Tag Archives: quantum states

Brain stuff: quantum entanglement and a multi-dimensional universe

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I have two brain news bits, one about neural networks and quantum entanglement and another about how the brain operates in* more than three dimensions.

Quantum entanglement and neural networks

A June 13, 2017 news item on phys.org describes how machine learning can be used to solve problems in physics (Note: Links have been removed),

Machine learning, the field that’s driving a revolution in artificial intelligence, has cemented its role in modern technology. Its tools and techniques have led to rapid improvements in everything from self-driving cars and speech recognition to the digital mastery of an ancient board game.

Now, physicists are beginning to use machine learning tools to tackle a different kind of problem, one at the heart of quantum physics. In a paper published recently in Physical Review X, researchers from JQI [Joint Quantum Institute] and the Condensed Matter Theory Center (CMTC) at the University of Maryland showed that certain neural networks—abstract webs that pass information from node to node like neurons in the brain—can succinctly describe wide swathes of quantum systems.

An artist’s rendering of a neural network with two layers. At the top is a real quantum system, like atoms in an optical lattice. Below is a network of hidden neurons that capture their interactions (Credit: E. Edwards/JQI)

A June 12, 2017 JQI news release by Chris Cesare, which originated the news item, describes how neural networks can represent quantum entanglement,

Dongling Deng, a JQI Postdoctoral Fellow who is a member of CMTC and the paper’s first author, says that researchers who use computers to study quantum systems might benefit from the simple descriptions that neural networks provide. “If we want to numerically tackle some quantum problem,” Deng says, “we first need to find an efficient representation.”

On paper and, more importantly, on computers, physicists have many ways of representing quantum systems. Typically these representations comprise lists of numbers describing the likelihood that a system will be found in different quantum states. But it becomes difficult to extract properties or predictions from a digital description as the number of quantum particles grows, and the prevailing wisdom has been that entanglement—an exotic quantum connection between particles—plays a key role in thwarting simple representations.

The neural networks used by Deng and his collaborators—CMTC Director and JQI Fellow Sankar Das Sarma and Fudan University physicist and former JQI Postdoctoral Fellow Xiaopeng Li—can efficiently represent quantum systems that harbor lots of entanglement, a surprising improvement over prior methods.

What’s more, the new results go beyond mere representation. “This research is unique in that it does not just provide an efficient representation of highly entangled quantum states,” Das Sarma says. “It is a new way of solving intractable, interacting quantum many-body problems that uses machine learning tools to find exact solutions.”

Neural networks and their accompanying learning techniques powered AlphaGo, the computer program that beat some of the world’s best Go players last year (link is external) (and the top player this year (link is external)). The news excited Deng, an avid fan of the board game. Last year, around the same time as AlphaGo’s triumphs, a paper appeared that introduced the idea of using neural networks to represent quantum states (link is external), although it gave no indication of exactly how wide the tool’s reach might be. “We immediately recognized that this should be a very important paper,” Deng says, “so we put all our energy and time into studying the problem more.”

The result was a more complete account of the capabilities of certain neural networks to represent quantum states. In particular, the team studied neural networks that use two distinct groups of neurons. The first group, called the visible neurons, represents real quantum particles, like atoms in an optical lattice or ions in a chain. To account for interactions between particles, the researchers employed a second group of neurons—the hidden neurons—which link up with visible neurons. These links capture the physical interactions between real particles, and as long as the number of connections stays relatively small, the neural network description remains simple.

Specifying a number for each connection and mathematically forgetting the hidden neurons can produce a compact representation of many interesting quantum states, including states with topological characteristics and some with surprising amounts of entanglement.

Beyond its potential as a tool in numerical simulations, the new framework allowed Deng and collaborators to prove some mathematical facts about the families of quantum states represented by neural networks. For instance, neural networks with only short-range interactions—those in which each hidden neuron is only connected to a small cluster of visible neurons—have a strict limit on their total entanglement. This technical result, known as an area law, is a research pursuit of many condensed matter physicists.

These neural networks can’t capture everything, though. “They are a very restricted regime,” Deng says, adding that they don’t offer an efficient universal representation. If they did, they could be used to simulate a quantum computer with an ordinary computer, something physicists and computer scientists think is very unlikely. Still, the collection of states that they do represent efficiently, and the overlap of that collection with other representation methods, is an open problem that Deng says is ripe for further exploration.

Here’s a link to and a citation for the paper,

Quantum Entanglement in Neural Network States by Dong-Ling Deng, Xiaopeng Li, and S. Das Sarma. Phys. Rev. X 7, 021021 – Published 11 May 2017

This paper is open access.

Blue Brain and the multidimensional universe

Blue Brain is a Swiss government brain research initiative which officially came to life in 2006 although the initial agreement between the École Politechnique Fédérale de Lausanne (EPFL) and IBM was signed in 2005 (according to the project’s Timeline page). Moving on, the project’s latest research reveals something astounding (from a June 12, 2017 Frontiers Publishing press release on EurekAlert),

For most people, it is a stretch of the imagination to understand the world in four dimensions but a new study has discovered structures in the brain with up to eleven dimensions – ground-breaking work that is beginning to reveal the brain’s deepest architectural secrets.

Using algebraic topology in a way that it has never been used before in neuroscience, a team from the Blue Brain Project has uncovered a universe of multi-dimensional geometrical structures and spaces within the networks of the brain.

The research, published today in Frontiers in Computational Neuroscience, shows that these structures arise when a group of neurons forms a clique: each neuron connects to every other neuron in the group in a very specific way that generates a precise geometric object. The more neurons there are in a clique, the higher the dimension of the geometric object.

“We found a world that we had never imagined,” says neuroscientist Henry Markram, director of Blue Brain Project and professor at the EPFL in Lausanne, Switzerland, “there are tens of millions of these objects even in a small speck of the brain, up through seven dimensions. In some networks, we even found structures with up to eleven dimensions.”

Markram suggests this may explain why it has been so hard to understand the brain. “The mathematics usually applied to study networks cannot detect the high-dimensional structures and spaces that we now see clearly.”

If 4D worlds stretch our imagination, worlds with 5, 6 or more dimensions are too complex for most of us to comprehend. This is where algebraic topology comes in: a branch of mathematics that can describe systems with any number of dimensions. The mathematicians who brought algebraic topology to the study of brain networks in the Blue Brain Project were Kathryn Hess from EPFL and Ran Levi from Aberdeen University.

“Algebraic topology is like a telescope and microscope at the same time. It can zoom into networks to find hidden structures – the trees in the forest – and see the empty spaces – the clearings – all at the same time,” explains Hess.

In 2015, Blue Brain published the first digital copy of a piece of the neocortex – the most evolved part of the brain and the seat of our sensations, actions, and consciousness. In this latest research, using algebraic topology, multiple tests were performed on the virtual brain tissue to show that the multi-dimensional brain structures discovered could never be produced by chance. Experiments were then performed on real brain tissue in the Blue Brain’s wet lab in Lausanne confirming that the earlier discoveries in the virtual tissue are biologically relevant and also suggesting that the brain constantly rewires during development to build a network with as many high-dimensional structures as possible.

When the researchers presented the virtual brain tissue with a stimulus, cliques of progressively higher dimensions assembled momentarily to enclose high-dimensional holes, that the researchers refer to as cavities. “The appearance of high-dimensional cavities when the brain is processing information means that the neurons in the network react to stimuli in an extremely organized manner,” says Levi. “It is as if the brain reacts to a stimulus by building then razing a tower of multi-dimensional blocks, starting with rods (1D), then planks (2D), then cubes (3D), and then more complex geometries with 4D, 5D, etc. The progression of activity through the brain resembles a multi-dimensional sandcastle that materializes out of the sand and then disintegrates.”

The big question these researchers are asking now is whether the intricacy of tasks we can perform depends on the complexity of the multi-dimensional “sandcastles” the brain can build. Neuroscience has also been struggling to find where the brain stores its memories. “They may be ‘hiding’ in high-dimensional cavities,” Markram speculates.

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…

About Blue Brain

The aim of the Blue Brain Project, a Swiss brain initiative founded and directed by Professor Henry Markram, is to build accurate, biologically detailed digital reconstructions and simulations of the rodent brain, and ultimately, the human brain. The supercomputer-based reconstructions and simulations built by Blue Brain offer a radically new approach for understanding the multilevel structure and function of the brain. http://bluebrain.epfl.ch

About Frontiers

Frontiers is a leading community-driven open-access publisher. By taking publishing entirely online, we drive innovation with new technologies to make peer review more efficient and transparent. We provide impact metrics for articles and researchers, and merge open access publishing with a research network platform – Loop – to catalyse research dissemination, and popularize research to the public, including children. Our goal is to increase the reach and impact of research articles and their authors. Frontiers has received the ALPSP Gold Award for Innovation in Publishing in 2014. http://www.frontiersin.org.

Here’s a link to and a citation for the paper,

Cliques of Neurons Bound into Cavities Provide a Missing Link between Structure and Function by Michael W. Reimann, Max Nolte, Martina Scolamiero, Katharine Turner, Rodrigo Perin, Giuseppe Chindemi, Paweł Dłotko, Ran Levi, Kathryn Hess, and Henry Markram. Front. Comput. Neurosci., 12 June 2017 | https://doi.org/10.3389/fncom.2017.00048

This paper is open access.

*Feb. 3, 2021: ‘on’ changed to ‘in’

Nanorods as multistate switches

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This research goes beyond the binary (0 or 1) and to an analog state that resembles quantum states. Fascinating, yes? An Oct. 10, 2016 news item on phys.org tells more,

Rice University scientists have discovered how to subtly change the interior structure of semi-hollow nanorods in a way that alters how they interact with light, and because the changes are reversible, the method could form the basis of a nanoscale switch with enormous potential.

“It’s not 0-1, it’s 1-2-3-4-5-6-7-8-9-10,” said Rice materials scientist Emilie Ringe, lead scientist on the project, which is detailed in the American Chemical Society journal Nano Letters. “You can differentiate between multiple plasmonic states in a single particle. That gives you a kind of analog version of quantum states, but on a larger, more accessible scale.”

Ringe and colleagues used an electron beam to move silver from one location to another inside gold-and-silver nanoparticles, something like a nanoscale Etch A Sketch. The result is a reconfigurable optical switch that may form the basis for a new type of multiple-state computer memory, sensor or catalyst.

An Oct. 10, 2016 Rice University news release, which originated the news item, describes the work in additional detail,

At about 200 nanometers long, 500 of the metal rods placed end-to-end would span the width of a human hair. However, they are large in comparison with modern integrated circuits. Their multistate capabilities make them more like reprogrammable bar codes than simple memory bits, she said.

“No one has been able to reversibly change the shape of a single particle with the level of control we have, so we’re really excited about this,” Ringe said.

Altering a nanoparticle’s internal structure also alters its external plasmonic response. Plasmons are the electrical ripples that propagate across the surface of metallic materials when excited by light, and their oscillations can be easily read with a spectrometer — or even the human eye — as they interact with visible light.

The Rice researchers found they could reconfigure nanoparticle cores with pinpoint precision. That means memories made of nanorods need not be merely on-off, Ringe said, because a particle can be programmed to emit many distinct plasmonic patterns.

The discovery came about when Ringe and her team, which manages Rice’s advanced electron microscopy lab, were asked by her colleague and co-author Denis Boudreau, a professor at Laval University in Quebec, to characterize hollow nanorods made primarily of gold but containing silver.

“Most nanoshells are leaky,” Ringe said. “They have pinholes. But we realized these nanorods were defect-free and contained pockets of water that were trapped inside when the particles were synthesized. We thought: We have something here.”

Ringe and the study’s lead author, Rice research scientist Sadegh Yazdi, quickly realized how they might manipulate the water. “Obviously, it’s difficult to do chemistry there, because you can’t put molecules into a sealed nanoshell. But we could put electrons in,” she said.

Focusing a subnanometer electron beam on the interior cavity split the water and inserted solvated electrons – free electrons that can exist in a solution. “The electrons reacted directly with silver ions in the water, drawing them to the beam to form silver,” Ringe said. The now-silver-poor liquid moved away from the beam, and its silver ions were replenished by a reaction of water-splitting byproducts with the solid silver in other parts of the rod.

“We actually were moving silver in the solution, reconfiguring it,” she said. “Because it’s a closed system, we weren’t losing anything and we weren’t gaining anything. We were just moving it around, and could do so as many times as we wished.”

The researchers were then able to map the plasmon-induced near-field properties without disturbing the internal structure — and that’s when they realized the implications of their discovery.

“We made different shapes inside the nanorods, and because we specialize in plasmonics, we mapped the plasmons and it turned out to have a very nice effect,” Ringe said. “We basically saw different electric-field distributions at different energies for different shapes.” Numerical results provided by collaborators Nicolas Large of the University of Texas at San Antonio and George Schatz of Northwestern University helped explain the origin of the modes and how the presence of a water-filled pocket created a multitude of plasmons, she said.

The next challenge is to test nanoshells of other shapes and sizes, and to see if there are other ways to activate their switching potentials. Ringe suspects electron beams may remain the best and perhaps only way to catalyze reactions inside particles, and she is hopeful.

“Using an electron beam is actually not as technologically irrelevant as you might think,” she said. “Electron beams are very easy to generate. And yes, things need to be in vacuum, but other than that, people have generated electron beams for nearly 100 years. I’m sure 40 years ago people were saying, ‘You’re going to put a laser in a disk reader? That’s crazy!’ But they managed to do it.

“I don’t think it’s unfeasible to miniaturize electron-beam technology. Humans are good at moving electrons and electricity around. We figured that out a long time ago,” Ringe said.

The research should trigger the imaginations of scientists working to create nanoscale machines and processes, she said.

“This is a reconfigurable unit that you can access with light,” she said. “Reading something with light is much faster than reading with electrons, so I think this is going to get attention from people who think about dynamic systems and people who think about how to go beyond current nanotechnology. This really opens up a new field.”

Here’s a link to and a citation for the paper,

Reversible Shape and Plasmon Tuning in Hollow AgAu Nanorods by Sadegh Yazdi, Josée R. Daniel, Nicolas Large, George C. Schatz, Denis Boudreau, and Emilie Ringe. Nano Lett., Article ASAP DOI: 10.1021/acs.nanolett.6b02946 Publication Date (Web): October 5, 2016

Copyright © 2016 American Chemical Society

This paper is behind a paywall.

The researchers have made this video available for the public,

Surviving 39 minutes at room temperature—recordbreaking for quantum materials

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There are two news releases about this work which brings quantum computing a step closer to reality. I’ll start with the Nov. 15, 2013 Simon Fraser University (SFU; located in Vancouver, Canada) news release (Note: A link has been removed),,

An international team of physicists led by Simon Fraser University professor Mike Thewalt has overcome a key barrier to building practical quantum computers, taking a significant step to bringing them into the mainstream.

In their record-breaking experiment conducted on SFU’s Burnaby campus, [part of Metro Vancouver] the scientists were able to get fragile quantum states to survive in a solid material at room temperature for 39 minutes. For the average person, it may not seem like a long time, but it’s a veritable eternity to a quantum physicist.

“This opens up the possibility of truly long-term coherent information storage at room temperature,” explains Thewalt.

Quantum computers promise to significantly outperform today’s machines at certain tasks, by exploiting the strange properties of subatomic particles. Conventional computers process data stored as strings of ones or zeroes, but quantum objects are not constrained to the either/or nature of binary bits.

Instead, each quantum bit – or qubit – can be put into a superposition of both one and zero at the same time, enabling them to perform multiple calculations simultaneously. For instance, this ability to multi-task could allow quantum computers to crack seemingly secure encryption codes.

“A powerful universal quantum computer would change technology in ways that we already understand, and doubtless in ways we do not yet envisage,” says Thewalt, whose research was published in Science today.

“It would have a huge impact on security, code breaking and the transmission and storage of secure information. It would be able to solve problems which are impossible to solve on any conceivable normal computer. It would be able to model the behaviour of quantum systems, a task beyond the reach of normal computers, leading, for example, to the development of new drugs by a deeper understanding of molecular interactions.”

However, the problem with attempts to build these extraordinary number-crunchers is that superposition states are delicate structures that can collapse like a soufflé if nudged by a stray particle, such as an air molecule.

To minimize this unwanted process, physicists often cool their qubit systems to almost absolute zero (-273 C) and manipulate them in a vacuum. But such setups are finicky to maintain and, ultimately, it would be advantageous for quantum computers to operate robustly at everyday temperatures and pressures.

“Our research extends the demonstrated coherence time in a solid at room temperature by a factor of 100 – and at liquid helium temperature by a factor of 60 (from three minutes to three hours),” says Thewalt.

“These are large, significant improvements in what is possible.”

The November 15, 2013 University of Oxford news release (also on EurekAlert), features their own researcher and more information (e.g., the previous record for maintaining coherence of a solid state at room temperature),

An international team including Stephanie Simmons of Oxford University report in this week’s Science a test performed as part of a project led by Mike Thewalt of Simon Fraser University, Canada, and colleagues. …

In the experiment, the team raised the temperature of a system, in which information is encoded in the nuclei of phosphorus atoms in silicon, from -269°C to 25°C and demonstrated that the superposition states survived at this balmy temperature for 39 minutes – outside of silicon the previous record for such a state’s survival at room temperature was around two seconds. [emphasis mine] The team even found that they could manipulate the qubits as the temperature of the system rose, and that they were robust enough for this information to survive being ‘refrozen’ (the optical technique used to read the qubits only works at very low temperatures).

‘Thirty-nine minutes may not seem very long but as it only takes one-hundred-thousandth of a second to flip the nuclear spin of a phosphorus ion – the type of operation used to run quantum calculations – in theory over two million operations could be applied in the time it takes for the superposition to naturally decay by 1%. Having such robust, as well as long-lived, qubits could prove very helpful for anyone trying to build a quantum computer,’ said Stephanie Simmons of Oxford University’s Department of Materials, an author of the paper.

…

The team began with a sliver of silicon doped with small amounts of other elements, including phosphorus. Quantum information was encoded in the nuclei of the phosphorus atoms: each nucleus has an intrinsic quantum property called ‘spin’, which acts like a tiny bar magnet when placed in a magnetic field. Spins can be manipulated to point up (0), down (1), or any angle in between, representing a superposition of the two other states.

The team prepared their sample at just 4°C above absolute zero (-269°C) and placed it in a magnetic field. Additional magnetic field pulses were used to tilt the direction of the nuclear spin and create the superposition states. When the sample was held at this cryogenic temperature, the nuclear spins of about 37% of the ions – a typical benchmark to measure quantum coherence – remained in their superposition state for three hours. The same fraction survived for 39 minutes when the temperature of the system was raised to 25°C.

…

There is still some work ahead before the team can carry out large-scale quantum computations. The nuclear spins of the 10 billion or so phosphorus ions used in this experiment were all placed in the same quantum state. To run calculations, however, physicists will need to place different qubits in different states. ‘To have them controllably talking to one another – that would address the last big remaining challenge,’ said Simmons.

Even for the uninitiated, going from a record of two seconds to 39 minutes has to raise an eyebrow.

Here’s a link to and a citation for the paper,

Room-Temperature Quantum Bit Storage Exceeding 39 Minutes Using Ionized Donors in Silicon-28.by Kamyar Saeedi, Stephanie Simmons, Jeff Z. Salvail, Phillip Dluhy, Helge Riemann, Nikolai V. Abrosimov, Peter Becker, Hans-Joachim Pohl, John J. L. Morton, & Mike L. W. Thewalt.  Science 15 November 2013: Vol. 342 no. 6160 pp. 830-833 DOI: 10.1126/science.1239584

This paper is behind a paywall.

ETA Nov. 18 ,2013:  The University College of London has also issued a Nov. 15, 2013 news release on EurekAlert about this work. While some of this is repetitive, I think there’s enough new information to make this excerpt worthwhile,

The team even found that they could manipulate the qubits as the temperature of the system rose, and that they were robust enough for this information to survive being ‘refrozen’ (the optical technique used to read the qubits only works at very low temperatures). 39 minutes may not sound particularly long, but since it only takes a tiny fraction of a second to run quantum computations by flipping the spin of phosphorus ions (electrically charged phosphorus atoms), many millions of operations could be carried out before a system like this decays.

“This opens up the possibility of truly long-term coherent information storage at room temperature,” said Mike Thewalt (Simon Fraser University), the lead researcher in this study.

The team began with a sliver of silicon doped with small amounts of other elements, including phosphorus. They then encoded quantum information in the nuclei of the phosphorus atoms: each nucleus has an intrinsic quantum property called ‘spin’, which acts like a tiny bar magnet when placed in a magnetic field. Spins can be manipulated to point up (0), down (1), or any angle in between, representing a superposition of the two other states.

The team prepared their sample at -269 °C, just 4 degrees above absolute zero, and placed it in a magnetic field. They used additional magnetic field pulses to tilt the direction of the nuclear spin and create the superposition states. When the sample was held at this cryogenic temperature, the nuclear spins of about 37 per cent of the ions – a typical benchmark to measure quantum coherence – remained in their superposition state for three hours. The same fraction survived for 39 minutes when the temperature of the system was raised to 25 °C.