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

Doubling down on Schrödinger’s cat

Yale physicists have given Schrödinger’s famous cat a second box to play in, and the result may help further the quest for reliable quantum computing.

Schrödinger’s cat is a well-known paradox that applies the concept of superposition in quantum physics to objects encountered in everyday life. The idea is that a cat is placed in a sealed box with a radioactive source and a poison that will be triggered if an atom of the radioactive substance decays. Quantum physics suggests that the cat is both alive and dead (a superposition of states), until someone opens the box and, in doing so, changes the quantum state.

This hypothetical experiment, envisioned by one of the founding fathers of quantum mechanics in 1935, has found vivid analogies in laboratories in recent years. Scientists can now place a wave-packet of light composed of hundreds of particles simultaneously in two distinctly different states. Each state corresponds to an ordinary (classical) form of light abundant in nature.

A team of Yale scientists created a more exotic type of Schrödinger’s cat-like state that has been proposed for experiments for more than 20 years. This cat lives or dies in two boxes at once, which is a marriage of the idea of Schrödinger’s cat and another central concept of quantum physics: entanglement. Entanglement allows a local observation to change the state of a distant object instantaneously. Einstein once called it “spooky action at a distance,” and in this case it allows a cat state to be distributed in different spatial modes.

The Yale team built a device consisting of two, 3D microwave cavities and an additional monitoring port — all connected by a superconducting, artificial atom. The “cat” is made of confined microwave light in both cavities.

“This cat is big and smart. It doesn’t stay in one box because the quantum state is shared between the two cavities and cannot be described separately,” said Chen Wang, a postdoctoral associate at Yale and first author of a study in the journal Science, describing the research. “One can also take an alternative view, where we have two small and simple Schrodinger’s cats, one in each box, that are entangled.”

The research also has potential applications in quantum computation. A quantum computer would be able to solve certain problems much faster than classical computers by exploiting superposition and entanglement. Yet one of the main problems in developing a reliable quantum computer is how to correct for errors without disturbing the information.

“It turns out ‘cat’ states are a very effective approach to storing quantum information redundantly, for implementation of quantum error correction. Generating a cat in two boxes is the first step towards logical operation between two quantum bits in an error-correctible manner,” said co-author Robert Schoelkopf, Sterling Professor of Applied Physics and Physics, and director of the Yale Quantum Institute.

Schoelkopf and his frequent collaborators, Michel Devoret and Steve Girvin, have pioneered the field of circuit quantum electrodynamics (cQED), providing one of the most widely used frameworks for quantum computation research. Devoret, Yale’s F.W. Beinecke Professor of Physics, and Girvin, Yale’s Eugene Higgins Professor of Physics and Applied Physics, are co-authors of the paper.

The research builds upon more than a decade of development in cQED architecture. The Yale team designed a variety of new features, including cylindrical 3D cavities with record quantum information storage time of more than 1 millisecond in superconducting circuits, and a measurement system that monitors certain aspects of a quantum state in a precise, non-destructive way. “We have combined quite a lot of recent technologies here,” Wang said.

Additional co-authors from the Yale Departments of Applied Physics and Physics include assistant professor Liang Jiang; senior research scientist Luigi Frunzio; postdoctoral associates Reinier Heeres and Nissim Ofek; graduate students Yvonne Gao, Philip Reinhold, Kevin Chou, Christopher Axline, Matthew Reagor, Jacob Blumoff, and Katrina Sliwa; and former Yale researcher Mazyar Mirrahimi.

Yale

Quantum experiments designed by machines

Algorithm does not work intuitive – just as quantum physics

Quantum physics is counterintuitive. Many of the phenomena in the quantum world do not have a classical analog: In the quantum world, a coin is not either heads or tails – but can have both properties at the same time. For a better understanding of such phenomena, laboratory experiments are indispensable. Quantum physicist Mario Krenn and his colleagues in the group of Anton Zeilinger from the Faculty of Physics at the University of Vienna and the Austrian Academy of Sciences have developed an algorithm which designs new useful quantum experiments. As the computer does not rely on human intuition, it finds novel unfamiliar solutions. The research has just been published in the journal Physical Review Letters.

The idea was developed when the physicists wanted to create new quantum states in the laboratory, but were unable to conceive of methods to do so. “After many unsuccessful attempts to come up with an experimental implementation, we came to the conclusion that our intuition about these phenomena seems to be wrong. We realized that in the end we were just trying random arrangements of quantum building blocks. And that is what a computer can do as well – but thousands of times faster”, explains Mario Krenn, PhD student in Anton Zeilinger’s group and first author research.

After a few hours of calculation, their algorithm – which they call Melvin – found the recipe to the question they were unable to solve, and its structure surprised them. Zeilinger says: “Suppose I want build an experiment realizing a specific quantum state I am interested in. Then humans intuitively consider setups reflecting the symmetries of the state. Yet Melvin found out that the most simple realization can be asymmetric and therefore counterintuitive. A human would probably never come up with that solution.”

The physicists applied the idea to several other questions and got dozens of new and surprising answers. “The solutions are difficult to understand, but we were able to extract some new experimental tricks we have not thought of before. Some of these computer-designed experiments are being built at the moment in our laboratories”, says Krenn.

Melvin not only tries random arrangements of experimental components, but also learns from previous successful attempts, which significantly speeds up the discovery rate for more complex solutions. In the future, the authors want to apply their algorithm to even more general questions in quantum physics, and hope it helps to investigate new phenomena in laboratories.

The research was supported by Austrian Academy of Sciences (ÖAW), the European Research Council (SIQS Grant No. 600645 EUFP7-ICT), the Austrian Science Fund (FWF) with SFB F40 (FOQUS).

Publication in "Physical Review Letters"
Mario Krenn, Mehul Malik, Robert Fickler, Radek Lapkiewicz, Anton Zeilinger: Automated Search for new Quantum Experiments, Physical Review Letters, 22 February, 2016
http://arxiv.org/abs/1509.02749

Universität Wien

Interacting Ion Qutrits

Enlisting symmetry to protect quantum states from disruptions

Symmetry permeates nature, from the radial symmetry of flowers to the left-right symmetry of the human body. As such, it provides a natural way of classifying objects by grouping those that share the same symmetry. This is particularly useful for describing transitions between phases of matter. For example, liquid and gas phases have translational symmetry, meaning the arrangement of molecules doesn’t change regardless of the direction from which they are observed. On the other hand, the density of atoms in a solid phase is not continuously the same — thus translational symmetry is broken.

In quantum mechanics, symmetry describes more than just the patterns that matter takes — it is used to classify the nature of quantum states. These states can be entangled, exhibiting peculiar connections that cannot be explained without the use of quantum physics. For some entangled states, the symmetry of these connections can offer a kind of protection against disruptions.

Here, the word protection indicates that the system is robust against non-symmetry breaking changes. Like an island in the middle of an ocean, there is not a direct road leading to a symmetry-protected phase or state. This means that the only way to access the state is to change the symmetry itself. Physicists are interested in exploring these classes of protected states because building a useful quantum device requires its building blocks to be robust against outside disturbances that may interfere with device operations.

Recently, JQI researchers under the direction of Christopher Monroe have used trapped atomic ions to construct a system that could potentially support a type of symmetry-protected quantum state. For this research they used a three-state system, called a qutrit, and demonstrated a proof-of-principle experiment for manipulating and controlling multiple qutrits. The result appeared in Physical Review X, an online open-access journal, and is the first demonstration of using multiple interacting qutrits for doing quantum information operations and quantum simulation of the behavior of real materials.

To date, almost all of the work in quantum information science has focused on manipulating "qubits," or so-called spin-1/2 particles that consist of just two energy levels.  In quantum mechanics, multilevel systems are analogous to the concept of "spin," where the number of energy levels corresponds to the number of possible states of spin. This group has used ion spins to explore a variety of topics, such as the physics of quantum magnetism and the transmission speed of quantum information across a spin-crystal. Increasingly, there is interest in moving beyond spin-½ to control and simulations of higher order spin systems, where the laws of symmetry can be radically altered. “One complication of spin-1 materials is that the added complexity of the levels often makes these systems much more difficult to model or understand. Thus, performing experiments in these higher [spin] dimensional systems may yield insight into difficult-to-calculate problems, and also give theorists some guidance on modeling such systems, ” explains Jake Smith, a graduate student in Monroe’s lab and author on the paper.

To engineer a spin-1 system, the researchers electromagnetically trapped a linear crystal of atomic ytterbium (Yb) ions, each atom a few micrometers from the next. Using a magnetic field, internal states of each ion are tailored to represent a qutrit, with a (+) state, (-) state and (0) state denoting the three available energy levels (see figure). With two ions, the team demonstrated the basic techniques necessary for quantum simulation: preparing initial states (placing the ions in certain internal states), observing the state of the system after some evolution, and verifying that the ions are entangled, here with 86% fidelity (fidelity is a measure of how much the experimentally realized state matches the theoretical target state).

To prepare the system in certain initial states, the team first lowers the system into its ground state, the lowest energy state in the presence of a large effective magnetic field. The different available spin chain configurations at a particular magnetic field value correspond to different energies. They observed how the spin chain reacted or evolved as the amplitude of the magnetic field was lowered. Changing the fields that the ions spins are exposed to causes the spins to readjust in order to remain in the lowest energy configuration.  

By adjusting the parameters (here laser amplitudes and frequencies) the team can open up and follow pathways between different energy levels. This is mostly true, but for some target states a simple trajectory that doesn’t break symmetries or pass through a phase transition does not exist. For instance, when the team added a third ion, they could not smoothly guide the system into its ground state, indicating the possible existence of a state with some additional symmetry protections.

“This result is a step towards investigating quantum phases that have special properties based on the symmetries of the system,” says Smith. Employing these sorts of topological phases may be a way to improve coherence times when doing quantum computation, even in the face of environmental disruptions. Coherence time is how long a state retains its quantum nature. Quantum systems are very sensitive to outside disturbances, and doing useful computation requires maintaining this quantum nature for longer than the time it takes to perform a particular calculation.

Monroe explains, "These symmetry-protected states may be the only way to build a large-scale stable quantum computer in many physical systems, especially in the solid-state.  With the exquisite control afforded atomic systems such as trapped ions demonstrated here, we hope to study and control how these very subtle symmetry effects might be used for quantum computing, and help guide their implementation in any platform."

To further investigate this protected phase, the researchers next intend to address the problem of creating antisymmetric ground states. Smith continues, “The next steps are to engineer more complicated interactions between the effective spins and implement a way to break the symmetries of the interactions.”

http://www.nanotechnologyworld.org/#!Interacting-Ion-Qutrits/c89r/55b65d7e0cf2d0bb156af20f