CAPTION This experiment at the Centre for Quantum Technologies in Singapore has made a record measurement of entanglement -- approaching the quantum limit with extreme precision. CREDIT Photo Credit: Alessandro Cerè / Centre for Quantum Technologies, National University of Singapore

An experiment in Singapore has pushed quantum weirdness close to its absolute limit.

Researchers from the Centre for Quantum Technologies (CQT) at the National University of Singapore and the University of Seville in Spain have reported the most extreme 'entanglement' between pairs of photons ever seen in the lab. The result was published 30 October 2015 in Physical Review Letters.

The achievement is evidence for the validity of quantum physics and will bolster confidence in schemes for quantum cryptography and quantum supercomputing designed to exploit this phenomenon.

"For some quantum technologies to work as we intend, we need to be confident that quantum physics is complete," says Poh Hou Shun, who carried out the experiment at CQT. "Our new result increases that confidence," he says.

Local realism

Entanglement says that two particles, such as photons, can be married into a joint state. Once in such a state, either particle observed on its own appears to behave randomly. But if you measure both particles at once, you notice they are perfectly synchronized.

Albert Einstein was famously troubled by this prediction of quantum physics. He didn't like the randomness that came with just one particle. He said "God does not play dice". He didn't like the correlations that came with two particles, either. He referred to this as "spooky action at a distance".

Experiments since the 1970s have been collecting evidence that quantum predictions are correct. Recently an experiment in the Netherlands became the first to do away with all assumptions in the data-gathering.

Technically known as a 'loophole-free Bell test', the experiment leaves no wiggle room in meaning: entangled particles do behave randomly, and they synchronize without exchanging signals. (The results appeared in Nature on 21 October 2015, doi:10.1038/nature15759).

Entangled to the max

In the lab in Singapore, Poh and his colleagues also performed a Bell test. But instead of closing loopholes, their setup pushes the entanglement towards its theoretical maximum.

They make entangled photons by shining a laser through a crystal. The photons interact with the crystal in such a way that occasionally, one splits into two and the pair emerges entangled. The team control the photons with an array of lenses, mirrors and other optical elements to optimize the effect.

The researchers looked at 33.2 million optimized photon pairs. Each pair was split up and the photons measured separately, then the correlation between the results quantified.

In such a Bell test, the strength of the correlation says whether or not the photons were entangled. The measures involved are complex, but can be reduced to a simple number. Any value bigger than 2 is evidence for quantum effects at work. But there is also an upper limit.

Quantum physics predicts the correlation measure cannot get any bigger than 2sqrt(2) ~2.82843. In the experiment at CQT, they measure 2.82759 ± 0.00051 - within 0.03% of the limit. If the peak value were the top of Everest, this would be only 2.6 metres below the summit.

No extensions

The record result also rules out a proposed extension to quantum theory. Earlier this year, Alexei Grinbaum with CEA in France put forward a model in which quantum physics is just an effective description of a more fundamental theory. He calculated a new limit on the correlation measure using tools from information theory. The calculations considered how much information an observer can hold about a two-particle system, and gave a limit on the correlation measure sitting just 0.1% under the quantum limit.

"You need a very precise measurement to be able to distinguish the quantum limit, and that was our achievement," says Christian Kurtsiefer, a Principal Investigator at CQT and co-author on the paper. The team's result exceeds the Grinbaum limit by enough to rule out the model behind it.

Entanglement doesn't allow faster-than-light communication, but it can be used for secret messaging and to speed up some calculations. Checking that it's possible to reach the quantum limit for correlations is valuable for these applications: their security and reliability depends on this limit being fundamental.

CAPTION Wolfgang Wernsdorfer studies fundamentals of molecular quantum computers. CREDIT Photo: Eric Lichtenscheidt

Wolfgang Wernsdorfer studies molecular nanomagnets for use as processors in future quantum computers -- Germany's highest award for researchers from abroad

Karlsruhe Institute of Technology (KIT) makes an internationally renowned experimental physicist move to Germany: Wolfgang Wernsdorfer was chosen for a Humboldt professorship. With this award, the Alexander von Humboldt Foundation honors internationally leading scientists that have been working abroad so far. With funds totaling up to EUR 5 million, the Humboldt professorship is Germany's highest award for international scientists. Wernsdorfer, a renowned expert for nanomagnets, will now continue his research at KIT's Physikalisches Institut. The Humboldt professorship will be awarded in Berlin in May 2016.

"We are extremely happy about the selection of Dr. Wolfgang Wernsdorfer for a Humboldt professorship," the President of KIT, Professor Holger Hanselka, says. "The decision of the Alexander von Humboldt Foundation in favor of Mr. Wernsdorfer, an internationally renowned expert in electronics, spin physics, and quantum computing, also is an honor for KIT and an appreciation of our strengths in research."

Presently, Dr. Wolfgang Wernsdorfer is working at the Institut NÉEL of the Centre National de la Recherche Scientifique (CNRS) in Grenoble / France. He has specialized in molecular quantum spintronics, an area of experimental solid-state physics at the interface to chemistry and materials sciences. Wernsdorfer is among the leading experts worldwide for molecular nanomagnets and their use in quantum computer systems. Already as a doctoral student at the Low-temperature Laboratory in Grenoble did he develop a nano-squid, a pioneering instrument to measure extremely small magnetic fields, by means of which he studied magnetic properties of individual nanostructures and molecules. Wernsdorfer found that molecular magnets behave according to the laws of quantum mechanics. Based on this finding, he was able to build electronic circuits with single molecules, in which electric current can be controlled by the magnetization of the molecule.

It is one of his defined new goals to integrate extremely small and quick molecular quantum processors into the highly advanced microelectronic chip technology. The challenge mainly lies in connecting Wernsdorfer's new switch elements based on magnetizable molecules with the so-called CMOS technology that is based on semiconductor components. In case of success, molecular nanomagnets coupled to semiconductor transistors might be used in future quantum supercomputers.

Wolfgang Wernsdorfer was born in 1966 in Germany and started to study physics at the University of Wuerzburg after he had passed vocational training to become an electrician and higher vocational school. He completed his studies at the renowned École Normale Supérieure in Lyon / France. In 1993, he became a doctoral researcher at the Low Temperature Laboratory and the Laboratoire de Magnetism in Grenoble, France - two of the institutes that formed today's Institut NEEL where he has held the position of Directeur de recherche 1ére classe since 2008. Wernsdorfer was granted a number of high-ranking honors and awards, such as the Agilent Europhysics Prize, the Olivier Kahn International Award, an ERC Advanced Grant, and the Prix Spécial of the Société Francaise de Physique. Now, he has received a call for a W3 professorship at the Physikalisches Institut (PHI) of KIT, where he wishes to establish a center for molecular quantum spintronics in cooperation with organic chemistry working groups of the KIT Institute of Nanotechnology (INT) and the KIT Institute for Theoretical Solid-state Physics.

Purdue University professor Gábor Csáthy and graduate student Katherine Schreiber inspect equipment used to cool samples to near absolute zero. The equipment was used in Purdue-led research that led to the observation of an unexpected phase transition in an ultrapure material. (Purdue University photo/Mark Simons)

An ultrapure material taken to pressures greater than that in the depths of the ocean and chilled to temperatures colder than outer space has revealed an unexpected phase transition that crosses two different phase categories.

Purdue University-led team of researchers observed electrons transition from a topologically ordered phase to a broken symmetry phase.

"To our knowledge, a transition across the two groups of phases had not been unambiguously demonstrated before, and existing theories cannot describe it," said Gábor Csáthy, an associate professor in Purdue's Department of Physics and Astronomy who led the research. "It is something like changing water from liquid to ice; except the two phases we saw were very different from one another."

A paper detailing the results of the Department of Energy and National Science Foundation-funded research will be published in an upcoming issue of Nature Physics and is currently available online.

A phase is a certain organization of matter. Most people know the ice, liquid and gas phases, and some are familiar with the different magnetic phases that store data in our electronic devices and the liquid crystalline phases that are used to create an image on certain electronic displays, but there are many other phases, Csáthy said.

In 1937 physicist Lev Landau established a theoretical framework that explained and classified all of the known phases, but in the late 1980s it was realized that there existed a second group of phases that occur at very low temperatures that do not fit in Landau's theory. The new phases were named topological phases, while the traditional phases described by Landau's theory are called broken symmetry phases, said Eduardo Fradkin, a professor of physics at the University of Illinois at Urbana-Champaign and director of the Institute for Condensed Matter Theory at the University of Illinois, who participated in the research and is a co-author of the paper.

Topological phases have been an area of focus in the field of condensed matter physics because of their special properties and potential technological applications, including quantum computing.

Csáthy specializes in the study of topological phases in semiconductors and works to discover and characterize rare topological phases. His team employs novel investigative techniques for the study of electrons freely flowing in ultrapure gallium-arsenide semiconductor crystals, which is a perfectly aligned lattice of gallium and arsenic atoms that can capture electrons on a two-dimensional plane.

Only a few groups in the world are able to grow the material, and the ultrapure crystals used in this research were grown by a group led byMichael Manfra, professor of physics and astronomy at Purdue. Manfra also is a professor of both materials engineering and electrical and computer engineering.

The gallium arsenide crystals grown using the molecular beam epitaxy technique serve as a model platform to explore the many phases that arise among strongly interacting electrons, said Manfra, who also participated in the research and is a co-author of the paper.

"Our gallium arsenide is unique among semiconductors and other novel materials due to its extremely low level of disorder," he said. "The extremes required for this science – extreme purity, extreme temperatures – are not easily achieved, but it is worth the effort to discover new phenomena involving the entire sea of electrons acting in concert. This is the biggest kick for scientists like us and why we try to push our experimental techniques to the absolute limit."

Material grown by the Manfra group was shown to have an electron mobility measurement of 35 million centimeters squared per volt-second, a measurement that puts it among the highest levels of purity achieved by any group in the world.

 "In most materials electrons are very restricted in what they can do because they bump into atomic-level defects that perturb them, scatter them and destroy fragile phases and correlated states," Csáthy said. "The material grown by the Manfra group is so pure and free from defects that it gives electrons the freedom to enter into more than 100 different phases, which is astonishing. Some of these phases simply couldn't exist in other materials."

Csáthy's team used unique equipment and techniques to take electricity to a temperature of 0.012 Kelvin, which is close to absolute zero and is about 460 degrees below zero Fahrenheit, and a pressure as high as 10,000 atmospheres, which is 10 times the pressure one would feel in the deepest part of the ocean, the Mariana Trench.

The extremely low temperature encourages the electrons to enter into exotic states where they no longer obey the laws of single particle physics, but instead are governed by their mutual interactions. A collective motion of the electrons is then possible that is described by the laws of quantum mechanics, instead of the laws of classical mechanics, he said. Purdue professor Michael Manfra holds a gallium-arsenide wafer as he stands next to the high-mobility gallium-arsenide molecular beam epitaxy system, or MBE, at the Birck Nanotechnology Center. Manfra leads a team of Purdue researchers that create ultrapure semiconductor heterostructures to study electrons in correlated states. (Purdue University photo/Andrew Hancock)

Csáthy's research team was focusing on the fractional quantum Hall state at quantum number 5/2, which is believed to be a non-Abelian topological phase. Non-Abelian states are different from anything known in nature, he said.

"Imagine eggs in an egg carton as electrons arranged in a certain formation," he said. "The eggs are identical just like the electrons are identical particles. If you swap one egg with another, nothing has changed. It is still a group of eggs in the same formation. If someone did not see the swap, he or she would never know it had happened. In non-Abelian states, if you swap two electrons, it causes a change to the entire group and the egg carton enters an entirely different state. This ability of a swap to affect the state of the entire group is a very special property."

The team was trying to induce an electron spin transition in this non-Abelian state, but before the desired state was reached, the electrons spontaneously transitioned into the so-called "stripe" phase that belongs to the traditional, broken symmetry phases group.

 "When we started the experiment we were trying to accomplish something else, but the stripes kept popping up and we would lose the fractional quantum Hall phase we were investigating," Csáthy said. "We were very surprised because it was thought that these two different categories of phases were far apart and such a transition was impossible, but the electrons went from deep in the topological phase to deep in the broken symmetry phase."

The team then changed the course of the experiment to go step by step through the new transition.

Rui-Rui Du, an expert in fractional quantum Hall experiments who has been in the field for more than 20 years, said the experiment presents something conceptually new in the change from a topological to a non-topological phase and also provides a new approach to better understand these phases.

"Through a clever experimental method, namely by applying pressure, the work demonstrates that such a topological phase transition can be fine-tuned by a new experimental knob," said Du, who is a professor of physics and astronomy at Rice University and did not participate in the research. "It is well known that the 5/2 fractional quantum Hall state may harbor so called non-Abelian quasiparticles, which are thought to be useful for topological quantum computers. This work offers a new method in further exploring the nature of the 5/2 state. It presents some of the most interesting work in the field for a long time."

The team next plans to characterize the new phase transition and establish its parameters so that the data can be compared to the developing theories, Csáthy said.

In addition to Csáthy, Manfra and Fradkin, co-authors of the paper include Nodar Samkharadze, a former research associate in the Department of Physics and Astronomy at Purdue who is now at Delft University of Technology; Katherine Schreiber, a graduate student in Purdue's Department of Physics and Astronomy; and Geoffrey Gardner, graduate student and senior research associate in Purdue's School of Materials Engineering.

CAPTION This picture shows from left to right Dr Matthew House, Sam Hile (seated), Scientia Professor Sven Rogge and Scientia Professor Michelle Simmons of the ARC Centre of Excellence for Quantum Computation and Communication Technology at UNSW. CREDIT Deb Smith, UNSW Australia

Physicists at UNSW Australia and the University of Melbourne have designed a scalable 3-D silicon chip architecture based on single atom quantum bits, providing a blueprint to build operational quantum computers

Australian scientists have designed a 3D silicon chip architecture based on single atom quantum bits, which is compatible with atomic-scale fabrication techniques - providing a blueprint to build a large-scale quantum computer.

Scientists and engineers from the Australian Research Council Centre of Excellence for Quantum Computation and Communication Technology (CQC2T), headquartered at the University of New South Wales (UNSW), are leading the world in the race to develop a scalable quantum computer in silicon - a material well-understood and favoured by the trillion-dollar computing and microelectronics industry.

Teams led by UNSW researchers have already demonstrated a unique fabrication strategy for realising atomic-scale devices and have developed the world's most efficient quantum bits in silicon using either the electron or nuclear spins of single phosphorus atoms. Quantum bits - or qubits - are the fundamental data components of quantum computers.

One of the final hurdles to scaling up to an operational quantum computer is the architecture. Here it is necessary to figure out how to precisely control multiple qubits in parallel, across an array of many thousands of qubits, and constantly correct for 'quantum' errors in calculations.

Now, the CQC2T collaboration, involving theoretical and experimental researchers from the University of Melbourne and UNSW, has designed such a device. In a study published today inScience Advances, the CQC2T team describes a new silicon architecture, which uses atomic-scale qubits aligned to control lines - which are essentially very narrow wires - inside a 3D design.

"We have demonstrated we can build devices in silicon at the atomic-scale and have been working towards a full-scale architecture where we can perform error correction protocols - providing a practical system that can be scaled up to larger numbers of qubits," says UNSW Scientia Professor Michelle Simmons, study co-author and Director of the CQC2T.

"The great thing about this work, and architecture, is that it gives us an endpoint. We now know exactly what we need to do in the international race to get there."

In the team's conceptual design, they have moved from a one-dimensional array of qubits, positioned along a single line, to a two-dimensional array, positioned on a plane that is far more tolerant to errors. This qubit layer is "sandwiched" in a three-dimensional architecture, between two layers of wires arranged in a grid.

By applying voltages to a sub-set of these wires, multiple qubits can be controlled in parallel, performing a series of operations using far fewer controls. Importantly, with their design, they can perform the 2D surface code error correction protocols in which any computational errors that creep into the calculation can be corrected faster than they occur.

"Our Australian team has developed the world's best qubits in silicon," says University of Melbourne Professor Lloyd Hollenberg, Deputy Director of the CQC2T who led the work with colleague Dr Charles Hill. "However, to scale up to a full operational quantum computer we need more than just many of these qubits - we need to be able to control and arrange them in such a way that we can correct errors quantum mechanically."

"In our work, we've developed a blueprint that is unique to our system of qubits in silicon, for building a full-scale quantum computer."

In their paper, the team proposes a strategy to build the device, which leverages the CQC2T's internationally unique capability of atomic-scale device fabrication. They have also modelled the required voltages applied to the grid wires, needed to address individual qubits, and make the processor work.

"This architecture gives us the dense packing and parallel operation essential for scaling up the size of the quantum processor," says Scientia Professor Sven Rogge, Head of the UNSW School of Physics. "Ultimately, the structure is scalable to millions of qubits, required for a full-scale quantum processor."

Within the last several years, considerable progress has been made in developing a quantum supercomputer, which holds the promise of solving problems a lot more efficiently than a classical computer. Physicists are now able to realize the basic building blocks, the quantum bits (qubits) in a laboratory, control them and use them for simple computations. For practical application, a particular class of quantum computers, the so-called adiabatic quantum computer, has recently generated a lot of interest among researchers and industry. It is designed to solve real-world optimization problems conventional computers are not able to tackle. All current approaches for adiabatic quantum supercomputation face the same challenge: The problem is encoded in the interaction between qubits; to encode a generic problem, an all-to-all connectivity is necessary, but the locality of the physical quantum bits limits the available interactions. "The programming language of these systems is the individual interaction between each physical qubit. The possible input is determined by the hardware. This means that all these approaches face a fundamental challenge when trying to build a fully programmable quantum computer," explains Wolfgang Lechner from the Institute for Quantum Optics and Quantum Information (IQOQI) at the Austrian Academy of Sciences in Innsbruck.

Fully programmable quantum supercomputer

Theoretical physicists Wolfang Lechner, Philipp Hauke and Peter Zoller have now proposed a completely new approach. The trio, working at the University of Innsbruck and the IQOQI, suggest overcoming the challenges by detaching the logical qubit from the physical implementation. Each physical qubit corresponds to one pair of logical qubits and can be tuned by local fields. These could be electrical fields when dealing with atoms and ions or magnetic fields in superconducting qubits. "Any generic optimization problem can be fully programmed via the fields," explains co-author Philipp Hauke from the Institute for Theoretical Physics at the University of Innsbruck, Austria. "By using this approach we are not only avoiding the limitations posed by the hardware but we also make the technological implementation scalable."

Integrated fault-tolerance

Because of the increased number of degrees of freedom, which could also lead to non-physical solutions, the physicists arrange the qubits in a way that four physical qubits interact locally. "In this way we guarantee that only physical solutions are possible," explains Wolfgang Lechner. The solution of the problem is encoded redundantly in the qubits. "With this redundancy our model has also a high fault-tolerance," says Lechner. The new architecture can be realized on various platforms ranging from superconducting circuits to ultracold gases in optical lattices. "Our approach allows for the application of technologies that have not been suitable for adiabatic quantum optimization until now," says the physicist. Lechner, Hauke and Zoller have introduced this new model in the journal Science Advances. The scientific community has also expressed great interest in the new model. Peter Zoller is convinced: "The step from mechanical calculators to fully programmable computers started the information technology age 80 years ago. Today we are approaching the age of quantum information."

A patent for the new quantum supercomputer architecture has been submitted this year. The scientists are financially supported by the Austrian Science Fund (FWF) and the European Research Council (ERC) among others.

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