MIT scientists find weird quantum effects, even over hundreds of miles

Neutrinos traveling 450 miles have no individual identities, according to MIT analysis.

In the world of quantum, infinitesimally small particles, weird and often logic-defying behaviors abound. Perhaps the strangest of these is the idea of superposition, in which objects can exist simultaneously in two or more seemingly counterintuitive states. For example, according to the laws of quantum mechanics, electrons may spin both clockwise and counter-clockwise, or be both at rest and excited, at the same time.

The physicist Erwin Schrödinger highlighted some strange consequences of the idea of superposition more than 80 years ago, with a thought experiment that posed that a cat trapped in a box with a radioactive source could be in a superposition state, considered both alive and dead, according to the laws of quantum mechanics. Since then, scientists have proven that particles can indeed be in superposition, at quantum, subatomic scales. But whether such weird phenomena can be observed in our larger, everyday world is an open, actively pursued question.

Now, MIT physicists have found that subatomic particles called neutrinos can be in superposition, without individual identities, when traveling hundreds of miles. Their results, to be published later this month in Physical Review Letters, represent the longest distance over which quantum mechanics has been tested to date. 

A subatomic journey across state lines

The team analyzed data on the oscillations of neutrinos — subatomic particles that interact extremely weakly with matter, passing through our bodies by the billions per second without any effect. Neutrinos can oscillate, or change between several distinct “flavors,” as they travel through the universe at close to the speed of light.

The researchers obtained data from Fermilab’s Main Injector Neutrino Oscillation Search, or MINOS, an experiment in which neutrinos are produced from the scattering of other accelerated, high-energy particles in a facility near Chicago and beamed to a detector in Soudan, Minnesota, 735 kilometers (456 miles) away. Although the neutrinos leave Illinois as one flavor, they may oscillate along their journey, arriving in Minnesota as a completely different flavor.

The MIT team studied the distribution of neutrino flavors generated in Illinois, versus those detected in Minnesota, and found that these distributions can be explained most readily by quantum phenomena: As neutrinos sped between the reactor and detector, they were statistically most likely to be in a state of superposition, with no definite flavor or identity.

What’s more, the researchers  found that the data was “in high tension” with more classical descriptions of how matter should behave. In particular, it was statistically unlikely that the data could be explained by any model of the sort that Einstein sought, in which objects would always embody definite properties rather than exist in superpositions.

“What’s fascinating is, many of us tend to think of quantum mechanics applying on small scales,” says David Kaiser, the Germeshausen Professor of the History of Science and professor of physics at MIT. “But it turns out that we can’t escape quantum mechanics, even when we describe processes that happen over large distances. We can’t stop our quantum mechanical description even when these things leave one state and enter another, traveling hundreds of miles. I think that’s breathtaking.”

Kaiser is a co-author on the paper, which includes MIT physics professor Joseph Formaggio, junior Talia Weiss, and former graduate student Mykola Murskyj.

A flipped inequality

The team analyzed the MINOS data by applying a slightly altered version of the Leggett-Garg inequality, a mathematical expression named after physicists Anthony Leggett and Anupam Garg, who derived the expression to test whether a system with two or more distinct states acts in a quantum or classical fashion.

Leggett and Garg realized that the measurements of such a system, and the statistical correlations between those measurements, should be different if the system behaves according to classical versus quantum mechanical laws.

“They realized you get different predictions for correlations of measurements of a single system over time, if you assume superposition versus realism,” Kaiser explains, where “realism” refers to models of the Einstein type, in which particles should always exist in some definite state.

Formaggio had the idea to flip the expression slightly, to apply not to repeated measurements over time but to measurements at a range of neutrino energies. In the MINOS experiment, huge numbers of neutrinos are created at various energies, where Kaiser says they then “careen through the Earth, through solid rock, and a tiny drizzle of them will be detected” 735 kilometers away.

According to Formaggio’s reworking of the Leggett-Garg inequality, the distribution of neutrino flavors — the type of neutrino that finally arrives at the detector — should depend on the energies at which the neutrinos were created. Furthermore, those flavor distributions should look very different if the neutrinos assumed a definite identity throughout their journey, versus if they were in superposition, with no distinct flavor.

“The big world we live in”

Applying their modified version of the Leggett-Garg expression to neutrino oscillations, the group predicted the distribution of neutrino flavors arriving at the detector, both if the neutrinos were behaving classically, according to an Einstein-like theory, and if they were acting in a quantum state, in superposition. When they compared both predicted distributions, they found there was virtually no overlap.

More importantly, when they compared these predictions with the actual distribution of neutrino flavors observed from the MINOS experiment, they found that the data fit squarely within the predicted distribution for a quantum system, meaning that the neutrinos very likely did not have individual identities while traveling over hundreds of miles between detectors.

But what if these particles truly embodied distinct flavors at each moment in time, rather than being some ghostly, neither-here-nor-there phantoms of quantum physics? What if these neutrinos behaved according to Einstein’s realism-based view of the world? After all, there could be statistical flukes due to defects in instrumentation, that might still generate a distribution of neutrinos that the researchers observed. Kaiser says if that were the case and “the world truly obeyed Einstein’s intuitions,” the chances of such a model accounting for the observed data would be “something like one in a billion.”  

So how do neutrinos do it? How do they maintain a quantum, identityless state for seemingly long distances? André de Gouvêa, professor of physics and astronomy at Northwestern University, says because neutrinos move so fast and interact with so little in the world, “relativistic effects — as in Einstein’s special theory of relativity —are huge, and conspire to make the very long distances appear [to the neutrinos] short.”

“The final result is that, like all other tests performed to date under very different circumstances, quantum mechanics appears to be the correct description of the world at all distance scales, weirdness not withstanding,” says Gouvêa, who was not involved in the research.

“What gives people pause is, quantum mechanics is quantitatively precise and yet it comes with all this conceptual baggage,” Kaiser says. “That’s why I like tests like this: Let’s let these things travel further than most people will drive on a family road trip, and watch them zoom through the big world we live in, not just the strange world of quantum mechanics, for hundreds of miles. And even then, we can’t stop using quantum mechanics. We really see quantum effects persist across macroscopic distances.”

MIT

UChicago physicists first to see behavior of quantum materials in curved space

Harnessing the shared wave nature of light and matter, researchers at the University of Chicago led by Neubauer Family Assistant Professor of Physics Jonathan Simon have used light to explore some of the most intriguing questions in the quantum mechanics of materials. The topic encompasses complex and non-intuitive phenomena that are often difficult to explain in non-technical language, but which carry important implications to specialists in the field.

In work published online June 6, 2016, in the journal Nature, Simon's group presents new experimental observations of a quantum Hall material near a singularity of curvature in space.

Quantum effects give rise to some of the most useful and promising properties of materials: they define standard units of measurement, give rise to superconductivity, and describe quantum computers. The quantum hall materials are one prominent example in which electrons are trapped in non-conducting circular orbits except at the edges of the material. There, electrons exhibit quantized resistance-free electrical conduction that is immune to disorder such as material impurities or surface defects.

Furthermore, electrons in quantum Hall materials do not transmit sound waves but instead have particle-like excitations, some of which are unlike any other particles ever discovered. Some of these materials also exhibit simultaneous quantum entanglement between millions of electrons, meaning that the electrons are so interconnected, the state of one instantly influences the state of all others. This combination of properties makes quantum Hall materials a promising platform for future quantum computation.

Researchers worldwide have spent the past 35 years delving into the mysteries of quantum Hall materials, but always in the same fundamental way. They use superconducting magnets to make very powerful magnetic fields and refrigerators to cool electronic samples to thousandths of a degree above absolute zero.

Trapping light...

In a new approach, Simon and his team demonstrated the creation of a quantum Hall material made up of light. "Using really good mirrors that are pointed at each other, we can trap light for a long time while it bounces back and forth many thousands of times between the mirrors," explained graduate student Nathan Schine.

In the UChicago experiment, photons travel back and forth between mirrors, while their side-to-side motion mimics the behavior of massive particles like electrons. To emulate a strong magnetic field, the researchers created a non-planar arrangement of four mirrors that makes the light twist as it completes a round trip. The twisting motion causes the photons to move like charged particles in a magnetic field, even though there is no actual magnet present.

"We make the photons spin, which leads to a force that has the same effect as a magnetic field," explained Schine. While the light is trapped, it behaves like the electrons in a quantum Hall material.

First, Simon's group demonstrated that they had a quantum Hall material of light. To do so, they shined infrared laser light at the mirrors. By varying the laser's frequency, Simon's team could map out precisely at which frequencies the laser was transmitted through the mirrors. These transmission frequencies, along with camera images of the transmitted light, gave a telltale signature of a quantum Hall state.

Next, the researchers took advantage of the precise control that advanced optical systems provide to place the photons in curved space, which has not been possible so far with electrons. In particular, they made the photons behave as if they resided on the surface of a cone.

...near a singularity

"We created a cone for light much like you might do by cutting a wedge of paper and taping the edges together," said postdoctoral fellow Ariel Sommer, also a co-author of the paper. "In this case, we imposed a three-fold symmetry on our light, which essentially divides the plane into three wedges and forces the light to repeat itself on each wedge."

The tip of a cone has infinite curvature--the singularity--so the researchers were able to study the effect of strong spatial curvature in a quantum Hall material. They observed that photons accumulated at the cone tip, confirming a previously untested theory of the quantum Hall effect in curved space.

Despite 20 years of interest, this is the first time an experiment has observed the behavior of quantum materials in curved space. "We are beginning to make our photons interact with each other," said Schine. "This opens up many possibilities, such as making crystalline or exotic quantum liquid states of light. We can then see how they respond to spatial curvature."

The researchers say this could be useful for characterizing a certain type of quantum computer that is built of quantum Hall materials.

"While quantum Hall materials were discovered in the eighties, they continue to reveal their fascinating secrets to this day," said Simon. "The final frontier is exploring the interplay of these beautiful materials with the curvature of space. That is what we've begun to explore with our photons."

Reference:

Synthetic Landau levels for photons
Nature (2016) doi:10.1038/nature17943

University of Chicago

A magnetic vortex to control electron spin

Researchers coupled a diamond nanoparticle with a magnetic vortex to control electron spin in nitrogen-vacancy defects. @ Case Western Reserve University

Researchers at Case Western Reserve University have developed a way to swiftly and precisely control electron spins at room temperature.

The technology, described in Nature Communications, offers a possible alternative strategy for building quantum computers that are far faster and more powerful than today's supercomputers.

"What makes electronic devices possible is controlling the movement of electrons from place to place using electric fields that are strong, fast and local," said physics Professor Jesse Berezovsky, leader of the research. "That's hard with magnetic fields, but they're what you need to control spin."

Other researchers have searched for materials where electric fields can mimic the effects of a magnetic field, but finding materials where this effect is strong enough and still works at room temperature has proven difficult.

"Our solution," Berezovsky said, "is to use a magnetic vortex."

Berezovsky worked with physics PhD students Michael S. Wolf and Robert Badea.

The researchers fabricated magnetic micro-disks that have no north and south poles like those on a bar magnet, but magnetize into a vortex. A magnetic field emanates from the vortex core. At the center point, the field is particularly strong and rises perpendicular to the disk.

The vortices are coupled with diamond nanoparticles. In the diamond lattice inside each nanoparticle, several individual spins are trapped inside of defects called nitrogen vacancies.

The scientists use a pulse from a laser to initialize the spin. By applying microwaves and a weak magnetic field, Berezovsky's team can move the vortex in nanoseconds, shifting the central point, which can cause an electron to change its spin.

In what's called a quantum coherent state, the spin can act as a quantum bit, or qubit--the basic unit of information in a quantum computer.

In current computers, bits of information exist in one of two states: zero or one. But in a superposition state, the spin can be up and down at the same time, that is, zero and one simultaneously. That capability would allow for more complex and faster computing.

"The spins are close to each other; you want spins to interact with their neighbors in quantum computing," Berezovsky said. "The power comes from entanglement."

The magnetic field gradient produced by a vortex proved sufficient to manipulate spins just nanometers apart.

In addition to computing, electrons controlled in coherent quantum states might be useful for extremely high-resolution sensors, the researchers say. For example, in an MRI, they could be used to sense magnetic fields in far more detail than with today's technology, perhaps distinguishing atoms.

Controlling the electron spins without destroying the coherent quantum states has proven difficult with other techniques, but a series of experiments by the group has shown the quantum states remain solid. In fact, "the vortex appears to enhance the microwave field we apply," Berezovsky said.

The scientists are continuing to shorten the time it takes to change the spin, which is a key to high-speed computing. They are also investigating the interactions between the vortex, microwave magnetic field and electron spin, and how they evolve together.

Case Western Reserve University

More stories on  Nanotechnology World Association

Nano 'hall of mirrors' causes molecules to mix with light

When a molecule emits a blink of light, it doesn't expect it to ever come back. However researchers have now managed to place single molecules in such a tiny optical cavity that emitted photons, or particles of light, return to the molecule before they have properly left. The energy oscillates back and forth between light and molecule, resulting in a complete mixing of the two.

Previous attempts to mix molecules with light have been complex to produce and only achievable at very low temperatures, but the researchers, led by the University of Cambridge, have developed a method to produce these 'half-light' molecules at room temperature.

These unusual interactions of molecules with light provide new ways to manipulate the physical and chemical properties of matter, and could be used to process quantum information, aid in the understanding of complex processes at work in photosynthesis, or even manipulate the chemical bonds between atoms. The results are reported in the journal Nature.

To use single molecules in this way, the researchers had to reliably construct cavities only a billionth of a metre (one nanometre) across in order to trap light. They used the tiny gap between a gold nanoparticle and a mirror, and placed a coloured dye molecule inside.

"It's like a hall of mirrors for a molecule, only spaced a hundred thousand times thinner than a human hair," said Professor Jeremy Baumberg of the NanoPhotonics Centre at Cambridge's Cavendish Laboratory, who led the research.

In order to achieve the molecule-light mixing, the dye molecules needed to be correctly positioned in the tiny gap. "Our molecules like to lie down flat on the gold, and it was really hard to persuade them to stand up straight," said Rohit Chikkaraddy, lead author of the study.

To solve this, the team joined with a team of chemists at Cambridge led by Professor Oren Scherman to encapsulate the dyes in hollow barrel-shaped molecular cages called cucurbiturils, which are able to hold the dye molecules in the desired upright position.

When assembled together correctly, the molecule scattering spectrum splits into two separated quantum states which is the signature of this 'mixing'. This spacing in colour corresponds to photons taking less than a trillionth of a second to come back to the molecule.

A key advance was to show strong mixing of light and matter was possible for single molecules even with large absorption of light in the metal and at room temperature. "Finding single-molecule signatures took months of data collection," said Chikkaraddy.

The researchers were also able to observe steps in the colour spacing of the states corresponding to whether one, two, or three molecules were in the gap.

###

The Cambridge team collaborated with theorists Professor Ortwin Hess at the Blackett Laboratory, Imperial College London and Dr Edina Rosta at Kings College London to understand the confinement and interaction of light in such tiny gaps, matching experiments amazingly well.

Reference:

Single-molecule strong coupling at room temperature in plasmonic nanocavities
Nature (2016) doi:10.1038/nature17974

University of Cambridge

Quantum Tunneling Creates A Never Before Seen Shape For Water Molecules

New state of water: Strange 6-sided molecule found

 

A strange new behavior of water molecules has been observed inside crystals of beryl, a type of emerald, caused by bizarre quantum-mechanical effects that let the water molecules face six different directions at the same time.

Under normal conditions, the two hydrogen atoms in each water molecule are arranged around the oxygen atom in an open "V" shape, sometimes compared to a boomerang or Mickey Mouse ears.

But in a new experiment, scientists have found that hydrogen atoms of some water molecules trapped in the crystal structure of the mineral beryl become "smeared out" into a six-sided ring. 

The ring shape is caused by the "quantum tunneling" of the molecules, a phenomenon that lets subatomic particles pass or "tunnel" through seemingly-impossible physical barriers.

In this scenario, the atoms of the water molecule are "delocalized" among six possible directions inside natural hexagonal pores or channels that run though the crystal structure of the beryl, so it partially exist in all six positions at the same time, the researchers said.

Inside crystals 

Scientists from Oak Ridge National Laboratory (ORNL) in Tennessee and the Rutherford Appleton Laboratory in the United Kingdom observed the newly discovered effect in blue aquamarine crystals purchased at a gem show. Blue aquamarine; green and red emerald; pink morganite; gold heliodor; and clear goshenite gemstones are all varieties of the mineral beryl (beryllium aluminum cyclosilicate) with traces of other chemicals that give the crystals their characteristic colors.

"We chose beryl because it has a crystal structure that has channels in it, about 5 angstroms [5 ten-millionths of a millimeter] across — a little bit bigger than a water molecule — and it's known from spectroscopic data that natural beryls have water in them," said Larry Anovitz, a geochemist at ORNL and one of the authors of a paper on the new research. "We already know from lots of other studies that as you put water in smaller and smaller pores it starts to affect the properties of the water — the freezing point drops, the density changes, all sorts of things. So, we wanted to know, if you made that pore so small that you only can get a single molecule of water into it, what would that would do to the properties of water?"

What happened next was unexpected, Anovitz told Live Science.

"We knew that natural beryl would have water in these channels in the structure, so we could go and look at that and see what the properties were," he said. "But we didn't know that the properties would turn out to be so strange when we looked." 

Seeing a new state 

At ORNL's Spallation Neutron Source facility, after cooling the beryl crystals to very low temperatures, the scientists measured the lowest-energy states of the atoms in the trapped water molecules with neutron-scattering experiments, which use a beam of subatomic neutron particles to chart the motion of atoms and molecules.

"When we started looking at peaks in the inelastic neutron spectrum for this sample, we saw a number of peaks in the spectrum that, instead of getting bigger with temperature — which is what is expected to happen — they got smaller with temperature," Anovitz said.

"There are two ways this could happen — either by quantum tunneling or magnetic transitions — and we were able to prove that this is actually the quantum tunneling of the water molecules," he said.

Interactions between water molecules and the walls of the hexagonal channels usually force the water into the center of the channel, with both hydrogen "mouse ears" facing the same one of the six sides.

In their lowest energy states, the water molecules do not have enough energy to rotate to one of the adjacent positions.

But in the areas where the channels narrow so just a single water molecule can fit, the atoms in the water molecule can "tunnel" through the energy barrier that prevents rotation. And the new experiments reveal that the molecules were forming a "double-top" shape, with the proton nucleus of each hydrogen atom delocalizing into a six-sided ring around the central oxygen atom, the researchers said.

Measuring the molecules 

Alexander Kolesnikov, a physicist at ORNL and the lead author of the new paper, said additional studies at the Rutherford Appleton Laboratory had determined that the kinetic energy of the hydrogen protons in the six-sided water molecules was about 30 percent lower than in molecules of water in its normal state, or "bulk water."

"That is a direct indication that this is a quantum property due to the tunneling of water in this beryl channel," Kolesnikov told Live Science. "In classical terms, the kinetic energy would be expected to be something comparable to all other bulk water.

"This is not a new phase of water [like ice or steam] — it's not completely in the gas phase, but it's close to a gas phase," he added. "But at low temperatures, due to quantum delocalization, the kinetic energy of the protons significantly decreases, and they propagate under this [energy] barrier. So, I would say this is kind of a new state of the water molecule."

Anovitz said that quantum tunneling was known to occur in other substances but that the effect was usually limited to subatomic particles rather than larger particles like water molecules.

Quantum tunneling was also known to take place among hydrogen atoms in methyl-group molecules, which are arranged in a triangular pyramid shape around a carbon atom, but the molecules looked the same shape after the tunneling transition, he said.

"With water, when it's moving around this six-fold axis in the beryl channel, it doesn't look the same anymore — and that's something that's never been seen before," Anovitz said. 

Reference:

The findings were published April 22 in the journal Physical Review Letters.

Quantum Tunneling of Water in Beryl: A New State of the Water

Phys. Rev. Lett. 116, 167802

ONRL

NIST’s Super Quantum Simulator ‘Entangles’ Hundreds of Ions

NIST physicists have built a quantum simulator made of trapped beryllium ions (charged atoms) that are proven to be entangled, a quantum phenomenon linking the properties of all the particles. The spinning crystal, about 1 millimeter wide, can contain anywhere from 20 to several hundred ions.

Credit: NIST

Physicists at the National Institute of Standards and Technology (NIST) have “entangled” or linked together the properties of up to 219 beryllium ions (charged atoms) to create a quantum simulator. The simulator is designed to model and mimic complex physics phenomena in a way that is impossible with conventional machines, even supercomputers. The techniques could also help improve atomic clocks.

The new NIST system can generate quantum entanglement in about 10 times as many ions as any previous simulators based on ions, a scale-up that is crucial for practical applications. The behavior of the entangled ions rotating in a flat crystal just 1 millimeter in diameter can also be tailored or controlled to a greater degree than before.

Described in the June 10, 2016, issue of Science, NIST’s latest simulator improves on the same research group’s 2012 version by removing most of the earlier system’s errors and instabilities, which can destroy fragile quantum effects.

“Here we get clear, indisputable proof the ions are entangled,” NIST postdoctoral researcher Justin Bohnet said. “What entanglement represents in this case is a useful resource for something else, like quantum simulation or to enhance a measurement in an atomic clock.”

In the NIST quantum simulator, ions act as quantum bits (qubits) to store information. Trapped ions are naturally suited to studies of quantum physics phenomena such as magnetism.

Quantum simulators might also help study problems such as how the universe began, how to engineer novel technologies (for instance, room-temperature superconductors or atom-scale heat engines), or accelerate the development of quantum computers. According to definitions used in the research community, quantum simulators are designed to model specific quantum processes, whereas quantum computers are universally applicable to any desired calculation.

Quantum simulators with hundreds of qubits have been made of other materials such as neutral atoms and molecules. But trapped ions offer unique advantages such as reliable preparation and detection of quantum states, long-lived states, and strong couplings among qubits at a variety of distances.

In addition to proving entanglement, the NIST team also developed the capability to make entangled ion crystals of varying sizes—ranging from 20 qubits up to hundreds. Even a slight increase in the number of particles makes simulations exponentially more complex to program and carry out. The NIST team is especially interested in modelling quantum systems of sizes just beyond the classical processing power of conventional computers.

“Once you get to 30 to 40 particles, certain simulations become difficult,” Bohnet said. “That’s the number at which full classical simulations start to fail. We check that our simulator works at small numbers of ions, then target the sweet spot in this midrange to do simulations that challenge classical simulations. Improving the control also allows us to more perfectly mimic the system we want our simulator to tell us about.”

The ion crystals are held inside a Penning trap, which confines charged particles by use of magnetic and electric fields. The ions naturally form triangular patterns, useful for studying certain types of magnetism. NIST is the only laboratory in the world generating two-dimensional arrays of more than 100 ions. Based on lessons learned in the 2012 experiment, NIST researchers designed and assembled a new trap to generate stronger and faster interactions among the ions. The interaction strength is the same for all ions in the crystal, regardless of the distances between them.

The researchers used lasers with improved position and intensity control, and more stable magnetic fields, to engineer certain dynamics in the “spin” of the ions’ electrons. Ions can be spin up (often envisioned as an arrow pointing up), spin down, or both at the same time, a quantum state called a superposition. In the experiments, all the ions are initially in independent superpositions but are not communicating with each other. As the ions interact, their spins collectively morph into an entangled state involving most, or all of the entire crystal.

Researchers detected the spin state based on how much the ions fluoresced, or scattered laser light. When measured, unentangled ions collapse from a superposition to a simple spin state, creating noise, or random fluctuations, in the measured results. Entangled ions collapse together when measured, reducing the detection noise. 

Crucially, the researchers measured a sufficient level of noise reduction to verify entanglement, results that agreed with theoretical predictions. This type of entanglement is called spin squeezing because it squeezes out (removes) noise from a target measurement signal and moves it to another, less important aspect of the system. The techniques used in the simulator might someday contribute to the development of atomic clocks based on large numbers of ions (current designs use one or two ions). 

“The reduction in the quantum noise is what makes this form of entanglement useful for enhancing ion and atomic clocks,” Bohnet said. “Here, spin squeezing confirms the simulator is working correctly, because it produces the quantum fluctuations we are looking for.”

The work was funded in part by the National Science Foundation, Army Research Office and Air Force Office of Scientific Research.

Reference: 

J.G. Bohnet, B.C. Sawyer, J.W. Britton, M.L. Wall, A.M. Rey, M. Foss-Feig, J.J. Bollinger. 2016. Quantum spin dynamics and entanglement generation with hundreds of trapped ions. Science, June 10, 2016. DOI: 10.1126/science.aad9958 

NIST