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

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The quantum Fredkin gate has been experimentally realised for the first time

Researchers from Griffith University and the University of Queensland have overcome one of the key challenges to quantum computing by simplifying a complex quantum logic operation. They demonstrated this by experimentally realising a challenging circuit -- the quantum Fredkin gate -- for the first time.

"The allure of quantum computers is the unparalleled processing power that they provide compared to current technology," said Dr Raj Patel from Griffith's Centre for Quantum Dynamics.

"Much like our everyday computer, the brains of a quantum computer consist of chains of logic gates, although quantum logic gates harness quantum phenomena."

The main stumbling block to actually creating a quantum computer has been in minimising the number of resources needed to efficiently implement processing circuits.

"Similar to building a huge wall out lots of small bricks, large quantum circuits require very many logic gates to function. However, if larger bricks are used the same wall could be built with far fewer bricks," said Dr Patel.

"We demonstrate in our experiment how one can build larger quantum circuits in a more direct way without using small logic gates."

At present, even small and medium scale quantum computer circuits cannot be produced because of the requirement to integrate so many of these gates into the circuits. One example is the Fredkin (controlled- SWAP) gate. This is a gate where two qubits are swapped depending on the value of the third.

Usually the Fredkin gate requires implementing a circuit of five logic operations. The research team used the quantum entanglement of photons -- particles of light -- to implement the controlled-SWAP operation directly.

"There are quantum computing algorithms, such as Shor's algorithm for factorising prime numbers, that require the controlled-SWAP operation.

The quantum Fredkin gate can also be used to perform a direct comparison of two sets of qubits (quantum bits) to determine whether they are the same or not. This is not only useful in computing but is an essential feature of some secure quantum communication protocols where the goal is to verify that two strings, or digital signatures, are the same," said Professor Tim Ralph from the University of Queensland.

Professor Geoff Pryde, from Griffith's Centre for Quantum Dynamics, is the project's chief investigator.

"What is exciting about our scheme is that it is not limited to just controlling whether qubits are swapped, but can be applied to a variety of different operations opening up ways to control larger circuits efficiently," said Professor Pryde.

"This could unleash applications that have so far been out of reach."

The team is part of the Australian Research Council's Centre for Quantum Computation and Communication Technology, an effort to exploit Australia's strong expertise in developing quantum information technologies.

The research has been published as A quantum Fredkin gate in Science Advances(DOI:10.1126/sciadv.1501531)

Griffith University

New quantum computer, based on five atoms

Researchers have designed and built a quantum computer from five atoms in an ion trap. The computer uses laser pulses to carry out Shor’s algorithm on each atom, to correctly factor the number 15. Image: Jose-Luis Olivares/MIT

The beginning of the end for encryption schemes?

What are the prime factors, or multipliers, for the number 15? Most grade school students know the answer — 3 and 5 — by memory. A larger number, such as 91, may take some pen and paper. An even larger number, say with 232 digits, can (and has) taken scientists two years to factor, using hundreds of classical computers operating in parallel.

Because factoring large numbers is so devilishly hard, this “factoring problem” is the basis for many encryption schemes for protecting credit cards, state secrets, and other confidential data. It’s thought that a single quantum computer may easily crack this problem, by using hundreds of atoms, essentially in parallel, to quickly factor huge numbers.

In 1994, Peter Shor, the Morss Professor of Applied Mathematics at MIT, came up with a quantum algorithm that calculates the prime factors of a large number, vastly more efficiently than a classical computer. However, the algorithm’s success depends on a computer with a large number of quantum bits. While others have attempted to implement Shor’s algorithm in various quantum systems, none have been able to do so with more than a few quantum bits, in a scalable way.

Now, in a paper published today in the journal Science, researchers from MIT and the University of Innsbruck in Austria report that they have designed and built a quantum computer from five atoms in an ion trap. The computer uses laser pulses to carry out Shor’s algorithm on each atom, to correctly factor the number 15. The system is designed in such a way that more atoms and lasers can be added to build a bigger and faster quantum computer, able to factor much larger numbers. The results, they say, represent the first scalable implementation of Shor’s algorithm.

“We show that Shor’s algorithm, the most complex quantum algorithm known to date, is realizable in a way where, yes, all you have to do is go in the lab, apply more technology, and you should be able to make a bigger quantum computer,” says Isaac Chuang, professor of physics and professor of electrical engineering and computer science at MIT. “It might still cost an enormous amount of money to build — you won’t be building a quantum computer and putting it on your desktop anytime soon — but now it’s much more an engineering effort, and not a basic physics question.”

Seeing through the quantum forest

In classical computing, numbers are represented by either 0s or 1s, and calculations are carried out according to an algorithm’s “instructions,” which manipulate these 0s and 1s to transform an input to an output. In contrast, quantum computing relies on atomic-scale units, or “qubits,” that can be simultaneously 0 and 1 — a state known as a superposition. In this state, a single qubit can essentially carry out two separate streams of calculations in parallel, making computations far more efficient than a classical computer.

In 2001, Chuang, a pioneer in the field of quantum computing, designed a quantum computer based on one molecule that could be held in superposition and manipulated with nuclear magnetic resonance to factor the number 15. The results, which were published in Nature, represented the first experimental realization of Shor’s algorithm. But the system wasn’t scalable; it became more difficult to control the system as more atoms were added.

“Once you had too many atoms, it was like a big forest — it was very hard to control one atom from the next one,” Chuang says. “The difficulty is to implement [the algorithm] in a system that’s sufficiently isolated that it can stay quantum mechanical for long enough that you can actually have a chance to do the whole algorithm.”

“Straightforwardly scalable”

Chuang and his colleagues have now come up with a new, scalable quantum system for factoring numbers efficiently. While it typically takes about 12 qubits to factor the number 15, they found a way to shave the system down to five qubits, each represented by a single atom. Each atom can be held in a superposition of two different energy states simultaneously. The researchers use laser pulses to perform “logic gates,” or components of Shor’s algorithm, on four of the five atoms. The results are then stored, forwarded, extracted, and recycled via the fifth atom, thereby carrying out Shor’s algorithm in parallel, with fewer qubits than is typically required.

The team was able to keep the quantum system stable by holding the atoms in an ion trap, where they removed an electron from each atom, thereby charging it. They then held each atom in place with an electric field.

“That way, we know exactly where that atom is in space,” Chuang explains. “Then we do that with another atom, a few microns away — [a distance] about 100th the width of a human hair. By having a number of these atoms together, they can still interact with each other, because they’re charged. That interaction lets us perform logic gates, which allow us to realize the primitives of the Shor factoring algorithm. The gates we perform can work on any of these kinds of atoms, no matter how large we make the system.”

Chuang’s team first worked out the quantum design in principle. His colleagues at the University of Innsbruck then built an experimental apparatus based on his methodology. They directed the quantum system to factor the number 15 — the smallest number that can meaningfully demonstrate Shor’s algorithm. Without any prior knowledge of the answers, the system returned the correct factors, with a confidence exceeding 99 percent.

“In future generations, we foresee it being straightforwardly scalable, once the apparatus can trap more atoms and more laser beams can control the pulses,” Chuang says. “We see no physical reason why that is not going to be in the cards.”

Mark Ritter, senior manager of physical sciences at IBM, says the group’s method of recycling qubits reduces the resources required in the system by a factor of 3 — a significant though small step towards scaling up quantum computing.

“Improving the state-of-the-art by a factor of 3 is good,” says Ritter. But truly scaling the system “requires orders of magnitude more qubits, and these qubits must be shuttled around advanced traps with many thousands of simultaneous laser control pulses.”

If the team can successfully add more quantum components to the system, Ritter says it will have accomplished a long-unrealized feat.

“Shor's algorithm was the first non-trivial quantum algorithm showing a potential of ‘exponential’ speed-up over classical algorithms,” Ritter says. “It captured the imagination of many researchers who took notice of quantum computing because of its promise of truly remarkable algorithmic acceleration. Therefore, to implement Shor's algorithm is comparable to the ‘Hello, World’ of classical computing.”

What will all this eventually mean for encryption schemes of the future?

“Well, one thing is that if you are a nation state, you probably don’t want to publicly store your secrets using encryption that relies on factoring as a hard-to-invert problem,” Chuang says. “Because when these quantum computers start coming out, you’ll be able to go back and unencrypt all those old secrets.”

MIT

Quantum computing is coming – are you prepared for it?

Quantum computing will change lives, society and the economy and a working system is expected to be developed by 2020 according to a leading figure in the world of quantum computing, who will talk tomorrow [21 January 2016] at the World Economic Forum (WEF) in Davos, Switzerland.

Professor O’Brien, Director of the Centre for Quantum Photonics at the University of Bristol and Visiting Fellow at Stanford University, is part of a European Research Council (ERC) Ideas Lab delegation who have been invited to talk at the annual meeting to industrial and political leaders of the world, including Prime Minister David Cameron.  The session will discuss the future of computing and how new fields of computer sciences are paving the way for the next digital revolution.

Quantum computing has the capability to unlock answers to some of humanity’s most pressing questions that are presently unsolvable with current computing technologies.  In 2014, the UK government invested over £270 million in the development of quantum technologies, ensuring that the UK becomes the epicentre of a technology revolution and Professor O’Brien has been leading the development of quantum computing using light in its quantum state – the photon- as the key ingredient. 

Professor O’Brien said: “In less than ten years quantum computers will begin to outperform everyday computers, leading to breakthroughs in artificial intelligence, the discovery of new pharmaceuticals and beyond.

“The very fast computing power given by quantum computers has the potential to disrupt traditional businesses and challenge our cyber-security. Businesses need to be ready for a quantum future because it’s coming.”

In his talk, Professor O’Brien will outline the current status of quantum computing and its potential applications and he will reveal his architectural blue-print for a manufacturable photonic quantum computer, showing all the components and a roadmap toward building a practical machine.

Quantum technologies offer ultra-secure communications, sensors of unprecedented precision and computers that are exponentially more powerful than any supercomputer for a given task. These technologies are destined to fundamentally change our lives and the first commercially available quantum devices are only now beginning to emerge.

As the holder of a prestigious Royal Academy of Engineering Chair in Quantum Engineering and an EPSRC RISE leader, Professor O’Brien has a ten year vision to engineer new quantum technologies that will inevitably disrupt todays ICT models, creating new businesses and valuable new markets.

The World Economic Forum (WEF) Annual Meeting of business and political leaders will take place from 20-23 January 2016 in Davos, Switzerland.

University of Bristol

A step towards quantum electronics

Work of physicists at the University of Geneva (UNIGE), Switzerland, and the Swiss Federal Institute of Technology in Zurich (ETH Zurich), in which they connected two materials with unusual quantum-mechanical properties through a quantum constriction, could open up a novel path towards both a deeper understanding of physics and future electronic devices. Their results have just been published in the journal Science.

The researchers work with atoms that are trapped in laser beams and thus isolated from any external disturbance. Lasers are also used to cool the atoms to temperatures lower than those found anywhere else in the entire Universe. These 'ultracold' temperatures then enable creating clean materials that possess intriguing quantum-mechanical properties, such as unusual superconductivity. Thierry Giamarchi, professor at the UNIGE and responsible for the theoretical part of the study, explains: "In a cold-atom superconductor, the particles interact very strongly, whereas the interaction is usually very weak. This brings out strong-interaction effects through cooling could be compared to freezing water: the basic system is the same, but the result after cooling is very different."

The experimental team in Zurich, led by Tilman Esslinger and Jean-Philippe Brantut, has now overcome the challenges to efficiently transport ultracold atoms between two quantum superconductors with strong interactions through a single quantum point, a so-called quantum point contact. "With this new quantum connection, we can now reveal new effects in these superconducting quantum systems. It is a fundamental breakthrough in the way we can use quantum physics with cold atoms", says Giamarchi, from UNIGE's Faculty of Science.

A collaboration serving innovation

In general, it is difficult to produce a clean junction between quantum materials. Thanks to the collaboration between the teams in Geneva and Zurich, an important step has now been taken towards developing efficient junctions. For their ultracold atoms, the researchers produced junctions with a transparency close to 100 %. This advance is a crucial step towards understanding quantum transport in ultracold atoms and will enable fundamental studies of superconductors and other quantum materials. But interconnecting quantum materials such as superconductors might bring also new possibilities for more efficient information processing, beyond what is possible with techniques currently available for connecting, in computers and electronic devices, active elements such as transistors to form electronic circuits.

Now that junctions between quantum materials with strong interactions can be produced, scientists might eventually create novel materials that can be used in everyday applications. The unconventional approach developed in Geneva and Zurich therefore establishes the first basis for new technologies and opens up a new research direction that might lead to creating ultrafast and robust electronic networks -- a dream that many physicists share.

University of Geneva

Nanotechnology World Association

Using Atoms to Turn Optical Nanofiber Guided Light On and Off

Researchers in the Light-Matter Interactions Unit led by Professor Síle Nic Chormaic at the Okinawa Institute of Science and Technology Graduate University (OIST) have developed an on-off switch with ultrathin optical fibers, which could be used for data transfer in the future. This research was published in the New Journal of Physics.

0101000001101000011110010111001101101001011000110111001100100000011010010111001100100000011001100111010101101110 means “Physics is fun” in binary code. Computers translate every letter, number, sign, space, image and sound to a set of 8 ones and zeros. For example, 01010000 corresponds to the letter P. While you type, your computer transfers your words to another distant computer by sending a series of ones and zeros encoded in light through standard optical fibers. Switching the light beam on and off very quickly generates the ones and zeros. These bits of information are converted to electronic signals at a node, usually a router or server, and finally appear as text on the screen of your recipient. While this is the classical way of transferring information online, OIST researchers are exploring more efficient ways of transferring data, using the quantum properties of light and matter. They have managed to create an on/off switch based on the quantum characteristics of rubidium atoms in the presence of light of different wavelengths. This proof-of-concept system could be used as a building block in a quantum network, the future of our internet.

The OIST team’s experimental setup consists of two lasers that produce light at different wavelengths, an optical nanofiber used to guide light, and rubidium atoms trapped around it. The peculiarity of optical nanofibers is their super-thin diameter. For this study the diameter was 350 nanometers, about 300 times thinner than the thickness of a sheet of paper. The diameter is even smaller than the wavelength of the light guided by the fiber. Some of the light, therefore, leaks outside the nanofiber and interacts with the rubidium atoms that are trapped around it. These atoms can function as a quantum node, a redistribution point of a network, the equivalent of today’s servers.

The off switch condition is obtained when only the laser producing 780 nm is on. In this case, at the point where light leaks outside of the optical nanofiber, the rubidium atoms absorb the maximum amount of light and almost no light can continue to pass along the fiber. In contrast, the switch is turned on when both 776 nm and 780 nm lights are present. In this situation, most of the light is transmitted through the optical nanofiber and the rubidium atoms absorb it only minimally.

Since the optical nanofiber is directly connected to a standard optical fiber, the light can, in principle, be transferred to another quantum system or node some distance away, in the same way you can send a message from your computer to that of your friend’s in another location. 

“Using optical nanofibers would allow us to fully integrate our system with existing fiber-based communication networks. While the current work is far from being a practical solution to quantum information, it brings the notion of using atoms and light to develop real devices based on quantum mechanics ever closer to fulfilment”, explains Professor Síle Nic Chormaic.

While the experiment at OIST currently only generates zeros/off and ones/on consecutively, further exploitation of the quantum behavior of atoms should allow the research team to send light as a combination of “on” and “off” at the same time. In this way, in the future, quantum networks will be able to process more data simultaneously, increase efficiency of information transfer and also provide better cyber security.

“It has been very exciting to work with optical nanofibers which can guide light extremely efficiently even if their diameter is much smaller than the wavelength of light itself. These systems are sure to give us significant progress in quantum networks in the years to come,” enthuses Ravi Kumar, one of the authors of this study and a PhD student at University College Cork in Ireland, doing his research work at OIST.

Okinawa Institute of Science and Technology Graduate University