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

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

Sensitive quantum particles

The quantum mechanical entanglement of particles plays an important role in many technical applications. To date, however, the effect has been difficult to measure experimentally.

Physicists from the Technical University of Munich (TUM), the University of Innsbruck and the Institute of Photonic Sciences (ICFO) in Barcelona have now developed a new protocol to detect entanglement of many-particle quantum states using established measuring methods.

In quantum theory, interactions between particles create fascinating correlations known as entanglement. They cannot be explained by any means known to the classical world.

Entanglement is a consequence of the probabilistic rules of quantum mechanics and seems to permit a peculiar instantaneous connection between particles over long distances that defies the laws of our macroscopic world – a phenomenon that Einstein referred to as “spooky action at a distance.”

Developing protocols to detect and quantify entanglement of many-particle quantum states is a key challenge for current experiments because entanglement becomes very difficult to study when many particles are involved. “We are able to control smaller particle ensembles well, where we can measure entanglement in a relatively straight forward way,” says quantum physicist Philipp Hauke. However, “when we are dealing with a large system of entangled particles, this measurement is extremely complex or rather impossible because the resources required scale exponentially with the system size.

”Markus Heyl from the Technical University of Munich, Philipp Hauke and Peter Zoller from the Department of Theoretical Physics at the University of Innsbruck and the Institute for Quantum Optics and Quantum Information (IQOQI) at the Austrian Academy of Sciences in collaboration with Luca Tagliacozzo from the Institute of Photonic Sciences in Barcelona (Spain) have found a new way to detect certain properties of many-particle entanglement independent of the size of the system and by using standard measurement tools.

Entanglement measurable via susceptibility

 

“When dealing with more complex systems, scientists had to carry out a large number of measurements to detect and quantify entanglement between many particles,” says Philipp Hauke. “Our protocol avoids this problem and can also be used for determining entanglement in macroscopic objects, which was nearly impossible until now.”

Using this new method, physicists can employ tools already well established experimentally. In their study published in Nature Physics the team of researchers gives explicit examples to demonstrate its framework: The entanglement of many-particle systems trapped in optical lattices can be determined using laser spectroscopy while the well-established technique of neutron scattering is utilized for measuring entanglement in solid-state systems.

The physicists successfully demonstrated that the quantum Fisher information, which can provide reliable proof for genuine multipartite entanglement, is in fact measurable. The researchers emphasize that entanglement can be detected by measuring the dynamic response of a system to a perturbation, which can be determined by comparing individual measurements.

“For example, when we move a sample through a time-dependent magnetic field, we can determine the system’s susceptibility towards the magnetic field through the measurement data and thereby detect and quantify internal entanglement,” explains Hauke.

Manifold applications

 

Quantum metrology, i.e. measurement techniques with increased precision exploiting quantum mechanics, is not the only important field of application of this protocol. It will also provide new perspectives for quantum simulations, where quantum entanglement is used as a resource for studying properties of quantum systems.

In solid-state physics, the protocol may be employed to investigate the role of entanglement in many-body systems, thereby providing a deeper understanding of quantum matter. The research work was supported by the European Community, the European Research Council (ERC), the Austrian Science Fund, the Spanish Government and the German National Academy of Sciences Leopoldina.

Publication:

Measuring multipartite entanglement via dynamic susceptibilities. Philipp Hauke, Markus Heyl, Luca Tagliacozzo, Peter Zoller. Advanced Online Publication, Nature Physics, on 21 March 2016. - DOI: 10.1038/nphys3700

Technical University of Munich

Quantum cryptography: Swedish researchers reveal security hole

Hacking the Bell Test using classical light in energy-time entanglement-based quantum key distribution.

Quantum cryptography is considered a fully secure encryption method, but researchers from Linköping University and Stockholm University have discovered that this is not always the case.

They found that energy-time entanglement - the method that today forms the basis for many systems of quantum cryptography - is vulnerable to attack. The results of their research have been published in Science Advances.

"With this security hole, it's possible to eavesdrop on traffic without being detected. We discovered this in our theoretical calculations, and our colleagues in Stockholm were subsequently able to demonstrate it experimentally," says Jan-Åke Larsson, professor at Linköping University's Division of Information Coding.

Quantum cryptography is considered a completely safe method for information transfer, and theoretically it should be impossible to crack. Many research groups around the world are working to make quantum cryptography resistant to various types of disturbance, and so far it has been possible to handle the disturbance that has been detected. Quantum cryptography technology is commercially available, but there is much doubt as to whether it is actually used.

"It's mostly rumours, I haven't seen any system in use. But I know that some universities have test networks for secure data transfer," says Prof Larsson.

The energy-time entanglement technology for quantum encryption studied here is based on testing the connection at the same time as the encryption key is created. Two photons are sent out at exactly the same time in different directions. At both ends of the connection is an interferometer where a small phase shift is added. This provides the interference that is used to compare similarities in the data from the two stations. If the photon stream is being eavesdropped there will be noise, and this can be revealed using a theorem from quantum mechanics - Bell's inequality.

On the other hand if the connection is secure and free from noise, you can use the remaining data, or photons, as an encryption key to protect your message.

What the LiU researchers Jan-Åke Larsson and his doctoral student Jonathan Jogenfors have revealed about energy-time entanglement is that if the photon source is replaced with a traditional light source, an eavesdropper can identify the key, the code string. Consequently they can also read the message without detection. The security test, which is based on Bell's inequality, does not react - even though an attack is underway.

Physicists at Stockholm University have subsequently been able to demonstrate in practical experiments that it is perfectly possible to replace the light source and thus also eavesdrop on the message.

But this problem can also be solved.

"In the article we propose a number of countermeasures, from simple technical solutions to rebuilding the entire machine," said Jonathan Jogenfors.

Linköping University