Through the quantum looking glass

An ultrathin invention could make future computing, sensing and encryption technologies remarkably smaller and more powerful by helping scientists control a strange but useful phenomenon of quantum mechanics, according to new research recently published in the journal Science.

Scientists at Sandia National Laboratories and the Max Planck Institute for the Science of Light have reported on a device that could replace a roomful of equipment to link photons in a bizarre quantum effect called entanglement. This device -- a kind of nano-engineered material called a metasurface -- paves the way for entangling photons in complex ways that have not been possible with compact technologies.

When scientists say photons are entangled, they mean they are linked in such a way that actions on one affect the other, no matter where or how far apart the photons are in the universe. It is an effect of quantum mechanics, the laws of physics that govern particles and other very tiny things.

Although the phenomenon might seem odd, scientists have harnessed it to process information in new ways. For example, entanglement helps protect delicate quantum information and correct errors in quantum computing, a field that could someday have sweeping impacts in national security, science and finance. Entanglement is also enabling new, advanced encryption methods for secure communication.

Research for the groundbreaking device, which is a hundred times thinner than a sheet of paper, was performed, in part, at the Center for Integrated Nanotechnologies, a Department of Energy Office of Science user facility operated by Sandia and Los Alamos national laboratories. Sandia's team received funding from the Office of Science, Basic Energy Sciences program.

Light goes in, entangled photons come out

The new metasurface acts as a doorway to this unusual quantum phenomenon. In some ways, it's like the mirror in Lewis Carrol's "Through the Looking-Glass," through which the young protagonist Alice experiences a strange, new world.

Instead of walking through their new device, scientists shine a laser through it. The beam of light passes through an ultrathin sample of glass covered in nanoscale structures made of a common semiconductor material called gallium arsenide.

"It scrambles all the optical fields," said Sandia senior scientist Igal Brener, an expert in a field called nonlinear optics who led the Sandia team. Occasionally, he said, a pair of entangled photons at different wavelengths emerge from the sample in the same direction as the incoming laser beam.

Brener said he is excited about this device because it is designed to produce complex webs of entangled photons -- not just one pair at a time, but several pairs all entangled together, and some that can be indistinguishable from each other. Some technologies need these complex varieties of so-called multi-entanglement for sophisticated information processing schemes.

Other miniature technologies based on silicon photonics can also entangle photons but without the much-needed level of complex, multi-entanglement. Until now, the only way to produce such results was with multiple tables full of lasers, specialized crystals and other optical equipment.

"It is quite complicated and kind of intractable when this multi-entanglement needs more than two or three pairs," Brener said. "These nonlinear metasurfaces essentially achieve this task in one sample when before it would have required incredibly complex optical setups."

The Science paper outlines how the team successfully tuned their metasurface to produce entangled photons with varying wavelengths, a critical precursor to generating several pairs of intricately entangled photons simultaneously.

However, the researchers note in their paper that the efficiency of their device -- the rate at which they can generate groups of entangled photons -- is lower than that of other techniques and needs to be improved.

What is a metasurface?

A metasurface is a synthetic material that interacts with light and other electromagnetic waves in ways conventional materials can't. Commercial industries, said Brener, are busy developing metasurfaces because they take up less space and can do more with light than, for instance, a traditional lens.

"You now can replace lenses and thick optical elements with metasurfaces," Brener said. "Those types of metasurfaces will revolutionize consumer products."

Sandia is one of the leading institutions in the world performing research in metasurfaces and metamaterials. Between its Microsystems Engineering, Science and Applications complex, which manufactures compound semiconductors, and the nearby Center for Integrated Nanotechnologies, researchers have access to all the specialized tools they need to design, fabricate and analyze these ambitious new materials.

"The work was challenging as it required precise nanofabrication technology to obtain the sharp, narrowband optical resonances that seeds the quantum process of the work," said Sylvain Gennaro, a former postdoctoral researcher at Sandia who worked on several aspects of the project.

The device was designed, fabricated and tested through a partnership between Sandia and a research group led by physicist Maria Chekhova, an expert in the quantum entanglement of photons at the Max Planck Institute for the Science of Light.

"Metasurfaces are leading to a paradigm shift in quantum optics, combining ultrasmall sources of quantum light with far reaching possibilities for quantum state engineering," said Tomás Santiago-Cruz, a member of the Max Plank team and first author on the paper.

Brener, who has studied metamaterials for more than a decade, said this newest research could possibly spark a second revolution -- one that sees these materials developed not just as a new kind of lens, but as a technology for quantum information processing and other new applications.

"There was one wave with metasurfaces that is already well established and on its way. Maybe there is a second wave of innovative applications coming," he said.

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Quantum mechanics could explain why DNA can spontaneously mutate

The molecules of life, DNA, replicate with astounding precision, yet this process is not immune to mistakes and can lead to mutations. Using sophisticated computer modelling, a team of physicists and chemists at the University of Surrey have shown that such errors in copying can arise due to the strange rules of the quantum world.

The two strands of the famous DNA double helix are linked together by subatomic particles called protons -?the nuclei of atoms of hydrogen -- which provide the glue that bonds molecules called bases together. These so-called hydrogen bonds are like the rungs of a twisted ladder that makes up the double helix structure discovered in 1952 by James Watson and Francis Crick based on the work of Rosalind Franklin and Maurice Wilkins.

Normally, these DNA bases (called A, C, T and G) follow strict rules on how they bond together: A always bonds to T and C always to G. This strict pairing is determined by the molecules' shape, fitting them together like pieces in a jigsaw, but if the nature of the hydrogen bonds changes slightly, this can cause the pairing rule to break down, leading to the wrong bases being linked and hence a mutation. Although predicted by Crick and Watson, it is only now that sophisticated computational modelling has been able to quantify the process accurately.

The team, part of Surrey's research programme in the exciting new field of quantum biology, have shown that this modification in the bonds between the DNA strands is far more prevalent than has hitherto been thought. The protons can easily jump from their usual site on one side of an energy barrier to land on the other side. If this happens just before the two strands are unzipped in the first step of the copying process, then the error can pass through the replication machinery in the cell, leading to what is called a DNA mismatch and, potentially, a mutation.

In a paper published this week in the journal Nature Communications Physics, the Surrey team based in the Leverhulme Quantum Biology Doctoral Training Centre used an approach called open quantum systems to determine the physical mechanisms that might cause the protons to jump across between the DNA strands. But, most intriguingly, it is thanks to a well-known yet almost magical quantum mechanism called tunnelling -- akin to a phantom passing through a solid wall -- that they manage to get across.

It had previously been thought that such quantum behaviour could not occur inside a living cell's warm, wet and complex environment. However, the Austrian physicist Erwin Schrödinger had suggested in his 1944 book What is Life? that quantum mechanics can play a role in living systems since they behave rather differently from inanimate matter. This latest work seems to confirm Schrödinger's theory.

In their study, the authors determine that the local cellular environment causes the protons, which behave like spread out waves, to be thermally activated and encouraged through the energy barrier. In fact, the protons are found to be continuously and very rapidly tunnelling back and forth between the two strands. Then, when the DNA is cleaved into its separate strands, some of the protons are caught on the wrong side, leading to an error.

Dr Louie Slocombe, who performed these calculations during his PhD, explains that:

" The protons in the DNA can tunnel along the hydrogen bonds in DNA and modify the bases which encode the genetic information. The modified bases are called "tautomers" and can survive the DNA cleavage and replication processes, causing "transcription errors" or mutations."

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Timekeeping theory combines quantum clocks and Einstein's relativity

 A phenomenon of quantum mechanics known as superposition can impact timekeeping in high-precision clocks, according to a theoretical study from Dartmouth College, Saint Anselm College and Santa Clara University.

Research describing the effect shows that superposition -- the ability of an atom to exist in more than one state at the same time -- leads to a correction in atomic clocks known as "quantum time dilation."

The research, published in the journal Nature Communications, takes into account quantum effects beyond Albert Einstein's theory of relativity to make a new prediction about the nature of time.

"Whenever we have developed better clocks, we've learned something new about the world," said Alexander Smith, an assistant professor of physics at Saint Anselm College and adjunct assistant professor at Dartmouth College, who led the research as a junior fellow in Dartmouth's Society of Fellows. "Quantum time dilation is a consequence of both quantum mechanics and Einstein's relativity, and thus offers a new possibility to test fundamental physics at their intersection."

In the early 1900s, Albert Einstein presented a revolutionary picture of space and time by showing that the time experienced by a clock depends on how fast it is moving -- as the speed of a clock increases, the rate at which it ticks decreases. This was a radical departure from Sir Isaac Newton's absolute notion of time.

Quantum mechanics, the theory of motion governing the atomic realm, allows for a clock to move as if it were simultaneously traveling at two different speeds: a quantum "superposition" of speeds. The research paper takes this possibility into account and provides a probabilistic theory of timekeeping, which led to the prediction of quantum time dilation.

To develop the new theory, the team combined modern techniques from quantum information science with a theory developed in the 1980s that explains how time might emerge out of a quantum theory of gravity.

"Physicists have sought to accommodate the dynamical nature of time in quantum theory for decades," said Mehdi Ahmadi, a lecturer at Santa Clara University who co-authored the study. "In our work, we predict corrections to relativistic time dilation which stem from the fact that the clocks used to measure this effect are quantum mechanical in nature."

In the same way that carbon dating relies on decaying atoms to determine the age of organic objects, the lifetime of an excited atom acts as a clock. If such an atom moves in a superposition of different speeds, then its lifetime will either increase or decrease depending on the nature of the superposition relative to an atom moving at a definite speed.

The correction to the atom's lifetime is so small that it would be impossible to measure in terms that make sense at the human scale. But the ability to account for this effect could enable a test of quantum time dilation using the most advanced atomic clocks.

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In a new quantum simulator, light behaves like a magnet

Riccardo Rota and Vincenzo Savona, the two EPFL physicists leading the study, working on the design of their quantum simulator.

When subject to the laws of quantum mechanics, systems made of many interacting particles can display behaviour so complex that its quantitative description defies the capabilities of the most powerful computers in the world. In 1981, the visionary physicist Richard Feynman argued we can simulate such complex behavior using an artificial apparatus governed by the very same quantum laws -- what has come to be known as a "quantum simulator."

One example of a complex quantum system is that of magnets placed at really low temperatures. Close to absolute zero (-273.15 degrees Celsius), magnetic materials may undergo what is known as a "quantum phase transition." Like a conventional phase transition (e.g. ice melting into water, or water evaporating into steam), the system still switches between two states, except that close to the transition point the system manifests quantum entanglement -- the most profound feature predicted by quantum mechanics. Studying this phenomenon in real materials is an astoundingly challenging task for experimental physicists.

But physicists led by Vincenzo Savona at EPFL have now come up with a quantum simulator that promises to solve the problem. "The simulator is a simple photonic device that can easily be built and run with current experimental techniques," says Riccardo Rota, the postdoc at Savona's lab who led the study. "But more importantly, it can simulate the complex behavior of real, interacting magnets at very low temperatures."

The simulator may be built using superconducting circuits -- the same technological platform used in modern quantum computers. The circuits are coupled to laser fields in such a way that it causes an effective interaction among light particles (photons). "When we studied the simulator, we found that the photons behaved in the same way as magnetic dipoles across the quantum phase transition in real materials," says Rota. In short, we can now use photons to run a virtual experiment on quantum magnets instead of having to set up the experiment itself.

"We are theorists," says Savona. "We came up with the idea for this particular quantum simulator and modelled its behavior using traditional computer simulations, which can be done when the quantum simulator addresses a small enough system. Our findings prove that the quantum simulator we propose is viable, and we are now in talks with experimental groups who would like to actually build and use it."

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