Monthly Archives: April 2013

Jim Al-Khalili – Quantum Life: How Physics Can Revolutionise Biology

Although many examples can be found in the scientific literature dating back half a century, there is still no widespread acceptance that quantum mechanics — that baffling yet powerful theory of the subatomic world — might play an important role in biological processes. Biology is, at its most basic, chemistry, and chemistry is built on the rules of quantum mechanics in the way atoms and molecules behave and fit together.

As Jim explains, biologists have until recently been dismissive of counter-intuitive aspects of the theory and feel it to be unnecessary, preferring their traditional ball-and-stick models of the molecular structures of life. Likewise, physicists have been reluctant to venture into the messy and complex world of the living cell – why should they when they can test their theories far more cleanly in the controlled environment of the physics lab?

But now, experimental techniques in biology have become so sophisticated that the time is ripe for testing ideas familiar to quantum physicists. Can quantum phenomena in the subatomic world impact the biological level and be present in living cells or processes – from the way proteins fold or genes mutate and the way plants harness light in photosynthesis to the way some birds navigate using the Earth’s magnetic field? All appear to utilise what Jim terms “the weirdness of the quantum world”.

The discourse explores multiple theories of quantum mechanics, from superposition to quantum tunnelling, and reveals why “the most powerful theory in the whole of science” remains incredibly mysterious. Plus, watch out for a fantastic explanation of the famous double slit experiment.

Photons run out of loopholes

A team led by the Austrian physicist Anton Zeilinger has now carried out an experiment with photons, in which they have closed an important loophole. The researchers have thus provided the most complete experimental proof that the quantum world is in conflict with our everyday experience. The results of this study appear this week in the renowned journal Nature (Advance Online Publication/AOP).
When we observe an object, we make a number of intuitive assumptions, among them that the unique properties of the object have been determined prior to the observation and that these properties are independent of the state of other, distant objects. In everyday life, these assumptions are fully justified, but things are different at the quantum level. In the past 30 years, a number of experiments have shown that the behaviour of quantum particles – such as atoms, electrons or photons – can be in conflict with our basic intuition. However, these experiments have never delivered definite answers. Each previous experiment has left open the possibility, at least in principle, that the observed particles ‘exploited’ a weakness of the experimental setup.

Quantum physics is an exquisitely precise tool for understanding the world around us at a very fundamental level. At the same time, it is a basis for modern technology: semiconductors (and therefore computers), lasers, MRI scanners, and numerous other devices are based on quantum-physical effects. However, even after more than a century of intensive research, fundamental aspects of quantum theory are not yet fully understood. On a regular basis, laboratories worldwide report results that seem at odds with our everyday intuition but that can be explained within the framework of quantum theory.

On the trail of the quantum entanglement mystery
The physicists in Vienna report not a new effect, but a deep investigation into one of the most fundamental phenomena of quantum physics, known as ‘entanglement.’ The effect of quantum entanglement is amazing: when measuring a quantum object that has an entangled partner, the state of the one particle depends on measurements performed on the partner. Quantum theory describes entanglement as independent of any physical separation between the particles. That is, entanglement should also be observed when the two particles are sufficiently far apart from each other that, even in principle, no information can be exchanged between them (the speed of communication is fundamentally limited by the speed of light). Testing such predictions regarding the correlations between entangled quantum particles is, however, a major experimental challenge.

Towards a definitive answer
The young academics in Anton Zeilinger’s group including Marissa Giustina, Alexandra Mech, Rupert Ursin, Sven Ramelow and Bernhard Wittmann, in an international collaboration with the National Institute of Standards and Technology/NIST (USA), the Physikalisch-Technische Bundesanstalt (Germany), and the Max-Planck-Institute of Quantum Optics (Germany), have now achieved an important step towards delivering definitive experimental evidence that quantum particles can indeed do things that classical physics does not allow them to do. For their experiment, the team built one of the best sources for entangled photon pairs worldwide and employed highly efficient photon detectors designed by experts at NIST. These technological advances together with a suitable measurement protocol enabled the researchers to detect entangled photons with unprecedented efficiency. In a nutshell: “Our photons can no longer duck out of being measured,” says Zeilinger.

This kind of tight monitoring is important as it closes an important loophole. In previous experiments on photons, there has always been the possibility that although the measured photons do violate the laws of classical physics, such non-classical behaviour would not have been observed if all photons involved in the experiment could have been measured. In the new experiment, this loophole is now closed. “Perhaps the greatest weakness of photons as a platform for quantum experiments is their vulnerability to loss – but we have just demonstrated that this weakness need not be prohibitive,” explains Marissa Giustina, lead author of the paper.

Now one last step
Although the new experiment makes photons the first quantum particles for which, in several separate experiments, every possible loophole has been closed, the grand finale is yet to come, namely, a single experiment in which the photons are deprived of all possibilities of displaying their counterintuitive behaviour through means of classical physics. Such an experiment would also be of fundamental significance for an important practical application: ‘quantum cryptography,’ which relies on quantum mechanical principles and is considered to be absolutely secure against eavesdropping. Eavesdropping is still theoretically possible, however, as long as there are loopholes. Only when all of these are closed is a completely secure exchange of messages possible.

An experiment without any loopholes, says Zeilinger, “is a big challenge, which attracts groups worldwide.” These experiments are not limited to photons, but also involve atoms, electrons, and other systems that display quantum mechanical behaviour. The experiment of the Austrian physicists highlights the photons’ potential. Thanks to these latest advances, the photon is running out of places to hide, and quantum physicists are closer than ever to conclusive experimental proof that quantum physics defies our intuition and everyday experience to the degree suggested by research of the past decades.

This work was completed in a collaboration including the following institutions: Institute for Quantum Optics and Quantum Information – Vienna / IQOQI Vienna (Austrian Academy of Sciences), Quantum Optics, Quantum Nanophysics and Quantum Information, Department of Physics (University of Vienna), Max-Planck-Institute of Quantum Optics, National Institute of Standards and Technology / NIST, Physikalisch-Technische Bundesanstalt, Berlin.

This work was supported by: ERC (Advanced Grant), Austrian Science Fund (FWF), grant Q-ESSENCE, Marie Curie Research Training Network EMALI, and John Templeton Foundation. This work was also supported by NIST Quantum Information Science Initiative (QISI).

Publication
Bell violation with entangled photons, free of the fair-sampling assumption: Marissa Giustina, Alexandra Mech, Sven Ramelow, Bernhard Wittmann, Johannes Kofler, Jörn Beyer, Adriana Lita, Brice Calkins, Thomas Gerrits, Sae Woo Nam, Rupert Ursin, Anton Zeilinger. In: Nature (Advance Online Publication/AOP). April 14, 2013. DOI: 10.1038/nature12012

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Nanoparticles loaded with bee venom kill HIV

Nanoparticles carrying a toxin found in bee venom can destroy human immunodeficiency virus (HIV) while leaving surrounding cells unharmed, researchers at Washington University School of Medicine in St. Louis have shown. The finding is an important step toward developing a vaginal gel that may prevent the spread of HIV, the virus that causes AIDS.

“Our hope is that in places where HIV is running rampant, people could use this gel as a preventive measure to stop the initial infection,” says Joshua L. Hood, MD, PhD, a research instructor in medicine.

The study appears in the current issue of Antiviral Therapy.

Bee venom contains a potent toxin called melittin that can poke holes in the protective envelope that surrounds HIV, and other viruses. Large amounts of free melittin can cause a lot of damage. Indeed, in addition to anti-viral therapy, the paper’s senior author, Samuel A. Wickline, MD, the J. Russell Hornsby Professor of Biomedical Sciences, has shown melittin-loaded nanoparticles to be effective in killing tumor cells.

The new study shows that melittin loaded onto these nanoparticles does not harm normal cells. That’s because Hood added protective bumpers to the nanoparticle surface. When the nanoparticles come into contact with normal cells, which are much larger in size, the particles simply bounce off. HIV, on the other hand, is even smaller than the nanoparticle, so HIV fits between the bumpers and makes contact with the surface of the nanoparticle, where the bee toxin awaits.

“Melittin on the nanoparticles fuses with the viral envelope,” Hood says. “The melittin forms little pore-like attack complexes and ruptures the envelope, stripping it off the virus.”

According to Hood, an advantage of this approach is that the nanoparticle attacks an essential part of the virus’ structure. In contrast, most anti-HIV drugs inhibit the virus’s ability to replicate. But this anti-replication strategy does nothing to stop initial infection, and some strains of the virus have found ways around these drugs and reproduce anyway.

“We are attacking an inherent physical property of HIV,” Hood says. “Theoretically, there isn’t any way for the virus to adapt to that. The virus has to have a protective coat, a double-layered membrane that covers the virus.”

Beyond prevention in the form of a vaginal gel, Hood also sees potential for using nanoparticles with melittin as therapy for existing HIV infections, especially those that are drug-resistant. The nanoparticles could be injected intravenously and, in theory, would be able to clear HIV from the blood stream.

“The basic particle that we are using in these experiments was developed many years ago as an artificial blood product,” Hood says. “It didn’t work very well for delivering oxygen, but it circulates safely in the body and gives us a nice platform that we can adapt to fight different kinds of infections.”

Since melittin attacks double-layered membranes indiscriminately, this concept is not limited to HIV. Many viruses, including hepatitis B and C, rely on the same kind of protective envelope and would be vulnerable to melittin-loaded nanoparticles.

While this particular paper does not address contraception, Hood says the gel easily could be adapted to target sperm as well as HIV. But in some cases people may only want the HIV protection.

“We also are looking at this for couples where only one of the partners has HIV, and they want to have a baby,” Hood says. “These particles by themselves are actually very safe for sperm, for the same reason they are safe for vaginal cells.”

While this work was done in cells in a laboratory environment, Hood and his colleagues say the nanoparticles are easy to manufacture in large enough quantities to supply them for future clinical trials.

Advancing secure communications: A better single-photon emitter for quantum cryptography

In a development that could make the advanced form of secure communications known as quantum cryptography more practical, University of Michigan researchers have demonstrated a simpler, more efficient single-photon emitter that can be made using traditional semiconductor processing techniques.

Single-photon emitters release one particle of light, or photon, at a time, as opposed to devices like lasers that release a stream of them. Single-photon emitters are essential for quantum cryptography, which keeps secrets safe by taking advantage of the so-called observer effect: The very act of an eavesdropper listening in jumbles the message. This is because in the quantum realm, observing a system always changes it.

For quantum cryptography to work, it’s necessary to encode the message—which could be a bank password or a piece of military intelligence, for example—just one photon at a time. That way, the sender and the recipient will know whether anyone has tampered with the message.

While the U-M researchers didn’t make the first single-photon emitter, they say their new device improves upon the current technology and is much easier to make.

“This thing is very, very simple. It is all based on silicon,” said Pallab Bhattacharya, the Charles M. Vest Distinguished University Professor of Electrical Engineering and Computer Science, and the James R. Mellor Professor of Engineering.

Bhattacharya, who leads this project, is a co-author of a paper on the work published in Nature Communications on April 9.

Tin nanocrystals for the battery of the future

Monodisperse tin nanodroplets in an electron microscopic image. (Image: Maksym Kovalenko / ETH Zürich)

Tin nanocrystals for the battery of the future

More powerful batteries could help electric cars achieve a considerably larger range and thus a breakthrough on the market. A new nanomaterial for lithium ion batteries developed in the labs of chemists at ETH Zurich and Empa could come into play here.

They provide power for electric cars, electric bicycles, smartphones and laptops; nowadays, rechargeable lithium ion batteries are the storage media of choice when it comes to supplying a large amount of energy in a small space and light weight. All over the world, scientists are currently researching a new generation of such batteries with an improved performance. Scientists headed by Maksym Kovalenko from the Laboratory of Inorganic Chemistry at ETH Zurich and Empa have now developed a nanomaterial which enables considerably more power to be stored in lithium ion batteries.

The nanomaterial is composed of tiny tin crystals, which are to be deployed at the minus pole of the batteries (anode). When charging the batteries, lithium ions are absorbed at this electrode; while discharging, they are released again (see box). “The more lithium ions the electrodes can absorb and release – the better they can breathe, as it were – the more energy can be stored in a battery,” explains Kovalenko.

Tin nanocrystals for the battery of the future.

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Scientists print self-assembling ‘living tissue’

Researchers have created networks of water droplets that mimic some properties of cells in biological tissues. Using a three-dimensional printer, a team at the University of Oxford, UK, assembled tiny water droplets into a jelly-like material that can flex like a muscle and transmit electric signals like chains of neurons. The work is published today inScience1.

Quantum Entanglement and Quantum Computing

In the spring of 1994 a mathematician at Bell Labs named Peter Shor [BS 1981] discovered an algorithm for factoring large numbers with a quantum computer. I got really excited by this, because it moved the boundary separating “easy” problems, which we can eventually expect to solve with advanced technologies, from truly hard problems that we may never be able to solve. There are problems we can solve using quantum physics that we couldn’t solve otherwise. The crucial problem is protecting a quantum computer from the various kinds of “noise” that could destroy quantum entanglement, and we’ve made a lot of progress on that.