their creation was consistent with > quantum field theory predictions. > > Researcher Pasi Lähteenmäki discussed the challenges he and his colleagues > – G. S. Paraoanu, Juha Hassel and Pertti J. Hakonen – encountered in their > study. Regarding their demonstration of the dynamical Casimir effect using > a Josephson metamaterial embedded in a microwave cavity at 5.4 GHz, > Lähteenmäki tells Phys.org that the main challenge in general is to get > high-quality samples. In addition, Lähteenmäki adds, they had to ensure > that the origin of the noise is quantum and not some unaccounted source of > excess noise, such as thermal imbalance between the environment and the > sample, or possibly leakage of external noise. > > Modulating the effective length of the cavity by flux-biasing the SQUID > (superconducting quantum interference device) metamaterial had its > challenges as well. "The pump signal needs to be rather strong, yet at the > same time one wants to be sure that no excess noise enters the system > through the pump line, Lähteenmäki notes, "and good filtering means high > attenuation, which is a requirement contradictory to a strong signal. > Also," Lähteenmäki continues, "at 10.8 GHz the pump frequency is rather > high – and at that frequency range both the sample and the setup is rather > prone to electrical resonances which can limit the usable frequencies." > > In short, the flux profile needs to be such that the pumping doesn't > counteract itself. In addition, trapping flux in SQUID loops can also > become a problem, limiting the range of optimal operating points and > causing excess loss. The researchers also showed that photons at > frequencies symmetric with respect to half the modulation frequency of the > cavity are generated in pairs. "In general, with frequency locked signal > analyzers today the extraction of this correlation is not particularly > problematic – especially since the low noise amplifier noise is not > correlated at different frequencies," Lähteenmäki explains. > > That said, issues related to data collection and averaging include > amplifier gain drift and phase randomization of the pump signal (relative > to the detection phase) if the state of the generator is changed. "The > noise temperature of the low noise amplifier sets some limits to the amount > of data that needs to be collected, especially in the case where one is > operating in the regime of low parametric gain." > > Lastly, the team also found that at large detunings of the cavity from > half the modulation frequency, they found power spectra that clearly showed > the theoretically-predicted hallmark of the dynamical Casimir effect. > "Large detunings imply low intensity of generated radiation," notes > Lähteenmäki. "This means long averaging times, so the system should be kept > stable for a long period. > > Also, the system needs to be fairly resonance-free over a large range of > frequencies to get decent data – and/or one needs to know the > characteristics of these resonances and noise temperature of the low noise > amplifier rather well." Lähteenmäki points out that addressing these issues > required a number of insights and innovations. "We combated amplifier drift > by continuously switching the pump on and off, and recording the difference > in the observed output power, suitable operating points were searched for > using trial and error, and trapping the photon flux was eliminated by > applying a heat pulse to the sample and letting it cool down again. > > The researchers also magnetically shielded the sample with a > superconductive shield, and minimized the effect of losses by changing the > coupling of the existing samples by making different valued vacuum coupling > capacitors with focused ion beam (FIB) cuts. "However," Lähteenmäki > stresses, "our biggest issue – ruling out the source of classical noise as > opposed to quantum noise – was accomplished primarily by characterizing the > sample and the environment well" Thermal imbalance was ruled out by the > symmetry of the sparrow-tail shape of the noise spectrum. > > It was essential for the scientists to clearly demonstrate that the > observed substantial photon flux could not be assigned to parametric > amplification of thermal fluctuations. "By characterizing the parametric > gain with a network analyzer," Lähteenmäki notes, "we found that in order > to explain the amount of noise one gets, the device would need to have > significantly higher gain than is observed if the only source of noise was > thermal." Moreover, confirming that photon pair creation is a direct > consequence of the noncommutativity structure of quantum field theory was > equally important. "Basically the experimental results fit the theoretical > predictions rather well – and in the absence of other sources of noise, the > theory implies that we should get no output from this sort of device. Since > we see output consistent with the theoretical predictions, the conclusion > was logical." > > Moving forward, Lähteenmäki describes next steps in their research. > "Instead of a continuous wave pump, we could have a straight flux line and > feed it with a step-like flux pulse," Lähteenmäki says. "This would allow > the creation of an analogue to a black hole event horizon. In fact," he > adds, "we're hoping to create an artificial event horizon in a metamaterial > similar to the one used in our current research and study Hawking radiation > originating from it. Also, it would be nice to be able to run experiments > on Bell's inequalities." > > His personal interests, Lähteenmäki says, are fundamental quantum > mechanics, quantum information and properties of the vacuum itself. "The > obvious applications for these devices," he notes, "come from quantum > computation, and in general they may serve as components for multitude of > sensitive measurements. I believe the interest towards low loss > metamaterials is high and the field is just getting started. Our results > show that these devices have potential and can offer a fruitful platform > for many experiments and perhaps practical devices as well. Improving such > devices – especially eliminating the losses and making them function more > robustly – would allow them to create a general purpose component suitable > for creating entangled microwave photon pairs, low noise amplification, > squeezed vacuum, and other functions that can be very useful for quantum > computation and general experiments in the quantum mechanics and in > studying the vacuum." > > Another possibility, Lähteenmäki adds, is to create a metamaterial which > would allow them to stop signal propagation in the material entirely and > allow them to resume it later. "This would act as a kind of slow light > memory that would store the photon for later use." Other areas of research > might benefit from their study as well, Lähteenmäki says. "There are some > connections to cosmology, the big bang, cosmic inflation, and other areas. > These metamaterials could possibly offer an analogy to such events and > serve as a platform to simulate the evolution of such conditions. Who > knows," he ponders, concluding that "perhaps we'd find clues to the > mysteries of dark matter and dark energy or other fundamental questions > from such systems." > > More information: Dynamical Casimir effect > in a Josephson metamaterial, PNAS Published online before print February > 12, 2013, doi:10.1073/pnas.1212705110Journal reference: Proceedings of the > National Academy of Sciences Copyright 2013 Phys.org All rights reserved. > This material may not be published, broadcast, rewritten or redistributed > in whole or part without the express written permission of Phys.org. > > Read more at: > http://phys.org/news/2013-03-nihilo-dynamical-casimir-effect-metamaterial.html#jCp > > >
