Tag Archives: University of Belgrade

A very sensitive graphene microphone

The researchers have not made a complete graphene-based microphone yet but they do have a proof-of-concept according a Nov. 26, 2015 Institute of Physics news release on EurekAlert,

Scientists have developed a graphene based microphone nearly 32 times more sensitive than microphones of standard nickel-based construction.

The researchers, based at the University of Belgrade, Serbia, created a vibrating membrane – the part of a condenser microphone which converts the sound to a current – from graphene, and were able to show up to 15 dB higher sensitivity compared to a commercial microphone, at frequencies up to 11 kHz.

“We wanted to show that graphene, although a relatively new material, has potential for real world applications” explains Marko Spasenovic, an author of the paper. “Given its light weight, high mechanical strength and flexibility, graphene just begs to be used as an acoustic membrane material.”

The graphene membrane, approximately 60 layers thick, was grown on a nickel foil using chemical vapour deposition, to ensure consistent quality across all the samples.

During membrane production, the nickel foil was etched away and the graphene membrane placed in the same housing as a commercial microphone for comparison. This showed a 15 dB higher sensitivity than the commercial microphone.

The researchers also simulated a 300-layer thick graphene membrane, which shows potential for performance far into the ultrasonic part of the spectrum.

“The microphone performed as well as we hoped it would” adds Spasenovic. “A thicker graphene membrane theoretically could be stretched further, enabling ultrasonic performance, but sadly we’re just not quite there yet experimentally.”

“At this stage there are several obstacles to making cheap graphene, so our microphone should be considered more a proof of concept” [emphasis mine] concludes Spasenovic. “The industry is working hard to improve graphene production – eventually this should mean we have better microphones at lower cost.”

There is a video abstract (the first time I’ve seen one of these)  for the research paper: http://bcove.me/5vfn17jb

Here’s a link to and a citation for the paper,

Multilayer graphene condenser microphone by Dejan Todorović, Aleksandar Matković, Marijana Milićević, Djordje Jovanović, Radoš Gajić, Iva Salom, and Marko Spasenović. 2D Materials, Volume 2, Number 4 doi.org/10.1088/2053-1583/2/4/045013  Published 26 November 2015

© 2015 IOP Publishing Ltd

This paper is behind a paywall.

Entangling thousands of atoms

Quantum entanglement as an idea seems extraordinary to me like something from of the fevered imagination made possible only with certain kinds of hallucinogens. I suppose you could call theoretical physicists who’ve conceptualized entanglement a different breed as they don’t seem to need chemical assistance for their flights of fancy, which turn out to be reality. Researchers at MIT (Massachusetts Institute of Technology) and the University of Belgrade (Serbia) have entangled thousands of atoms with a single photon according to a March 26, 2015 news item on Nanotechnology Now,

Physicists from MIT and the University of Belgrade have developed a new technique that can successfully entangle 3,000 atoms using only a single photon. The results, published today in the journal Nature, represent the largest number of particles that have ever been mutually entangled experimentally.

The researchers say the technique provides a realistic method to generate large ensembles of entangled atoms, which are key components for realizing more-precise atomic clocks.

“You can make the argument that a single photon cannot possibly change the state of 3,000 atoms, but this one photon does — it builds up correlations that you didn’t have before,” says Vladan Vuletic, the Lester Wolfe Professor in MIT’s Department of Physics, and the paper’s senior author. “We have basically opened up a new class of entangled states we can make, but there are many more new classes to be explored.”

A March 26, 2015 MIT news release by Jennifer Chu (also on EurekAlert but dated March 25, 2015), which originated the news item, describes entanglement with particular attention to how it relates to atomic timekeeping,

Entanglement is a curious phenomenon: As the theory goes, two or more particles may be correlated in such a way that any change to one will simultaneously change the other, no matter how far apart they may be. For instance, if one atom in an entangled pair were somehow made to spin clockwise, the other atom would instantly be known to spin counterclockwise, even though the two may be physically separated by thousands of miles.

The phenomenon of entanglement, which physicist Albert Einstein once famously dismissed as “spooky action at a distance,” is described not by the laws of classical physics, but by quantum mechanics, which explains the interactions of particles at the nanoscale. At such minuscule scales, particles such as atoms are known to behave differently from matter at the macroscale.

Scientists have been searching for ways to entangle not just pairs, but large numbers of atoms; such ensembles could be the basis for powerful quantum computers and more-precise atomic clocks. The latter is a motivation for Vuletic’s group.

Today’s best atomic clocks are based on the natural oscillations within a cloud of trapped atoms. As the atoms oscillate, they act as a pendulum, keeping steady time. A laser beam within the clock, directed through the cloud of atoms, can detect the atoms’ vibrations, which ultimately determine the length of a single second.

“Today’s clocks are really amazing,” Vuletic says. “They would be less than a minute off if they ran since the Big Bang — that’s the stability of the best clocks that exist today. We’re hoping to get even further.”

The accuracy of atomic clocks improves as more and more atoms oscillate in a cloud. Conventional atomic clocks’ precision is proportional to the square root of the number of atoms: For example, a clock with nine times more atoms would only be three times as accurate. If these same atoms were entangled, a clock’s precision could be directly proportional to the number of atoms — in this case, nine times as accurate. The larger the number of entangled particles, then, the better an atomic clock’s timekeeping.

It seems weak lasers make big entanglements possible (from the news release),

Scientists have so far been able to entangle large groups of atoms, although most attempts have only generated entanglement between pairs in a group. Only one team has successfully entangled 100 atoms — the largest mutual entanglement to date, and only a small fraction of the whole atomic ensemble.

Now Vuletic and his colleagues have successfully created a mutual entanglement among 3,000 atoms, virtually all the atoms in the ensemble, using very weak laser light — down to pulses containing a single photon. The weaker the light, the better, Vuletic says, as it is less likely to disrupt the cloud. “The system remains in a relatively clean quantum state,” he says.

The researchers first cooled a cloud of atoms, then trapped them in a laser trap, and sent a weak laser pulse through the cloud. They then set up a detector to look for a particular photon within the beam. Vuletic reasoned that if a photon has passed through the atom cloud without event, its polarization, or direction of oscillation, would remain the same. If, however, a photon has interacted with the atoms, its polarization rotates just slightly — a sign that it was affected by quantum “noise” in the ensemble of spinning atoms, with the noise being the difference in the number of atoms spinning clockwise and counterclockwise.

“Every now and then, we observe an outgoing photon whose electric field oscillates in a direction perpendicular to that of the incoming photons,” Vuletic says. “When we detect such a photon, we know that must have been caused by the atomic ensemble, and surprisingly enough, that detection generates a very strongly entangled state of the atoms.”

Vuletic and his colleagues are currently using the single-photon detection technique to build a state-of-the-art atomic clock that they hope will overcome what’s known as the “standard quantum limit” — a limit to how accurate measurements can be in quantum systems. Vuletic says the group’s current setup may be a step toward developing even more complex entangled states.

“This particular state can improve atomic clocks by a factor of two,” Vuletic says. “We’re striving toward making even more complicated states that can go further.”

This research was supported in part by the National Science Foundation, the Defense Advanced Research Projects Agency, and the Air Force Office of Scientific Research.

Here’s a link to and a citation for the paper,

Entanglement with negative Wigner function of almost 3,000 atoms heralded by one photon by Robert McConnell, Hao Zhang, Jiazhong Hu, Senka Ćuk & Vladan Vuletić. Nature 519 439–442 (26 March 2015) doi:10.1038/nature14293 Published online 25 March 2015

This article is behind a paywall but there is a free preview via ReadCube Access.

This image illustrates the entanglement of a large number of atoms. The atoms, shown in purple, are shown mutually entangled with one another. Image: Christine Daniloff/MIT and Jose-Luis Olivares/MIT

This image illustrates the entanglement of a large number of atoms. The atoms, shown in purple, are shown mutually entangled with one another.
Image: Christine Daniloff/MIT and Jose-Luis Olivares/MIT

Serbia’s nanotechnology efforts

There’s not a lot of information which means this is not a comprehensive overview of nanotechnology research in Serbia but (as far as I can recall) this is the first time I’ve seen anything about their research, from the Oct. 4, 2013 news item on Nanowerk (Note: A link has been removed),

Nanostructured materials are at the forefront of research and development of novel devices in a variety of fields. Serbian scientists exploited EU funding to significantly enhance their research capacity in applied nanotechnology.

The Nanotechnology and Functional Materials Centre at the Faculty of Technology and Metallurgy, University of Belgrade (NFMC FTM UB) is actively investigating inorganic materials, composites and polymers with an eye to applications. The centre was established from small independent research groups or individuals to create a critical mass to move these topics forward. NFMC FTM UB recently made a step change to its research capacity with EU funding of the project ‘Reinforcing of Nanotechnology and Functional Materials Centre’ (NANOTECH FTM).

The Oct. 3, 2013 CORDIS news release, which originated the news item, describes the hopes that researchers and the Serbian government have for this centre,

New researchers, both experienced and young scientists, were brought onboard, ensuring an influx of scientific management expertise as well as cutting-edge research techniques. The incoming senior scientists have established important relations with top European and international investigators in related fields. Such networking with both labs and industry including EU-based industrial partners will continue to foster both high-level research in the European Research Area (ERA) and also increased visibility for NFMC FTM UB in the regional and international arena. The young scientists actively participated in research exchange programmes with partner institutions, receiving training and mentorship from the world’s leading scientists.

Together with the purchase of new equipment, the team is in an excellent position to conduct novel experiments previously not possible at NFMC FTM UB. Funding was also exploited to communicate progress and activities. Two workshops and a conference were organised and scientists presented their work in peer-reviewed and popular science journals as well as through flyers and television presentations.

EU support of the NFMC FTM UB in Serbia has enabled an important increase in its research capacity. Enhancements encompass not only investigative techniques and equipment but also project management, scientific communication and networking. The quality and specificity of results has increased, as has the centre’s position on the international stage. Numerous joint projects initiated within the context of the current one ensure continued progress and success.

It’s a little hard to determine when the centre actually opened as the date is not mentioned in the news release or on the NFMC FTM UB website.