Tag Archives: quantum

Quantum Computation: Spooky Arithmetic

Quantum computation holds the promise of vastly superior performance over traditional digital systems based on bits that are either “on” or “off”. Yet for all the theory, quantum computation still remains very much a research enterprise in its very infancy. And, because of the peculiarities of the quantum world — think Schrödinger’s cat, both dead and alive — it’s even difficult to measure a quantum computer at work.

From Wired:

In early May, news reports gushed that a quantum computation device had for the first time outperformed classical computers, solving certain problems thousands of times faster. The media coverage sent ripples of excitement through the technology community. A full-on quantum computer, if ever built, would revolutionize large swaths of computer science, running many algorithms dramatically faster, including one that could crack most encryption protocols in use today.

Over the following weeks, however, a vigorous controversy surfaced among quantum computation researchers. Experts argued over whether the device, created by D-Wave Systems, in Burnaby, British Columbia, really offers the claimed speedups, whether it works the way the company thinks it does, and even whether it is really harnessing the counterintuitive weirdness of quantum physics, which governs the world of elementary particles such as electrons and photons.

Most researchers have no access to D-Wave’s proprietary system, so they can’t simply examine its specifications to verify the company’s claims. But even if they could look under its hood, how would they know it’s the real thing?

Verifying the processes of an ordinary computer is easy, in principle: At each step of a computation, you can examine its internal state — some series of 0s and 1s — to make sure it is carrying out the steps it claims.

A quantum computer’s internal state, however, is made of “qubits” — a mixture (or “superposition”) of 0 and 1 at the same time, like Schrödinger’s fabled quantum mechanical cat, which is simultaneously alive and dead. Writing down the internal state of a large quantum computer would require an impossibly large number of parameters. The state of a system containing 1,000 qubits, for example, could need more parameters than the estimated number of particles in the universe.

And there’s an even more fundamental obstacle: Measuring a quantum system “collapses” it into a single classical state instead of a superposition of many states. (When Schrödinger’s cat is measured, it instantly becomes alive or dead.) Likewise, examining the inner workings of a quantum computer would reveal an ordinary collection of classical bits. A quantum system, said Umesh Vazirani of the University of California, Berkeley, is like a person who has an incredibly rich inner life, but who, if you ask him “What’s up?” will just shrug and say, “Nothing much.”

“How do you ever test a quantum system?” Vazirani asked. “Do you have to take it on faith? At first glance, it seems that the obvious answer is yes.”

It turns out, however, that there is a way to probe the rich inner life of a quantum computer using only classical measurements, if the computer has two separate “entangled” components.

In the April 25 issue of the journal Nature, Vazirani, together with Ben Reichardt of the University of Southern California in Los Angeles and Falk Unger of Knight Capital Group Inc. in Santa Clara, showed how to establish the precise inner state of such a computer using a favorite tactic from TV police shows: Interrogate the two components in separate rooms, so to speak, and check whether their stories are consistent. If the two halves of the computer answer a particular series of questions successfully, the interrogator can not only figure out their internal state and the measurements they are doing, but also issue instructions that will force the two halves to jointly carry out any quantum computation she wishes.

“It’s a huge achievement,” said Stefano Pironio, of the Université Libre de Bruxelles in Belgium.

The finding will not shed light on the D-Wave computer, which is constructed along very different principles, and it may be decades before a computer along the lines of the Nature paper — or indeed any fully quantum computer — can be built. But the result is an important proof of principle, said Thomas Vidick, who recently completed his post-doctoral research at the Massachusetts Institute of Technology. “It’s a big conceptual step.”

In the short term, the new interrogation approach offers a potential security boost to quantum cryptography, which has been marketed commercially for more than a decade. In principle, quantum cryptography offers “unconditional” security, guaranteed by the laws of physics. Actual quantum devices, however, are notoriously hard to control, and over the past decade, quantum cryptographic systems have repeatedly been hacked.

The interrogation technique creates a quantum cryptography protocol that, for the first time, would transmit a secret key while simultaneously proving that the quantum devices are preventing any potential information leak. Some version of this protocol could very well be implemented within the next five to 10 years, predicted Vidick and his former adviser at MIT, the theoretical computer scientist Scott Aaronson.

“It’s a new level of security that solves the shortcomings of traditional quantum cryptography,” Pironio said.

Spooky Action

In 1964, the Irish physicist John Stewart Bell came up with a test to try to establish, once and for all, that the bafflingly counterintuitive principles of quantum physics are truly inherent properties of the universe — that the decades-long effort of Albert Einstein and other physicists to develop a more intuitive physics could never bear fruit.

Einstein was deeply disturbed by the randomness at the core of quantum physics — God “is not playing at dice,” he famously wrote to the physicist Max Born in 1926.

In 1935, Einstein, together with his colleagues Boris Podolsky and Nathan Rosen, described a strange consequence of this randomness, now called the EPR paradox (short for Einstein, Podolsky, Rosen). According to the laws of quantum physics, it is possible for two particles to interact briefly in such a way that their states become “entangled” as “EPR pairs.” Even if the particles then travel many light years away from each other, one particle somehow instantly seems to “know” the outcome of a measurement on the other particle: When asked the same question, it will give the same answer, even though quantum physics says that the first particle chose its answer randomly. Since the theory of special relativity forbids information from traveling faster than the speed of light, how does the second particle know the answer?
To Einstein, these “spooky actions at a distance” implied that quantum physics was an incomplete theory. “Quantum mechanics is certainly imposing,” he wrote to Born. “But an inner voice tells me that it is not yet the real thing.”

Over the remaining decades of his life, Einstein searched for a way that the two particles could use classical physics to come up with their answers — hidden variables that could explain the behavior of the particles without a need for randomness or spooky actions.

But in 1964, Bell realized that the EPR paradox could be used to devise an experiment that determines whether quantum physics or a local hidden-variables theory correctly explains the real world. Adapted five years later into a format called the CHSH game (after the researchers John Clauser, Michael Horne, Abner Shimony and Richard Holt), the test asks a system to prove its quantum nature by performing a feat that is impossible using only classical physics.

The CHSH game is a coordination game, in which two collaborating players — Bonnie and Clyde, say — are questioned in separate interrogation rooms. Their joint goal is to give either identical answers or different answers, depending on what questions the “detective” asks them. Neither player knows what question the detective is asking the other player.

If Bonnie and Clyde can use only classical physics, then no matter how many “hidden variables” they share, it turns out that the best they can do is decide on a story before they get separated and then stick to it, no matter what the detective asks them, a strategy that will win the game 75 percent of the time. But if Bonnie and Clyde share an EPR pair of entangled particles — picked up in a bank heist, perhaps — then they can exploit the spooky action at a distance to better coordinate their answers and win the game about 85.4 percent of the time.

Bell’s test gave experimentalists a specific way to distinguish between quantum physics and any hidden-variables theory. Over the decades that followed, physicists, most notably Alain Aspect, currently at the École Polytechnique in Palaiseau, France, carried out this test repeatedly, in increasingly controlled settings. Almost every time, the outcome has been consistent with the predictions of quantum physics, not with hidden variables.

Aspect’s work “painted hidden variables into a corner,” Aaronson said. The experiments had a huge role, he said, in convincing people that the counterintuitive weirdness of quantum physics is here to stay.

If Einstein had known about the Bell test, Vazirani said, “he wouldn’t have wasted 30 years of his life looking for an alternative to quantum mechanics.” He simply would have convinced someone to do the experiment.

Read the whole article here.

The Promise of Quantum Computation

Advanced in quantum physics and in the associated realm of quantum information promise to revolutionize computing. Imagine a computer several trillions of times faster than the present day supercomputers — well, that’s where we are heading.

[div class=attrib]From the New York Times:[end-div]

THIS summer, physicists celebrated a triumph that many consider fundamental to our understanding of the physical world: the discovery, after a multibillion-dollar effort, of the Higgs boson.

Given its importance, many of us in the physics community expected the event to earn this year’s Nobel Prize in Physics. Instead, the award went to achievements in a field far less well known and vastly less expensive: quantum information.

It may not catch as many headlines as the hunt for elusive particles, but the field of quantum information may soon answer questions even more fundamental — and upsetting — than the ones that drove the search for the Higgs. It could well usher in a radical new era of technology, one that makes today’s fastest computers look like hand-cranked adding machines.

The basis for both the work behind the Higgs search and quantum information theory is quantum physics, the most accurate and powerful theory in all of science. With it we created remarkable technologies like the transistor and the laser, which, in time, were transformed into devices — computers and iPhones — that reshaped human culture.

But the very usefulness of quantum physics masked a disturbing dissonance at its core. There are mysteries — summed up neatly in Werner Heisenberg’s famous adage “atoms are not things” — lurking at the heart of quantum physics suggesting that our everyday assumptions about reality are no more than illusions.

Take the “principle of superposition,” which holds that things at the subatomic level can be literally two places at once. Worse, it means they can be two things at once. This superposition animates the famous parable of Schrödinger’s cat, whereby a wee kitty is left both living and dead at the same time because its fate depends on a superposed quantum particle.

For decades such mysteries were debated but never pushed toward resolution, in part because no resolution seemed possible and, in part, because useful work could go on without resolving them (an attitude sometimes called “shut up and calculate”). Scientists could attract money and press with ever larger supercolliders while ignoring such pesky questions.

But as this year’s Nobel recognizes, that’s starting to change. Increasingly clever experiments are exploiting advances in cheap, high-precision lasers and atomic-scale transistors. Quantum information studies often require nothing more than some equipment on a table and a few graduate students. In this way, quantum information’s progress has come not by bludgeoning nature into submission but by subtly tricking it to step into the light.

Take the superposition debate. One camp claims that a deeper level of reality lies hidden beneath all the quantum weirdness. Once the so-called hidden variables controlling reality are exposed, they say, the strangeness of superposition will evaporate.

Another camp claims that superposition shows us that potential realities matter just as much as the single, fully manifested one we experience. But what collapses the potential electrons in their two locations into the one electron we actually see? According to this interpretation, it is the very act of looking; the measurement process collapses an ethereal world of potentials into the one real world we experience.

And a third major camp argues that particles can be two places at once only because the universe itself splits into parallel realities at the moment of measurement, one universe for each particle location — and thus an infinite number of ever splitting parallel versions of the universe (and us) are all evolving alongside one another.

These fundamental questions might have lived forever at the intersection of physics and philosophy. Then, in the 1980s, a steady advance of low-cost, high-precision lasers and other “quantum optical” technologies began to appear. With these new devices, researchers, including this year’s Nobel laureates, David J. Wineland and Serge Haroche, could trap and subtly manipulate individual atoms or light particles. Such exquisite control of the nano-world allowed them to design subtle experiments probing the meaning of quantum weirdness.

Soon at least one interpretation, the most common sense version of hidden variables, was completely ruled out.

At the same time new and even more exciting possibilities opened up as scientists began thinking of quantum physics in terms of information, rather than just matter — in other words, asking if physics fundamentally tells us more about our interaction with the world (i.e., our information) than the nature of the world by itself (i.e., matter). And so the field of quantum information theory was born, with very real new possibilities in the very real world of technology.

What does this all mean in practice? Take one area where quantum information theory holds promise, that of quantum computing.

Classical computers use “bits” of information that can be either 0 or 1. But quantum-information technologies let scientists consider “qubits,” quantum bits of information that are both 0 and 1 at the same time. Logic circuits, made of qubits directly harnessing the weirdness of superpositions, allow a quantum computer to calculate vastly faster than anything existing today. A quantum machine using no more than 300 qubits would be a million, trillion, trillion, trillion times faster than the most modern supercomputer.

[div class=attrib]Read the entire article after the jump.[end-div]

[div class=attrib]Image: Bloch sphere representation of a qubit, the fundamental building block of quantum computers. Courtesy of Wikipedia.[end-div]

Quantum Computer Leap

The practical science behind quantum computers continues to make exciting progress. Quantum computers promise, in theory, immense gains in power and speed through the use of atomic scale parallel processing.

[div class=attrib]From the Observer:[end-div]

The reality of the universe in which we live is an outrage to common sense. Over the past 100 years, scientists have been forced to abandon a theory in which the stuff of the universe constitutes a single, concrete reality in exchange for one in which a single particle can be in two (or more) places at the same time. This is the universe as revealed by the laws of quantum physics and it is a model we are forced to accept – we have been battered into it by the weight of the scientific evidence. Without it, we would not have discovered and exploited the tiny switches present in their billions on every microchip, in every mobile phone and computer around the world. The modern world is built using quantum physics: through its technological applications in medicine, global communications and scientific computing it has shaped the world in which we live.

Although modern computing relies on the fidelity of quantum physics, the action of those tiny switches remains firmly in the domain of everyday logic. Each switch can be either “on” or “off”, and computer programs are implemented by controlling the flow of electricity through a network of wires and switches: the electricity flows through open switches and is blocked by closed switches. The result is a plethora of extremely useful devices that process information in a fantastic variety of ways.

Modern “classical” computers seem to have almost limitless potential – there is so much we can do with them. But there is an awful lot we cannot do with them too. There are problems in science that are of tremendous importance but which we have no hope of solving, not ever, using classical computers. The trouble is that some problems require so much information processing that there simply aren’t enough atoms in the universe to build a switch-based computer to solve them. This isn’t an esoteric matter of mere academic interest – classical computers can’t ever hope to model the behaviour of some systems that contain even just a few tens of atoms. This is a serious obstacle to those who are trying to understand the way molecules behave or how certain materials work – without the possibility to build computer models they are hampered in their efforts. One example is the field of high-temperature superconductivity. Certain materials are able to conduct electricity “for free” at surprisingly high temperatures (still pretty cold, though, at well but still below -100 degrees celsius). The trouble is, nobody really knows how they work and that seriously hinders any attempt to make a commercially viable technology. The difficulty in simulating physical systems of this type arises whenever quantum effects are playing an important role and that is the clue we need to identify a possible way to make progress.

It was American physicist Richard Feynman who, in 1981, first recognised that nature evidently does not need to employ vast computing resources to manufacture complicated quantum systems. That means if we can mimic nature then we might be able to simulate these systems without the prohibitive computational cost. Simulating nature is already done every day in science labs around the world – simulations allow scientists to play around in ways that cannot be realised in an experiment, either because the experiment would be too difficult or expensive or even impossible. Feynman’s insight was that simulations that inherently include quantum physics from the outset have the potential to tackle those otherwise impossible problems.

Quantum simulations have, in the past year, really taken off. The ability to delicately manipulate and measure systems containing just a few atoms is a requirement of any attempt at quantum simulation and it is thanks to recent technical advances that this is now becoming possible. Most recently, in an article published in the journal Nature last week, physicists from the US, Australia and South Africa have teamed up to build a device capable of simulating a particular type of magnetism that is of interest to those who are studying high-temperature superconductivity. Their simulator is esoteric. It is a small pancake-like layer less than 1 millimetre across made from 300 beryllium atoms that is delicately disturbed using laser beams… and it paves the way for future studies into quantum magnetism that will be impossible using a classical computer.

[div class=attrib]Read the entire article after the jump.[end-div]

[div class=attrib]Image: A crystal of beryllium ions confined by a large magnetic field at the US National Institute of Standards and Technology’s quantum simulator. The outermost electron of each ion is a quantum bit (qubit), and here they are fluorescing blue, which indicates they are all in the same state. Photograph courtesy of Britton/NIST, Observer.[end-div]

When the multiverse and many-worlds collide

[div class=attrib]From the New Scientist:[end-div]

TWO of the strangest ideas in modern physics – that the cosmos constantly splits into parallel universes in which every conceivable outcome of every event happens, and the notion that our universe is part of a larger multiverse – have been unified into a single theory. This solves a bizarre but fundamental problem in cosmology and has set physics circles buzzing with excitement, as well as some bewilderment.

The problem is the observability of our universe. While most of us simply take it for granted that we should be able to observe our universe, it is a different story for cosmologists. When they apply quantum mechanics – which successfully describes the behaviour of very small objects like atoms – to the entire cosmos, the equations imply that it must exist in many different states simultaneously, a phenomenon called a superposition. Yet that is clearly not what we observe.

Cosmologists reconcile this seeming contradiction by assuming that the superposition eventually “collapses” to a single state. But they tend to ignore the problem of how or why such a collapse might occur, says cosmologist Raphael Bousso at the University of California, Berkeley. “We’ve no right to assume that it collapses. We’ve been lying to ourselves about this,” he says.

In an attempt to find a more satisfying way to explain the universe’s observability, Bousso, together with Leonard Susskind at Stanford University in California, turned to the work of physicists who have puzzled over the same problem but on a much smaller scale: why tiny objects such as electrons and photons exist in a superposition of states but larger objects like footballs and planets apparently do not.

This problem is captured in the famous thought experiment of Schrödinger’s cat. This unhappy feline is inside a sealed box containing a vial of poison that will break open when a radioactive atom decays. Being a quantum object, the atom exists in a superposition of states – so it has both decayed and not decayed at the same time. This implies that the vial must be in a superposition of states too – both broken and unbroken. And if that’s the case, then the cat must be both dead and alive as well.

[div class=attrib]More from theSource here.[end-div]

Is Quantum Mechanics Controlling Your Thoughts?

[div class=attrib]From Discover:[end-div]

Graham Fleming sits down at an L-shaped lab bench, occupying a footprint about the size of two parking spaces. Alongside him, a couple of off-the-shelf lasers spit out pulses of light just millionths of a billionth of a second long. After snaking through a jagged path of mirrors and lenses, these minus­cule flashes disappear into a smoky black box containing proteins from green sulfur bacteria, which ordinarily obtain their energy and nourishment from the sun. Inside the black box, optics manufactured to billionths-of-a-meter precision detect something extraordinary: Within the bacterial proteins, dancing electrons make seemingly impossible leaps and appear to inhabit multiple places at once.

Peering deep into these proteins, Fleming and his colleagues at the University of California at Berkeley and at Washington University in St. Louis have discovered the driving engine of a key step in photosynthesis, the process by which plants and some microorganisms convert water, carbon dioxide, and sunlight into oxygen and carbohydrates. More efficient by far in its ability to convert energy than any operation devised by man, this cascade helps drive almost all life on earth. Remarkably, photosynthesis appears to derive its ferocious efficiency not from the familiar physical laws that govern the visible world but from the seemingly exotic rules of quantum mechanics, the physics of the subatomic world. Somehow, in every green plant or photosynthetic bacterium, the two disparate realms of physics not only meet but mesh harmoniously. Welcome to the strange new world of quantum biology.

On the face of things, quantum mechanics and the biological sciences do not mix. Biology focuses on larger-scale processes, from molecular interactions between proteins and DNA up to the behavior of organisms as a whole; quantum mechanics describes the often-strange nature of electrons, protons, muons, and quarks—the smallest of the small. Many events in biology are considered straightforward, with one reaction begetting another in a linear, predictable way. By contrast, quantum mechanics is fuzzy because when the world is observed at the subatomic scale, it is apparent that particles are also waves: A dancing electron is both a tangible nugget and an oscillation of energy. (Larger objects also exist in particle and wave form, but the effect is not noticeable in the macroscopic world.)

[div class=attrib]More from theSource here.[end-div]

[div class=attrib]Image courtesy of Dylan Burnette/Olympus Bioscapes Imaging Competition.[end-div]

Raiders of the lost dimension

[div class=attrib]From Los Alamos National Laboratory:[end-div]

A team of scientists working at the National High Magnetic Field Laboratory’s Pulsed Field Facility at Los Alamos has uncovered an intriguing phenomenon while studying magnetic waves in barium copper silicate, a 2,500-year-old pigment known as Han purple. The researchers discovered that when they exposed newly grown crystals of the pigment to very high magnetic fields at very low temperatures, it entered a rarely observed state of matter. At the threshold of that matter state–called the quantum critical point-the waves actually lose a dimension. That is, the magnetic waves go from a three-dimensional to a two-dimensional pattern. The discovery is yet another step toward understanding the quantum mechanics of the universe.

Writing about the work in today’s issue of the scientific journal Nature, the researchers describe how they discovered that at high magnetic fields (above 23 Tesla) and at temperatures between 1 and 3 degrees Kelvin (or roughly minus 460 degrees Fahrenheit), the magnetic waves in Han purple crystals “exist” in a unique state of matter called a Bose Einstein condensate (BEC). In the BEC state, magnetic waves propagate simultaneously in all of three directions (up-down, forward-backward and left-right). At the quantum critical point, however, the waves stop propagating in the up-down dimension, causing the magnetic ripples to exist in only two dimensions, much the same way as ripples are confined to the surface of a pond.

“The reduced dimensionality really came as a surprise,” said Neil Harrison, an experimental physicist at the Los Alamos Pulsed Field Facility, “just when we thought we had reached an understanding of the quantum nature of its magnetic BEC.”

[div class=attrib]More from theSource here.[end-div]

Computing with Quantum Knots

[div class=attrib]From Scientific American:[end-div]

A machine based on bizarre particles called anyons that represents a calculation as a set of braids in spacetime might be a shortcut to practical quantum computation.

Quantum computers promise to perform calculations believed to be impossible for ordinary computers. Some of those calculations are of great real-world importance. For example, certain widely used encryption methods could be cracked given a computer capable of breaking a large number into its component factors within a reasonable length of time. Virtually all encryption methods used for highly sensitive data are vulnerable to one quantum algorithm or another.

The extra power of a quantum computer comes about because it operates on information represented as qubits, or quantum bits, instead of bits. An ordinary classical bit can be either a 0 or a 1, and standard microchip architectures enforce that dichotomy rigorously. A qubit, in contrast, can be in a so-called superposition state, which entails proportions of 0 and 1 coexisting together. One can think of the possible qubit states as points on a sphere. The north pole is a classical 1, the south pole a 0, and all the points in between are all the possible superpositions of 0 and 1 [see “Rules for a Complex Quantum World,” by Michael A. Nielsen; Scientific American, November 2002]. The freedom that qubits have to roam across the entire sphere helps to give quantum computers their unique capabilities.

[div class=attrib]More from theSource here.[end-div]

Quantum Trickery: Testing Einstein’s Strangest Theory

[div class=attrib]From the New York Times:[end-div]

Einstein said there would be days like this.

This fall scientists announced that they had put a half dozen beryllium atoms into a “cat state.”

No, they were not sprawled along a sunny windowsill. To a physicist, a “cat state” is the condition of being two diametrically opposed conditions at once, like black and white, up and down, or dead and alive.

These atoms were each spinning clockwise and counterclockwise at the same time. Moreover, like miniature Rockettes they were all doing whatever it was they were doing together, in perfect synchrony. Should one of them realize, like the cartoon character who runs off a cliff and doesn’t fall until he looks down, that it is in a metaphysically untenable situation and decide to spin only one way, the rest would instantly fall in line, whether they were across a test tube or across the galaxy.

The idea that measuring the properties of one particle could instantaneously change the properties of another one (or a whole bunch) far away is strange to say the least – almost as strange as the notion of particles spinning in two directions at once. The team that pulled off the beryllium feat, led by Dietrich Leibfried at the National Institute of Standards and Technology, in Boulder, Colo., hailed it as another step toward computers that would use quantum magic to perform calculations.

But it also served as another demonstration of how weird the world really is according to the rules, known as quantum mechanics.

The joke is on Albert Einstein, who, back in 1935, dreamed up this trick of synchronized atoms – “spooky action at a distance,” as he called it – as an example of the absurdity of quantum mechanics.

“No reasonable definition of reality could be expected to permit this,” he, Boris Podolsky and Nathan Rosen wrote in a paper in 1935.

Today that paper, written when Einstein was a relatively ancient 56 years old, is the most cited of Einstein’s papers. But far from demolishing quantum theory, that paper wound up as the cornerstone for the new field of quantum information.

Nary a week goes by that does not bring news of another feat of quantum trickery once only dreamed of in thought experiments: particles (or at least all their properties) being teleported across the room in a microscopic version of Star Trek beaming; electrical “cat” currents that circle a loop in opposite directions at the same time; more and more particles farther and farther apart bound together in Einstein’s spooky embrace now known as “entanglement.” At the University of California, Santa Barbara, researchers are planning an experiment in which a small mirror will be in two places at once.

[div class=attrib]More from theSource here.[end-div]