Quantum Attraction

English

(l-r) Prof. Gershon Kurizki and Ephraim Shahmoon

Even a perfect vacuum is not truly empty. If you could observe that vacuum on the quantum, atomic or subatomic scale, you would see a “bubbling soup” of  "ghost" particles. Known as “virtual particles,” they randomly pop in and out of existence in the empty space; and they cause a phenomenon known as vacuum fluctuation. So, in a space that is completely devoid of any detectable radiation – that is, a vacuum existing at the temperature of absolute- zero – fluctuations in electromagnetic fields will still be taking place on a microscopic, quantum scale.

 
 
 
Though they are called virtual, these particles create real forces between atoms. If you place two atoms close together, they will change the local vacuum between them, creating fluctuations through virtual photons – light particles. The attraction between these close-together atoms is called the van der Waals force. Place the atoms farther apart, and you will still observe a slight pull between them. This is the Casimir force. Both of these forces are weak and often hard to measure.

Prof. Gershon Kurizki and research student Ephraim Shahmoon, of the Weizmann Institute’s Chemical Physics Department, together with Dr. Igor Mazets of the Vienna University of Technology recently suggested a way of enormously enhancing these forces – until they become a sort of “quantum glue” holding atoms together. In their paper, recently published in the Proceedings of the National Academy of Sciences (PNAS), the researchers considered atoms placed near a line of conducting material, similar to an ordinary coaxial cable used to hook a TV to a satellite dish. In the setup they envision, a  virtual photon that is emitted from one atom would be confined so that it propagates in one dimension to the nearest atom down the line, where it would be absorbed, then reemitted back to the first atom and so on.
Possible "quantum glue" setup: Coaxial line: two concentric metallic cylinders, the inner one with radius a and the outer (hollow) one with radius b. Two dipoles represented by black circles are placed in between the cylinders, along the wave propagation direction z. They interact via modes of the coaxial line that are in the vacuum state, giving rise to a vdW-like interaction energy
 

Having this exchange of virtual photons occur under one-dimensional confinement, rather than in everyday three-dimensional space, greatly increases the odds that the process will take place. The researchers’ calculations suggest that the attraction between atoms via virtual photons in the electric coaxial cable could be millions of times greater than that in three-dimensional space, transforming a  normally weak force into potent " glue." This research was highlighted in the journals Physics Today and Nature Photonics.

If scientists manage to demonstrate such one-dimensional vacuum forces, their experiments could help us understand the phenomena surrounding virtual particles.

 
Prof. Gershon Kurizki’s research is supported by the Mary and Tom Beck-Canadian Center for Alternative Energy Research. Prof. Kurizki is the incumbent of the George W. Dunne Professorial Chair of Chemical Physics.
 

 

 
 

 
 
(l-r) Prof. Gershon Kurizki and Ephraim Shahmoon
Space & Physics
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The World’s First Photonic Router

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llustration of the photonic router the Weizmann Institute scientists created. At the center is the single atom (orange) that routes photons (yellow) in different directions

 

 

 

 

 

 

 

 

 


 


Weizmann Institute scientists have demonstrated for the first time a photonic router – a quantum device based on a single atom that enables routing of single photons by single photons. This achievement, as reported in Science magazine, is another step toward overcoming the difficulties in building quantum computers.


At the core of the device is an atom that can switch between two states. The state is set just by sending a single particle of light – or photon – from the right or the left via an optical fiber. The atom, in response, then reflects or transmits the next incoming photon, accordingly. For example, in one state, a photon coming from the right continues on its path to the left, whereas a photon coming from the left is reflected backwards, causing the atomic state to flip. In this reversed state, the atom lets photons coming from the left continue in the same direction, while any photon coming from the right is reflected backwards, flipping the atomic state back again. This atom-based switch is solely operated by single photons – no additional external fields are required.
 
 
Dr. barak Dayan
 
“In a sense, the device acts as the photonic equivalent to electronic transistors, which switch electric currents in response to other electric currents,” says Dr. Barak Dayan, head of the Weizmann Institute’s Quantum Optics group, including Itay Shomroni, Serge Rosenblum, Yulia Lovsky, Orel Bechler and Gabriel Guendleman of the Chemical Physics Department in the Faculty of Chemistry. The photons are not only the units comprising the flow of information, but also the ones that control the device. 

This achievement was made possible by the combination of two state-of-the-art technologies. One is the laser cooling and trapping of atoms. The other is the fabrication of chip-based, ultra-high quality miniature optical resonators that couple directly to the optical fibers. Dayan’s lab at the Weizmann Institute is one of a handful worldwide that has mastered both these technologies. 

The main motivation behind the effort to develop quantum computers is the quantum phenomenon of superposition, in which particles can exist in many states at once, potentially being able to process huge amounts of data in parallel. Yet superposition can only last as long as nothing observes or measures the system otherwise it collapses to a single state. Therefore, photons are the most promising candidates for communication between quantum systems as they do not interact with each other at all, and interact very weakly with other particles.

Dayan: “The road to building quantum computers is still very long, but the device we constructed demonstrates a simple and robust system, which should be applicable to any future architecture of such computers. In the current demonstration a single atom functions as a transistor – or a two-way switch – for photons, but in our future experiments, we hope to expand the kinds of devices that work solely on photons, for example new kinds of quantum memory or logic gates.”
 
Dr. Barak Dayan's group members: (l-r) Serge Rosenblum, Yulia Lovsky, Orel Bechler and Itay Shomroni

 
Dr. Barak Dayan’s research is supported by the Benoziyo Endowment Fund for the Advancement of Science. Dr. Dayan is the incumbent of the Joseph and Celia Reskin Career Development Chair.
 
llustration of the photonic router the Weizmann Institute scientists created. At the center is the single atom (orange) that routes photons (yellow) in different directions
Space & Physics
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Acting Locally, Reporting Globally

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Mapping of size distributions of a mouse’s gray matter by quantum-controlled proton MRI. (l) Brain proton MRI; (c) mean cellular size; (r) distribution peak

The smallest devices for storing information are qubits – quantum bits. Qubits are all around us – they are in the nuclei of atoms, for example. The trick is to learn to read and use the information they contain.

One technology that reads the information in quantum systems is magnetic resonance (MR). MR – including the familiar MRI scan – exploits the quantum nature of the protons found, for example, in the water molecules in our bodies. These protons have a property known as “spin”: states of rotation that are measured in one of two discrete values, often referred to as “up” or “down.” Spin states endow the protons in a molecule with magnetism, transforming them into “quantum compass needles” that “point” along one of two orientations. These micro-magnets are what enable magnetic resonance: Protons, placed in certain conditions in an external magnetic field, will rotate like a top, emitting very weak – but observable – radio waves. MR equipment detects the distinctive radio waves emitted from the protons in such molecules as water, using their locations to build an image.
 
 
Prof. Lucio Frydman
 
Worlds of information, in addition to the spatial location of protons, could be extracted from MRI if the protons’ emitting states could be efficiently preserved. Unfortunately, the quantum states of these micro-magnet protons are exquisitely delicate; the information they record is rapidly destroyed by interference from the tiny magnetic fluctuations that occur spontaneously in their surroundings.   

How can one protect these delicate systems, on the one hand, and get them to reveal the subtle information they carry about their environment, on the other? Prof. Lucio Frydman, together with visiting scientist Dr. Gonzalo Alvarez and postdoctoral fellow Dr. Noam Shemesh, all in the Institute’s Chemical Physics Department, illustrated a new approach to answering this question, showing that some forms of interference can help, rather than hurt, in preserving this information. Their research, as reported in Physical Review Letters, reveals a new family of MR-based methods that may be used, among other things, to probe the size and shape of small pores, cells or tiny nanostructures.

The new methods operate by exposing nuclear spins to one kind of environmental interference while protecting them from all the others. The interference chosen by Frydman and his group comes from fluctuating magnetic fields associated with the random, Brownian motions that arise from regular quantum particle collisions. This manipulation turns the nuclei into “spies” that, during their random walks, can “scout out” the confines of a cell or the boundaries of their local microstructures. Combined with the spatial imaging ability of a standard MRI, the result is a completely new way to measure microscopic structural architectures at extremely high resolution. As an added plus, the technique is noninvasive: Shemesh, Alvarez and Frydman demonstrated its ability to perform complex biological imaging – in this case, “virtual histologies” – mapping cell-size distribution in whole brains.

Possible additional applications of this method range from solid-state physics to materials sciences and, of course, to the life sciences – where they could, for example, measure the changes in size and shape that the nerve cells’ axons experience due to neural stimulation or maturation. It could also lead to the development of new contrast mechanisms for medical MRI scans, and to better diagnostic methods for observing physiological changes in diseased tissues or those caused by neurodegeneration.
 
Prof. Lucio Frydman’s research is supported by the Ilse Katz Institute for Material Sciences and Magnetic Resonance Research; the Helen and Martin Kimmel Award for Innovative Investigation; the Helen and Martin Kimmel Institute for Magnetic Resonance Research, which he heads; the Adelis Foundation; the Mary Ralph Designated Philanthropic Fund of the Jewish Community Endowment Fund; Gary and Katy Leff, Calabasas, CA; Paul and Tina Gardner, Austin TX; and the European Research Council.

 

 
Mapping of size distributions of a mouse’s gray matter by quantum-controlled proton MRI. (l) Brain proton MRI; (c) mean cellular size; (r) distribution peak
Space & Physics
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Liquid Phase

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(l-r) Dr. Vladimir Umansky, Prof. Israel Bar-Joseph and Dr. Michael Stern
 

 

When we lose something, its memory can stay with us, granting the object an ephemeral existence. We know this psychologically and even physiologically – for instance, in the phenomenon of phantom pain from an amputated limb. But in the material world of atoms and electrons, the void created by a thing’s disappearance can be every bit as real as the thing itself.
 
Take, for instance, electrons in a semiconductor material. When light hits them, they may undergo optical excitation that causes them to jump to a higher energy level, leaving behind a hole. That hole, however, is not a lack of substance but rather an opposite: It is a physical entity in all respects, with a positive charge, as opposed to the electron’s negative one. Sometimes the electron, as it jumps to the higher energy level, briefly orbits the hole – something like an atom with its positively charged nucleus – and the electron-hole system now becomes an “exciton.”

An exciton, in contrast to a natural atom, is a fleeting flash in the pan. Within mere nanoseconds, the electron has dropped down to its former energy state, and the hole disappears. As the electron is reabsorbed, a photon – a particle of light – is emitted. That emission, tiny but measurable, has become a tool much used by researchers to understand the lifestyle of electrons, as well as having many practical applications.
 
 
When it comes to excitons, scientists ask: Is it possible to produce complex structures with large numbers of excitons? Could one create, for example, a crystal or a liquid from these atom-like objects? Prof. Israel Bar-Joseph of the Condensed Matter Physics Department, together with Drs. Michael Stern and Vladimir Umansky took up this challenge; they succeeded in observing, for the first time, the formation of an exciton liquid. The results of their research were recently published in Science.
Microscope image of the sample, with electrical contacts
 
The primary obstacle to creating such complex structures has been the excitons’ short lifespan. They vanish the instant they are formed, making it difficult to amass enough of them in a sufficiently dense pack to flow as a liquid. The idea then, was to extend the survival of excitons. To do this, the researchers used multilayered materials composed of sheets just a few atoms thick. Growing such finely layered materials is one of the specialties of the submicron center at the Weizmann Institute. Within the layers, the scientists created “quantum wells” to trap the electrons, along with their holes.
 
By designing the structure of the material so that the quantum wells were very close together and then applying an electric field, the researchers managed to produce a situation in which the electrons were trapped in one layer while their holes were stuck in the next layer. Though the mutual attraction was still there, and thus an exciton still formed, the separation of the layers made it much harder for the electron to reunite with its hole. The scientists found that they could use the intensity of the electric field to control the life span of the excitons.  
 
Two phases: The inner blue regions are the liquid phase and the others – the gas phase
 
When the system reached very low temperatures (necessary for certain quantum phenomena to occur) and the exciton density rose to a critical level, the researchers observed a sharp transition. The system was now separated into two areas: The first area resembled a gas, in which the excitons moved randomly with no relation to the others; in the second the movement was organized, as in a liquid.
 
Although the excitons flowed in liquid fashion, the underlying physics was actually the opposite of the forces that produce a liquid state in ordinary substances. Both liquids and crystals form through natural attraction between atoms over small distances. But excitons repel one another; in the thin layers, the repelling force between electron and electron, or hole and hole was just a tad stronger than the attracting force acting on the hole and its electron in the next layer over. The weak residual repulsion caused the now long-lived excitons to array themselves in formation – each equidistant from its neighbors – to produce an organized, dynamic structure resembling a liquid state.

 

Prof. Israel Bar-Joseph’s research is supported by the Carolito Stiftung; and the estate of Toni Fox. Prof. Bar-Joseph is the incumbent of the Jane and Otto Morningstar Professorial Chair of Physics.
 

 
(l-r) Dr. Vladimir Umansky, Prof. Israel Bar-Joseph and Dr. Michael Stern
Space & Physics
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Atoms Take a Bath

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Drawing by Ron Weinstock, a student at the Henry Ronson ORT High School in Ashkelon
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
The laws of thermodynamics – the basic laws of energy exchange in physical systems – apply to all machinery. Until now, most physicists would have said that a machine built of a single atom, though it may operate according to the strange rules of quantum mechanics, is subject to the same thermodynamic restrictions as a steam engine. But until recently, no one could actually prove this assumption. Now, research by Prof. Gershon Kurizki in the Institute’s Faculty of Chemistry, conducted together with scientists in the Czech Republic and Poland, suggests that quantum machines may operate under somewhat different sets of rules. Although the basic laws of thermodynamics remain valid in the quantum realm, some of the restrictions they entail may differ from those encountered in large, “classical” machines. In theoretical work described in Physical Review Letters and Physical Review, the group revealed new facets of quantum machinery for converting heat energy to work, or work to cooling.
 
Prof. Gershon Kurizki and David Gelbwaser-Klimovsky
 
The starting point for the present research was a surprising effect that had previously been discovered in Kurizki’s group: Measuring (which is a form of observing) a quantum system – for example a single atom – causes it to heat up or cool down. Whether it heats or cools depends on the rate of measurement. Normally an atom placed in a larger system exchanges heat with its environment – known in scientific terms as a “heat bath.” Such a system should reach an equilibrium in which the temperature of the atom is the same as that of the heat bath. But from the point of view of the atom, energy is still being passed back and forth periodically between it and its surroundings.

In 2008, Kurizki’s group showed, in an article that appeared in Nature, that repeated measurements can temporarily decouple this oscillation so that increasing or lowering the temperature of the atom depends only on the rate of measurement. This means that the atom can be heated or cooled beyond the temperature of the surrounding bath by just observing it – in apparent violation of the laws previously believed to be imposed by thermodynamics. That finding was corroborated in an experiment conducted by the Weizmann Institute’s Dr. Gonzalo Alvarez and Prof. Lucio Frydman, which was published in 2010 in Physical Review Letters.

Kurizki and research student David Gelbwaser-Klimovsky have now found that the same quantum phenomenon gives the atom the ability to influence its energy exchange with the outside world in the form of useful work. If the atom, after being immersed in a bath, is then hooked up to an oscillator, and the measurements are performed on the atom at the right rate, the atom will amplify the oscillations and thereby execute work. This finding has implications for current theories – for example, the widely accepted principle proposed in 1961 by Rolf Landauer which states that work obtainable from a measurement must not exceed the energy cost of erasing its record from the observer’s memory. Here, no such record is required, so Landauer’s cost does not apply to the work extracted from the measurement.
 
Heat engine
 
 
That setup led the scientists to invent the world’s smallest refrigerator: They showed that if the atom is coupled to two baths – one heating the atom and the other cooling it – part of the heat will be converted to work that can have an amplifying effect on the oscillator. When the frequency of those oscillations gets high enough, the machine becomes a quantum refrigerator, lowering the temperature of the already-cold bath.
 
quantum fridge
 
 
On a basic level, these findings may help bridge the gap between classical thermodynamics and quantum mechanics. On a more practical level, they may have implications for the cooling of tiny electronic components and nanodevices. Today, the technical difficulties in cooling such devices are one of the main limitations on chip miniaturization.
 
According to Kurizki, the findings may call into question one version of the third law of thermodynamics as it was first formulated by Walther Nernst in 1908. As Nernst put it, it is impossible for any procedure to lead to an absolute-zero temperature in a finite number of steps. Indeed, the classical idea of cooling involves energy or heat flowing from a warmer to a cooler substance, implying one could never actually get down to absolute zero. But the new findings suggest otherwise. Kurizki: “It may be possible to attain absolute cooling if the bath is a chain of spins (magnetized particles) connected to a single-atom ‘refrigerator.’ That chain of spins could fully freeze. Not a few physicists who have been trained to accept the third law as Gospel will be uncomfortable with this prediction. But that, ultimately, is what drives science – surprises and debate.” As for experimental proof, Kurizki thinks the theory may be tested by experiments in ultra-cold gases.
 
Prof. Gershon Kurizki is the incumbent of the George W. Dunne Professorial Chair of Chemical Physics.


 
 
 
 
Drawing by Ron Weinstock, a student at the Henry Ronson ORT High School in Ashkelon
Space & Physics
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Quantum Effects in Cold Chemistry

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(l-r)Sasha Gersten, Alon Henson, Dr. Ed Narevicius, Etay Lavert-Ofir, Julia Narevicius and Yuval Shagam

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
At very low temperatures, close to absolute zero, chemical reactions may proceed at a much higher rate than classical chemistry says they should: In this extreme chill, quantum effects enter the picture. A Weizmann Institute team recently confirmed this experimentally. Their results should not only provide insight into processes in the intriguing quantum world in which particles act as waves, they might explain how chemical reactions occur in the vast frigid regions of interstellar space.

Long-standing predictions say that quantum effects should allow the formation of a transient bond – one that will force colliding atoms and molecules to orbit each other instead of separating after the collision. Such a state would be very important, as orbiting atoms and molecules would then have multiple chances to interact chemically. In this theory, a reaction that would seem to have a very low probability of occurring would, at certain energies, proceed very rapidly.

Dr. Ed Narevicius and his team in the Institute’s Chemical Physics Department managed, for the first time, to experimentally confirm this elusive process in a reaction they performed at temperatures just a fraction of a degree above absolute zero – 0.01 K. Their results appeared recently in Science.
 
The experimental system: two supersonic valves followed by two skimmers. The blue beam passes through a curved magnetic quadrupole guide, and the merged beam (purple) enters a quadrupole mass spectrometer. B is a front view of the quadrupole guide
“The problem,” says Narevicius, “is that in classical chemistry, we think of reactions on the molecular level in terms of colliding billiard balls held together by springs. In the classical picture, reaction barriers sometimes block those billiard balls from approaching one another, whereas in the quantum physics world, reaction barriers can be penetrated by particles, as these acquire wave-like qualities at ultra-low temperatures.”

The quest to observe quantum effects in chemical reactions started over half a century ago with pioneering experiments by Dudley Herschbach and Yuan T. Lee, who later received a Nobel Prize for their work. They succeeded in observing chemical reactions at unprecedented resolution by colliding two low-temperature, supersonic beams. However, the collisions took place at relative speeds that were much too high to resolve many quantum effects: When two fast beams collide, the relative velocity sets the collision temperature at above 100 K, much too warm for quantum effects to play a significant role. Over the years, researchers had used various ingenious techniques, including changing the angle of the beams and slowing them down to a near-halt. These managed to bring the temperatures down to around 5 K – close, but still a miss for those seeking to observe chemical reactions in quantum conditions.

The innovation that Narevicius and his team, including Alon B. Henson, Sasha Gersten, Yuval Shagam and Julia Narevicius, introduced was to merge the beams rather than collide them. One beam was produced in a straight line, and the second beam was bent using a magnetic device until it was parallel to the first. Even though the beams were racing at high speed, the relative speed of the particles in relation to the others was zero. Thus a much lower collision temperature of only 0.01 K could be achieved. One beam contained helium atoms in an excited state, the other either argon atoms or hydrogen molecules. In the ensuing chemical reaction, the argon or hydrogen molecules became ionized – releasing electrons.

To see if quantum phenomena were in play, the researchers looked at reaction rates – a measure of how fast a reaction proceeds – at different collision energies. At high collision energies, classical effects dominated and the reaction rates slowed down gradually as the temperature dropped. But below about 3 K, the reaction rate in the merged beams suddenly took on peaks and valleys. This is a sign that a quantum phenomenon known as “scattering resonances due to tunneling” was occurring in the reactions. At low energies, particles started behaving as waves: Those waves that were able to tunnel through the potential barrier interfered constructively with the reflected waves upon collision. This created a standing wave that corresponded to particles trapped in orbits around one another. Such interference occurs at particular energies and is marked by a dramatic increase in reaction rates.

Narevicius: “Our experiment is the first proof that the reaction rate can change dramatically in the cold reaction regime. Beyond the surprising results, we have shown that such measurements can serve as an ultrasensitive probe for reaction dynamics. Our observations already prove that our understanding of even the simplest ionization reaction is far from complete; it requires a thorough rethinking and the construction of better theoretical models. We expect that our method will be used to solve many puzzles in reactions that are especially relevant to interstellar chemistry, which generally occurs at ultra-low temperatures.”
 
Dr. Edvardas Narevicius is the incumbent of the Ernst and Kaethe Ascher Career Development Chair.


 
 
The experimental system: two supersonic valves followed by two skimmers. The blue beam passes through a curved magnetic quadrupole guide, and the merged beam (purple) enters a quadrupole mass spectrometer. B is a front view of the quadrupole guide
Space & Physics
English

Tiny Vibrations

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(l-r) Shlomi Kotler, Yinnon Glickman, Nitzan Akerman and Dr. Roee Ozeri. Separating the signal from the noise
 

 

 

 
The lab, though it may seem quiet and insulated, can be as full of background noise as a crowded train station when we’re trying to catch the announcements. Our brains can filter out the noise and focus on the message up to a certain point, but turning up the volume on the loudspeakers – improving the signal-to-noise ratio – helps as well.
 
Separating out the signal from the noise – increasing one while reducing the other – is so basic that much of scientific research could not take place without it. One common method, developed by the physicist Robert Dicke at Princeton University, is based on a principle similar to the one that enables radio broadcasts to pass through the noisy atmosphere. In short, one modulates electric waves (which correspond to the sound waves) one wishes to send over long distances, adding them on top of a high-frequency wave. Someone wanting to listen to the broadcast, must have a receiver that is tuned to the frequency of the carrier wave (the numbered band on the FM dial), which then splits the two waves apart and amplifies the second, “rider” wave – the music or talk the listener wants to hear.
 
The method used by the physics labs is called “locked-in amplification.” Here, too, a low-frequency, measured signal “rides” a high-frequency wave. A locked-in amplifier singles out the specific wave from the rest of the noise, “locking on” to the required signal and enabling scientists to make a great variety of accurate measurements.
 
To obtain good spatial resolution, one should measure with the smallest possible detector, and one can’t get much smaller than a single atom. The world of single atoms, however, is governed by the laws of quantum physics, and any sort of observation in the quantum world is a complex undertaking. The Heisenberg uncertainty principle, one of the cornerstones of quantum theory, sets limits on our ability to measure with any kind of precision. But that very theory contains some clues as to how these limits can be approached.


Dr. Roee Ozeri and research students Shlomi Kotler, Nitzan Akerman, Yinnon Glickman and Anna Keselman in the Physics of Complex Systems Department applied the rules of quantum mechanics to a single atomic-ion detector, building a quantum version of a locked-in amplifier. Using the ion’s spin as a sensor, they were able to measure magnetic vibrations with a spatial resolution of just a few nanometers (a few billionths of a meter). The sensitivity of this measurement was extremely high: around 100 times better than any previous such measurement. This technique, says Ozeri, could be used in physics labs around the world to improve the sensitivity of all kinds of quantum sensors.
 
Ion trap in the lab of Dr. Roee Ozeri
 


Dr. Roee Ozeri’s research is supported by the Yeda-Sela Center for Basic Research; the Wolfson Family Charitable Trust; David Dickstein, France; Martin Kushner Schnur, Mexico; and the Crown Photonics Center.

Ion trap in the lab of Dr. Roee Ozeri
Space & Physics
English

Listening with One Atom

English
 
The lab, though it may seem quiet and insulated, can be as full of background noise as a crowded train station when we’re trying to catch the announcements. Our brains can filter out the noise and focus on the message up to a certain point, but turning up the volume on the loudspeakers – improving the signal-to-noise ratio – helps as well.
 
Separating out the signal from the noise – increasing one while reducing the other – is so basic that much of scientific research could not take place without it. One common method, developed by the physicist Robert Dicke at Princeton University, is based on a principle similar to the one that enables radio broadcasts to pass through the noisy atmosphere. In short, one modulates electric waves (which correspond to the sound waves) one wishes to send over long distances, adding them on top of a high-frequency wave. To listen to the broadcast, one must have a receiver that is tuned to the frequency of the carrier wave (that numbered band on the FM dial), which then splits the two waves apart and amplifies the second “rider” wave – the music or talk we want to hear.
 
The method used by the physics labs is called “locked-in amplification.” Here, too, a low-frequency, measured signal “rides” a high-frequency wave. A locked-in amplifier singles out the specific wave from the rest of the noise, “locking” onto the required signal and enabling scientists to make all sorts of accurate measurements.
 
To obtain good spatial resolution, one should measure with the smallest possible detector; one can’t get much smaller than a single atom. The world of single atoms, however, is governed by the laws of quantum physics, and any sort of observation in the quantum world is a complex undertaking. The Heisenberg uncertainty principle, one of the cornerstones of quantum theory, sets limits on our ability to measure with any kind of precision. But that very theory contains some clues as to how these limits can be approached.
 
Dr. Roee Ozeri and research students Shlomi Kotler, Nitzan Akerman, Yinnon Glickman and Anna Keselman in the Weizmann Institute’s Physics of Complex Systems Department applied the rules of quantum mechanics to a single atomic-ion detector, building a quantum version of a locked-in amplifier. Using the ions’ spin as a sensor, they were able to measure magnetic vibrations with a spatial resolution of a just few nanometers (a few billionths of a meter). The sensitivity of this measurement was extremely high: around 100 times better than any previous such measurement. This technique, says Ozeri, could be used in physics labs around the world to improve the sensitivity of all sorts of quantum sensors.
 
 
Ion trap in the lab of Dr. Roee Ozeri
 

Dr. Roee Ozeri’s research is supported by the Yeda-Sela Center for Basic Research; the Wolfson Family Charitable Trust; David Dickstein, France; and Martin Kushner Schnur, Mexico. 

Ion trap in the lab of Dr. Roee Ozeri
Space & Physics
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Biological Molecules Select Their Spin

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New findings could help build better biomedical devices

 
Do the principles of quantum mechanics apply to biological systems? Until now, says Prof. Ron Naaman of the Institute’s Chemical Physics Department (Faculty of Chemistry), both biologists and physicists have considered quantum systems and biological molecules to be like apples and oranges. But research he conducted together with scientists in Germany, which appeared recently in Science, definitively shows that a biological molecule – DNA – can discern between quantum states known as spin.

Quantum phenomena, it is generally agreed, take place in extremely tiny systems – single atoms, for instance, or very small molecules. To investigate them, scientists must usually cool their material down to temperatures approaching absolute zero. Once such a system exceeds a certain size or temperature, its quantum properties collapse, and “every day” classical physics takes over. Naaman: “Biological molecules are quite large, and they work at temperatures that are much warmer than the temperatures at which most quantum physics experiments are conducted. One would expect that the quantum phenomenon of spin, which exists in two opposing states, would be scrambled in these molecules – and thus irrelevant to their function.”

But biological molecules have another property: they are chiral. In other words, they exist in either “left-” or “right-handed” forms that can’t be superimposed on one another. Double-stranded DNA molecules are doubly chiral – both in the arrangement of the individual strands and in the direction of the helices’ twist. Naaman knew from previous studies that some chiral molecules can interact in different ways with the two different spins. Together with Prof. Zeev Vager of the Particle Physics and Astrophysics Department, research student Tal Markus, and Prof. Helmut Zacharias and his research team at the University of Münster, Germany, he set out to discover whether DNA might show some spin-selective properties.

The researchers fabricated self-assembling, single layers of DNA attached to a gold substrate. They then exposed the DNA to mixed groups of electrons with both directions of spin. Indeed, the team’s results surpassed expectations: The biological molecules reacted strongly with the electrons carrying one of those spins, and hardly at all with the others. The longer the molecule, the more efficient it was at choosing electrons with the desired spin, while single strands and damaged bits of DNA did not exhibit this property. These findings imply that the ability to pick and choose electrons with a particular spin stems from the chiral nature of the DNA molecule, which somehow “sets the preference” for the spin of electrons moving through it.

In fact, says Naaman, DNA turns out to be a superb “spin filter,” and the team’s findings could have relevance for both biomedical research and the field of spintronics. If further studies, for instance, bear out the finding that DNA only sustains damage from spins pointing in one direction, then exposure might be reduced and medical devices designed accordingly. On the other hand, DNA and other biological molecules could become a central feature of new types of spintronic devices, which will work on particle spin rather than electric charge, as they do today.
 
 
Prof. Ron Naaman is head of the Nancy and Stephen Grand Research Center for Sensors and Security, and his research is supported by Rachel Schwartz, Canada. Prof. Naaman is the incumbent of the Aryeh and Mintzi Katzman Professorial Chair.
 
Space & Physics
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Black Holes, Quantum Information and Fuzzballs

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Today’s guest post is by Weizmann Institute physicist, Prof. Micha Berkooz. Berkooz, a string theorist, recently organized a conference at the Institute on “Black Holes and Quantum Information Theory.” We asked him about Hawking’s recent proposal, reported in Nature under the headline: There are no black holes.
 
 
Celebrated theoretical physicist Stephen Hawking has opened a can of worms in his 1976 paper on black holes. In a recent article, he is trying to put the worms back into the can. It may prove a little trickier than expected.
 
Black holes are solutions of Einstein’s equations of general relativity which have the unique property that they possess a closed surface in space which is the ultimate “point of no return.” Even the fastest things that nature allows us – light rays – will not escape if they cross this surface, dubbed the horizon. Despite their strange properties, black holes are not really exotic objects. In the theoretical realm, if one takes enough matter, throws it all to a point and fast forwards using Einstein’s equations, one ends up with a black hole. In the observational realm, there is ample astrophysical evidence that massive stars end their life as black holes, and that there are mega black holes at the center of many galaxies (including our own).
 
Black Holes, Quantum Information and Fuzzballs
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But black holes don’t just gobble things up. Rather they quite effortlessly chew things up – thoroughly and completely – and spit them out in a completely undecipherable form. This is the main point of Hawking’s work from 1976, in which he showed that a black hole in empty space emits precisely thermal (black body) radiation when quantum mechanics is taken into account. In fact it emits thermal radiation until it completely evaporates, and any initial state of the system will end up exactly the same, in the form of thermal radiation.
 
Very loosely we can understand this as follows: Consider creating an electron and positron pair, in a specific quantum state, just outside the horizon. Each of these particles can have either spin up or spin down so there are a total of four states. Suppose the black hole now gobbles up the positron, never to be seen again, and that the electron makes it back to our lab. The electron has only two states. So we started with a system which had four states and ended with a system that has two states – we lost information! Equivalently we can say that quantum mechanics is not unitary (reversible) in the presence of black holes, or in more technical terms we can say that the emitted electron is in a density matrix and not a pure state, just the same as exactly thermal radiation.
 
Hawking’s computation is extremely elegant and robust – it only uses 1) quantum field theory on 2) curved space. The former is well tested and verified in just about any high energy physics experiment, and the latter is just Einstein’s general relativity (as a classical theory). Furthermore, a very similar set of computations is successful in the context of generating the structures in the universe from primordial quantum fluctuations after the big bang. Yet around the black hole, the synthesis of these two sets of ideas leads to a bewildering result, since any high-enough energy experiment will create a black hole, and end in information-free thermal radiation.The universe just can’t help losing the information of where the keys are. This is unlike any other quantum mechanical system whose time evolution does not lose any information.
 
Interestingly, the surprising prediction for a flux of thermal radiation from a black hole fits very nicely with other properties of the black hole. Shortly before Hawking’s article, Jacob Bekenstein suggested that black holes have entropy. Bekenstein’s entropy, Hawking’s temperature and the mass of the black hole, which is the same as its energy, satisfy the ordinary laws of thermodynamics.
 
The synthesis of quantum mechanics and general relativity has been an outstanding problem for quite some time. Using string theory, and more specifically Maldacena’s AdS/CFT correspondence, it was finally established that evolution of black holes is unitary and that we do not lose any information, since we can embed black holes in standard quantum theories which we know are completely unitary. In these theories, black holes seem no different than lumps of coal that burn. The issue remains, however: Where exactly does the synthesis of field theory and classical general relativity fail, and which of their well tested properties are we forced to modify?
 
There has been a renewed interest in this question in recent years. An elegant argument from Almheiri, Marolf, Polchinski and Sully suggested that one needs to quantum-mechanically modify the horizon of a black hole into a hot membrane (whose nature is not clear). This solution has been called the “firewall” solution. In this solution, one gives up some aspects of Einstein’s equivalence principle, as well as parts of the black hole solution in classical general relativity, where it naively seems that quantum effects should be small.
 
In another solution, suggested a few years ago by Mathur, one replaces the black hole by a large set of horizon-free solutions of string theory – this is the “fuzzball” solution. This solution is quite attractive, but so far no one has been able to construct enough “fuzzballs” to account for the black hole entropy. Other solutions suggest some non-locality in space-time, which allows information to be transported from the interior of the black hole to its exterior, or replacing space-time itself by an algebraic construction, keeping only quantum mechanics. Hawking conceded already 10 years ago that black holes do not really lose information, and his recent paper provides evidence for the “fuzzball” proposal for the description of black holes.
 
This topic is one of the topics of research of the String theory group at the Weizmann Institute, Profs. Ofer Aharony, Micha Berkooz, Zohar Komargodski and Adam Schwimmer, who hosted a workshop on “Black Holes and Quantum Information” earlier this month. The workshop explored the role of entanglement entropy and quantum information theory in the resolution of the black hole information paradox, and in the very emergence of space-time as a derived concept, which seems to appear in a way similar to how thermodynamics is derived from statistical physics.
Black Holes, Quantum Information and Fuzzballs
Space & Physics
English

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