How Quantum Mechanics Helps Us Breathe

New insights explain how respiration does not result in asphyxiation.

 Why don't we suffocate whenever we try to take a breath? An international team of scientists has used quantum mechanics – the science that usually deals with events at the level of the ultra-small – to solve this human-sized mystery.

Quantum mechanics has long proved its value in understanding such phenomena as the behavior of electrons and in classifying subatomic particles. But in recent years theorists have increasingly shown how it applies to all facets of life, large and small.

The new research, led by Cédric Weber of Kings College, London and reported in the journalProceedings of the National Academy of Sciences, confirms that point.

"This work," said team member David O'Regan, a physicist at Ireland's Trinity College, Dublin, "helps to illustrate the fact that quantum-mechanical effects, which may sometimes be viewed as somehow very exotic or only relevant under extreme conditions, are at play in the day-to-day regimes where biology, chemistry, and materials science operate."

The conundrum elucidated by Weber's team stems from the way in which carbon monoxide reacts with proteins that carry oxygen around our bodies.

Those proteins, which contain iron atoms, transport oxygen molecules through the bloodstream to wherever the body needs them. According to conventional theory, the proteins should typically link up more often with molecules of carbon monoxide – from inside and outside the body – than with oxygen molecules.

If that happened regularly, it would result in asphyxiation, killing off humans and animals.

The small amounts of carbon monoxide naturally produced in our bodies would not be enough to displace oxygen fully, even if it had a greater binding ability. But we would be more vulnerable to poisoning by carbon monoxide in the atmosphere than experience shows to be the case. The fact that this doesn't happen means that oxygen molecules bind more effectively to proteins than theory forecasts.

"The problem that scientists have had is explaining how the proteins achieve this discrimination in favor of oxygen," said team member Daniel Cole, a chemist at Yale University in New Haven, Connecticut.

To do so, the team applied a computer simulation technique based on quantum mechanics to reactions of oxygen and carbon monoxide with myoglobin, the main oxygen-carrying protein in muscle tissue.

The technique, called density-functional theory, or DFT, won the 1998 Nobel Prize in Chemistry for its progenitor, Walter Kohn of the University of California, Santa Barbara. Since then it has become a bulwark of theoretical chemistry and physics.

"DFT has been the standard tool for simulating electronic properties of materials and molecules for a number of years," O'Regan said.

The team used the technique to study reactions between the iron atom inside myoglobin and a molecule of oxygen or carbon monoxide. These reactions involve electrostatics, the arrangement of electric charges in atoms and molecules. When the iron atom transfers negative electric charges to an oxygen or carbon monoxide molecule, it enables the molecule to attach itself to the entire myoglobin protein.

Unfortunately, the theory consistently predicted that carbon monoxide should bind to myoglobin much more readily than oxygen.

"Our previous DFT calculations had shown that there is a transfer of about half an electron to the oxygen molecule," Cole noted. "Although this provided some stabilization, it was not enough; the calculations predicted that carbon monoxide should be much, much more strongly bound than oxygen."

In response, the team employed two fresh approaches to the problem.

Because myoglobin molecules contain more than 1,000 atoms, the scientists used a special variety of DFT that, according to O'Regan, "is designed for dealing with larger systems without compromising on accuracy."

They also applied another extension of DFT, called dynamical mean-field theory.

"Using DMFT, we showed that, in fact, close to one electron is transferred to the oxygen molecule," Cole explained. "This provides much greater electrostatic stabilization than previously thought. It means that our estimate of the relative binding of oxygen and carbon dioxide is now in excellent agreement with experiment."

The analysis revealed that an effect called entanglement plays a critical role in binding oxygen molecules to the protein. Entanglement is a quintessential characteristic of quantum mechanics that links pairs of electrons so strongly that they no longer act independently. The process also involves Hund's exchange, another quantum-mechanical property that previous simulations had ignored.

"These effects strengthen the direct bonding between iron and oxygen, and also enhance electrostatic interactions with the protein," Cole explained.

The overall result: "We significantly improve the agreement of theory with experiment in terms of the relative tendencies to bind oxygen and carbon monoxide," O'Regan said.

The research has potential uses beyond understanding the molecular basis of breathing. According to Cole, the better understanding of how molecules bind to iron-containing proteins could help the drug-development process and possibly facilitate the design of artificial photosynthesis devices that would capture and store energy from the sun.

First Schrödinger's Cat ...and now Paradoxical Pigeons

First there was Schrödinger's cat, now an international team of physicists has come up with a new animal-related paradox involving "quantum pigeons".

For nearly a century students have struggled to understand the many counter-intuitive implications of quantum physics. Perhaps the most famous paradox is Schrödinger's cat, whereby a cat being both dead and alive at the same time illustrates the fact that a particle can exist simultaneously in two quantum states.

Now, Jeff Tollaksen of Chapman University in California and colleagues in Israel, Italy and the UK have proposed an equally bizarre scenario dubbed the "quantum-pigeonhole effect". The paradox begins with the observation that when you put three pigeons in two pigeonholes, there will always be at least two pigeons in the same hole. But according to the team's quantum analysis, it is possible for none of the pigeons to share a hole.

"It's one of those things that seem to be impossible," says Tollaksen. But it is a direct consequence of quantum mechanics and, he adds, "It really has immense implications."

Nondeterministic measurements

Classical physics is deterministic. This means that measuring the initial state of a system will, in principle, tell you everything you need to determine the final state. But in 1964 Yakir Aharonov of Chapman University and Tel Aviv University helped discover that in quantum mechanics, you can choose initial and final states that are entirely independent, Tollaksen says.

Now Aharonov has teamed up with Tollaksen and colleagues to use this and other concepts of quantum mechanics to postulate the quantum-pigeonhole effect. They reckon that the effect will arise when an observer makes a sequence of measurements while trying to fit three particles in two boxes. First, you make an initial, "pre-selection" measurement of the locations of the particles. Next, you can perform an intermediate measurement to see whether two particles share a box. Finally, you make a final, "post-selection" measurement of the locations. You can make the pre-selection and post-selection measurements such that they are completely independent. In the intermediate step, you can make what's called a weak measurement to look at all three particles simultaneously. And when you do, it turns out that no two particles share a box.

Spooky and profound

The implications of these results, Tollaksen says, complement the well-known Einstein–Podolsky–Rosen (EPR) paradox. In this scenario, two particles that start in the same place can become intimately correlated, a relationship called entanglement. Measuring the state of the first particle seems to influence the state of the second one, even if they are subsequently separated by distances so great that it would be impossible to explain the influence using classical physics. This unsettling conclusion led Einstein to call entanglement "spooky action at a distance".

"EPR is one of the most profound discoveries in science," Tollaksen says. "But that's only half the story." The quantum-pigeonhole principle creates a somewhat opposite situation, he explains. Three particles can begin separated with no connections or correlations at all. You bring them together and force them to interact by squeezing them in two boxes. During this intermediate stage, they are more strongly correlated than classically possible. But in the final stage, they are not correlated at all.

The implications of the EPR paradox are important and shape our understanding of information and the fundamental physics of matter. Although it is too early to predict every implication, he believes that the quantum-pigeonhole principle could prove to be just as influential – if not more so. "This is at least as equally profound, if not more profound," he says. It implies a new concept of correlation that is surprising.

Electronic pigeons

To verify their conclusions, Tollaksen and colleagues propose an experiment in which three electrons travel through an interferometer. This is essentially a beam splitter that creates two separate paths for the electrons, which then meet again.

Because there are only two possible paths, you would expect at least two electrons to share a path. If so, then the two will be close together and interact: their identical electric charges will repel each other, slightly deflecting their trajectories. Then physicists will be able to detect these deflections when all three electrons reunite after the paths converge. But, Tollaksen says, because their calculations show that no two of the three electrons will actually follow the same path, no deflections will be observed.

Physicists have not done these experiments yet, but Tollaksen is confident in their results. "I'm sure it will be confirmed experimentally very soon," he says.

The new results seem "fascinating," says Leonard Susskind of Stanford University. "I would guess that the new effect is a serious step in understanding quantum correlations."

Quantum Compass : Successor to GPS

Virtually every mobile device on the planet has a GPS chip built-in that lets you find your way around, but GPS is getting old and doesn’t work well in all situations. The British Ministry of Defence is hard at work developing a so-called “quantum compass” that could become the successor to GPS, and just like GPS, it might land in your pocket one day.

The UK military is investing millions of pounds in the quantum compass mainly for use in submarines. GPS systems require a view of the sky, or at least very little obstruction to get a location fix. A tube of metal sliding through the water 100 meters beneath the waves can’t really get a GPS lock. Subs currently use a type of inertial location system based on accelerometers. Each twist and turn a submarine makes is recorded and used to calculate its position based on the last known coordinates. This is called dead reckoning navigation.

A quantum compass takes advantage of the 1997 Nobel-winning discovery that lasers can be used to cool atoms to within fractions of a degree of absolute zero. Atoms in this frozen state are extremely sensitive to the magnetic and gravitational field of the Earth. Thus, they can be used to track movement with amazing accuracy. To be clear, this is still a type of inertial “dead reckoning” navigation. The difference is that after getting a solid GPS lock, a sub could go underwater and be exactly on target when it surfaces days or even weeks later.

The prototype compass build by the Ministry of Defence resembles a 1-meter-long shoe box. Inside is an array of lasers cooling a tiny cloud of rubidium atoms. Three such devices could be placed at right angles to each other to measure movement along all three axes. The next step is to miniaturize the technology, perhaps making it compact enough to be integrated in a single chip.

But why go to all this trouble for submarines? Well, that’s not the only reason for governments to move away from satellite positioning systems. The US has long warned that the signals from GPS satellites could be interfered with or hijacked to corrupt location information. Additionally, as international tensions mount over space, several countries (most notably China) have tested satellite-busting weapons that could cripple GPS and leave space strewn with junk. And of course, the US could always cut off other countries and civilians from using its GPS network.

Dropletons : New State of Matter

There was a time when states of matter were simple: Solid, liquid, gas. Then came plasma, Bose -Einstein condensate, supercritical fluid and more. Now the list has grown by one more, with the unexpected discovery of a new state dubbed “dropletons” that bear some resemblance to liquids but occur under very different circumstances.

The discovery occurred when a team at the University of Colorado Joint Institute for Lab Astrophysics were focusing laser light on gallium arsenide (GaAs) to create excitons.

Excitons are formed when a photon strikes a material, particularly a semiconductor. If an electron is knocked loose, or excited, it leaves what is termed an “electron hole” behind. If the forces of other charges nearby keep the electron close enough to the hole to feel an attraction, a bound state forms known as an exciton. Excitons are called quasiparticles because the electrons and holes behave together as if they were a single particle.

If this all sounds a bit hard to relate to, consider that solar cells are semiconductors, and the formation of excitons is one possible step to the production of electricity. A better understanding of how excitons form and behave could produce ways to harvest sunlight more efficiently.

Graduate student Andrew Almand-Hunter was forming biexcitons – two excitons that behave like a molecule, by focusing the laser to a dot 100nm across and leaving it on for shorter and shorter fractions of a second.

“But the experiment didn’t behave at all in the way we expected,” Almand-Hunter said. When the pulses were lasting less than 100 millionths of a second exciton density reached a critical threshold. “We expected to see the energy of the biexcitons increase as the laser generated more electrons and holes. But, what we saw when we did the experiment was that the energy actually decreased!” 

The team figured that they had created something other than biexcitons, but were not sure what. They contacted theorists at Philipps-University, Marburg who suggested they had made droplets of 4, 5 or 6 electrons and holes, and constructed a model of these dropletons' behavior. 

The dropletons are small enough to behave quantum mechanically, but the electrons and holes are not in pairs, as they would be if the dropleton was just a group of excitons. Instead they form a “quantum fog” of electrons and holes that flow around each other and even ripple like a liquid, rather than existing as discrete pairs. However, unlike liquids we are familiar with, dropletons a finite size, outside which the electron/hole association breaks down.

The discovery has been published in Nature. Perhaps the most remarkable thing is that the dropletons are stable, by the standards of quantum physics. While they can only survive inside solid materials, they last around 25 trillionths of a second, which is actually long enough for scientists to study the way their behavior is shaped by the environment. At 200nm wide the dropletons are as large as very small bacteria – a size that can be seen by conventional microscopes.

Quantum Teleportation

One of the hurdles to teleportation has been overcome, with the reliable movement of quantum information between two objects separated by a short distance. The achievement is still a very, very long way from the movements familiar from science fiction, but strengthens our confidence in the theory of quantum entanglement, one of the most controversial aspects of modern physics. It may, moreover, assist the much closer goal of quantum computing.

Certain subatomic particles always exist in paired states. For example, two electrons may have opposite spins. This is fine initially, but creates a famous paradox if one particle is interfered with in such a way that its spin is changed. According to entanglement theory the other particle will instantly respond to the changes wrought on its pair so that the two remain opposite. 

However, the distance between the two this means that the information of what has happened to the one particle must be transmitted infinitely fast – faster than the speed of light. Einstein famously mocked the idea as “spooky action at a distance”, and suggested our understanding of quantum mechanics must be in error. However, with quantum theory's subsequent success physicists have grown more comfortable with the idea that entanglement exists, although many argue it cannot be used to transmit information.

In 1964 physicist John Stewart Bell came up with an idea for an experiment to test whether entanglement is real. At the time the test was impractical, but with publication in Science a team from the Delft University of Technology, Netherlands have got close to conducting Bell's test.

The Delft team trapped electrons in very low temperature diamonds, which team leader Ronald Hanson describes as “miniprisons”. This allowed them to measure the spin for each electron very reliably. Alterations to this spin were reflected in the spin of an entangled electron trapped in a similar diamond prison on the next bench.

The small distance between the two diamonds makes it hard to demonstrate that the transfer of information is occurring instantly, rather than at light speed. Consequently, the next step will be to entangle caged electrons and expand their separation across town or around the world. Entanglement between islands more than 100km apart has already been demonstrated, but only statistically, rather than with 100% success. 

Besides finally settling one of 20th Century physics greatest debates, reliable quantum teleportation could make possible the ultimate in secure communication channels, which would also be infinitely fast as well. Although this is starting to sound awfully close to Ursula Le Guin's ansible, most physicists dispute the possibility of such a device.

As usual, the result does not come out of nowhere. Other teams have also been able to teleport quantum information, but only in a minority of cases. Last year Hanson's team announced they had achieved quantum teleportation using diamond entrapment, but without the 100% reliability of the most recent work.