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

New method of quantum entanglement vastly increases how much information can be carried in a photon

Led by UCLA researchers, research could have applications in finance, health care, government and military communications

A team of researchers led by UCLA electrical engineers has demonstrated a new way to harness light particles, or photons, that are connected to each other and act in unison no matter how far apart they are — a phenomenon known as quantum entanglement.

In previous studies, photons have typically been entangled by one dimension of their quantum properties — usually the direction of their polarization.

In the new study, researchers demonstrated that they could slice up and entangle each photon pair into multiple dimensions using quantum properties such as the photons’ energy and spin. This method, called hyperentanglement, allows each photon pair to carry much more data than was possible with previous methods.

Quantum entanglement could allow users to send data through a network and know immediately whether that data had made it to its destination without being intercepted or altered. With hyperentanglement, users could send much denser packets of information using the same networks.

The research, published today in Nature Photonics, was led by Zhenda Xie, a research scientist in the lab of Chee Wei Wong, a UCLA associate professor of electrical engineering who was the research project’s principal investigator. Researchers from MIT, Columbia University, the University of Maryland and the National Institute of Standards and Technology were also part of the team.

Albert Einstein famously described quantum entanglement as “spooky action at a distance” because it seems so improbable that what happens to one particle in an entangled pair also happens instantly to the other particle, even over great distances. The phenomenon exceeds the speed of light.

In the new study, researchers sent hyperentangled photons in a shape known as a biphoton frequency comb, essentially breaking up entangled photons into smaller parts.

In secure data transfer, photons sent over fiber optic networks can be encrypted through entanglement. With each dimension of entanglement, the amount of information carried on a photon pair is doubled, so a photon pair entangled by five dimensions can carry 32 times as much data as a pair entangled by only one. The result greatly extends from wavelength multiplexing, the method for carrying many videos over a single optical fiber.

“We show that an optical frequency comb can be generated at single photon level,” Xie said. “Essentially, we’re leveraging wavelength division multiplexing concepts at the quantum level.”

Potential applications for the research include secure communication and information processing, in particular for high-capacity data transfer with minimal error. This could be useful for medical servers, government data communications, financial markets and military communication channels, as well as quantum cloud communications and distributed quantum computing.

“We are fortunate to verify a decades-old theoretical prediction by Professor Jeff Shapiro of MIT, that quantum entanglement can be observed in a comb-like state,” Wong said. “With the help of state-of-the-art high-speed single photon detectors at NIST and support from Dr. Franco Wong, Dr. Xie was able to verify the high-dimensional and multi-degrees-of-freedom entanglement of photons. These observations demonstrate a new fundamentally secure approach for dense information processing and communications.”

Co-authors on the paper are Sajan Shrestha, XinAn Xu and Junlin Liang, prior students and postdoctoral scientists at Columbia with Wong; Tian Zhong, professors Jeffrey Shapiro and Franco N.C. Wong of MIT; Yan-Xiao Gong of Southeast University in Nanjing, China; and Joshua Bienfang and Alessandro Restelli, affiliated with both the University of Maryland and the NIST.

The work was funded by the Defense Advanced Research Projects Agency.

http://www.nanotechnologyworld.org/#!New-method-of-quantum-entanglement-vastly-increases-how-much-information-can-be-carried-in-a-photon/c89r/5592af200cf2585ebcda12a4 

Record quantum entanglement of multiple dimensions

An international team of researchers, with participation from the UAB, has managed to create an entanglement of 103 dimensions with only two photons. The record had been established at 11 dimensions. The discovery could represent a great advance toward the construction of quantum computers with much higher processing speeds than current ones, and toward a better encryption of information.

The states in which elementary particles, such as photons, can be found have properties which are beyond common sense. Superpositions are produced, such as the possibility of being in two places at once, which defies intuition. In addition, when two particles are entangled a connection is generated: measuring the state of one (whether they are in one place or another, or spinning one way or another, for example) affects the state of the other particle instantly, no matter how far away from each other they are.

Scientists have spent years combining both properties to construct networks of entangled particles in a state of superposition. This in turn allows constructing quantum computers capable of operating at unimaginable speeds, encrypting information with total security and conducting experiments in quantum mechanics which would be impossible to carry out otherwise.

Until now, in order to increase the "computing" capacity of these particle systems, scientists have mainly turned to increasing the number of entangled particles, each of them in a two-dimensional state of superposition: a qubit (the quantum equivalent to an information bit, but with values which can be 1, 0 or an overlap of both values). Using this method, scientists managed to entangle up to 14 particles, an authentic multitude given its experimental difficulty.

The research team was directed by Anton Zeilinger and Mario Krenn from the Institute for Quantum Optics and Quantum Information of the Austrian Academy of Sciences. It included the participation of Marcus Huber, researcher from the Group of Quantum Information and Quantum Phenomena from the UAB Department of Physics, as well as visiting researcher at the Institute of Photonic Sciences (ICFO). The team has advanced one more step towards improving entangled quantum systems.

In an article published this week in the journal Proceedings (PNAS), scientists described how they managed to achieve a quantum entanglement with a minimum of 103 dimensions with only two particles. "We have two Schrödinger cats which could be alive, dead, or in 101 other states simultaneously", Huber jokes, “plus, they are entangled in such a way that what happens to one immediately affects the other”. The results implies a record in quantum entanglements of multiple dimensions with two particles, established until now at 11 dimensions.

Instead of entangling many particles with a qubit of information each, scientists generated one single pair of entangled photons in which each could be in more than one hundred states, or in any of the superpositions of theses states; something much easier than entangling many particles. These highly complex states correspond to different modes in which photons may find themselves in, with a distribution of their characteristic phase, angular momentum and intensity for each mode.

"This high dimension quantum entanglement offers great potential for quantum information applications. In cryptography, for example, our method would allow us to maintain the security of the information in realistic situations, with noise and interference. In addition, the discovery could facilitate the experimental development of quantum computers, since this would be an easier way of obtaining high dimensions of entanglement with few particles", explains UAB researcher Marcus Huber.

Now that the results demonstrate that obtaining high dimension entanglements is accessible, scientists conclude in the article that the next step will be to search how they can experimentally control these hundreds of spatial modes of the photons in order to conduct quantum computer operations.

Source: http://www.uab.es/servlet/Satellite/latest-news/news-detail/record-quantum-entanglement-of-multiple-dimensions-1096476786473.html?noticiaid=1345668721554

Quarks Linked by Wormholes?

Hypothetical shortcuts through the universe, wormholes link
separate points in space-time.

Quantum entanglement may explain gravity.

Quantum entanglement is one of the more bizarre theories to come out of the study of quantum mechanics—so strange, in fact, that Albert Einstein famously referred to it as “spooky action at a distance.”

Essentially, entanglement involves two particles, each occupying multiple states at once, for example simultaneously spinning clockwise and counterclockwise. But neither has a definite state until one is measured, causing the other particle to instantly assume a corresponding state. The resulting correlations between the particles are preserved even if they reside on opposite ends of the universe.

But what enables particles to communicate instantaneously—seemingly faster than the speed of light—over such vast distances?

Now an MIT physicist looking at entanglement through the lens of string theory has proposed an answer: the creation of two entangled quarks—the building blocks of matter—simultaneously gives rise to a wormhole connecting the pair.

The theoretical results bolster the relatively new and exciting idea that the laws of gravity holding together the universe may not be fundamental but arise instead from quantum entanglement.

Julian Sonner, a senior postdoc in MIT’s Laboratory for Nuclear Science and Center for Theoretical Physics, has published his results in the journal Physical Review Letters.

To see what emerges from two entangled quarks, he first created a theoretical model of quarks based on the Schwinger effect—a concept in quantum theory that makes it possible to create particles out of nothing. Once extracted from a vacuum, these particles are considered entangled.

Sonner mapped the entangled quarks onto a four-dimensional space, considered a representation of space-time. In contrast, gravity is thought to exist in the next dimension, where, according to Einstein’s laws, it acts to “bend” and shape space-time.

To see what geometry may emerge in the fifth dimension from entangled quarks in the fourth, Sonner employed the string theory concept of holographic duality, used to derive a more complex dimension from the next-lowest dimension.

He found that what emerged was a wormhole connecting the two entangled quarks, implying that the creation of quarks simultaneously creates a wormhole. More fundamentally, he says, gravity itself may be a result of entanglement. What’s more, the universe’s geometry as described by classical gravity may be a consequence of entanglement—pairs of particles strung together by tunneling wormholes.

“It’s the most basic representation yet that we have where entanglement gives rise to some sort of geometry,” Sonner says. “What happens if some of this entanglement is lost, and what happens to the geometry? There are many roads that can be pursued.”

Source: http://www.technologyreview.com/article/524191/quarks-linked-by-wormholes/