Profs. Lucio Frydman and Gershon Kurizki and D. D. Bhaktavatsala Rao. Minimal force
“In the everyday world, energy must be directly pumped into an object if we want to heat it. But for such quantum-sized objects as atoms or atomic nuclei, all one has to do is ‘take their temperature’,” says Prof. Gershon Kurizki of the Institute’s Chemical Physics Department in the Faculty of Chemistry. Recently, Kurizki and Prof. Lucio Frydman of the same department demonstrated this principle. The results of their experiment may, in the future, lead to new applications in magnetic resonance, as well as open new ways of storing information.
In 2008, Kurizki and his colleagues Noam Erez and Goren Gordon predicted in an article appearing in Nature that measuring certain quantum systems could coax them into more- or less-ordered states. (The more ordered a system, the colder one can say it is.) This prediction was based on a fundamental effect of quantum physics: Many consecutive measurements or observations will change the system. The key, according to Kurizki’s research, is in the timing – how frequently these measurements are repeated. Extremely frequent measurements, for example, might heat a system up, whereas slightly slower rates could cool it down.
This is where Frydman, an experimental scientist who enjoys pushing the limits of nuclear magnetic resonance (NMR), came into the picture. Together with Kurizki and postdoctoral researchers Gonzalo Alvarez and D. D. Bhaktavatsala Rao, he came up with a way to test the prediction using NMR. “NMR turns out to be an ideal technology for conducting such experiments. The long radio waves it employs make it many times slower and often much more accurate than what is feasible with other methods,” says Frydman. “This gives us a great level of control.”
Kurizki’s work was based on an open quantum system: one in which a small ensemble of quantum particles interacts with a surrounding “bath” consisting of many particles. Just as a normal-sized object and a bath of water in which it is placed will exchange heat, eventually causing their temperatures to equalize, quantum objects in a particle bath tend to reach an equilibrium point with their surroundings. On the quantum level, that equilibrium may entail more than balancing heat: It can involve changes in certain quantum characteristics, for instance, in the intrinsic angular momentum carried by many atomic nuclei – a property called spin. Particle spins that are ordered – i.e., aligned with one another – will resemble a system that has been “cooled”; less-ordered, more random spin arrangements will make the system appear “hotter.” According to Kurizki’s prediction, measurements can disrupt the equilibration between the quantum ensemble and its bath, a finding that contradicts the expectations of classical thermodynamics. In other words, measurements can effectively free the particles from some of their bath’s influence, allowing one to “reset” their temperature.
In the experiment assayed by this team, the bath was composed of a large number of protons (hydrogen nuclei), while the quantum particles consisted of scattered 13C nuclei associated with carbon atoms. To mimic the measurement process, the scientists applied short magnetic pulses and, as they did so, they looked at the alignment of the 13C spins. Initially, these nuclei were in a disordered state, with their spins pointing every which way. But using varied timing for the magnetic pulses – at the rate of about one to ten per millisecond – the spins could be lined up parallel or anti-parallel to the magnetic field. “It’s like a person wandering back and forth along a path,” says Frydman. “By deciding when and where we stop him, thus ‘resetting’ his walk, we control where he ends up. With our experimental system, we found that we could align groups of quantum particles’ spins ‘upwards’ or ‘downwards’ by this approach – and in certain instances end up obtaining higher alignments than those achievable by other methods for manipulating such systems.”
The scientists were surprised at the extent to which the experimental results matched the theory, and they have begun to envision possible applications. Frydman, for instance, believes that a method for controlling the spins of quantum particles could increase the effectiveness of certain NMR and MRI experiments. Kurizki, meanwhile, intends to investigate how the principle could help overcome one of the stumbling blocks to building quantum computers. “To create a quantum memory register,” he says, “one needs to begin with all the particles’ spins aligned in the same direction. Our method could do this with minimal ‘brute force’. Generating the necessary order might be as simple as finding the right timing for the repeating of a measurement.”
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 Fritz Haber Center for Physical Chemistry, which he heads; the Willner Family Leadership Institute for the Weizmann Institute of Science; the estate of Hilda Jacoby-Schaerf; and the estate of Lela London.
Eight atomic ions trapped in a quantum state in the lab of Dr. Roee Ozeri
As large objects, we’re limited to existing in one place, and one state, at a time. Quantum particles have a much more interesting existence: According to quantum theory, they can be in different places, in different states, doing different things – all at the same time.
Computers based on quantum mechanics might complete, reasonably quickly, calculations that would take today’s computers a million years. One necessary step to creating such a quantum computer is to design a switch that can be in two states at once (i.e., zero and one). Scientists in the Institute’s Faculty of Physics are on the cutting edge of this field. Prof. Adi Stern has invented a method to check whether a type of system based on the movement of composite particles – arising from the collective behavior of electrons in a magnetic field – can be one such switch, called a topological quantum switch.
And how will we overcome one of the basic tenets of quantum theory – that observation changes the results? Dr. Roee Ozeri’s advanced experimental system, one of only a few in the world, addresses this and other questions. He holds strontium ions in a special trap, in which he can “observe” them existing in two states at once.
Prof. Yaron Silberberg investigates quantum phenomena occurring between light particles (photons). His studies of entanglement between particles emitted from the same source, and of changes that take place as light passes through various materials, may lead to the development of microscopes that can focus on organelles and even molecules in living cells. Prof. Nir Davidson traps cold atoms with lasers; these might be able to store quantum information accurately for extended periods. His research could aid in the development of robust quantum communications and applications in navigation and space travel.
(l-r) Dr. Roee Ozeri, Profs. Adi Stern, Yaron Silberberg and Nir Davidson
Prof. Nir Davidson is the incumbent of the Peter and Carola Kleeman Professorial Chair of Optical Sciences.
Dr. Roee Ozeri’s research is supported by the Wolfson Family Charitable Trust; the Lord Sieff of Brimpton Memorial Fund; and David Dickstein, France.
Prof. Yaron Silberberg is Head of the Crown Photonics Center; his research is supported by the Wolfson Family Charitable Trust; and the Cymerman- Jakubskind Prize. Prof. Silberberg is the incumbent of the Harry Weinrebe Professorial Chair of Laser Physics.
Prof. Adi Stern’s research is supported by the Wolfson Family Charitable Trust.
“God doesn’t play dice with the world,” remarked Albert Einstein, referring to one of the pillars of quantum theory – randomness. In the quantum world, this property can take a unique twist: Two separate events may be random, yet perfectly linked. It’s as though dice rolled in two different cities always fall at the exact same instant on the same number. Einstein felt this should be impossible, but such “entangled systems” have been observed in a number of labs around the world.
Dr. Barak Dayan, who recently joined the Institute’s Chemical Physics Department, is attracted by the seemingly illogical nature of entangled systems. Though they fly in the face of intuition, such systems form the basis of quantum optics, a field that might revolutionize computing and communication sciences. Quantum information processing, as the new technology is called, will work in profoundly different ways from those that we’re used to. For instance, using entanglement, quantum computers could carry out parallel computations that are impossible in today’s conventional computers. Dayan compares such computing to scanning a large number of suitcases for a bomb. Rather than check them one at a time, a quantum setup could address the whole set of suitcases as an entangled system, revealing the location of the bomb after a drastically smaller number of trials.
But quantum entanglement shares a drawback with other quantum systems: Measurement (or any observation) affects the final result. In practical terms, this means that during the quantum process such systems must be strictly isolated to prevent interactions with the outside world – any such interaction could lead to information “leaking out.” In the systems Dayan devises, an atom is the “computer” and single photons (light particles) are the “input and output” – vehicles for transferring quantum information to and from the atom. “The minimalist design of the system curtails information escape,” he says.
The trick is to control every aspect of the interactions between the atoms and photons. To do this, Dayan gets a single atom to interact with a single photon. This is no trivial feat: Individual photons are quite weak and an exceptionally strong sensor is needed to discern them. And, if this were not enough, the lone atom the photon must aim for is an extremely tiny target. Dayan also means to control the outcome of this get-together – the exact path of the photon, whether the atom absorbs or scatters the photon, how it reacts to the information carried in the light particle, etc. He uses novel technology he helped to develop at the California Institute of Technology (Caltech), consisting of lasers and magnetic fields for manipulating the individual atoms and bringing them into contact with the photons, which are confined within tiny, chip-based ring-like resonators smaller than one-third the width of a human hair.
Dayan aims to investigate interactions between individual photons in which the atom acts as a go-between. This presents a great challenge, as photons normally don’t “speak” to one another. In his postdoctoral work at Caltech, Dayan used the atom-photon interaction in the resonator to create a “revolving door.” Photons could enter the resonator singly, in pairs or threesomes, but they had to leave in single file, one at a time. This study, which appeared in Science, is one of the very few demonstrations to date of interactions between single photons.
An understanding of how to let single photons interact with one another could lead to all-optical switches. In such a switch, one photon could signal to the others “wait a sec, I’m going through.” More importantly, such atom-mediated photon-photon interactions could serve as the basis of logic gates for quantum computers.
A quantum optic experimental system developed by Dr. Barak Dayan at Caltech. An optic fiber (horizontal line, left) brings light to resonators (light dots appearing in a vertical row on the optic chip, center). Lasers above the chip are used to cool and manipulate single atoms
Entangled in the Lab
Dr. Barak Dayan was born in Tel Aviv in 1970. Through the elite “Talpiot” academic army project, he completed a B.Sc. in physics and mathematics and an M.Sc. in physics at the Hebrew University of Jerusalem. After leaving the Israel Defense Forces in 1999, he started Ph.D. studies in the group of Prof. Yaron Silberberg of the Weizmann Institute’s Physics of Complex Systems Department. There he took part in initiating and setting up a brand new quantum optics lab. One of his experiments provided some of the first examples ever of interactions between pairs of entangled photons, demonstrating the counter-intuitive mathematical rules that govern entangled systems. During this time, he also planned and taught courses in the Institute’s Young@Science programs.
Following postdoctoral research in the quantum optics group of Prof. Jeff Kimble at Caltech, Dayan returned to Israel and joined the Weizmann Institute’s Chemical Physics Department in 2008. Outside the lab, his efforts at science education are aimed at his children, Tamar, aged eight, and Oded, aged three, with whom he loves spending most of his free time.
Dr. Barak Dayan’s research is supported by the estate of David Guerchenson.
(l-r) Dr. Roee Ozeri, Prof. Adi Stern and Prof. Moty Heiblum. Pushing quantum limits
Tomorrow's computer might be a quantum one based on the physics of particles smaller than atoms. No one is quite sure what a quantum computer should look like, or even whether it's possible to build a functional one; but scientists at the Weizmann Institute have been working on some of the basic questions that will need to be answered before we can begin to create this new kind of computer.
Is it a qubit?
At the level of atomic and subatomic particles, things work very differently from the macro world of everyday objects. For instance, there's wave-particle duality: The basic bits of matter and light behave sometimes as discrete particles and sometimes as waves, which can be in many locations at once. And there's quantum superposition – particles existing simultaneously in more than one state at a time – which could, theoretically, provide a dramatic increase in computing power. An electronic computer bit can be in only one of two states (0 or 1), whereas a quantum bit (called a qubit) can exist simultaneously in both 0 and 1, in an infinite number of different superpositions. The challenge for scientists is to connect these fragile quantum states to the larger world without destroying them.
Several years ago, Prof. Adi Stern of the Condensed Matter Physics Department came up with a way to test a system to see whether it could be used as a special kind of qubit – a topological quantum bit. The system in question involves, on the one hand, electrons moving in a very cold two-dimensional plane, with a strong magnetic field applied at right angles to the plane and, on the other, quasiparticles. These "imaginary" particles – which have electrical charges of one-third, one-quarter or one-fifth of an electron – don't exist in nature, but they have been created and measured in the lab of Prof. Moty Heiblum of the Condensed Matter Physics Department.
Such a system must meet several criteria before it can be considered a possible qubit. The particles must be able to exchange places, and this exchange must leave a sort of trail that can be traced – that is, it must preserve information. In Stern's theoretical experiment, two parallel lines of current run through such a system with a separation "wall" containing quasiparticles between them. An odd number of quasiparticles should cause the electrons in the current to behave as particles, flowing in line through the material. But if they are separated by an even number, the electrons in the system should act as waves, producing interference patterns at the end of the current pathways.
In addition to the number of fractionally charged particles in a system, the fraction itself is relevant. The quasiparticles Heiblum measured in the 1990s had odd denominators, and these don't leave traces when they exchange places in the plane, making them unfit for storing information. Even-denominator fractions might be better for the purpose, but they're harder to produce. This past year, Heiblum and Stern, together with research student Merav Dolev and Drs. Vladimir Umansky and Diana Mahalu, all of the Condensed Matter Physics Department, succeeded in creating a nanoscopic device in which quasiparticles with one-quarter the charge of an electron were measured for the first time. They are now continuing their experiments to find out if quarter-charge quasiparticles are truly suitable candidates for quantum bits.
Can quantum errors be corrected?
Quantum "weirdness" – the strange reality that rules the world of ultra-tiny particles – presents some unique challenges. For instance, how can one perform calculations in a system in which the very act of measurement changes the basic configuration of that system?
Quantum superposition has been demonstrated in particles such as electrons, but it has never been observed in larger objects composed of many particles. The reason, scientists believe, is that in larger groups the particles interact with one another and with their environment, forcing the quantum superposition of the system into a single classical state. (Measurement is one form of interaction.) This transition is called decoherence. One could conceivably build a very simple quantum computer with only a few qubits, but how to create one that has millions?
Since joining the Weizmann Institute in 2007, Dr. Roee Ozeri and his students Nitzan Akerman, Yinnon Glickman, Shlomi Kotler, Yoni Dallal and Anna Keselman have been setting up a lab in the Physics of Complex Systems Department, and they have recently begun to conduct experiments that may one day help overcome the limitations. Ozeri is especially interested in error correction in quantum computing. Today's electronic computers compensate for possible errors by building in redundancy and using error-correction protocols. In analog quantum protocols, different kinds of error correction may help overcome decoherence and keep superpositions of many particle states "alive." Ozeri is also investigating ways of creating complex quantum logic gates – the basic operations of quantum computing – in which actions performed on one qubit can, under the right conditions, change the state of a second. Because quantum systems can't be measured directly without affecting the result, Ozeri must use roundabout methods that ascertain whether there are errors in the qubits' final state.
His experimental quantum system is based on ions – specifically, atoms of the element strontium that have undergone "laser surgery" to remove some of their electrons. Several of these ions are fired into a vacuum chamber, where they're trapped in an array of electrical fields, while another laser cools them to within a few millionths of a degree of absolute zero. Although Ozeri's experiments trap just a few ions at a time, he can examine the effects of decoherence by applying an electromagnetic field to create noise in the ions' environment. For "communicating" with the ions, he uses yet more lasers, which are precisely tuned to interact with various transitions between strontium ion states.
While the challenge of creating the basis of future computers is compelling, it is ultimately the questions of basic physics that Ozeri finds most fascinating: "We've been exploring the physics of the quantum world for around 100 years, and those of macro systems for much longer, but we still don't know much about the point at which one takes over from the other, how the transition happens or whether it's possible to push the limits and extend the quantum superposition principle into many-particle systems. This research might help provide answers to some of these very basic mysteries."
Do quantum codes communicate better?
Assuming quantum computers become a reality one day, what will they be used for? Will they be more efficient for every type of operation? For example, factoring large numbers – a process that could be used to break some encryption codes – is believed to take an impossibly long time on today's computers, but it could be done quite efficiently on a quantum computer. Prof. Ran Raz of the Computer Science and Applied Mathematics Department investigates whether communication between computers would be better with quantum methods. Quantum computers may be far in the future, but quantum communication has already been successfully demonstrated in experiments.
An example of a problem involving communication is a program for setting up a two-person meeting. The minimum number of bits needed to be communicated today to find a common free hour in each participant's network calendar equals the number of calendar slots that must be checked (n). But a quantum communication protocol could perform the same task using just the square root of (n) bits. Raz found that the difference for some other communication problems could be even greater: The improvement would be logarithmic. In other words, as the value of n rises, quantum communication protocols should quickly leave classic ones behind in the dust.
Prof. Moty Heiblum's research is supported by the Joseph H. and Belle R. Braun Center for Submicron Research. Prof. Heiblum is the incumbent of the Alex and Ida Sussman Professorial Chair of Submicron Electronics.
Dr. Roee Ozeri's research is supported by the Chais Family Fellows Program for New Scientists.
An artist’s impression of the quantum Hall device in which even fractional charges were measured
When is dividing by four more complicated than dividing by three or five? When the thing divided is the charge of an electron. A decade after they discovered “quasiparticles” with charges of one-third and one-fifth, Weizmann Institute physicists have succeeded in demonstrating, for the first time, the existence of quasi-particles with one-quarter the charge of an electron. This finding could be a first step toward creating exotic types of quantum computers that might be powerful, yet highly stable.
Fractional electron charges were first predicted over 20 years ago, and were found by the Weizmann group some ten years ago. Although electrons are indivisible, if they are confined to a two-dimensional layer inside a semiconductor, chilled down to a fraction of a degree above absolute zero and exposed to a strong magnetic field that is perpendicular to the layer, they effectively behave as independent particles, called quasiparticles, with charges smaller than that of an electron.
The experiment done by research student Merav Dolev in Prof. Moty Heiblum’s group, in collaboration with Drs. Vladimir Umansky and Diana Mahalu and Prof. Adi Stern, all of the Condensed Matter Physics Department, owes the finding of quarter-charge quasiparticles to an extremely precise setup and unique material properties: The gallium arsenide material they produced for the semiconductor was among the purest in the world.
Prof. Moty Heiblum’s research is supported by the Joseph H. and Belle R. Braun Center for Submicron Research. Prof. Heiblum is the incumbent of the Alex and Ida Sussman Professorial Chair of Submicron Electronics.
Dr. Ehud Altman. Disruptions in the particle dance
If classical physics called the tune, particles would become as still and silent as statues when the temperature reached absolute zero; but these particles actually continue to dance to the music of quantum mechanics, down to the very lowest degree. Is it possible to detect the hum of this so-called zero-point motion?
Advances in technology have facilitated the cooling of large collections of atoms to the dazzlingly low temperature of only one billionth of a degree above absolute zero – even colder than the outer limits of the universe. This breakthrough led to the observation of a new kind of quantum matter: the Bose-Einstein Condensate (BEC). Recently, Dr. Ehud Altman of the Weizmann Institute’s Condensed Matter Physics Department, in collaboration with Prof. Eugene Demler of Harvard University and Prof. Anatoli Polkovnikov of Boston University, USA, developed a novel idea: Experiments with ultracold atoms could serve as a means to directly observe the quantum fluctuations inside matter and detect new phases of that matter.
A BEC contains an enormous number of non-interacting atoms that, at very low temperatures, start to behave less like particles and more like waves. Each atomic wave starts to overlap with that of its neighbor, and they go through a sort of collective quantum “identity crisis,” losing their individuality to join in one big wave. When two separate BECs are allowed to expand and overlap each other, a striped pattern emerges in the gas cloud. This phenomenon – like overlapping ripples when two stones are thrown into a pond – is a direct testament to the wave nature of quantum particles. A BEC in which the waves “dance in unison” is said to be a very orderly state of matter. (In physicists’ language: The phase of the waves is not fluctuating.) Thanks to this property, the interference pattern is so incredibly sharp that the exact shape of the stripes is reproducible from one experiment to the next.
But what happens in a system in which the interactions between particles become increasingly important? This occurs, for example, when the particles can move in only a single line or a two-dimensional plane. The waves associated with different particles become entangled with one another, resulting in quantum fluctuations that disrupt the stately “dance in unison,” turning it into more of a frenetic jitterbug. In recent work published in the Proceedings of the National Academy of Sciences (PNAS), USA, Altman and his colleagues explained that these fluctuations should be observable in the interference patterns in one- or two-dimensional condensates. The stripes appearing in the interference patterns will exhibit twists and wiggles that change from one experiment to the next. Like the discordant vibrations of badly tuned guitar strings, these wiggles appear to be noise. Altman and his colleagues showed however, that they have some interesting statistical properties and thus are not noise at all, but the footprints left by quantum fluctuations. The team calculated the properties of these wiggles and explained how they could enable experimentalists to directly observe the workings of quantum mechanics in matter. Indeed, the theory developed by Altman and his collaborators has successfully been applied experimentally by a group from ENS, Paris, who observed, for the first time, an elusive phase transition in a system of ultracold atoms.
In another paper, published recently in Nature Physics, Altman and his colleagues investigated in greater detail the random fluctuations in the sharpness of the interference pattern. They showed that the fluctuations follow a remarkable probability distribution – one that usually describes rare but catastrophic events, such as stock market crashes and earthquakes.
These findings have practical value: They could facilitate the development of measurement devices that would be able to measure very slight changes in the gravitational field by tracking phase changes between two BECs. These could be used, for example, in mapping geological layers in search of oil, or as gravity wave detectors for cosmological experiments.
Dr. Ehud Altman’s research is supported by the Asher and Jeannette Alhadeff Research Award.
A gas of ultracold atoms is released from two separate traps (gray squares). The expanding clouds interfere with one another like waves (light gray ellipses), casting shadows with light and dark stripes on a detector screen. Scientists can learn about collective quantum phenomena from the statistical properties of these patterns
Crowd Behavior
Transitions between different forms of organization in matter are often driven by changes in temperature, which is a measure of the random motion of atoms inside the material. Take water, for example: As we lower the temperature, the particles slow down until they organize into the crystal structure we know as ice. When temperatures fall to near absolute zero, we might imagine that matter comes to a complete standstill, cutting short opportunities for change. But in this extreme deep freeze, the quantum mechanical nature of the particles comes into play. Quantum fluctuations – a consequence of quantum uncertainty, the principle stating that it is impossible to know the exact position of an atom and its speed simultaneously – open up possibilities for matter to organize in dramatically new and unforeseen ways.
Under the laws of quantum mechanics, the motion and behavior of single particles, strange as they may appear to us, are by now well understood. But how do quantum mechanical principles such as particle-wave duality and the uncertainty principle affect the collective behavior of the hordes of interacting particles that make up matter? This question, which Altman addresses in his research, is one of the more significant challenges facing physics today.
Prof. Amir Yacoby and Ph.D. students Merav Dolev and Sandra Foletti. Quantum states
Quantum computers might have the ability to work millions of times faster than today’s computers. For this reason, scientists around the world cherish the dream of creating practical quantum computers, even though no one is quite sure the undertaking is really feasible. But this much is already clear: to construct a quantum computer, they must devise a way of storing and processing information that is encoded in quantum bits.
A regular bit is an entity that can exist in one of two states, usually described by the digits zero (0) and one (1). In contrast, a quantum bit can exist simultaneously in many more than two states. Several candidates might perform the function of a quantum bit. One is the electron, which forms the basis of modern electronics. Electronics takes advantage of the electron’s property of carrying an electric charge; but electrons are also characterized by a kind of a spinning motion, or spin. This motion can have two opposite directions. Thus two electrons carrying the same electric charge can differ in the direction of their spins. The differences in spin direction, as well as a unique state called superposition, are likely to provide the basis for quantum computation technology.
Superposition is a quantum phenomenon first described by Erwin Schroedinger, who demonstrated it through a thought experiment known as "Schroedinger’s cat." In this experiment, a cat is placed in a closed box together with a bottle of poison whose cork is connected to a trigger made of radioactive material. When this material decays at some point, the accompanying radiation will activate the trigger, which will then open the bottle and release the poison that will kill the cat. An observer watching the closed box can’t know whether or not the radioactive trigger has already been activated, and therefore cannot be sure whether the cat inside is dead or alive. What, then, is the cat’s real state? According to quantum theory, before the observer opens the box or measures the cat’s condition in any way, the cat exists in a state of superposition: It is simultaneously dead and alive. This is not a trick but a formulation of principle: In quantum theory, an unevaluated cat is truly both dead and alive at the same time. (It’s worth noting that quantum theory is the best proven of all scientific theories.)
Prof. Amir Yacoby of the Weizmann Institute’s Condensed Matter Physics Department and a team headed by Prof. Charles Marcus of Harvard University have recently conducted an experiment showing how the spin of electrons in a state of superposition can be used as a quantum bit. The idea is based on the fact that superposition exists only until someone has observed or measured the system. At the moment of measurement, the "magic" vanishes - the cat can only be either dead or alive; it can no longer exist "in both worlds." The question is: How long can the "magic" last before somebody or something measures it, thereby ending its existence?
In the new experiment, the scientists created systems in which the electrons’ spin was in superposition, and they managed to measure, for the first time ever, the duration of this superposition in a single electron. In other words, they measured how long a situation in which an electron is characterized by spins pointed simultaneously in two directions can last - a situation, that is, which ends the moment something or someone performs a measurement on the electron. To conduct the experiment, the scientists developed a way to measure the spin directions of a single electron inside a "box." Their approach is based on a system that "translates" spin into an electric charge. The position of the charge can be measured by well-known methods, so that the system measures electric charges and derives the spins from these measurements. A report on this research has recently been published in the journal Science.
Essentially, the new approach allows scientists to point a particle’s spin in any desired direction, not only the usual two. This means that the scientists can control the spin, designing different superposition states on demand. Each such superposition can represent different information, making it possible to encode information in quantum systems based on the electrons' spin. That’s a major advance toward developing quantum computers that will be able to perform computational tasks which are difficult for regular computers - such as dismantling a product of two large primes into its original components. The ability to perform such tasks has great practical significance in several areas, including encoding and decoding information.
Prof. Yacoby and his Ph.D. students Merav Dolev and Sandra Foletti are now studying ways of grouping quantum bits, moving them from one location to another, creating logical quantum gates and performing a number of other tasks that may advance the practical application of quantum computers.
Prof. Amir Yacoby's research is supported by the Rosa and Emilio Segre Fund; and the Joseph H. and Belle R. Braun Center for Submicron Research.
Prof. Gershon Kurizki. A quick glimpseurizki. Anti-Zeno effect
For over 2,500 years, scientists and philosophers have been grappling with Zeno of Elea's famous paradox. More recently, scientists believed that the counterpart of this paradox, known as the quantum Zeno paradox, is realizable in the microscopic world governed by quantum physics. Now scientists from the Weizmann Institute have shown that in most cases, the quantum Zeno paradox should not occur. An article describing the calculations that led to this surprising conclusion appeared in Nature and was surveyed in the journal's "News and Views" section.
The Greek philosopher Zeno, who lived in the 5th Century B.C., decades before Socrates, dedicated his life's work to showing the logical paradoxes inherent in the idea of the indefinite divisibility of space and time (i.e., that every line is composed of an infinite number of points). One of these paradoxes is known as the arrow paradox: if the motion of a flying arrow is divided ad infinitum, then during each of these infinitesimal moments, the arrow is at rest. The sum of an infinity of zeros remains zero, and therefore the arrow cannot move. One can imagine how someone giving a flying arrow repeated quick glimpses, can actually freeze it in place. Zeno inferred from this that movement cannot happen. Indeed, he was a true follower of Parmenides, his teacher and mentor, who advocated that any change in nature is but an illusion.
This philosophical view was rejected by Aristotle, as well as by scientists and philosophers of the 19th century, who resolved Zeno's paradox by showing that non-zero velocity can exist in the limit of infinitesimal divisions of a trajectory. The paradox was bolstered in the 1960s, however, by the physicist Leonid A. Khalfin, working in the former USSR, and by physicists E.C.G. Sudarshan and Baidyanath Misra, working in the U.S. during the 1970s. Using quantum theory, they concluded that if an "observer" makes repeated observations of a microscopic object undergoing changes in time, it is highly probable that the object will indeed stop changing. The frequent observations divide the trajectory along which the object evolves into infinitesimal segments in which there is no change. In other words, in the quantum world an observer can freeze the evolution of an object, in accordance with Zeno's paradox.
Skeptics who doubted those calculations must have been genuinely surprised when, in 1990, Colorado University physicist John Wineland proved that "freezing glimpses" do work in the real world (or at least in a "simple" world with only two energy levels). Ever since, physicists have been struggling to understand the implications of the experiment. Can the Zeno paradox, for example, "glimpse-freeze" radioactive nuclear decay, thus stopping radiation? The prevailing answer during the past thirty years has been that such a freeze should be possible, provided the successive observations are made frequently enough.
Prof. Gershon Kurizki and Dr. Abraham Kofman of the Chemical Physics Department have shown that, for better or worse, such "freezing" does not take place in reality, and decay cannot actually be stopped by "bombarding" the system with glimpses. According to their calculations, the ability to "freeze" changes with quick glimpses depends on the ratio of the decay's memory to the time interval between successive observations. Every process of decay has a memory time. In the case of radioactive decay, for instance, this is the period in which the radiation has not yet escaped from the atom, allowing the system to "remember" its state prior to the decay. The memory time in the radiative decay process of an excited atom (an atom occupying an unstable energy level) is less than a billionth of a billionth of a second. To "freeze" this decay, the observations would have to be at intervals of much less than a billionth of a billionth of a second.
However, a sequence of observations so close in time would cause the appearance of new particles, changing the system completely and destroying it; the question of stopping the decay would thus become meaningless. On the other hand, if the time interval between observations is longer than the decay's memory time, the rate of decay and radiation is actually increased. Not only does Zeno's paradox not take effect in such a case but there is actually an opposite effect: the "anti-Zeno effect."
Kurizki: "In other words, if we make an analogy between an object undergoing changes in time -- for example, a decaying nucleus or an excited atom -- and Zeno's moving arrow, the arrow will increase its speed as the rate of the 'glimpses' increases. The surprising conclusion of this research is that the anti-Zeno effect (i.e., the increase of decay through frequent observations) can occur in all processes of decay, while the original Zeno effect, which would slow down and even stop decay, requires conditions that only rarely exist in such processes."
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Taking Quantum Particles’ Temperature
“In the everyday world, energy must be directly pumped into an object if we want to heat it. But for such quantum-sized objects as atoms or atomic nuclei, all one has to do is ‘take their temperature’,” says Prof. Gershon Kurizki of the Institute’s Chemical Physics Department in the Faculty of Chemistry. Recently, Kurizki and Prof. Lucio Frydman of the same department demonstrated this principle. The results of their experiment may, in the future, lead to new applications in magnetic resonance, as well as open new ways of storing information.