Tag Archives: decoherence

Seeing the future with quantum computing

Researchers at the University of Sydney (Australia) have demonstrated the ability to see the ‘quantum future’ according to a Jan. 16, 2017 news item on ScienceDaily,

Scientists at the University of Sydney have demonstrated the ability to “see” the future of quantum systems, and used that knowledge to preempt their demise, in a major achievement that could help bring the strange and powerful world of quantum technology closer to reality.

The applications of quantum-enabled technologies are compelling and already demonstrating significant impacts — especially in the realm of sensing and metrology. And the potential to build exceptionally powerful quantum computers using quantum bits, or qubits, is driving investment from the world’s largest companies.

However a significant obstacle to building reliable quantum technologies has been the randomisation of quantum systems by their environments, or decoherence, which effectively destroys the useful quantum character.

The physicists have taken a technical quantum leap in addressing this, using techniques from big data to predict how quantum systems will change and then preventing the system’s breakdown from occurring.

A Jan. 14, 2017 University of Sydney press release (also on EurekAlert), which originated the news item, expands on the theme,

“Much the way the individual components in mobile phones will eventually fail, so too do quantum systems,” said the paper’s senior author Professor Michael J.  Biercuk.

“But in quantum technology the lifetime is generally measured in fractions of a second, rather than years.”

Professor Biercuk, from the University of Sydney’s School of Physics and a chief investigator at the Australian Research Council’s Centre of Excellence for Engineered Quantum Systems, said his group had demonstrated it was possible to suppress decoherence in a preventive manner. The key was to develop a technique to predict how the system would disintegrate.

Professor Biercuk highlighted the challenges of making predictions in a quantum world: “Humans routinely employ predictive techniques in our daily experience; for instance, when we play tennis we predict where the ball will end up based on observations of the airborne ball,” he said.

“This works because the rules that govern how the ball will move, like gravity, are regular and known.  But what if the rules changed randomly while the ball was on its way to you?  In that case it’s next to impossible to predict the future behavior of that ball.

“And yet this situation is exactly what we had to deal with because the disintegration of quantum systems is random. Moreover, in the quantum realm observation erases quantumness, so our team needed to be able to guess how and when the system would randomly break.

“We effectively needed to swing at the randomly moving tennis ball while blindfolded.”

The team turned to machine learning for help in keeping their quantum systems – qubits realised in trapped atoms – from breaking.

What might look like random behavior actually contained enough information for a computer program to guess how the system would change in the future. It could then predict the future without direct observation, which would otherwise erase the system’s useful characteristics.

The predictions were remarkably accurate, allowing the team to use their guesses preemptively to compensate for the anticipated changes.

Doing this in real time allowed the team to prevent the disintegration of the quantum character, extending the useful lifetime of the qubits.

“We know that building real quantum technologies will require major advances in our ability to control and stabilise qubits – to make them useful in applications,” Professor Biercuk said.

Our techniques apply to any qubit, built in any technology, including the special superconducting circuits being used by major corporations.

“We’re excited to be developing new capabilities that turn quantum systems from novelties into useful technologies. The quantum future is looking better all the time,” Professor Biercuk said.

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

Prediction and real-time compensation of qubit decoherence via machine learning by Sandeep Mavadia, Virginia Frey, Jarrah Sastrawan, Stephen Dona, & Michael J. Biercuk. Nature Communications 8, Article number: 14106 (2017) doi:10.1038/ncomms14106 Published online: 16 January 2017

This paper is open access.

Nanodevices and quantum entanglement

A May 30, 2016 news item on phys.org introduces a scientist with an intriguing approach to quantum computing,

Creating quantum computers which some people believe will be the next generation of computers, with the ability to outperform machines based on conventional technology—depends upon harnessing the principles of quantum mechanics, or the physics that governs the behavior of particles at the subatomic scale. Entanglement—a concept that Albert Einstein once called “spooky action at a distance”—is integral to quantum computing, as it allows two physically separated particles to store and exchange information.

Stevan Nadj-Perge, assistant professor of applied physics and materials science, is interested in creating a device that could harness the power of entangled particles within a usable technology. However, one barrier to the development of quantum computing is decoherence, or the tendency of outside noise to destroy the quantum properties of a quantum computing device and ruin its ability to store information.

Nadj-Perge, who is originally from Serbia, received his undergraduate degree from Belgrade University and his PhD from Delft University of Technology in the Netherlands. He received a Marie Curie Fellowship in 2011, and joined the Caltech Division of Engineering and Applied Science in January after completing postdoctoral appointments at Princeton and Delft.

He recently talked with us about how his experimental work aims to resolve the problem of decoherence.

A May 27, 2016 California Institute of Technology (CalTech) news release by Jessica Stoller-Conrad, which originated the news item, proceeds with a question and answer format,

What is the overall goal of your research?

A large part of my research is focused on finding ways to store and process quantum information. Typically, if you have a quantum system, it loses its coherent properties—and therefore, its ability to store quantum information—very quickly. Quantum information is very fragile and even the smallest amount of external noise messes up quantum states. This is true for all quantum systems. There are various schemes that tackle this problem and postpone decoherence, but the one that I’m most interested in involves Majorana fermions. These particles were proposed to exist in nature almost eighty years ago but interestingly were never found.

Relatively recently theorists figured out how to engineer these particles in the lab. It turns out that, under certain conditions, when you combine certain materials and apply high magnetic fields at very cold temperatures, electrons will form a state that looks exactly as you would expect from Majorana fermions. Furthermore, such engineered states allow you to store quantum information in a way that postpones decoherence.

How exactly is quantum information stored using these Majorana fermions?

The fascinating property of these particles is that they always come in pairs. If you can store information in a pair of Majorana fermions it will be protected against all of the usual environmental noise that affects quantum states of individual objects. The information is protected because it is not stored in a single particle but in the pair itself. My lab is developing ways to engineer nanodevices which host Majorana fermions. Hopefully one day our devices will find applications in quantum computing.

Why did you want to come to Caltech to do this work?

The concept of engineered Majorana fermions and topological protection was, to a large degree, conceived here at Caltech by Alexei Kiteav [Ronald and Maxine Linde Professor of Theoretical Physics and Mathematics] who is in the physics department. A couple of physicists here at Caltech, Gil Refeal [professor of theoretical physics and executive officer of physics] and Jason Alicea [professor of theoretical physics], are doing theoretical work that is very relevant for my field.

Do you have any collaborations planned here?

Nothing formal, but I’ve been talking a lot with Gil and Jason. A student of mine also uses resources in the lab of Harry Atwater [Howard Hughes Professor of Applied Physics and Materials Science and director of the Joint Center for Artificial Photosynthesis], who has experience with materials that are potentially useful for our research.

How does that project relate to your lab’s work?

There are two-dimensional, or 2-D, materials that are basically very thin sheets of atoms. Graphene [emphasis mine]—a single layer of carbon atoms—is one example, but you can create single layer sheets of atoms with many materials. Harry Atwater’s group is working on solar cells made of a 2-D material. We are thinking of using the same materials and combining them with superconductors—materials that can conduct electricity without releasing heat, sound, or any other form of energy—in order to produce Majorana fermions.

How do you do that?

There are several proposed ways of using 2-D materials to create Majorana fermions. The majority of these materials have a strong spin-orbit coupling—an interaction of a particle’s spin with its motion—which is one of the key ingredients for creating Majoranas. Also some of the 2-D materials can become superconductors at low temperatures. One of the ideas that we are seriously considering is using a 2-D material as a substrate on which we could build atomic chains that will host Majorana fermions.

What got you interested in science when you were young?

I don’t come from a family of scientists; my father is an engineer and my mother is an administrative worker. But my father first got me interested in science. As an engineer, he was always solving something and he brought home some of the problems he was working. I worked with him and picked it up at an early age.

How are you adjusting to life in California?

Well, I like being outdoors, and here we have the mountains and the beach and it’s really amazing. The weather here is so much better than the other places I’ve lived. If you want to get the impression of what the weather in the Netherlands is like, you just replace the number of sunny days here with the number of rainy days there.

I wish Stevan Nadj-Perge good luck!

D-Wave upgrades Google’s quantum computing capabilities

Vancouver-based (more accurately, Burnaby-based) D-Wave systems has scored a coup as key customers have upgraded from a 512-qubit system to a system with over 1,000 qubits. (The technical breakthrough and concomitant interest from the business community was mentioned here in a June 26, 2015 posting.) As for the latest business breakthrough, here’s more from a Sept. 28, 2015 D-Wave press release,

D-Wave Systems Inc., the world’s first quantum computing company, announced that it has entered into a new agreement covering the installation of a succession of D-Wave systems located at NASA’s Ames Research Center in Moffett Field, California. This agreement supports collaboration among Google, NASA and USRA (Universities Space Research Association) that is dedicated to studying how quantum computing can advance artificial intelligence and machine learning, and the solution of difficult optimization problems. The new agreement enables Google and its partners to keep their D-Wave system at the state-of-the-art for up to seven years, with new generations of D-Wave systems to be installed at NASA Ames as they become available.

“The new agreement is the largest order in D-Wave’s history, and indicative of the importance of quantum computing in its evolution toward solving problems that are difficult for even the largest supercomputers,” said D-Wave CEO Vern Brownell. “We highly value the commitment that our partners have made to D-Wave and our technology, and are excited about the potential use of our systems for machine learning and complex optimization problems.”

Cade Wetz’s Sept. 28, 2015 article for Wired magazine provides some interesting observations about D-Wave computers along with some explanations of quantum computing (Note: Links have been removed),

Though the D-Wave machine is less powerful than many scientists hope quantum computers will one day be, the leap to 1000 qubits represents an exponential improvement in what the machine is capable of. What is it capable of? Google and its partners are still trying to figure that out. But Google has said it’s confident there are situations where the D-Wave can outperform today’s non-quantum machines, and scientists at the University of Southern California [USC] have published research suggesting that the D-Wave exhibits behavior beyond classical physics.

A quantum computer operates according to the principles of quantum mechanics, the physics of very small things, such as electrons and photons. In a classical computer, a transistor stores a single “bit” of information. If the transistor is “on,” it holds a 1, and if it’s “off,” it holds a 0. But in quantum computer, thanks to what’s called the superposition principle, information is held in a quantum system that can exist in two states at the same time. This “qubit” can store a 0 and 1 simultaneously.

Two qubits, then, can hold four values at any given time (00, 01, 10, and 11). And as you keep increasing the number of qubits, you exponentially increase the power of the system. The problem is that building a qubit is a extreme difficult thing. If you read information from a quantum system, it “decoheres.” Basically, it turns into a classical bit that houses only a single value.

D-Wave claims to have a found a solution to the decoherence problem and that appears to be borne out by the USC researchers. Still, it isn’t a general quantum computer (from Wetz’s article),

… researchers at USC say that the system appears to display a phenomenon called “quantum annealing” that suggests it’s truly operating in the quantum realm. Regardless, the D-Wave is not a general quantum computer—that is, it’s not a computer for just any task. But D-Wave says the machine is well-suited to “optimization” problems, where you’re facing many, many different ways forward and must pick the best option, and to machine learning, where computers teach themselves tasks by analyzing large amount of data.

It takes a lot of innovation before you make big strides forward and I think D-Wave is to be congratulated on producing what is to my knowledge the only commercially available form of quantum computing of any sort in the world.

ETA Oct. 6, 2015* at 1230 hours PST: Minutes after publishing about D-Wave I came across this item (h/t Quirks & Quarks twitter) about Australian researchers and their quantum computing breakthrough. From an Oct. 6, 2015 article by Hannah Francis for the Sydney (Australia) Morning Herald,

For decades scientists have been trying to turn quantum computing — which allows for multiple calculations to happen at once, making it immeasurably faster than standard computing — into a practical reality rather than a moonshot theory. Until now, they have largely relied on “exotic” materials to construct quantum computers, making them unsuitable for commercial production.

But researchers at the University of New South Wales have patented a new design, published in the scientific journal Nature on Tuesday, created specifically with computer industry manufacturing standards in mind and using affordable silicon, which is found in regular computer chips like those we use every day in smartphones or tablets.

“Our team at UNSW has just cleared a major hurdle to making quantum computing a reality,” the director of the university’s Australian National Fabrication Facility, Andrew Dzurak, the project’s leader, said.

“As well as demonstrating the first quantum logic gate in silicon, we’ve also designed and patented a way to scale this technology to millions of qubits using standard industrial manufacturing techniques to build the world’s first quantum processor chip.”

According to the article, the university is looking for industrial partners to help them exploit this breakthrough. Fisher’s article features an embedded video, as well as, more detail.

*It was Oct. 6, 2015 in Australia but Oct. 5, 2015 my side of the international date line.

ETA Oct. 6, 2015 (my side of the international date line): An Oct. 5, 2015 University of New South Wales news release on EurekAlert provides additional details.

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

A two-qubit logic gate in silicon by M. Veldhorst, C. H. Yang, J. C. C. Hwang, W. Huang,    J. P. Dehollain, J. T. Muhonen, S. Simmons, A. Laucht, F. E. Hudson, K. M. Itoh, A. Morello    & A. S. Dzurak. Nature (2015 doi:10.1038/nature15263 Published online 05 October 2015

This paper is behind a paywall.

What is a diamond worth?

A couple of diamond-related news items have crossed my path lately causing me to consider diamonds and their social implications. I’ll start first with the news items, according to an April 4, 2012 news item on physorg.com a quantum computer has been built inside a diamond (from the news item),

Diamonds are forever – or, at least, the effects of this diamond on quantum computing may be. A team that includes scientists from USC has built a quantum computer in a diamond, the first of its kind to include protection against “decoherence” – noise that prevents the computer from functioning properly.

I last mentioned decoherence in my July 21, 2011 posting about a joint (University of British Columbia, University of California at Santa Barbara and the University of Southern California) project on quantum computing.

According to the April 5, 2012 news item by Robert Perkins for the University of Southern California (USC),

The multinational team included USC professor Daniel Lidar and USC postdoctoral researcher Zhihui Wang, as well as researchers from the Delft University of Technology in the Netherlands, Iowa State University and the University of California, Santa Barbara. The findings were published today in Nature.

The team’s diamond quantum computer system featured two quantum bits, or qubits, made of subatomic particles.

As opposed to traditional computer bits, which can encode distinctly either a one or a zero, qubits can encode a one and a zero at the same time. This property, called superposition, along with the ability of quantum states to “tunnel” through energy barriers, some day will allow quantum computers to perform optimization calculations much faster than traditional computers.

Like all diamonds, the diamond used by the researchers has impurities – things other than carbon. The more impurities in a diamond, the less attractive it is as a piece of jewelry because it makes the crystal appear cloudy.

The team, however, utilized the impurities themselves.

A rogue nitrogen nucleus became the first qubit. In a second flaw sat an electron, which became the second qubit. (Though put more accurately, the “spin” of each of these subatomic particles was used as the qubit.)

Electrons are smaller than nuclei and perform computations much more quickly, but they also fall victim more quickly to decoherence. A qubit based on a nucleus, which is large, is much more stable but slower.

“A nucleus has a long decoherence time – in the milliseconds. You can think of it as very sluggish,” said Lidar, who holds appointments at the USC Viterbi School of Engineering and the USC Dornsife College of Letters, Arts and Sciences.

Though solid-state computing systems have existed before, this was the first to incorporate decoherence protection – using microwave pulses to continually switch the direction of the electron spin rotation.

“It’s a little like time travel,” Lidar said, because switching the direction of rotation time-reverses the inconsistencies in motion as the qubits move back to their original position.

Here’s an image I downloaded from the USC webpage hosting Perkins’s news item,

The diamond in the center measures 1 mm X 1 mm. Photo/Courtesy of Delft University of Technolgy/UC Santa Barbara

I’m not sure what they were trying to illustrate with the image but I thought it would provide an interesting contrast to the video which follows about the world’s first purely diamond ring,

I first came across this ring in Laura Hibberd’s March 22, 2012 piece for Huffington Post. For anyone who feels compelled to find out more about it, here’s the jeweller’s (Shawish) website.

What with the posting about Neal Stephenson and Diamond Age (aka, The Diamond Age Or A Young Lady’s Illustrated Primer; a novel that integrates nanotechnology into a story about the future and ubiquitous diamonds), a quantum computer in a diamond, and this ring, I’ve started to wonder about role diamonds will have in society. Will they be integrated into everyday objects or will they remain objects of desire? My guess is that the diamonds we create by manipulating carbon atoms will be considered everyday items while the ones which have been formed in the bowels of the earth will retain their status.

Darwin theme: Rap about Darwin & evolutionary biology and Darwinism in quantum dots

You wouldn’t expect someone with this pedigree,

… Professor of Microbial Genomics at the University of Birmingham since July 2001. … is dually qualified as a scientist (PhD) and as a medic/clinical bacteriologist (MBBS, MRCPath), and benefits from Research-Council funding for both bioinformatics and laboratory-based molecular bacteriology projects. His interests focus on bacterial pathogenesis and the exploitation of sequence data, particularly genome sequence data.

to commission a piece of rap music but that’s just what Professor Mark Pallen did last year to honour Darwin’s anniversary (150 years since the publication of Darwin’s theory and 200 years isnce his birth). He contacted Baba Brinkman, a Vancouver, Canada -based rap artist, to commission a series of raps about Darwin and evolutionary biology. The project has become The Rap Guide to Evolution. You can find more about the work at Pasco Phronesis (thank you for the pointer) which also features a number of Brinkman’s videos. There’s also a Brinkman ‘evolutionary’ video on the CBC 3 (Canadian Broadcasting Corporation) site. In that video, Brinkman spontaneously adds some lines to his rap. I don’t know if you’ve ever tried to improvise while you’re presenting for any length of time but it’s not easy and Brinkman actually manages to do this while rhyming about evolutionary biology in front of an audience that’s somewhere between 200 and 500 people (I can’t be sure of the number).

There are some places I don’t expect to see any mention of the theory of evolution and quantum theory is one of those places. From the news item on physorg.com,

Physicists have found new evidence that supports the theory of quantum Darwinism, the idea that the transition from the quantum to the classical world occurs due to a quantum form of natural selection. By explaining how the classical world emerges from the quantum world, quantum Darwinism could shed light on one of the most challenging questions in physics of the past century.

The basis of almost any theoretical quantum-to-classical transition lies in the concept of decoherence. In the quantum world, many possible quantum states “collapse” into a single state due to interactions with the environment. To quantum Darwinists, decoherence is a selection process, and the final, stable state is called a “pointer state.” Although pointer states are quantum states, they are “fit enough” to be transmitted through the environment without collapsing and can then make copies of themselves that can be observed on the macroscopic scale. Although everything in our world is quantum at its core, our classical view of the universe is ultimately determined by these pointer states.

How researchers have used quantum dots  to provide evidence of quantum Darwinism and the link from quantum physics to classical physics is covered in the rest of the news item. The researchers’ study is published here,

A.M. Burke. “Periodic Scarred States in Open Quantum Dots as Evidence of Quantum Darwinism.” Physical Review Letters 104, 176801 (2010). Doi:10.1103/PhysRevLett.104.176801