Posts Tagged ‘quantum physics’

By Rafi Letzter

Giant molecules can be in two places at once, thanks to quantum physics.

That’s something that scientists have long known is theoretically true based on a few facts: Every particle or group of particles in the universe is also a wave — even large particles, even bacteria, even human beings, even planets and stars. And waves occupy multiple places in space at once. So any chunk of matter can also occupy two places at once. Physicists call this phenomenon “quantum superposition,” and for decades, they have demonstrated it using small particles.

But in recent years, physicists have scaled up their experiments, demonstrating quantum superposition using larger and larger particles. Now, in a paper published Sept. 23 in the journal Nature Physics, an international team of researchers has caused molecule made up of up to 2,000 atoms to occupy two places at the same time.

To pull it off, the researchers built a complicated, modernized version of a series of famous old experiments that first demonstrated quantum superposition.

Researchers had long known that light, fired through a sheet with two slits in it, would create an interference pattern, or a series of light and dark fringes, on the wall behind the sheet. But light was understood as a massless wave, not something made of particles, so this wasn’t surprising. However, in a series of famous experiments in the 1920s, physicists showed that electrons fired through thin films or crystals would behave in a similar way, forming patterns like light does on the wall behind the diffracting material.

If electrons were simply particles, and so could occupy only one point in space at a time, they would form two strips, roughly the shape of the slits, on the wall behind the film or crystal. But instead, the electrons hit that wall in complex patterns suggesting the electrons had interfered with themselves . That is a telltale sign of a wave; in some spots, the peaks of the waves coincide, creating brighter regions, while in other spots, the peaks coincide with troughs, so the two cancel each other out and create a dark region. Because physicists already knew that electrons had mass and were definitely particles, the experiment showed that matter acts both as individual particles and as waves.

But it’s one thing to create an interference pattern with electrons. Doing it with giant molecules is a lot trickier. Bigger molecules have less-easily detected waves, because more massive objects have shorter wavelengths that can lead to barely-perceptible interference patterns. And these 2,000-atom particles have wavelengths smaller than the diameter of a single hydrogen atom, so their interference pattern is much less dramatic.

To pull off the double-slit experiment for big things, the researchers built a machine that could fire a beam of molecules (hulking things called “oligo-tetraphenylporphyrins enriched with fluoroalkylsulfanyl chains,” some more than 25,000 times the mass of a simple hydrogen atom) through a series of grates and sheets bearing multiple slits. The beam was about 6.5 feet (2 meters) long. That’s big enough that the researchers had to account for factors like gravity and the rotation of the Earth in designing the beam emitter, the scientists wrote in the paper. They also kept the molecules fairly warm for a quantum physics experiment, so they had to account for heat jostling the particles.

But still, when the researchers switched the machine on, the detectors at the far end of the beam revealed an interference pattern. The molecules were occupying multiple points in space at once.

It’s an exciting result, the researchers wrote, proving quantum interference at larger scales than had ever before been detected.

“The next generation of matter-wave experiments will push the mass by an order of magnitude,” the authors wrote.

https://www.space.com/2000-atoms-in-two-places-at-once.html?utm_source=notification

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by MIKE MCRAE

For around a century it’s been thought that particles don’t have defined properties until we nail them down with a measurement.

That kind of quantum madness opens up a whole world of counter-intuitive paradoxes. Take this one, for example – it’s possible for a single particle to experience two sequences of events at the same time, making it impossible to know which came first.

Physicists from the University of Queensland designed a race course for light that forced a single particle to traverse two pathways at once, making it impossible to say in which order it completed a pair of operations.

In boring old everyday life you could roll a single ball down a ramp and have it ring bell A and then ring bell B. Or, if you’d prefer, you could roll it down another ramp and have it ring B before A.

If you want to get fancy you could even set up a rig so one bell causes the other bell to ring.

None of this is mind blowing, since we’re used to events in the Universe having a set order, where one thing precedes another in such a way that we presume an order of causation.

But nothing is so simple when we accept that reality is a blur of possibility prior to it being measured.

To demonstrate this, the physicists created a physical equivalent of something called a quantum switch, where multiple operations occur while a particle is in a superposition of all its possible locations.

Keeping it simple, the team set up a pathway that split apart and converged again in an interferometer, with access to each fork dependent on the polarisation of the light entering it.

Light waves travelling down each fork in the pathway would then merge and interfere to create a distinctive pattern depending on its properties.

In this particular case, the two light waves were actually the same photon taking both paths at the same time.

Before being measured, a photon can be either vertically or horizontally polarised. Or, more precisely, it’s polarised both vertical and horizontal at the same time until a measurement confirms one over the other.

Since this undefined photon’s polarisation is both vertical and horizontal, it enters both pathways, with the vertically polarised version of the photon barrelling down one channel and the horizontally polarised version heading down the second.

Following the two paths, the team had the quantum equivalent of those bells we mentioned earlier – in the form of lenses that subtly changed the shape of the photon.

The horizontal polarisation would hit ‘bell’ A before striking B, while the vertical polarisation would strike ‘bell’ B, and then A.

An analysis of the interference pattern of the reunited photon revealed signs of this mess of possible sequences.

On one hand, it’s easy to imagine two separate light particles – one horizontally polarised, the other vertically polarised – passing each lens in separate orders.

That’s not what happened, though. This was a single photon with two possible histories, neither of which set in reality until they’re measured.

While the events A and B were independent in this quantum switch, they could be linked to affect one another. A could cause B, or B could cause A … all depending on which history you wanted after the event.

Putting aside daydreams of travelling back in time to undo that big mistake (what were you thinking?!), this does have one possible practical application in the emerging field of quantum communications.

Transmitting photons down a noisy channel could be disastrous for their quantum information, quickly making a mess of their precious superposition. Sending them down channels fitted with a quantum switch, however, could in principle give the quantum information an opportunity to get through.

A paper the team published on the pre-peer review website arxiv.org back in July shows how a quantum switch applied to two noisy channels can allow a superposition to survive.

Whatever weird clockwork is going on in reality’s basement, we won’t pretend to understand it. But the very fact physicists are able to craft it into new technology is truly mindblowing in itself.

This research was published in Physical Review Letters.

https://www.sciencealert.com/quantum-switch-causation-superposition-applied-technology

Academics are challenging the foundations of quantum science with a radical new theory on parallel universes. Scientists now propose that parallel universes really exist, and that they interact. They show that such an interaction could explain everything that is bizarre about quantum mechanics.

Griffith University academics are challenging the foundations of quantum science with a radical new theory based on the existence of, and interactions between, parallel universes.

In a paper published in the journal Physical Review X, Professor Howard Wiseman and Dr Michael Hall from Griffith’s Centre for Quantum Dynamics, and Dr Dirk-Andre Deckert from the University of California, take interacting parallel worlds out of the realm of science fiction and into that of hard science.
The team proposes that parallel universes really exist, and that they interact. That is, rather than evolving independently, nearby worlds influence one another by a subtle force of repulsion. They show that such an interaction could explain everything that is bizarre about quantum mechanics.

Quantum theory is needed to explain how the universe works at the microscopic scale, and is believed to apply to all matter. But it is notoriously difficult to fathom, exhibiting weird phenomena which seem to violate the laws of cause and effect.

As the eminent American theoretical physicist Richard Feynman once noted: “I think I can safely say that nobody understands quantum mechanics.”

However, the “Many-Interacting Worlds” approach developed at Griffith University provides a new and daring perspective on this baffling field.

“The idea of parallel universes in quantum mechanics has been around since 1957,” says Professor Wiseman.

“In the well-known “Many-Worlds Interpretation,” each universe branches into a bunch of new universes every time a quantum measurement is made. All possibilities are therefore realised — in some universes the dinosaur-killing asteroid missed Earth. In others, Australia was colonised by the Portuguese.

“But critics question the reality of these other universes, since they do not influence our universe at all. On this score, our “Many Interacting Worlds” approach is completely different, as its name implies.”

Professor Wiseman and his colleagues propose that:

•The universe we experience is just one of a gigantic number of worlds. Some are almost identical to ours while most are very different;
•All of these worlds are equally real, exist continuously through time, and possess precisely defined properties;
•All quantum phenomena arise from a universal force of repulsion between ‘nearby’ (i.e. similar) worlds which tends to make them more dissimilar.
Dr Hall says the “Many-Interacting Worlds” theory may even create the extraordinary possibility of testing for the existence of other worlds.

“The beauty of our approach is that if there is just one world our theory reduces to Newtonian mechanics, while if there is a gigantic number of worlds it reproduces quantum mechanics,” he says.

“In between it predicts something new that is neither Newton’s theory nor quantum theory.

“We also believe that, in providing a new mental picture of quantum effects, it will be useful in planning experiments to test and exploit quantum phenomena.”

The ability to approximate quantum evolution using a finite number of worlds could have significant ramifications in molecular dynamics, which is important for understanding chemical reactions and the action of drugs.

Professor Bill Poirier, Distinguished Professor of Chemistry at Texas Tech University, has observed: “These are great ideas, not only conceptually, but also with regard to the new numerical breakthroughs they are almost certain to engender.”

Journal Reference:

1.Michael J. W. Hall, Dirk-André Deckert, Howard M. Wiseman. Quantum Phenomena Modeled by Interactions between Many Classical Worlds. Physical Review X, 2014; 4 (4) DOI: 10.1103/PhysRevX.4.041013

http://www.sciencedaily.com/releases/2014/10/141030101654.htm