Quantum channel made of light

A point contact through which neutral, ultracold atoms flow. According to the laws of quantum physics, conductivity can only change in discrete steps.

Two vessels filled with gas and connected by a channel -- this is the basic setting for the experiments carried out by the physicists at the ETH Institute for Quantum Electronics. As one vessel contains more gas than the other, particles flow through the channel from one side to the other. "The question now is how the conductivity changes as we gradually make the channel narrower," says physics professor Tilman Esslinger. Initially, the conductivity decreases smoothly. However, at some point an amazing phenomenon appears: the conductivity does not change continuously anymore, but in steps, and the size of the steps is determined by a fundamental entity known as the Planck constant. "This is an immediate consequence of quantum physics," explains Esslinger.

The phenomenon has been observed before, but only in electronic systems, such as in quantum point contacts in specific semiconductor structures. "We have now observed for the first time quantisation of conductivity in neutral matter; that is, for particles that are not charged," says Esslinger. "This experiment is certainly something for the quantum-physics textbooks."

This basic-research project, which is supported by the Swiss National Science Foundation (SNF) and the European Union, might be of relevance for the design and construction of the next generation of electronic devices, as it enables the future study of effects that currently cannot be explored with electronic systems.

Cooling to almost absolute zero

The group led by Tilman Esslinger works with ultracold atoms. In the experiment described by the researchers in the current issue of the journal Nature, they used a gas consisting of lithium atoms at a temperature of merely 35 billionths of a degree above absolute zero. "Cooling is the main focus of our work in the lab," says Dr Jean-Philippe Brantut, SNF Ambizione Fellow at the Institute for Quantum Electronics. "99 percent of our equipment, which we developed in house, serves that purpose." Cooled to such low temperatures, the lithium atoms behave similarly to electrons in a solid-state material, even if -- in contrast to electrons -- the atoms are not charged.

The centrepieces of the complex experimental setup are a high-vacuum glass cell and two ultra-high-resolution microscopes. The lithium gas sits in the cell between the microscopes, in a cigar-shaped cloud with a diameter of approximately 300 micrometres. A laser beam divides this cloud into two reservoirs, connected by a narrow two-dimensional channel. A second laser beam passes through a lithographically produced mask and then through a projection system made of a lens and one of the microscopes. In this way, the pattern defined on the mask is reduced to the size of the channel. As a result, a quantum point contact with a width of just one micrometre is created, as can be validated using the other microscope.

Microscopic flow requires a stable system

The channel structure is sufficiently narrow that the laws of quantum mechanics come into play. This means that for atoms flowing through the channel, the conductivity should change not continuously but in steps, whose size are given by Planck's quantum of action, which is a fundamental constant of nature. This behaviour is precisely what the research group has observed. Ten atoms are in the channel at a time, says Brantut. To make the microscopic flow visible, the channel had to be kept open until 1,000 or so atoms have passed through it. This took some 1.5 seconds, which is a rather long time for an experiment of this type. "The experiment can only work if the atoms are very stable -- that is, extremely cold -- and nothing else changes," explains the physicist.

The atoms transverse the experimental setup like small bullets, without being thrown off course by collisions. The physicists therefore refer to this as a ballistic system. The electronics industry is looking to develop in the future ballistic transistors, in which electrical resistance is extremely low. The experiments involving neutral atoms and laser light might contribute to these developments, as they enable scientists to systematically study theoretical models and compare results directly, which often is not possible with electronic systems, due to the current inability to produce suitable samples. "Until now, we have been carrying out measurements based on predictions from theoretical models," says Brantut. "Now we are venturing into uncharted territory."

Selengkapnya »»  

Neutrinos can deliver not only full-on hits but also 'glancing blows'

In what they call a "weird little corner" of the already weird world of neutrinos, physicists have found evidence that these tiny particles might be involved in a surprising reaction.

Neutrinos are famous for almost never interacting. As an example, ten trillion neutrinos pass through your hand every second, and fewer than one actually interacts with any of the atoms that make up your hand. However, when neutrinos do interact with another particle, it happens at very close distances and involves a high-momentum transfer.

And yet a new paper, published in Physical Review Letters this week, shows that neutrinos sometimes can also interact with a nucleus but leave it basically untouched -- inflicting no more than a "glancing blow" -- resulting in a particle being created out of a vacuum.

Professor Kevin McFarland is a scientific co-spokesperson with the international MINERvA collaboration, which carries out neutrino scattering experiments at Fermilab McFarland, who also heads up the Rochester team that was primarily responsible for the analysis of the results, compares neutrino interactions to the firing of a bullet at a bubble, only to find the bubble was left intact.

"The bubble -- a carbon nucleus in the experiment -- deflects the neutrino 'bullet' by creating a particle from the vacuum," McFarland explains. "This effectively shields the bubble from getting blasted apart and instead the bullet only delivers a gentle bump to the bubble."

Producing an entirely new particle -- in this case a charged pion -- requires much more energy than it would take to blast the nucleus apart -- which is why the physicists are always surprised that the reaction happens as often as it does. McFarland adds that even painstakingly detailed theoretical calculations for this reaction "have been all over the map."

"The production of pions from this reaction had not been observed consistently in other experiments," McFarland said. By using a new technique, they were able to measure how much momentum and energy were transferred to the carbon nucleus -- showing that it remained undisturbed -- and the distribution of the pions that were created.

"After analyzing the results, we now have overwhelming evidence for the process," McFarland says.

The two members of the collaboration who were primarily responsible for analyzing the results were Aaron Higuera, at the time a postdoc at Rochester and now at the University of Houston, and Aaron Mislivec, one of McFarland's Ph.D. students.

Working with Higuera, Mislivec wrote the computer code that allowed them to sift through the results and get a picture of the reaction. "Our detector gave us access to the full information of exactly what was happening in this reaction," Mislivec explains. "Our data was consistent with the unique fingerprint of this reaction and determined how these interactions happen and how often." The key to identifying the reaction was finding undisturbed carbon nuclei and then studying the two resulting particles -- the pion, which is responsible for shielding the nucleus, and the muon.

Understanding this reaction, McFarland states, "is not going to make a better mousetrap, but it is exciting to learn that this weird reaction really does take place."

Researchers in the MINERvA collaboration measure low energy neutrino interactions both to support neutrino oscillation experiments and study the strong dynamics of the nucleon and nucleus that affect the interactions.

Selengkapnya »»  

Quantum physics just got less complicated: Wave-particle duality and quantum uncertainty are same thing

Quantum physics says that particles can behave like waves, and vice versa. Researchers have now shown that this 'wave-particle duality' is simply the quantum uncertainty principle in disguise.

Here's a nice surprise: quantum physics is less complicated than we thought. An international team of researchers has proved that two peculiar features of the quantum world previously considered distinct are different manifestations of the same thing. The result is published 19 December in Nature Communications.

Patrick Coles, Jedrzej Kaniewski, and Stephanie Wehner made the breakthrough while at the Centre for Quantum Technologies at the National University of Singapore. They found that 'wave-particle duality' is simply the quantum 'uncertainty principle' in disguise, reducing two mysteries to one.

"The connection between uncertainty and wave-particle duality comes out very naturally when you consider them as questions about what information you can gain about a system. Our result highlights the power of thinking about physics from the perspective of information," says Wehner, who is now an Associate Professor at QuTech at the Delft University of Technology in the Netherlands.

The discovery deepens our understanding of quantum physics and could prompt ideas for new applications of wave-particle duality.

Wave-particle duality is the idea that a quantum object can behave like a wave, but that the wave behaviour disappears if you try to locate the object. It's most simply seen in a double slit experiment, where single particles, electrons, say, are fired one by one at a screen containing two narrow slits. The particles pile up behind the slits not in two heaps as classical objects would, but in a stripy pattern like you'd expect for waves interfering. At least this is what happens until you sneak a look at which slit a particle goes through -- do that and the interference pattern vanishes.

The quantum uncertainty principle is the idea that it's impossible to know certain pairs of things about a quantum particle at once. For example, the more precisely you know the position of an atom, the less precisely you can know the speed with which it's moving. It's a limit on the fundamental knowability of nature, not a statement on measurement skill. The new work shows that how much you can learn about the wave versus the particle behaviour of a system is constrained in exactly the same way.

Wave-particle duality and uncertainty have been fundamental concepts in quantum physics since the early 1900s. "We were guided by a gut feeling, and only a gut feeling, that there should be a connection," says Coles, who is now a Postdoctoral Fellow at the Institute for Quantum Computing in Waterloo, Canada.

It's possible to write equations that capture how much can be learned about pairs of properties that are affected by the uncertainty principle. Coles, Kaniewski and Wehner are experts in a form of such equations known as 'entropic uncertainty relations', and they discovered that all the maths previously used to describe wave-particle duality could be reformulated in terms of these relations.

"It was like we had discovered the 'Rosetta Stone' that connected two different languages," says Coles. "The literature on wave-particle duality was like hieroglyphics that we could now translate into our native tongue. We had several eureka moments when we finally understood what people had done," he says.

Because the entropic uncertainty relations used in their translation have also been used in proving the security of quantum cryptography -- schemes for secure communication using quantum particles -- the researchers suggest the work could help inspire new cryptography protocols.

In earlier papers, Wehner and collaborators found connections between the uncertainty principle and other physics, namely quantum 'non-locality' and the second law of thermodynamics. The tantalising next goal for the researchers is to think about how these pieces fit together and what bigger picture that paints of how nature is constructed.

Selengkapnya »»