Most Invisibility Cloak Schemes Would Make Objects More Visible, Not Less

"Making an object invisible to red light, for instance, may actually make it bright blue, increasing its overall visibility."

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Phys. Rev. X 3, 041005 (2013): From ancient times, humanity has been fascinated by the concept of invisibility, and recently, scientists have moved a step closer to bringing this idea to reality by exploiting engineered artificial materials, or metamaterials. Several recent studies have indeed shown that a properly tailored metamaterial cover can, in principle, render an object invisible when illuminated by an electromagnetic wave oscillating at the specific frequency of interest. Yet, experimental realizations and theoretical investigations have consistently shown that reducing the visibility of an object with a passive cloak in a specific window of the electromagnetic spectrum is generally accompanied by a drastic increase of its visibility in other frequency ranges. Making an object invisible to red light, for instance, may actually make it bright blue, increasing its overall visibility.

In this paper, we quantitatively assess the potentials and limitations of passive cloaks in terms of overall visibility, integrated over the entire frequency spectrum. Quite surprisingly, our results show that any linear, causal, and passive invisibility cloak, without special superconducting features, is deemed to increase the scattering and visibility of the original uncloaked object, when integrated over all frequencies. This result confirms that the most popular cloaking devices actually scatter more, not less, when considered over a sufficiently broad frequency range, allowing easy detection using, e.g., pulsed excitation.

Our general theorem holds a relevant exception if specific covers with a strong static diamagnetism are considered, and, based on this principle, we propose a technique to reduce the global scattering, as well as the local response around a frequency of interest, using diamagnetic and superconducting thin cloaking layers. More generally, our results provide a quantitative measure to compare the overall performance of different cloaking devices and generally assess their detectability. These findings may open important research directions in the quest for invisibility, not only in the electromagnetic domain but also for acoustic, mechanical, and matter waves.

Contact:
James Riordon
riordon@aps.org
301-209-3238 

APS Physics Tip Sheet – Oct 29, 2013

In this issue: Planet Search Finds No Dark-Matter Black Holes, Secure Quantum Commitment, A Single-Atom Switch, and More

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Planet Search Finds No Dark-Matter Black Holes 

(Image credit: NASA/Kepler mission/Wendy Stenzel)

Using data from a planet-hunting mission, scientists place new limits on a supposed population of moon-sized black holes that could act as dark matter in our galaxy

According to recent theoretical studies, small, primordial black holes (PBHs) formed during the universe’s early expansion could be responsible for the gravitational effects attributed to dark matter. A team of researchers from the US and Taiwan has searched for PBHs using observations by Kepler - a NASA satellite designed to look for Earth-like planets orbiting other stars. Over 4 years, Kepler monitored ~150,000 stars in the Milky Way. If a primordial black hole passed in front of one of these stars, the star would become temporarily brighter due to the hole’s gravitational lensing. The analysis revealed no black-hole lensing events, ruling out the possibility that primordial black holes could account for a major fraction of the dark matter that is known to exist in our galaxy.

* Kim Griest (contact author), AM Cieplak, and MJ Lehner, “New limits on primordial black-hole dark matter from an analysis of Kepler source microlensing data”, Physical Review Letters (expected publication date: Oct 31)

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Secure Quantum Commitment

Researchers have demonstrated a secure quantum commitment scheme - a cryptographic protocol that allows one to commit to a chosen value, while keeping it secret until the value is revealed.

Secure commitment schemes could find use in bidding or voting systems, in which no one should learn anyone else's intent before an appointed time. Inspired by the success of quantum cryptography techniques like Quantum Key Distribution, researchers have turned to quantum mechanics to develop inherently secure commitment protocols. Some theoretical work seemed to demonstrate that a secure quantum commitment scheme was impossible, but according to a 2012 proposal, a solution could be found if relativistic effects were present. Now, a group of researchers from Switzerland, Singapore, the UK and Canada has experimentally realized this proposal. In an optical fiber link between Geneva and Singapore, the team showed that a committed bit could be kept secret for up to 15 milliseconds - a time that may be sufficient for high-speed applications such as stock trading.

* T Lunghi, Felix Bussières (contact author) et al, “Experimental bit commitment based on quantum communication and special relativity”, Physical Review Letters (expected publication date: Nov 1)

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A Single-Atom Switch

A research team at the Vienna Center for Quantum Science and Technology has demonstrated an optical switch made of a single rubidium atom placed in a microscopic cavity. Controlled by the atom position, the switch can be used to reroute optical signals from an optical fiber to different output fiber ports.

* D O’Shea, C Junge, J Volz, Arno Rauschenbeutel (contact author), “Fiber-optical switch controlled by a single atom”, Physical Review Letters (expected publication date: Nov 4)

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More from the APS Physics News Ticker:

- Physicists Detect the "Kick" from Light Hitting a Mirror

- Twisting Ribbons: Researchers have developed a model that can be used to design devices made of elastic materials likegraphene sheets, semiconductor nanoribbons and biomaterials.

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Journal articles and preprints are available to journalists on request. 

Contact: Matteo Rini Tel: +1 631 591 4224 (office), +1 646 288 5441 (cell), email: mrini@aps.org

Physicists Detect the "Kick" from Light Hitting a Mirror

From the authors of manuscript LF14494:

When a reflective object is pushed by a pulse of light, various types of mechanical waves are launched from the illuminated surface. In terms of decreasing amplitude, a high-intensity laser pulse gives rise to the following: ablation-induced waves (AIWs) resulting from material recoil, thermoelastic waves (TEWs) caused by light absorption and the subsequent thermal expansion, and the ubiquitous, but weakest, light-pressure-induced elastic waves (LIWs) emanating solely from the linear momentum transfer during the photon recoil. Even though the macroscopic motion of the object is the superposition of all these waves, only AIWs and LIWs are capable of displacing the object’s center of mass. Until now, LIWs were indiscernible from the dominating AIWs and TEWs. However, we have succeeded, for the first time, in detecting LIW by striking a defocused laser pulse with a fluence of 1 J/cm² on the front surface of a 99.999% ultra-high-reflectivity mirror. Its reverberations, causing picometer-large displacements of the mirror’s rear facet, were observed with a calibrated piezoelectric sensor. Again, the light heating had to be suppressed, as was the case over a century ago, when the radiation pressure was experimentally validated with a delicate vane, similar to a light mill.