quantum mechanics – Australian Science

Schrödinger’s cat: the quantum world not so absurd after all?

Kevin Orrman-Rossiter — Fri, 02 Aug 2013 00:10:37 +0000

Diagram of Schrödinger’s cat theory. Image credit: Dhatfield

Since Erwin Schrödinger’s famous 1935 cat thought experiment, physicists around the world have tried to create large scale systems to test how the rules of quantum mechanics apply to everyday objects. Scientists have only managed to recreate quantum effects on much smaller scales, resulting in a nagging possibility that quantum mechanics, by itself, is not sufficient to describe reality.

Researchers Alex Lvovsky and Christoph Simon from the University of Calgary recently made a significant step forward in this direction by creating a large system that displays quantum behaviour, publishing their results in Nature Physics.

Understanding Schrödinger’s cat

Quantum mechanics is without doubt one of the most successful physics theories to date. Without it the world we live in would be remarkably different: driving and shaping our modern world making possible everything from computers, mobile phones, nuclear weapons, solar cells and our everyday appliances. At the same time it presents us with conundrums that are at the far end of reason; challenging even the greatest minds to comprehend.

In contrast to our everyday experience, quantum physics allows for particles to be in two states at the same time — so-called quantum superpositions. A radioactive nucleus, for example, can simultaneously be in a decayed and non-decayed state.

Schrödinger’s Cat;
visualization of the separation of the universe due to two superposed and entangled quantum mechanical states. (image credit: Christian Schirm)

Applying these quantum rules to large objects leads to paradoxical and even bizarre consequences. To emphasize this, Erwin Schrödinger, one of the founding fathers of quantum physics, proposed in 1935 a thought experiment involving a cat that could be killed by a mechanism triggered by the decay of a single atomic nucleus. If the nucleus is in a superposition of decayed and non-decayed states, and if quantum physics applies to large objects, the belief is that the cat will be simultaneously dead and alive.

Schrödinger’s thought experiment involves a (macroscopic) cat whose quantum state becomes entangled with that of a (microscopic) decaying nucleus. While quantum systems with properties akin to ‘Schrödinger’s cat’ have been achieved at a micro level, the application of this principle to everyday macro objects has proved to be difficult to demonstrate. The experimental creation of such micro-macro entanglement is what these authors successfully achieved.

Photons help to illuminate the paradox

The breakthrough achieved by Calgary quantum physicists is that they were able to contrive a quantum state of light that consists of a hundred million photons and can even be seen by the naked eye. In their state, the “dead” and “alive” components of the “cat” correspond to quantum states that differ by tens of thousands of photons.

While the findings are promising, study co-author Simon admits that many questions remain unanswered.

“We are still very far from being able to do this with a real cat,” he says. “But this result suggests there is ample opportunity for progress in that direction.”

Seeing quantum effects requires extremely precise measurements. In order to see the quantum nature of this state, one has to be able to count the number of photons in it perfectly. This becomes more and more difficult as the total number of photons is increased. Distinguishing one photon from two photons is within reach of current technology, but distinguishing a million photons from a million plus one is not.

Decoherence: the emergence of the classical world from the quantum

Why don’t we see quantum effects in everyday life? The current explanation is that it is to do with decoherence.

Physicists see quantum systems as fragile. When a photon interacts with its environment, even just a tiny bit, the superposition is destroyed. This interaction, could be as a result of measurement or an observation, or just a random interaction. Superposition is a fundamental principle of quantum physics that says that systems can exist in all their possible states simultaneously. But when measured, only the result of one of the states is given.

This effect is known as decoherence and it has been studied intensively over the last few decades. The idea of decoherence as a thought experiment was raised by Erwin Schrödinger, in his famous cat paradox. Unfortunately for non-physicists decoherence only provides an explanation for the observance of wave function collapse, as the quantum nature of the system “leaks” into the environment. It does not tell us where the line is, if one does exist, between the quantum and everyday worlds.

Although Schrodinger’s thought experiment was originally intended to convey the absurdity of applying quantum mechanics to macroscopic objects, this experiment and related ones suggest that it may apply on all scales.

If you are interested in the history and foundation of quantum mechanics then I highly recommend Quantum: Einstein, Bohr and the great debate about the nature of reality, by Manjit Kumar (2009), and The Age of Entanglement: when quantum physics was reborn, by Louisa Gilder (2008). Both are well-researched and captivating brilliant accounts of science science and scientists.

Cite this article:
Orrman-Rossiter K (2013-08-02 00:10:37). Schrödinger's cat: the quantum world not so absurd after all?. Australian Science. Retrieved: May 05, 2024, from http://australianscience.com.au/physics/schrodingers-cat-the-quantum-world-not-so-absurd-after-all/

test

The post Schrödinger’s cat: the quantum world not so absurd after all? appeared first on Australian Science.

Connecting the Quantum Dots

Danica Radovanovic — Fri, 22 Feb 2013 00:01:24 +0000

Last week, after I spent a couple of days in Brest, Brittany at a ESF, EU workshop/seminar brainstorming with other internet and scientific researchers on interesting topics related to internet science and innovation, I got myself back to Paris. I visited a French national institute with an international reputation for scientific excellence – ESPCI (École supérieure de physique et de chimie industrielles) and the CNRS department of Physics, Quantum Foundations – a group dedicated to research on quantum effects in materials. Also, I took the opportunity to meet up with two Australian Science writers who reside in Paris: Rayna, and Charles.

ESPCI Paris Tech stands for Physics and Chemistry Higher Educational Institution (a French “Grande École d’ingénieurs”). Founded in 1882, ESPCI is a major institution of higher education – an internationally renowned research center, gathering leading scientific innovators like Nobel Prize laureates Pierre and Marie Curie, Paul Langevin, Frédéric Joliot-Curie, Pierre-Gilles de Gennes, and Georges Charpak.

ESPCI ParisTech

At ESPCI, I met with Arjen Dijksman, a physicist and researcher interested in tiny semiconductive nanoparticles, known as “quantum dots

Cite this article:
Radovanovic D (2013-02-22 00:01:24). Connecting the Quantum Dots. Australian Science. Retrieved: May 05, 2024, from http://australianscience.com.au/science-2/connecting-quantum-dots/

test

The post Connecting the Quantum Dots appeared first on Australian Science.

Turn Back Time

Kelly Burnes — Fri, 23 Nov 2012 00:26:42 +0000

Electron-positron collisions at SLAC produce a Υ(4s) resonance that results in an entangled pair of B mesons. Source: http://physics.aps.org/articles/v5/129

Recall the outrageously cool movie from the 1980s, Back to the Future? Marty McFly and Doc Brown were stretching scientific boundaries, righting the future, all while making sure not to meet their future selves. Fast-forward and time travel may be closer than we think. (Close, relatively speaking of course.)

If you like exceptions to the rule, well, you’re in luck because the Department of Energy’s (DOE) SLAC National Accelerator Laboratory has achieved something quite impressive – the BaBar experiment.

The BaBar experiment investigates the most fundamental questions about the universe by getting back to basics with elementary particles. It’s a global collaboration of physicists delving into the nature of antimatter, the relationship of quarks and leptons and crossing over into areas of physics yet to be explored.

Earlier this week, scientists working on the BaBar project made the first direct observation of a violation to the concept known as time reversal symmetry. Their work was published in Physical Review Letters if you want to journey into the world of B mesons.

How did they do it?

Data from billions of particle collisions, nearly 10 years worth, were poured over and sifted through by researchers on this project. They examined a chain of particle transformations in which B mesons flipped between two different states called B-zero and B-even. This quantum entanglement of B mesons enables information about the first decaying particle to be used to determine the state of its partner at the time of the decay. This allowed the team to find that these transformations happened six times more often in one direction than the other.

Largely by thinking there was something wrong with the picture they were looking at was how missing pieces of the puzzle were filled in, according to the physics coordinator for BaBar, Fabio Anulli of the National Institute for Nuclear Physics in Rome. The BaBar data they had showed evidence of change-parity symmetry violation, so this was a good place to start looking. In fact, just looking at the data in a slightly different way allowed them to the see time violation.

Given the abundance of data the BaBar team had to work with, they were able to measure the T-violation to the 14 sigma level – a high level of statistical significance. Basically meaning there is only 1 chance in 10⁴³ that this effect is not real. Recall the Higgs boson discovery this past summer; that was granted a 5 sigma level. The results demonstrate that the direction of time matters, at least for some elementary particle processes. This provides a strong confirmation that a few subatomic processes like to do things on their own schedule – they have a preferred direction of time, changing into one another much more often in one way, than they change in the other. Time does not run the same forwards as backwards.

The findings – full of futuristic possibilities

What does this mean for the future? This discovery may rock our world in ways we haven’t even conceived of yet. It could have implications for business, for communications, for me getting to Brisbane instantaneously without first stopping for a layover at LAX. We may have perhaps inched closer to that time travelling ease marvelously displayed in Star Trek. Stay tuned.

Discoveries such as this make us think about the beauty of science. The vectors and numbers and B mesons – scientists work that stuff out to provide the beautiful possibilities that are so important to the human race. It may be small now, but we’re definitely on to something bigger. Think how many picture frames we may just be able to tilt to get the answers we seek.

test

The post Turn Back Time appeared first on Australian Science.