Latest Advances toward Quantum Computing, Part 2
January 24, 2014
In Part 1 of this series, I discussed, inter alia, how researchers are working to create more stable qubits. Philip Ball explains why stable qubits are important. “Quantum phenomena such as superpositions are generally very delicate,” he writes, “They get easily disrupted or destroyed by disturbances from the surrounding environment, particularly the randomizing effects of heat. So to make such states usually requires very low temperatures. This fragility of quantum effects means that, while the question of what you could do with a quantum computer has been explored extensively already by physicists and mathematicians, actually building a device that can do any of it is taxing electrical engineers and applied physicists to the limit.” [“Quantum computers: when, what, who and why,” homunculus, 8 December 2013]
The fragility of quantum computing systems is one reason that researchers are experimenting with a number of different materials, including graphene. Tina Casey reports, “One key to quantum computing, according to our friends over at Lawrence Berkeley National Laboratory, is to develop a ‘fault-tolerant’ material from an exotic class of materials called topological conductors, which have an insulating interior but are conductors on the surface.” [“Dirty Or Clean, Graphene Could Make A Nice Little Quantum Computer,” CleanTechnica, 23 December 2013] Lynn Yarris is one of the researchers involved in the field of fault-tolerant materials. He notes, “If the enormous potential of quantum computing is to be fully realized, scientists must learn to create ‘fault-tolerant’ quantum computers.” [“On the Road to Fault-Tolerant Quantum Computing,” Lawrence Berkeley National Laboratory News Center, 16 September 2013] Working with scientists in China, Yarris is concentrating on “interfacing a topological insulator, bismuth selenide, with a high temperature superconductor, BSCCO (bismuth strontium calcium copper oxide).” Casey reports, however, that scientists at MIT are working with another material, graphene.
Graphene is looking more and more like a wonder material. Casey calls it “the nanomaterial of the new millennium.” She continues:
“An international team of researchers based at MIT has figured out how to make the edges of the two-dimensional wonder material graphene behave like one-dimensional electronic wires. I know, right? To ice the cake, the edges don’t have to be perfectly formed. They can be irregular or ‘dirty’ and those electrons would still go zipping along in the right direction. In terms of quantum computers, that’s an important advantage for graphene. … Graphene is cheap compared to other materials with quantum computer potential but it is notoriously difficult to fabricate perfect examples in bulk, so if a measure of imperfection does not interfere with its efficiency, finding applications for it would be that much more likely.”
It turns out, Casey reports, that running electrons along the edges of graphene basically turns it into a kind of topological insulator. She explains:
“As described by MIT writer David Chandler, in order to get graphene to behave like a topological conductor, the research team subjected a flake of graphene to a 35-tesla magnetic field (think of an MRI machine, times ten) under a temperature of just 0.3 degrees Celsius [update: that’s 0.3 degrees Celsius above absolute zero, not just plain old 0.3 degrees Celsius]. Here’s what happened when they turned the field perpendicular to the flake, keeping in mind that normally graphene is a conductor throughout its structure:
…the behavior changes: Current flows only along the edge, while the bulk remains insulating. Moreover, this current flows only in one direction — clockwise or counterclockwise, depending on the orientation of the magnetic field — in a phenomenon known as the quantum Hall effect.
By exposing the flake to another magnetic field in the same plane, the researchers got electrons to move around the edges in different directions. Combine that with switchability (the edge states can be turned on and off at will), there you have the makings of atom-scale circuits and transistors.”
The problem, if there is one, is that the conditions described for using graphene as a topological conductor are pretty extreme (meaning any computer using that material would be expensive to build and maintain). Casey reports, however, that “the research team is already working on a system that requires less extreme conditions.”
Researchers at Yale University believe that “light might be able to play a bigger, more versatile role in the future of quantum computing.” [“In quantum computing, light may lead the way,” Yale News, 7 October 2013] The article explains:
“A team of Yale physicists has coaxed an unprecedented number of light particles, or photons, to behave quantum mechanically, or to assume more than one state simultaneously, such as ‘alive’ and ‘dead.’ In this case, the light is in the form of trapped microwave photons. Control over a greater number of photons — more than 100 in this case — raises the possibility that such states of light could play the part of several quantum bits (qubits), the building blocks typically found in a quantum computer. This could potentially minimize the physical scale and cost of building one. … The photon states generated in the Yale experiment mimic the metaphorical ‘Schrödinger’s cat,’ which describes the counterintuitive idea that objects we encounter every day should also exhibit the strange behaviors of quantum mechanics — a housecat that could be alive and dead at the same time, for example. In current quantum computing models, scientists typically describe systems built of many artificial quantum components known as qubits. Photons are a good tool for transferring information between qubits, but their ability to serve as qubits is limited, due to difficulty controlling them. The new research, led by Sterling Professor of Applied Physics and Physics Robert Schoelkopf, shows that large numbers of photons can be controlled with the help of a lone qubit. This suggests the possibility that a collection of photons may soon play the role of many qubits, potentially minimizing the cost and scale of quantum computing devices.”
Another research group, this one at the University of Sussex, also believes that microwaves will play an important role in building affordable quantum computers. Those scientists are “working to produce the world’s fastest, most powerful computers [by] creating a practical prototype using microwaves – by shielding the atoms driving this new generation of computers from the harmful effects of noise.” [“Scientists take a quieter step closer to first practical quantum computer,” US, October 2013] The article explains:
“A new generation of quantum computers is now being devised utilising microwaves, … which are easier to use and which should bring the construction of a large-scale ion-trap quantum-information processor much closer. But there is a problem. The quantum effects that give a quantum computer its tremendous power (such as quantum superposition, where a single object can be at two different places simultaneously) are easily destroyed by any external noise. Now, Dr Winfried Hensinger, along with postdoctoral fellow Simon Webster and PhD students Seb Weidt, Kim Lake and James McLoughlin, who form part of Dr Hensinger’s Sussex Ion Quantum Technology Group, have come up with an extremely efficient and easy way to shield the quantum computer from external noise, effectively enabling large-scale operation of a microwave quantum computer. … By applying a special combination of microwaves and radio frequency fields, the team were able to modify the atoms so that they became more resilient to external noise.”
Another breakthrough using light was recently made by researchers in Australia and Japan. They managed to create ten thousand entangled modes using lasers. [“Ten thousand entangles modes,” by Brian Wang, Next Big Future, 4 December 2013] The article reports:
“Australia National University PhD student Seiji Armstrong has made a quantum leap towards next-generation computing. Working with a team in Tokyo, Seiji has created the largest cluster of quantum systems ever – a milestone on the way to super-powerful, super-fast quantum computers. ‘The more quantum systems you have in the cluster, the more powerful your quantum computer will be,’ he says. ‘Previously the world record was 14. But in our experiment we went to more than 10,000 at once.’ Each quantum system can encode a quantum ‘bit’ of information, like the binary system that a traditional computer uses, explains Seiji. The researchers employed a split laser that contained all 10,000 individually addressable quantum wave packets — photons, essentially. Each photon in the system has an entangled partner in the other half of the beam, which makes it theoretically easier to take measurements. This experimental setup allowed the team to more easily entangle large numbers of quantum bits, which is one of the necessary elements of a quantum computer.”
As evidenced by the MIT research discussed above, magnetism is another force that is being explored for use in quantum computing. European researchers are exploring how magnetism can be used. to manipulate atoms. [“A fresh step towards quantum computing,” by Irati Kortabitarte, Basque Research, 19 November 2013]. Kortabitarte reports:
“One of the aims of the Ikerbasque researcher José Ignacio Pascual of nanoGUNE and his collaborators at the Free University of Berlin is to explore under which conditions one can write and read information by manipulating the magnetism of atoms. By studying the behaviour of tiny magnetic molecules in contact with a superconductor surface they have shown that it is possible to find a regime in which the superconductivity of the surface ‘assists’ the magnetism of the atom and facilitate the processes of writing and reading its magnetic state. The study is contributing towards the ultimate goal of computation with individual atoms, as it shows that it is possible to manipulate the magnetic state in which an atom finds itself and make the latter last long enough to be ‘read’.”
In another recent breakthrough, “scientists at IBM Research have demonstrated a complex quantum mechanical phenomenon known as Bose-Einstein condensation (BEC), using a luminescent polymer (plastic) similar to the materials in light emitting displays used in many of today’s smartphones.” [“IBM Scientists Demonstrate Quantum Phenomenon for the First Time Using a Plastic Film,” by John Galvez, Hispanic Business, 11 December 2013] Galvez explains the importance of this achievement:
“This discovery has potential applications in developing novel optoelectronic devices including energy-efficient lasers and ultra-fast optical switches — critical components for powering future computer systems to process massive Big Data workloads. The use of a polymer material and the observation of BEC at room temperature provides substantial advantages in terms of applicability and cost. … The phenomenon only lasts for a few picoseconds (one trillionth of a second), but the scientists believe this is already long enough to use the bosons to create a source of laser-like light and/or an optical switch for future optical interconnects. These components are important building blocks to control the flow of information in the form of zeroes and ones between future chips and can significantly speed up their performance while using much less energy.”
Taken individually, each of the breakthroughs discussed above is just a small step towards building a quantum computer. Most scientists believe that such a computer won’t be fully developed for another 30 years. Nevertheless, each small step is brings us that much closer to the day when quantum computers will be used to solve some very interesting problems.